CA1157968A - Device for determining the density distribution in an object - Google Patents
Device for determining the density distribution in an objectInfo
- Publication number
- CA1157968A CA1157968A CA000363596A CA363596A CA1157968A CA 1157968 A CA1157968 A CA 1157968A CA 000363596 A CA000363596 A CA 000363596A CA 363596 A CA363596 A CA 363596A CA 1157968 A CA1157968 A CA 1157968A
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- Canada
- Prior art keywords
- primary beam
- detector
- radiation
- primary
- zone
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
- A61B6/48—Diagnostic techniques
- A61B6/483—Diagnostic techniques involving scattered radiation
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
- A61B6/06—Diaphragms
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
- A61B6/58—Testing, adjusting or calibrating apparatus or devices for radiation diagnosis
- A61B6/582—Calibration
- A61B6/583—Calibration using calibration phantoms
Abstract
PHD.79.129 11.6.80 "Device for determining the density distribution in an object" In a device for determining the density distribution on a straight line by means of a narrow penetrating beam, the measuring accuracy for the centre of an object to be examined is increased in that the primary beam is not only displaced perpendicularly to its direction, but is also rotated around a point in this centre. To this end, a radiation source and a detector device are mounted on a supporting device which can rotate the path of the primary beam around a central point, preferably the centre of the object, around an axis which intersects the path of the primary beam at right angles.
Description
1 ~.S7~68 P~lD.79.12g The invention relates to a device for deter-minlng the density distribution in an object, compris-ing at least one radiation source for generating a narrow primary beam which irradiates the object, a detector device which is arranged adjacent the primary beam for detecting scattered radiation produced by the primary beam, a diaphragm device which is arranged between the detector device and the object and which is shaped so that the detector device measures a set of scattered radiation values from the path of the primary beam through the object, and a drive device for the displacement of the path of the primary beam through the object.
A device of this kind is known from our Canadian Patent 1,101,133 which issued on May 12, 1981.
The diaphragm device thereof comprises a slit-shaped aperture, the principal dimension of which extends in a direction approximately perpendicularly to the prim-ary beam. The detector device comprises a series of detectors which are arranged behind the diaphragm device.
Each detector can be struck only by the scattered radi-ation which is generated in a given part of the primary beam, so that a given part of the object is associated with each detector. The detectors together measure the path of the primary beam in the object. The invention can also be used in devices where the diaphragm device is a multi-channel diaphragm. Instead of a detector device consisting of many separate detectors, use can also be made of a different type of location-sensitive detection device, for example, a gamma camera or an X-ray image intensifier. It is only important that an unambiguous spatial relationship exists between parts of the path of the primary 1 15796~
PHD.79.129 2 11.6.80 beam on the one side and separately readable parts of the detector device on the other side.
For a given path of the primary beam through the object to be examined, only the scattered radiation S in this path can be measured. For determining the scattered radiation in a slice of the object, therefore, it is necessary to change the path of the primary beam with respect to the object. This is done transversely of the primary beam direction in all known devices.
The invention is based on the following consi-derations: in this type of device, on the one hand the primary radiation and on the other hand the scattered radia-tion produced in the body is attenuated by photo~
absorption or by Compton dispersion. This attenuation is l~ greater as the distance to be travelled by the primary beam until it reaches -the point wherefrom the scattered beam to be measured departs is greater, and as the dis-tance to be travelled through the body by the scattered radiation generated in this point is greater. In general, therefore, the intensity of the scattered radiation reaching -the detector device is lowest when it originates from the centre of a body to be examined. However, the lower the radiation in-tensity measured by the detector device, the greater the inaccuracy of the measured values will be, so that the density distribution in the centre of the body to be examined can be recons-tructed only comparatively inaccurately. However, -the zones which are interesting for the di~osis are often situated in the centre of the body.
In order to increase the accuracy, the intensity of the primary beam can be increased. However, this increases the radiation dose applied -to the patient, that is to say to the same extent in all areas of -the body, withou-t the accuracy being substantially increased in the outer zones of the body where the primary radia-tion and/or -the scattered radiation is only comparatively little attenua-ted.
l 157968 PHD.79.129 3 11.6.80 The accuracy of the measurements of the central zone can also be impro~ed by making the primary beam pass through the centre in different direc-tions, without its intensity being increased, and by summing the measuring values obtained in the overlapping zone of the beams, the sum value thus formed being multiplied by a factor which is proportional to the coverage of density in this zone; this results in a mean value of the measuring values determined in this zone. Even though in addition to the accuracy the dose in the centre of the body is also increased, the dose in the outer zones of the body is not substantially increased.
In view of these considerations, the invention has for its object to construct a device of the described kind so that the centre of the object is irradiated more often by the primary beam than the outer zones.
This object is achieved in that the radiation source and the diaphragm device are mounted on a suppor-ting device which leaves an object space free and which is rotatable around an axis which intersects the path of the primary beam and which is directed transversely there-of.
In accordance with the invention, a circular examination zone is scanned by means of a primary beam which is directed onto the centre thereof (the axis of rotation of the supporting device). During the rotation of the supporting body, the radiation source is rotated along an arc of a circle around the cen-tre of the examina-tion zone so that, ignoring the attenuation of the radiation in the object, a concentration is obtained which increases in the direction of the centre. Together with the intensity attenuation, an approximately uniform concentra-tion is then obtained~ ~ecause the accuracy with which a scattered radiation coefficient can be calculated is dependent of the measured number of photons dispersed in this point, the device in accordance with the invention offers a substantially higher image quality 1 1 5~6.~
PHD.79.129 4 for the centre of the object than the known devices where displacement always takes place perpendicularly to the direction of the primary beam.
It is to be noted that our Canadian Patent 1/134,066 which issued on October 19, 1982 discloses a device for determining internal body structures by means of scattered radiation which corresponds to the so-called transmission computer tomography apparatus, however, therein the scattered radiation produced along the stopped primary beam is also measured by means of two hollow cylindrical detectors which are arranged one on each side of the examination plane and which enclose the body being examined. The hollow cylindrical detectors for measuring the scattered radiation, however, can meas-ure only the scattered radiation generated by the primarybeam throughout the examination zone, so not the scattered radiation which is generated by individual points struck by the primary beam. Moreover, therein the primary beam is displaced perpendicularly to its direction and the device is rotated through a small angular increment only after the complete examination zone has thus been scanned.
Ignoring the intensity attenuation by the body being examined, the concentration of primary radiation in the examination zone is homogeneous, which is contrary to the invention where the concentration is higher in the centre.
In a further embodiment in accordance with the invention, the detector device is also mounted on the supporting device. In all positions of the supporting device, each element of the detector device measures only the scattered radiation originating from a given zone of the primary beam. Thanks to this unambiguous spatial assignment and the fact that this zone describes a circle around the axis of rotation when the supporting body is rotated, each detector measures the scattered radiation in an arc of a circ~e around the axis of rotation.
In a further embodiment in accordance with the 1 15796~
PHD.79.129 5 11.6.80 invention, the detector device forms a s-tationary arc of a circle which is concentric to the axis of rotation of the supporting device. Between this embodiment and the previously described embodiment there is a difference which is analogous to the difference between a transmission computer tomography apparatus of -the fourth generation and one of the third generation. The detector device in general re~uires a larger number of detector elements than the previously described embodiment. Therefore, the assignment of a detector element to the primary beam changes as the supporting device is rota-ted, so that the scattered radiation generated in a given zone of -the primary beam is each t~me measured by a different element of the detector device in each rotary position of the i5 supporting device. Differences in sensitivity of the various elements of the detector device, therefore~ have only a small effect on the reconstruction of the density distribution. Ring-shaped artefacts which are liable to occur in the former embodiment when the sensitivity of a detector element deviates from that of the other detect~ elements are not intensified.
The accuracy and the response sensitivity can be further increased by arranging different diaphragm devices on both sidQs of the primary beam, each elemen-t of the detector devices then measuring the scattered radiation generated in a given zone of the primary beam through said diaphragm devices. For example, when two diaphragm devices are arranged one on each side of the primary beam, the scattered radiation produced in a given zone of the primary beam is measured by a total of four elements of the detector device. The four measuring values are added in order -to form a signal which corres-ponds to the density in this zone.
As is known, devices of the described kind, including the device in accordance with the invention, enable a simpler reconstruction of the density dis-tribll-tion in the slice than so-called -transmission computer ~ 157968 PHD.79.129 6 11.6.80 tomography apparatus, because each detector already measures the density generated in a given ~int in a given zone, rather than the line integral of the density over the relevant radiation path like in transmission compu-ter tomography apparatus. On the other hand, the reconstructed density distribution gives only a qualitative result if neither the attenuation of the primary beam up to the point in which the scattered radiation is generated nor -the attenuation of the scattered radiation on its way to the relevant detector element is -taken into account. This can in principle be realized by means of a suitable computer program in which given assumptions as regards the mean density and the dimensions of the object to be examined are made. Obviously, the density distributions thus obtained can be further improved by suitable itera-tion methods, but they generally cannot be accurate in view of these eaoh assumptions.
A more accurate determination of the density distribution is possible in a further embodiment in accor-dance with the invention which comprises an X-ray source whose radiation can be stopped by a collimator so that it passes through the total examination zone and is measured by a detector device which is arranged behind the examination zone. Therein, the X-ray source forms part, together with said detector deviceg of a computer tomo- -graphy apparatus of the third or fourth generation (depending on whether the detector device is moved or not), which is capable of determining the absorp-tion distribu-tion in the examination plane by means of a computer.
The additional costs are low, because the detector device and the mechanism for rota-ting the source around the examination zone (supporting device) are present anyway.
From the absorption density distribu-tion thus 3 obtained the attenuation can be calculated for each individual measuring value, because the paths of the prima-ry beam up to the scat-ter point and from the sca-tter point to the relevant detec-tor elemen-t are fixed. It is merely PllD.79.129 7 11.6.80 necessary to add the attenuation coefficients obtained along these paths, so that the total attenuation can be calculated. The values measured by the individual detector elements must then be weighted by a weighting factor which is proportional to the attenuation factor thus calculated for the total beam path. The dose applied to the patient during such a determination of the absorp-tion distribution can be substantially smaller than in transmission computer tomography apparatus. The absorp-tion coefficients measured in the individual points are 1ess accurate, but these reconstruction errors do not exert their full effect, because for the determination of the attenuation of a primary or scattered beam on its way through the examination zone -the attenuation coeffi-cients of many points have to be added and errors are at least partly eliminated by averaging. ~oreover, the X-ray source can efficiently operate with the same high voltage (for example, 350 kV) such as during the determination of the density distribution, so that the dose is also reduced on the basis of this fact.
However, in order to obtain the absorption distribution it is also possible to operate the X-ray source with the same tube voltage and the same inte~sity as customarily used for transmission computer tomography~
On the basis of the absorption distribution thus obtained, the density distribution can be accurately reconstructed.
Using the density distributions thus obtained, the scatter contributions to the transmission computer tomo-gram (the absorption in an individual point of the transmission computer tomogram is composed of an attenua-tion component by photoabsorption and an attenuation component by scattering) can be determined and subtracted from the values obtained; this resul-ts in the photo-absorption. At the end of these calculations ? two density distributions are then obtained: on the one hancl the density distribution due to radiation scattering in the examina-tion zone and on the other hand the photoabsorption 1 1579~8 PHD.79.129 8 11.6.80 therein. The radiologist can derive extra information therefrom.
The invention will be described in detail herein-after with r~ference to the accompanying diagrammatic drawing.
~igure 1 shows first embodiment in accordance with the invention, comprising two oppositely arranged ~-ray sources and detector devices mounted on the suppor-ting device, 13 Figure 2 shows a similar device, which, however, offers a better probability of detection of scattered radiation, Figure 3a shows a device whereby at the same time a transmission computer tomogram can be made, lS Figure 3b shows this device during the measure-ment required for the transmission computer tomogram, Figure 4 shows a device in which the elements of the detector device are arranged to be stationary on an arc of a circle around the axis of rotation, 2D Figure 5 shows a corresponding device which, however, is also suitable for making transmission computer tomograms, and Figure 6 shows a device for recons-tructing the density distribution on the basis of the measuring values ZS
obtained.
The reference numeral 10Z in Figure 1 denotes a sta-tionary housing in which a supporting device 10 is journalled to be rotatable around an axis 3 by way of roller bearings 101. The drive units for rotating the supporting device 10 are no-t shown. The supporting device comprises an aperture 4 which defines the examination zone within which the scattered radiation can be deter-mined. On an examination table 2 inside this examination zone there is arranged the patient body 1 in a given slice of which the density distribution has -to be deter-mined. To this end, on either side of the examination ~one which is concentricall~ situated with respect to the l 1579~8 PHD.79.129 9 axis of rotation 3, there are arranged X-ray sources 51 and 52, in front of which there are arranged collimators 61 and 62 which stop registering primary beams 31 which intersect the axis of rotation.
In front of each of the two X-ray sources there is arranged a transmission detector 71, 72 which is pro-vided with holes (not shown~ in order to allow unobstruc-ted passage of the primary beam from its relevant radi-ation source, whilst the primary beam from the oppositely situated radiation source, attenuated by the examination zone, can be detected as a result of the dispersion inside the body. Using these detectors, any assumptions concerning the attenuation inside the body can be cor-rected as described in our Canadian Patent 1,101,133 which issued on May 12, 1981.
On either side of the primary beam 31 and out-side the examination zone 4 on the rotatable supporting device 10 there are provided two detector devices 91 and 92 which consist of a large number of adjacently arranged detector elements whose largest dimension extends in the direction perpendicular to the plane of the drawing, as described in our Canadian Patent 1,101,133. Between each of the two detector devices 91 and 92 and the exam-ination zone 4 there is arranged a slit-like diaphragm 81, 82 which provides an unambiguous spatial assignment of a point or zone, for example, the zone 11, of the primary beam to the detector device 91, 92, so that the scattered radiation generated in this point 11 of the primary beam is measured by the detector elements which are present at the locations 93 and 94. The scattered radiation generated at other locations in the primary beam within the examination zone 4 is measured by other elements of the detector device 91, 92, for example, like in the device described in our Canadian Patent 1,101,133.
In the latter device the primary beam is dis-placed perpendicularly to its direction with respect to ~ 1$798.~
PHD.79.129 10 11.6.80 the examination zone in order to determine the density distribution in a slice; however, in the present case the complete slice to be examined is covered in that the supporting device 10 is first rotated through a small angular increment, after which the two detector devices record new sets of measuring values, after which a further rotation through a small angular increment takes place etc. until the device has been rotated -through a total angle of 180 . The angular increments are chosen so that the complete contour is also completely co~ered by the various primary beams. The primary beam remains directed onto the axis of rotation 3.
It is alternatively possible to use only a single detector and diaphragm device; however, the proba-bility of detection is then lower if the detector sur-faces are not increased. Instead of using two X-ray sources, it is alternatively possible to use only a single source. However, for a complete measurement a rotation of the supporting device through 360 is then required in order to expose each point on the contour of the examination zone 4 once to the unattenuated primary beam.
Figure 2 shows a part of a further embodiment in which two detector systems are arranged on each side of the primary beam 31. For the sake of eimplicity, the two detector systems to the left of -the primary beam 31 and the two X-ray sources generating the primary beam are not shown. The two detector devices are advantageously combined to form a single detector device 95, and a diaphragm device 89, comprising two slits 85 and 86, is arranged between each detector device and -the examination zone. The slits are proportioned and situated so that the lower part of the elements of the detector device can be struck by scattered radiation which is generated in the primary beam 31 and which passes through the slit 85, whilst the upper part of the elements of the detector device can be s-truck by scat-tered radiation ~hich is 1 157gg8 PHD.79.129 11 11.6.80 generated in the primary beam within the examination zone and which passes through the slit 860 No element of the detector device canthen measure radiation which passes through the slit 86 as well as through the slit 85 and which originates from the primary beam within the examination zone 4.
This embodiment of the detector and diaphragm device increases the probabili-ty of detection, thus improving the reconstruction accuracy because the scatte-red radiation from each point on the primary beam 31 can be measured by two different detector elements, as is shown in Figure 2 for the scattered beams 96 and 97 which originate from the point 11 on the primary beam 31 and which pass through the slits 85 and 86. ~oreover, the two diaphragm and detector devices complement each other, i.e. the scattered radiation originating from the lower left half of the primary beam in Figure 2 is measured better by the detector elements associated with the slit 85, whilst scattered radia-tion from the upper right half of the primary beam 31 is measured better by the elements associated with the slit 86. This is because the scattered radiation produced in the lower left half of the primary beam 31 is attenuated less when it passes through the slit 85, whilst the scattered radiation produced in the upper right half of the primary beam is attenuated less when it passes through the upper slit 86; for example, the scattered beam 97, being produced in the point 11 in the upper right half of the primary beam 31 and passing through the slit 86, is attenuation less by -the body I than the scattered beam 96 which is produced in the same point and which passes through the slit 85.
The diaphragm device 89 in figure 2 comprises a further slit 87 which, however, is covered by a shield ~J
88 during normal operation. When the cross-section of the body 1 is so small -tllat it is situated within the circle 41 which is concentric with respect to the axis ~ 1~7~6~
PIID.79.129 12 11.6.80 of rotation 3 and whose diameter is smaller than the diameter of the examination zone 4, the shield 88 can be moved out of the beam path by means of a drive unit (not shown). A further scattered radiation path 8g is then released, thus increasing the probability of detection of scattered radiation and hence the measuring speed.
The circle 41 is proportioned so that each element of the detector device can "see" each time only one point on the primary beam within the circle 41 through the slits 85, 86 and 87, so that disturbing superposition of scattered radiation which can reach a detector element via at least two different scattered radiation beam paths, is precluded.
Figure 3a also shows a device comprising two X-ray sources 51 and 52 which are arranged on the supporting device 10 with collimators 61 and 62 which are arranged in front ~ the sources and wherethrough the primary beam 31 passes. On both sides of the primary beam two detector devices are arranged again. However, whilst on the left half the elements of the two detector devices 92 adjoin directly and are covered by a common diaphragm device 82 comprising two slits, the detector devices 91 and 93 are separated on the right side and each detector device comprises a slit diaphragm device 81, 83. For example, the scattered radiation produced in the point ~1 reaches, along the lines 94, 95, 96 and 97, an element in the different detector devices. Between the two detec-tors and diaphragm devices 81, 91 and 83, 93 there is arranged an X-ray source 53, i.e. opposite the detector device 92. In fron-t of the source there is provided a collimator 63 which is shaped so tha-t the radiation generated by the source 53 passes through the examination zone 4 in the same plane as the primary beam 31.
Figure 3a shows the device in an examination phase where the primary beam 31 passes through the examination zone 4 in order to determine the density distribution, and Figure 3_ shows the phase in which the ~ 157968 PHD.79.129 13 11.6.80 X-rays generated by the source 53 irradiate the complete ex~mination zone between -the rays 99 in order to determine the attenuation distribution within the examination zone 4. In this phase, a drive and displacement device (not shown) displaces the slit diaphragm device 82 perpendicu-larly to the plane of examination, so that the elements of the detector device 92 can measure the radiation beam of the radiation source 53.
The two examination phases shown in the Figures 3a and 3b may be performed in direct succession, the tube voltage which is initially applied to the X-ray sources 51 and 53 for at least one half revolution, subsequently being switched over to the X-ray source 53 until the attenuation distribution within the examination zone 4 has been comple-tely measured by means of the detector device.
It will be clear tha~ the cost of the additional execution of transmission computer tomography are compara-tively low, because the detector device 9 2 for measuring the transmission measuring values is already required for the scattered radiation measurement and so is the rotatable supporting device.
Figure ~ shows an embodiment which differs from the embodiment described thus far in that the detector device is connected to the housing 102 and is constructed as a stationary circle of separate detector elements which concentrically encloses the axis of rotation 3 and the supporting device 10. Only the X-ray source 51 with the collimator 61 for the formation of -the primary beam 31 and the diaphragm device 80, comprising a total of seven slits, are arranged on the supporting device 10 which in this embodiment, comprising only one X-ray source, has to be rotated through 360 . The primary beam 31 passes through a slit which is situated diametri-cally opposite the X-ray source 52, so that the element each time presen-t behind this slit can measure the attenuation of the primary beam 31. The six other slits ~ 1. 5 7~ G ~
PHD.79.129 14 11.6.80 in the diaphragm device 80 are distributed on both sides of the primary beam so -that in each angular position of the supporting device 10, each element of the detector device can be reached through a slit only by scattered radiation which is generated within the examination zone 4 and in the primary beam 31. Figure 4 again shows the six beam paths 901 to 906 along which the scattered radiation generated in the point 11 reaches six different elements of the detector device.
The embodiment shown in Figure 4 usually requires more detector elements than the previously described embodiments, but these elements need not be moved. Moreover, a detector element is not permanently assigned to an arc of a circle around the axis of rotation 3; this means on the one hand that ring-like artefacts which are liable to occur in the described devices when the sensitivi-ty of a detector element deviates from that of the other elements, cannot occur; on the other hand, it also means that for the determination of the density distribution on an arc of a circle around the axis of rotation 3, the measuring values of a large number of different detector elements must be taken into account. Moreover, the journalling elements for the supporting device 10 (not shown in Figure 4) must be arranged so that they do not shield the-scattered radiation paths to the detector elements.
Figure 5_ shows an embodiment comprising stationary detector elements which also enables measure~
ment of transmission measuring values. Like in Figure 4, the detector device which consists of a stationary ring of detector elernents which is concentric to the axis of rotation 3 is mounted on the supporting frame of the device (not shol~n). Like in ~igure 2, the diaphragm device 80 is shaped so that the radiation beam wllich co-vers the entire examination zone 4 between the ex-treme rays 32 and 33 is not influenced by the diaphragm device 80.
1 157~6~
PHD.79.129 15 11.6.80 In fron-t of the radiation source 51 there is arranged a collimator plate which is displaceable perpen-dicularly to the plane of the drawing and which comprises two apertures 62 and 63 which are situated perpendicularly one above the other (see Figure ~b). When the opening aperture 62 is slid into the examination plane 40 ~ a narrow collimated primary beam 31 is stopped and the density distribu-tion can be measured. However, when the aperture 63 is slid into the examination plane 40, a fan-shaped radiation beam with the extreme rays 32 and 33 is stopped and transmission measurements can be performed, The measurements can be consecutively performed, i.e. during a rotation through 360 first the density distribution is determined and then the attenuation distribution, or vice versa. However, during operation, that is to say during the rotation of the supporting device 10, it is alternatively possible to slide the diaphragm 61 quickly to and fro on -the stationary rails 64, extending perpendicularly to the plane of the drawing of Figure 5a, so that the primary beam 31 and the radia-tion beam with the outer rays 32 and 33 are alternately emitted. It is thus ensured that the scattered radiation and the transmission radiation are measured in neighbou-ring angular positions of the supporting device 10 25without substantial delay.
In the device shown in Figure 5a, the additional cost for executing the computer tomography are only for the collimator plate 61 which is displaceable perpendi-cularly to the plane of examination.
Figure 6 shows a device for reconstructing the density distribution from measuring values obtained by means of a device as shown in the Figures 3a, 3b or 5a, 5b, comprising a memory 100 whereto the scat-tered radiation 3 measuring values are applied. rhese measuring values are weighted with different weighting factors in the arithme-tic unit 400. It is thus taken into account that for a point in the cen-tre of -tt-le examina-tion ~one a substantial-1 157~
PHD.7~.129 16 1 1 .6.~o ly larger number of measuring values is present than for a point outside the centre, so that the measuring values for a point in the centre must be weighted with a corres-pondingly lower factor before or after the summing.
For example, if it is assumed that the number z of scanning directions and the dimensions of the primary beam are selected so that the primary beams cover each location on the periphery of the examination zone 4 exact-ly once, and if it is also assumed that the density in lG a polar coordinate system is to be reconstructed in points which are situated on straight lines through the centre whose angular positions correspond -to those of the primary beam with respect to the examination zone, the distance a between the cen-tres of two adjacent points on a straight line corresponding to the width of the primary beam, the weighting factors are formed as follows:
The measuring values for the point in the centre, or the sum thereof, are weighted by a weighting fact~r l/z, because the primary beam passes through this point in all z angular posi-tions. For a poin-t outside the centre, the measuring values, or the sum thereof, are weighted by a factor n/N,Nbeing z/2jl and n being an integer value which indicates how many times the distance between -the centre 3 of the examination zone and the centre of the relevant point is larger than the distance a between two adjacent points.
The weighting factor distribution is thus formed for a polar coordinate system. However, if desired, after application of known transformation rules, the weighting factors for points in a cartesian coordinate system can be calculated therefrom.
Simultaneously with this weighting factor, other weighting fac-tors which are dependent of the geometry of the device, for example, the angular dependency of the scattered radiation~ and the differen-t de-tector sensitivities can be taken into account. The resultant weighting factors are stored in the memory 3OO.
1 15796~
PHD.79.129 17 11.6.80 The reference numeral 200 deno-tes a sorting unit which, in the case of stationary detectors (Figure 5a), sorts the measuring values of permanently stored coordinate transformations so as if the measuring values were measured by rotating detector elements (like in Figure 3a). In a further arithmetic unit 150, the values thus weighted are weighted by a factor which corresponds to the at-tenuation of the radiation by the body. The higher the attenuation, the larger the weighting factor will be.
These values are derived from the transmission computer tomogram. To this end, the transmission measuring values are stored for the time being in the memory 110 and the attenuation density distribution in the examina-tion zone is reconstructed therefrom in the arithmetic unit 180. For each separate point in the examination zone it can be calculated to what extent the primary radiation has been attenuated on its way to this point and how much the scattered radiation has been attenua~ed in order to reach a given detector element from this point. The attenuation coefficient is the line integral over the attenuation coefficients along the primary beam as far as the relevant point and further over the relevan-t scattered radiation path to the detector element. These values are calculated in one point in an arithmetic unit 130 and are intermediately stored in the memory 140 and used for the weighting of the values, supplied by the arithmetic unit 400, in the arithmetic unit 150. From the values supplied by the arithmetic unit 150, the density distribution is calculated in the reconstruction unit 400, each element of the detec-tor device, assumed to have rotated along, being associated with a concen-tric circle in the reconstruction plane, and each measuring value on a poin-t of this circle being -transferred under a pole angle whicl-l corresponds to tlle angle of ro-tation with which this measuring value has been obtained. ~fter completion of these calculations, the image is s-tored in ~ 15796~
PHD.79.129 18 11.6.80 the memory 600 and displayed on a display apparatus 700.
The weighting factors corresponding to the relevant attenuation cannot only be determined by measure-ment by means of a transmission computer tomogram, but also by way of attenuation measurements which are performed on a suitable phantom. For different body cross-sections of patients of different size, use must be made of diffe-rent phantoms whereto each time a set of weighting factors must be assigned in the memory 140. This enables only an approximate determination of the relevant weight-ing factor, which is better as the selected phantom cross-section corresponds better to the body cross-section examined.
It is in principle also possible to determine the attenuation of the scattered radiation, or the weighting factors taking into account this attenuation, by calculation. To this end, first the contour of the body slice to be examined is determined by comparing each measuring value with a threshold value which corresponds approximately to the scatter coefficient of water or which is slightly lower. When a measuring value is lower than the threshold value, it is assigned to the part of the examination zone situated outside the body (air);
if it is larger, it is assigned to the body slice.
The slice of the body thus obtained is first assigned a homogeneous density distribution, this density being assumed to be equal to that of water. This is a suitable approximation of the actual conditions, because a human body largely consists of water. The attenuation of the primary beam occurring for -the indicated density distribution is calculated for the individual scanning directions and is compared with the attenuation values resulting from the measuring values from the elements 71 and 72 (Figure 1). The difference is used for correcting the assumed density distribution.
From the density distribution -thus corrected, tlle a-ttenuation of -the beam along the primary beam path 1 1579B~
PHD.79.129 19 11.6.80 to the scatter point (for exampleg 11, ~igure 2) and therefrom to the detector via the scattered beam path (for example, 97) is determined. This calculation is successively performed for all scanning directions or angular positions and all detector positions, and the results are stored. Using the beam attenuation values stored, being each time the reciprocal values of the weigh-ting factors, the measured scattered radiation measuring values are corrected. Therefrom a corrected density distribution can be determined.
The measured density distribution can be further improved by comparing the density values deter-mined for the individual points again with the scatter coefficient of water in order to determine not only the contour of the body, like in the previously described comparison cycle, but also areas inside the body whose scatter coefficient deviates substantially from that of water (for example, air or bone). Subsequently, the described cycle is completed again; if neces~ary, it may be repeated again.
A device of this kind is known from our Canadian Patent 1,101,133 which issued on May 12, 1981.
The diaphragm device thereof comprises a slit-shaped aperture, the principal dimension of which extends in a direction approximately perpendicularly to the prim-ary beam. The detector device comprises a series of detectors which are arranged behind the diaphragm device.
Each detector can be struck only by the scattered radi-ation which is generated in a given part of the primary beam, so that a given part of the object is associated with each detector. The detectors together measure the path of the primary beam in the object. The invention can also be used in devices where the diaphragm device is a multi-channel diaphragm. Instead of a detector device consisting of many separate detectors, use can also be made of a different type of location-sensitive detection device, for example, a gamma camera or an X-ray image intensifier. It is only important that an unambiguous spatial relationship exists between parts of the path of the primary 1 15796~
PHD.79.129 2 11.6.80 beam on the one side and separately readable parts of the detector device on the other side.
For a given path of the primary beam through the object to be examined, only the scattered radiation S in this path can be measured. For determining the scattered radiation in a slice of the object, therefore, it is necessary to change the path of the primary beam with respect to the object. This is done transversely of the primary beam direction in all known devices.
The invention is based on the following consi-derations: in this type of device, on the one hand the primary radiation and on the other hand the scattered radia-tion produced in the body is attenuated by photo~
absorption or by Compton dispersion. This attenuation is l~ greater as the distance to be travelled by the primary beam until it reaches -the point wherefrom the scattered beam to be measured departs is greater, and as the dis-tance to be travelled through the body by the scattered radiation generated in this point is greater. In general, therefore, the intensity of the scattered radiation reaching -the detector device is lowest when it originates from the centre of a body to be examined. However, the lower the radiation in-tensity measured by the detector device, the greater the inaccuracy of the measured values will be, so that the density distribution in the centre of the body to be examined can be recons-tructed only comparatively inaccurately. However, -the zones which are interesting for the di~osis are often situated in the centre of the body.
In order to increase the accuracy, the intensity of the primary beam can be increased. However, this increases the radiation dose applied -to the patient, that is to say to the same extent in all areas of -the body, withou-t the accuracy being substantially increased in the outer zones of the body where the primary radia-tion and/or -the scattered radiation is only comparatively little attenua-ted.
l 157968 PHD.79.129 3 11.6.80 The accuracy of the measurements of the central zone can also be impro~ed by making the primary beam pass through the centre in different direc-tions, without its intensity being increased, and by summing the measuring values obtained in the overlapping zone of the beams, the sum value thus formed being multiplied by a factor which is proportional to the coverage of density in this zone; this results in a mean value of the measuring values determined in this zone. Even though in addition to the accuracy the dose in the centre of the body is also increased, the dose in the outer zones of the body is not substantially increased.
In view of these considerations, the invention has for its object to construct a device of the described kind so that the centre of the object is irradiated more often by the primary beam than the outer zones.
This object is achieved in that the radiation source and the diaphragm device are mounted on a suppor-ting device which leaves an object space free and which is rotatable around an axis which intersects the path of the primary beam and which is directed transversely there-of.
In accordance with the invention, a circular examination zone is scanned by means of a primary beam which is directed onto the centre thereof (the axis of rotation of the supporting device). During the rotation of the supporting body, the radiation source is rotated along an arc of a circle around the cen-tre of the examina-tion zone so that, ignoring the attenuation of the radiation in the object, a concentration is obtained which increases in the direction of the centre. Together with the intensity attenuation, an approximately uniform concentra-tion is then obtained~ ~ecause the accuracy with which a scattered radiation coefficient can be calculated is dependent of the measured number of photons dispersed in this point, the device in accordance with the invention offers a substantially higher image quality 1 1 5~6.~
PHD.79.129 4 for the centre of the object than the known devices where displacement always takes place perpendicularly to the direction of the primary beam.
It is to be noted that our Canadian Patent 1/134,066 which issued on October 19, 1982 discloses a device for determining internal body structures by means of scattered radiation which corresponds to the so-called transmission computer tomography apparatus, however, therein the scattered radiation produced along the stopped primary beam is also measured by means of two hollow cylindrical detectors which are arranged one on each side of the examination plane and which enclose the body being examined. The hollow cylindrical detectors for measuring the scattered radiation, however, can meas-ure only the scattered radiation generated by the primarybeam throughout the examination zone, so not the scattered radiation which is generated by individual points struck by the primary beam. Moreover, therein the primary beam is displaced perpendicularly to its direction and the device is rotated through a small angular increment only after the complete examination zone has thus been scanned.
Ignoring the intensity attenuation by the body being examined, the concentration of primary radiation in the examination zone is homogeneous, which is contrary to the invention where the concentration is higher in the centre.
In a further embodiment in accordance with the invention, the detector device is also mounted on the supporting device. In all positions of the supporting device, each element of the detector device measures only the scattered radiation originating from a given zone of the primary beam. Thanks to this unambiguous spatial assignment and the fact that this zone describes a circle around the axis of rotation when the supporting body is rotated, each detector measures the scattered radiation in an arc of a circ~e around the axis of rotation.
In a further embodiment in accordance with the 1 15796~
PHD.79.129 5 11.6.80 invention, the detector device forms a s-tationary arc of a circle which is concentric to the axis of rotation of the supporting device. Between this embodiment and the previously described embodiment there is a difference which is analogous to the difference between a transmission computer tomography apparatus of -the fourth generation and one of the third generation. The detector device in general re~uires a larger number of detector elements than the previously described embodiment. Therefore, the assignment of a detector element to the primary beam changes as the supporting device is rota-ted, so that the scattered radiation generated in a given zone of -the primary beam is each t~me measured by a different element of the detector device in each rotary position of the i5 supporting device. Differences in sensitivity of the various elements of the detector device, therefore~ have only a small effect on the reconstruction of the density distribution. Ring-shaped artefacts which are liable to occur in the former embodiment when the sensitivity of a detector element deviates from that of the other detect~ elements are not intensified.
The accuracy and the response sensitivity can be further increased by arranging different diaphragm devices on both sidQs of the primary beam, each elemen-t of the detector devices then measuring the scattered radiation generated in a given zone of the primary beam through said diaphragm devices. For example, when two diaphragm devices are arranged one on each side of the primary beam, the scattered radiation produced in a given zone of the primary beam is measured by a total of four elements of the detector device. The four measuring values are added in order -to form a signal which corres-ponds to the density in this zone.
As is known, devices of the described kind, including the device in accordance with the invention, enable a simpler reconstruction of the density dis-tribll-tion in the slice than so-called -transmission computer ~ 157968 PHD.79.129 6 11.6.80 tomography apparatus, because each detector already measures the density generated in a given ~int in a given zone, rather than the line integral of the density over the relevant radiation path like in transmission compu-ter tomography apparatus. On the other hand, the reconstructed density distribution gives only a qualitative result if neither the attenuation of the primary beam up to the point in which the scattered radiation is generated nor -the attenuation of the scattered radiation on its way to the relevant detector element is -taken into account. This can in principle be realized by means of a suitable computer program in which given assumptions as regards the mean density and the dimensions of the object to be examined are made. Obviously, the density distributions thus obtained can be further improved by suitable itera-tion methods, but they generally cannot be accurate in view of these eaoh assumptions.
A more accurate determination of the density distribution is possible in a further embodiment in accor-dance with the invention which comprises an X-ray source whose radiation can be stopped by a collimator so that it passes through the total examination zone and is measured by a detector device which is arranged behind the examination zone. Therein, the X-ray source forms part, together with said detector deviceg of a computer tomo- -graphy apparatus of the third or fourth generation (depending on whether the detector device is moved or not), which is capable of determining the absorp-tion distribu-tion in the examination plane by means of a computer.
The additional costs are low, because the detector device and the mechanism for rota-ting the source around the examination zone (supporting device) are present anyway.
From the absorption density distribu-tion thus 3 obtained the attenuation can be calculated for each individual measuring value, because the paths of the prima-ry beam up to the scat-ter point and from the sca-tter point to the relevant detec-tor elemen-t are fixed. It is merely PllD.79.129 7 11.6.80 necessary to add the attenuation coefficients obtained along these paths, so that the total attenuation can be calculated. The values measured by the individual detector elements must then be weighted by a weighting factor which is proportional to the attenuation factor thus calculated for the total beam path. The dose applied to the patient during such a determination of the absorp-tion distribution can be substantially smaller than in transmission computer tomography apparatus. The absorp-tion coefficients measured in the individual points are 1ess accurate, but these reconstruction errors do not exert their full effect, because for the determination of the attenuation of a primary or scattered beam on its way through the examination zone -the attenuation coeffi-cients of many points have to be added and errors are at least partly eliminated by averaging. ~oreover, the X-ray source can efficiently operate with the same high voltage (for example, 350 kV) such as during the determination of the density distribution, so that the dose is also reduced on the basis of this fact.
However, in order to obtain the absorption distribution it is also possible to operate the X-ray source with the same tube voltage and the same inte~sity as customarily used for transmission computer tomography~
On the basis of the absorption distribution thus obtained, the density distribution can be accurately reconstructed.
Using the density distributions thus obtained, the scatter contributions to the transmission computer tomo-gram (the absorption in an individual point of the transmission computer tomogram is composed of an attenua-tion component by photoabsorption and an attenuation component by scattering) can be determined and subtracted from the values obtained; this resul-ts in the photo-absorption. At the end of these calculations ? two density distributions are then obtained: on the one hancl the density distribution due to radiation scattering in the examina-tion zone and on the other hand the photoabsorption 1 1579~8 PHD.79.129 8 11.6.80 therein. The radiologist can derive extra information therefrom.
The invention will be described in detail herein-after with r~ference to the accompanying diagrammatic drawing.
~igure 1 shows first embodiment in accordance with the invention, comprising two oppositely arranged ~-ray sources and detector devices mounted on the suppor-ting device, 13 Figure 2 shows a similar device, which, however, offers a better probability of detection of scattered radiation, Figure 3a shows a device whereby at the same time a transmission computer tomogram can be made, lS Figure 3b shows this device during the measure-ment required for the transmission computer tomogram, Figure 4 shows a device in which the elements of the detector device are arranged to be stationary on an arc of a circle around the axis of rotation, 2D Figure 5 shows a corresponding device which, however, is also suitable for making transmission computer tomograms, and Figure 6 shows a device for recons-tructing the density distribution on the basis of the measuring values ZS
obtained.
The reference numeral 10Z in Figure 1 denotes a sta-tionary housing in which a supporting device 10 is journalled to be rotatable around an axis 3 by way of roller bearings 101. The drive units for rotating the supporting device 10 are no-t shown. The supporting device comprises an aperture 4 which defines the examination zone within which the scattered radiation can be deter-mined. On an examination table 2 inside this examination zone there is arranged the patient body 1 in a given slice of which the density distribution has -to be deter-mined. To this end, on either side of the examination ~one which is concentricall~ situated with respect to the l 1579~8 PHD.79.129 9 axis of rotation 3, there are arranged X-ray sources 51 and 52, in front of which there are arranged collimators 61 and 62 which stop registering primary beams 31 which intersect the axis of rotation.
In front of each of the two X-ray sources there is arranged a transmission detector 71, 72 which is pro-vided with holes (not shown~ in order to allow unobstruc-ted passage of the primary beam from its relevant radi-ation source, whilst the primary beam from the oppositely situated radiation source, attenuated by the examination zone, can be detected as a result of the dispersion inside the body. Using these detectors, any assumptions concerning the attenuation inside the body can be cor-rected as described in our Canadian Patent 1,101,133 which issued on May 12, 1981.
On either side of the primary beam 31 and out-side the examination zone 4 on the rotatable supporting device 10 there are provided two detector devices 91 and 92 which consist of a large number of adjacently arranged detector elements whose largest dimension extends in the direction perpendicular to the plane of the drawing, as described in our Canadian Patent 1,101,133. Between each of the two detector devices 91 and 92 and the exam-ination zone 4 there is arranged a slit-like diaphragm 81, 82 which provides an unambiguous spatial assignment of a point or zone, for example, the zone 11, of the primary beam to the detector device 91, 92, so that the scattered radiation generated in this point 11 of the primary beam is measured by the detector elements which are present at the locations 93 and 94. The scattered radiation generated at other locations in the primary beam within the examination zone 4 is measured by other elements of the detector device 91, 92, for example, like in the device described in our Canadian Patent 1,101,133.
In the latter device the primary beam is dis-placed perpendicularly to its direction with respect to ~ 1$798.~
PHD.79.129 10 11.6.80 the examination zone in order to determine the density distribution in a slice; however, in the present case the complete slice to be examined is covered in that the supporting device 10 is first rotated through a small angular increment, after which the two detector devices record new sets of measuring values, after which a further rotation through a small angular increment takes place etc. until the device has been rotated -through a total angle of 180 . The angular increments are chosen so that the complete contour is also completely co~ered by the various primary beams. The primary beam remains directed onto the axis of rotation 3.
It is alternatively possible to use only a single detector and diaphragm device; however, the proba-bility of detection is then lower if the detector sur-faces are not increased. Instead of using two X-ray sources, it is alternatively possible to use only a single source. However, for a complete measurement a rotation of the supporting device through 360 is then required in order to expose each point on the contour of the examination zone 4 once to the unattenuated primary beam.
Figure 2 shows a part of a further embodiment in which two detector systems are arranged on each side of the primary beam 31. For the sake of eimplicity, the two detector systems to the left of -the primary beam 31 and the two X-ray sources generating the primary beam are not shown. The two detector devices are advantageously combined to form a single detector device 95, and a diaphragm device 89, comprising two slits 85 and 86, is arranged between each detector device and -the examination zone. The slits are proportioned and situated so that the lower part of the elements of the detector device can be struck by scattered radiation which is generated in the primary beam 31 and which passes through the slit 85, whilst the upper part of the elements of the detector device can be s-truck by scat-tered radiation ~hich is 1 157gg8 PHD.79.129 11 11.6.80 generated in the primary beam within the examination zone and which passes through the slit 860 No element of the detector device canthen measure radiation which passes through the slit 86 as well as through the slit 85 and which originates from the primary beam within the examination zone 4.
This embodiment of the detector and diaphragm device increases the probabili-ty of detection, thus improving the reconstruction accuracy because the scatte-red radiation from each point on the primary beam 31 can be measured by two different detector elements, as is shown in Figure 2 for the scattered beams 96 and 97 which originate from the point 11 on the primary beam 31 and which pass through the slits 85 and 86. ~oreover, the two diaphragm and detector devices complement each other, i.e. the scattered radiation originating from the lower left half of the primary beam in Figure 2 is measured better by the detector elements associated with the slit 85, whilst scattered radia-tion from the upper right half of the primary beam 31 is measured better by the elements associated with the slit 86. This is because the scattered radiation produced in the lower left half of the primary beam 31 is attenuated less when it passes through the slit 85, whilst the scattered radiation produced in the upper right half of the primary beam is attenuated less when it passes through the upper slit 86; for example, the scattered beam 97, being produced in the point 11 in the upper right half of the primary beam 31 and passing through the slit 86, is attenuation less by -the body I than the scattered beam 96 which is produced in the same point and which passes through the slit 85.
The diaphragm device 89 in figure 2 comprises a further slit 87 which, however, is covered by a shield ~J
88 during normal operation. When the cross-section of the body 1 is so small -tllat it is situated within the circle 41 which is concentric with respect to the axis ~ 1~7~6~
PIID.79.129 12 11.6.80 of rotation 3 and whose diameter is smaller than the diameter of the examination zone 4, the shield 88 can be moved out of the beam path by means of a drive unit (not shown). A further scattered radiation path 8g is then released, thus increasing the probability of detection of scattered radiation and hence the measuring speed.
The circle 41 is proportioned so that each element of the detector device can "see" each time only one point on the primary beam within the circle 41 through the slits 85, 86 and 87, so that disturbing superposition of scattered radiation which can reach a detector element via at least two different scattered radiation beam paths, is precluded.
Figure 3a also shows a device comprising two X-ray sources 51 and 52 which are arranged on the supporting device 10 with collimators 61 and 62 which are arranged in front ~ the sources and wherethrough the primary beam 31 passes. On both sides of the primary beam two detector devices are arranged again. However, whilst on the left half the elements of the two detector devices 92 adjoin directly and are covered by a common diaphragm device 82 comprising two slits, the detector devices 91 and 93 are separated on the right side and each detector device comprises a slit diaphragm device 81, 83. For example, the scattered radiation produced in the point ~1 reaches, along the lines 94, 95, 96 and 97, an element in the different detector devices. Between the two detec-tors and diaphragm devices 81, 91 and 83, 93 there is arranged an X-ray source 53, i.e. opposite the detector device 92. In fron-t of the source there is provided a collimator 63 which is shaped so tha-t the radiation generated by the source 53 passes through the examination zone 4 in the same plane as the primary beam 31.
Figure 3a shows the device in an examination phase where the primary beam 31 passes through the examination zone 4 in order to determine the density distribution, and Figure 3_ shows the phase in which the ~ 157968 PHD.79.129 13 11.6.80 X-rays generated by the source 53 irradiate the complete ex~mination zone between -the rays 99 in order to determine the attenuation distribution within the examination zone 4. In this phase, a drive and displacement device (not shown) displaces the slit diaphragm device 82 perpendicu-larly to the plane of examination, so that the elements of the detector device 92 can measure the radiation beam of the radiation source 53.
The two examination phases shown in the Figures 3a and 3b may be performed in direct succession, the tube voltage which is initially applied to the X-ray sources 51 and 53 for at least one half revolution, subsequently being switched over to the X-ray source 53 until the attenuation distribution within the examination zone 4 has been comple-tely measured by means of the detector device.
It will be clear tha~ the cost of the additional execution of transmission computer tomography are compara-tively low, because the detector device 9 2 for measuring the transmission measuring values is already required for the scattered radiation measurement and so is the rotatable supporting device.
Figure ~ shows an embodiment which differs from the embodiment described thus far in that the detector device is connected to the housing 102 and is constructed as a stationary circle of separate detector elements which concentrically encloses the axis of rotation 3 and the supporting device 10. Only the X-ray source 51 with the collimator 61 for the formation of -the primary beam 31 and the diaphragm device 80, comprising a total of seven slits, are arranged on the supporting device 10 which in this embodiment, comprising only one X-ray source, has to be rotated through 360 . The primary beam 31 passes through a slit which is situated diametri-cally opposite the X-ray source 52, so that the element each time presen-t behind this slit can measure the attenuation of the primary beam 31. The six other slits ~ 1. 5 7~ G ~
PHD.79.129 14 11.6.80 in the diaphragm device 80 are distributed on both sides of the primary beam so -that in each angular position of the supporting device 10, each element of the detector device can be reached through a slit only by scattered radiation which is generated within the examination zone 4 and in the primary beam 31. Figure 4 again shows the six beam paths 901 to 906 along which the scattered radiation generated in the point 11 reaches six different elements of the detector device.
The embodiment shown in Figure 4 usually requires more detector elements than the previously described embodiments, but these elements need not be moved. Moreover, a detector element is not permanently assigned to an arc of a circle around the axis of rotation 3; this means on the one hand that ring-like artefacts which are liable to occur in the described devices when the sensitivi-ty of a detector element deviates from that of the other elements, cannot occur; on the other hand, it also means that for the determination of the density distribution on an arc of a circle around the axis of rotation 3, the measuring values of a large number of different detector elements must be taken into account. Moreover, the journalling elements for the supporting device 10 (not shown in Figure 4) must be arranged so that they do not shield the-scattered radiation paths to the detector elements.
Figure 5_ shows an embodiment comprising stationary detector elements which also enables measure~
ment of transmission measuring values. Like in Figure 4, the detector device which consists of a stationary ring of detector elernents which is concentric to the axis of rotation 3 is mounted on the supporting frame of the device (not shol~n). Like in ~igure 2, the diaphragm device 80 is shaped so that the radiation beam wllich co-vers the entire examination zone 4 between the ex-treme rays 32 and 33 is not influenced by the diaphragm device 80.
1 157~6~
PHD.79.129 15 11.6.80 In fron-t of the radiation source 51 there is arranged a collimator plate which is displaceable perpen-dicularly to the plane of the drawing and which comprises two apertures 62 and 63 which are situated perpendicularly one above the other (see Figure ~b). When the opening aperture 62 is slid into the examination plane 40 ~ a narrow collimated primary beam 31 is stopped and the density distribu-tion can be measured. However, when the aperture 63 is slid into the examination plane 40, a fan-shaped radiation beam with the extreme rays 32 and 33 is stopped and transmission measurements can be performed, The measurements can be consecutively performed, i.e. during a rotation through 360 first the density distribution is determined and then the attenuation distribution, or vice versa. However, during operation, that is to say during the rotation of the supporting device 10, it is alternatively possible to slide the diaphragm 61 quickly to and fro on -the stationary rails 64, extending perpendicularly to the plane of the drawing of Figure 5a, so that the primary beam 31 and the radia-tion beam with the outer rays 32 and 33 are alternately emitted. It is thus ensured that the scattered radiation and the transmission radiation are measured in neighbou-ring angular positions of the supporting device 10 25without substantial delay.
In the device shown in Figure 5a, the additional cost for executing the computer tomography are only for the collimator plate 61 which is displaceable perpendi-cularly to the plane of examination.
Figure 6 shows a device for reconstructing the density distribution from measuring values obtained by means of a device as shown in the Figures 3a, 3b or 5a, 5b, comprising a memory 100 whereto the scat-tered radiation 3 measuring values are applied. rhese measuring values are weighted with different weighting factors in the arithme-tic unit 400. It is thus taken into account that for a point in the cen-tre of -tt-le examina-tion ~one a substantial-1 157~
PHD.7~.129 16 1 1 .6.~o ly larger number of measuring values is present than for a point outside the centre, so that the measuring values for a point in the centre must be weighted with a corres-pondingly lower factor before or after the summing.
For example, if it is assumed that the number z of scanning directions and the dimensions of the primary beam are selected so that the primary beams cover each location on the periphery of the examination zone 4 exact-ly once, and if it is also assumed that the density in lG a polar coordinate system is to be reconstructed in points which are situated on straight lines through the centre whose angular positions correspond -to those of the primary beam with respect to the examination zone, the distance a between the cen-tres of two adjacent points on a straight line corresponding to the width of the primary beam, the weighting factors are formed as follows:
The measuring values for the point in the centre, or the sum thereof, are weighted by a weighting fact~r l/z, because the primary beam passes through this point in all z angular posi-tions. For a poin-t outside the centre, the measuring values, or the sum thereof, are weighted by a factor n/N,Nbeing z/2jl and n being an integer value which indicates how many times the distance between -the centre 3 of the examination zone and the centre of the relevant point is larger than the distance a between two adjacent points.
The weighting factor distribution is thus formed for a polar coordinate system. However, if desired, after application of known transformation rules, the weighting factors for points in a cartesian coordinate system can be calculated therefrom.
Simultaneously with this weighting factor, other weighting fac-tors which are dependent of the geometry of the device, for example, the angular dependency of the scattered radiation~ and the differen-t de-tector sensitivities can be taken into account. The resultant weighting factors are stored in the memory 3OO.
1 15796~
PHD.79.129 17 11.6.80 The reference numeral 200 deno-tes a sorting unit which, in the case of stationary detectors (Figure 5a), sorts the measuring values of permanently stored coordinate transformations so as if the measuring values were measured by rotating detector elements (like in Figure 3a). In a further arithmetic unit 150, the values thus weighted are weighted by a factor which corresponds to the at-tenuation of the radiation by the body. The higher the attenuation, the larger the weighting factor will be.
These values are derived from the transmission computer tomogram. To this end, the transmission measuring values are stored for the time being in the memory 110 and the attenuation density distribution in the examina-tion zone is reconstructed therefrom in the arithmetic unit 180. For each separate point in the examination zone it can be calculated to what extent the primary radiation has been attenuated on its way to this point and how much the scattered radiation has been attenua~ed in order to reach a given detector element from this point. The attenuation coefficient is the line integral over the attenuation coefficients along the primary beam as far as the relevant point and further over the relevan-t scattered radiation path to the detector element. These values are calculated in one point in an arithmetic unit 130 and are intermediately stored in the memory 140 and used for the weighting of the values, supplied by the arithmetic unit 400, in the arithmetic unit 150. From the values supplied by the arithmetic unit 150, the density distribution is calculated in the reconstruction unit 400, each element of the detec-tor device, assumed to have rotated along, being associated with a concen-tric circle in the reconstruction plane, and each measuring value on a poin-t of this circle being -transferred under a pole angle whicl-l corresponds to tlle angle of ro-tation with which this measuring value has been obtained. ~fter completion of these calculations, the image is s-tored in ~ 15796~
PHD.79.129 18 11.6.80 the memory 600 and displayed on a display apparatus 700.
The weighting factors corresponding to the relevant attenuation cannot only be determined by measure-ment by means of a transmission computer tomogram, but also by way of attenuation measurements which are performed on a suitable phantom. For different body cross-sections of patients of different size, use must be made of diffe-rent phantoms whereto each time a set of weighting factors must be assigned in the memory 140. This enables only an approximate determination of the relevant weight-ing factor, which is better as the selected phantom cross-section corresponds better to the body cross-section examined.
It is in principle also possible to determine the attenuation of the scattered radiation, or the weighting factors taking into account this attenuation, by calculation. To this end, first the contour of the body slice to be examined is determined by comparing each measuring value with a threshold value which corresponds approximately to the scatter coefficient of water or which is slightly lower. When a measuring value is lower than the threshold value, it is assigned to the part of the examination zone situated outside the body (air);
if it is larger, it is assigned to the body slice.
The slice of the body thus obtained is first assigned a homogeneous density distribution, this density being assumed to be equal to that of water. This is a suitable approximation of the actual conditions, because a human body largely consists of water. The attenuation of the primary beam occurring for -the indicated density distribution is calculated for the individual scanning directions and is compared with the attenuation values resulting from the measuring values from the elements 71 and 72 (Figure 1). The difference is used for correcting the assumed density distribution.
From the density distribution -thus corrected, tlle a-ttenuation of -the beam along the primary beam path 1 1579B~
PHD.79.129 19 11.6.80 to the scatter point (for exampleg 11, ~igure 2) and therefrom to the detector via the scattered beam path (for example, 97) is determined. This calculation is successively performed for all scanning directions or angular positions and all detector positions, and the results are stored. Using the beam attenuation values stored, being each time the reciprocal values of the weigh-ting factors, the measured scattered radiation measuring values are corrected. Therefrom a corrected density distribution can be determined.
The measured density distribution can be further improved by comparing the density values deter-mined for the individual points again with the scatter coefficient of water in order to determine not only the contour of the body, like in the previously described comparison cycle, but also areas inside the body whose scatter coefficient deviates substantially from that of water (for example, air or bone). Subsequently, the described cycle is completed again; if neces~ary, it may be repeated again.
Claims (6)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A device for determining the density distribu-tion in an object, comprising at least one radiation source for generating a narrow primary beam which irradi-ates the object, a detector device which is arranged adjacent the primary beam for detecting scattered radi-ation produced by the primary beam, a diaphragm device which is arranged between the object and the detector device and which is shaped so that the detector device measures a set of scattered radiation values from the path of the primary beam through the object, and also comprising a drive device for the displacement of the path of the primary beam through the object, character-ized in that the radiation source and the diaphragm device are mounted on a supporting device which leaves an object space free and which is rotatable around an axis which intersects the path of the primary beam and which is directed transversely thereof.
2. A device as claimed in Claim 1, characterized in that the detector device is also mounted on the sup-porting device.
3. A device as claimed in Claim 1, characterized in that the detector device forms a stationary arc of a circle which is concentric with respect to the axis of rotation of the supporting device.
4. A device as claimed in Claim 1, characterized in that on both sides of the primary beam there is arranged more than one diaphragm device wherethrough each time an element of the detector devices measures the scattered radiation produced in a given zone of the prim-ary beam.
5. A device as claimed in Claim 1, characterized in that an X-ray source has associated with it a collim-ator wherethrough the primary radiation, after having passed through the object, can be measured by a detector device which is arranged behind the object.
PHD.79.129 21
PHD.79.129 21
6. A device as claimed in Claim 3 or Claim 5, char-acterized in that in front of the radiation source there is arranged a collimator device which is displaceable per-pendicularly to the plane of examination, which can be locked in two positions and which comprises two apertures, the one aperture in one position stopping the primary beam and the other aperture in the other position stopping a radiation beam which covers the complete examination zone.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19792944147 DE2944147A1 (en) | 1979-11-02 | 1979-11-02 | ARRANGEMENT FOR DETERMINING THE SPREAD DENSITY DISTRIBUTION IN A LEVEL EXAMINATION AREA |
| DEP2944147.2 | 1979-11-02 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1157968A true CA1157968A (en) | 1983-11-29 |
Family
ID=6084907
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000363596A Expired CA1157968A (en) | 1979-11-02 | 1980-10-30 | Device for determining the density distribution in an object |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4375695A (en) |
| EP (1) | EP0028431B1 (en) |
| JP (1) | JPS5674644A (en) |
| AU (1) | AU6388780A (en) |
| CA (1) | CA1157968A (en) |
| DE (2) | DE2944147A1 (en) |
| ES (1) | ES496430A0 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6556653B2 (en) | 2000-05-25 | 2003-04-29 | University Of New Brunswick | Non-rotating X-ray system for three-dimensional, three-parameter imaging |
| US6563906B2 (en) | 2000-08-28 | 2003-05-13 | University Of New Brunswick | X-ray compton scattering density measurement at a point within an object |
Families Citing this family (26)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3023263C2 (en) * | 1980-06-21 | 1986-08-14 | Philips Patentverwaltung Gmbh, 2000 Hamburg | Arrangement for determining the internal structure of a body by means of monoenergetic radiation |
| DE3031949A1 (en) * | 1980-08-25 | 1982-04-01 | Philips Patentverwaltung Gmbh, 2000 Hamburg | SCREEN EXAMINATION ARRANGEMENT FOR DETERMINING THE INNER STRUCTURE OF A BODY |
| US4426578A (en) * | 1980-10-08 | 1984-01-17 | Technicare Corporation | Support structure for rotatable scintillation detector |
| EP0105618B1 (en) * | 1982-09-07 | 1989-10-25 | The Board Of Trustees Of The Leland Stanford Junior University | X-ray imaging system having radiation scatter compensation and method |
| DE3406905A1 (en) * | 1984-02-25 | 1985-09-05 | Philips Patentverwaltung Gmbh, 2000 Hamburg | ROENTGENGERAET |
| DE3534702A1 (en) * | 1985-09-28 | 1987-04-09 | Philips Patentverwaltung | METHOD FOR DETERMINING THE PHOTOS LEAKAGE IN A AREA OF AN EXAMINATION EXAMPLES AND ARRANGEMENT FOR IMPLEMENTING THE METHOD |
| US4809312A (en) * | 1986-07-22 | 1989-02-28 | American Science And Engineering, Inc. | Method and apparatus for producing tomographic images |
| DE3630651A1 (en) * | 1986-09-09 | 1988-03-17 | Philips Patentverwaltung | METHOD FOR TWO-DIMENSIONAL COMPTON PROFILE IMAGE |
| GB2309368B (en) * | 1996-01-18 | 1999-09-15 | Hamamatsu Photonics Kk | An optical computer tomographic apparatus and image reconstruction method using optical computer tomography |
| US7664223B1 (en) * | 2001-02-12 | 2010-02-16 | Sectra Mamea Ab | Collimator element |
| US8243876B2 (en) | 2003-04-25 | 2012-08-14 | Rapiscan Systems, Inc. | X-ray scanners |
| US8451974B2 (en) | 2003-04-25 | 2013-05-28 | Rapiscan Systems, Inc. | X-ray tomographic inspection system for the identification of specific target items |
| US8223919B2 (en) | 2003-04-25 | 2012-07-17 | Rapiscan Systems, Inc. | X-ray tomographic inspection systems for the identification of specific target items |
| US9113839B2 (en) | 2003-04-25 | 2015-08-25 | Rapiscon Systems, Inc. | X-ray inspection system and method |
| US8837669B2 (en) | 2003-04-25 | 2014-09-16 | Rapiscan Systems, Inc. | X-ray scanning system |
| US7949101B2 (en) | 2005-12-16 | 2011-05-24 | Rapiscan Systems, Inc. | X-ray scanners and X-ray sources therefor |
| GB0525593D0 (en) * | 2005-12-16 | 2006-01-25 | Cxr Ltd | X-ray tomography inspection systems |
| JP3919724B2 (en) * | 2003-09-19 | 2007-05-30 | ジーイー・メディカル・システムズ・グローバル・テクノロジー・カンパニー・エルエルシー | Radiation calculation tomographic image apparatus and tomographic image data generation method |
| US7203276B2 (en) * | 2004-08-27 | 2007-04-10 | University Of New Brunswick | X-ray scatter image reconstruction by balancing of discrepancies between detector responses, and apparatus therefor |
| US20070025514A1 (en) * | 2005-06-06 | 2007-02-01 | Ruediger Lawaczeck | X-ray arrangement for graphic display of an object under examination and use of the x-ray arrangement |
| US8238513B2 (en) * | 2005-09-19 | 2012-08-07 | Feng Ma | Imaging system and method utilizing primary radiation |
| US20100183115A1 (en) * | 2006-08-11 | 2010-07-22 | Koninklijke Philips Electronics N.V. | System and method for acquiring image data |
| CN101506688B (en) * | 2006-08-23 | 2011-12-21 | 美国科技工程公司 | scatter attenuation tomography |
| DE102007045798B4 (en) * | 2007-09-25 | 2010-12-02 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Arrangement and method for recording X-ray scattering images |
| JP5081556B2 (en) * | 2007-09-28 | 2012-11-28 | 株式会社リガク | X-ray diffraction measurement apparatus equipped with a Debye-Scherrer optical system and X-ray diffraction measurement method therefor |
| NL2010267C2 (en) * | 2013-02-07 | 2014-08-11 | Milabs B V | High energy radiation detecting apparatus and method. |
Family Cites Families (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3106640A (en) * | 1960-10-06 | 1963-10-08 | William H Oldendorf | Radiant energy apparatus for investigating selected areas of the interior of objectsobscured by dense material |
| FR2232294B1 (en) * | 1973-06-05 | 1978-01-13 | Emi Ltd | |
| US3922552A (en) * | 1974-02-15 | 1975-11-25 | Robert S Ledley | Diagnostic X-ray systems |
| DE2461877A1 (en) * | 1974-12-30 | 1976-07-01 | Alexander Dipl Phys Dr R Krebs | X-ray or gamma radio diagnostic scattered radiation appts - for medical radiodiagnosis or investigating internal organ structures |
| DE2655230A1 (en) * | 1976-12-06 | 1978-06-15 | Siemens Ag | Gamma-ray and X=ray tomography - using planar bundle of rays whose absorption in the body and dispersion is measured |
| FR2383648A1 (en) * | 1977-03-17 | 1978-10-13 | Askienazy Serge | Transverse tomography irradiation appts. - has rotary irradiation devices to determine sectioning plane and topographical detection devices with focal axis perpendicular to that plane |
| DE2713581C2 (en) * | 1977-03-28 | 1983-04-14 | Philips Patentverwaltung Gmbh, 2000 Hamburg | Arrangement for the representation of a plane of a body with gamma or X-rays |
| FR2405696A1 (en) * | 1977-10-11 | 1979-05-11 | Radiologie Cie Gle | TRANSVERSE AXIAL TOMOGRAPHY METHOD AND APPARATUS |
| DE2831311C2 (en) * | 1978-07-17 | 1986-10-30 | Philips Patentverwaltung Gmbh, 2000 Hamburg | Device for determining internal body structures by means of scattered radiation |
-
1979
- 1979-11-02 DE DE19792944147 patent/DE2944147A1/en not_active Withdrawn
-
1980
- 1980-10-20 US US06/198,740 patent/US4375695A/en not_active Expired - Lifetime
- 1980-10-23 EP EP80201007A patent/EP0028431B1/en not_active Expired
- 1980-10-23 DE DE8080201007T patent/DE3067961D1/en not_active Expired
- 1980-10-30 CA CA000363596A patent/CA1157968A/en not_active Expired
- 1980-10-31 AU AU63887/80A patent/AU6388780A/en not_active Abandoned
- 1980-10-31 ES ES496430A patent/ES496430A0/en active Granted
- 1980-11-01 JP JP15455780A patent/JPS5674644A/en active Granted
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6556653B2 (en) | 2000-05-25 | 2003-04-29 | University Of New Brunswick | Non-rotating X-ray system for three-dimensional, three-parameter imaging |
| US6563906B2 (en) | 2000-08-28 | 2003-05-13 | University Of New Brunswick | X-ray compton scattering density measurement at a point within an object |
Also Published As
| Publication number | Publication date |
|---|---|
| DE2944147A1 (en) | 1981-05-14 |
| JPH027421B2 (en) | 1990-02-19 |
| EP0028431A1 (en) | 1981-05-13 |
| ES8107396A1 (en) | 1981-10-01 |
| EP0028431B1 (en) | 1984-05-23 |
| US4375695A (en) | 1983-03-01 |
| DE3067961D1 (en) | 1984-06-28 |
| JPS5674644A (en) | 1981-06-20 |
| AU6388780A (en) | 1981-05-07 |
| ES496430A0 (en) | 1981-10-01 |
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| Date | Code | Title | Description |
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| MKEX | Expiry |