CA1073123A - Tomography using an out-fanning beam of rays - Google Patents

Abstract

Abstract of the Disclosure A method and apparatus for pre-processing radiological information obtained by exposing a subject at various spaced positions on one side of this subject to an out-fanning, substantially flat beam of penetrating radiation. The resultant ray transmissions on the other side of the subject are recorded on a record medium along a collection of recording paths each having a predetermined trajectory and a spatial orientation related to an associated position of exposure. The radiological information thus recorded is then scanned and read-out along another collection of adjacent scanning paths each having a predetermined trajectory which intersects different recording paths and likewise having a spatial orientation related to the associated position of exposure. The information thus read out represents radiological information resulting from a virtual exposure to substantially parallel rays of penetrating radiation, and permits the reconstruction of desired laminal planes of the subject by means of a process involving a single convolution function.

Description

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The invention relates to a method and apparatus for forming an image of a lamina of a subject under examination by means of penetrating radiation, such as X-rays, employing a source for producing a substantial- ~-ly flat, out-fanning beam of the penetrating radiation, a support for ;
supporting the subject under examination so that this subject is irra-diated by this beam, and a detector device for producing and processing signal profiles which are indicative of the radiation absorption in the lamina of the subject as determined by the beam and are the result of a ~ `
relative movement between the subject and an assembly including the source of radiation and the detector device.
In the technique known as tomography it is possible to form a density image (tomogram~ of a subject under examination that is represen- ~
tative of a "plan view" of a slice-like cross-section (lamina) of this ~`
subject. To this end, a relative movement is established between the subject and the assembly including the source of radiation and the de-tector device, and the subject under examination is irradiated from a series of positions, each position corresponding with a signal profile derived by the detector device. For example, the assembly is rotated about a vertical axis extending through the subject; alternatively, the assembly is stationary and the subject is rotated about this axis.
A tomogram can be reconstructed rrom a series of signal profiles thus derived by the detector device. It appears that in such a reconstruc-tion the so-called point spread function degrades the definition or ~ ~`
- resolution of the tomogram. To eliminate this drawback, it has been pro- ~`
posed to preceed the superpositioning of signal profile information, as ~-~
carried out during the processing of signal profiles provided by the detector device, by a pre-processing of these signal profiles such that ,,

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~73~Z3 the interfering point spread function is eLiminated. In this prooedureJ
each original signal profile is opto-electronically pre-processed to provide a new signal profile which~ during the reproducing projection9 provides a ~ ;
tomogram that is free from the interfering point spread function. It appears that, when use is made of a source of radiation irradiating the subject with a beam of parallel rays, one and the same point spread function applies to all points of the image field. However, the point spread function appears to be position-dependent when the irradiation is performed by means of an out-fanning beam. In other words, when a beam of parallel rays is used an exact correction with respect to the point spread function can be achieved as each original signal profile can be multiplied, fragment by fragment, by a given convolution integral. Such an exact correction and hence optimaliz-ation of the resolution cannot be achieved, however, when such a correcting operation is simply applied to a situation in which an out-fanning beam is employed.
; For techno-economical reasons, it is unacceptable in the art of tomography to use a beam of parallel rays in a configuration in which the source of radiation is mounted at a relatively great distance from the sub-ject under examination. Gne should rather contemplate a configuration in which a source for producing a substantially flat, out-fanning beam of radiation is mounted optimally close to the subject.
Computerized tomography systems are known which by means of digital processing techniques can transform digital data representative of radiation absorption signals obtained by out-fanning beams into signals representative of signal profiles obtained by beams of parallel rays. In such computerized tomography system the detector device comprises a plurality of discrete detector elements. The output signals of these detector elements are subjected to an analogue-to-digital conversion and are stored in a digital memory system with information regarding the directions of the corresp~nding pencil beams o~ penetrating radiation. A~ter all data ob~ained after one complete rotatlon of the object under examination reLati~e to the apparatus are collected, a re-ordering process of these data is carried out

3-.., ~C~731Z3 to obtain signal profiles representative of a fictitious beams of parallel rays. Back projection of these so-called parallellized profiles eventually yields a tomogram which is free from the blurring effects of a point-spread function. Such a computerized tomography system, however, cannot be applied for analogue tomography. This becausein analogue tomography it is essential that continuous and complete profiles are processed, so that any system wherein profile-elements corresponding to discrete detector elements are individually processed is ruled out as a practical possibility. In order to be able in analogue tomography to apply the spatial filtering processes which are necessary to remove the effects o the point-spread function, it ~ ~;
is necessary to find a method based on analogue electronic techniques, where-in from a collection of primary profiles resulting from out-anning beams of radiation another collection of profiles representative of fictitious beams of parallel rays is obtained. All these parallellized profiles can be sub-sequently subjected to the same spatial filtering process, since the back projection of parallel profiles gives rise to a point-spread function which is constant through out the plane.

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To achieve this object, the apparatus according to the invention is characterized by first means for recording in an analogue mode and in de-pendence upon a series of first positions successively taken by the source of radiation relative to the subject under examination, a collection of con-tinuous primary signal profiles in analogue form corresponding therewith;
and second means for scanning, in dependence upon a series of second positions related to said first positions, the collection of primary signal profiles in accordance with a collection of paths corresponding with this series of second positions, in such a manner that the signal profile fragments each corresponding with such a path, together form a secondary signal profile that is representative of the radiation absorption of the subject as a result of an imaginary beam of parallel rays incident from a direction corresponding with one of the first positions.
; Due to its simple organization, an apparatus according to the invention can be manufactured at a relatively low cost price. Further ad-vantages are, moreover, that relatively little space is required to accommo-date the apparatus, and that, as parallel beams are reconstructed from out-fanning beams, the source of radiation can be mounted close to the subject, which permits efficient use of such a source of radiation and a low radi-ation dose while optimal resolution of the ultimate tomogram is achieved.
The invention will be described in greater detail hereinafter with reference to the accompanying drawings, in which:
Figure 1 shows a diagram with reference to which the principle of the present invention will be elucidated;
Figure 2 shows a diagram illustrating a configuration lacking a drawback inherent in the configuration shown in Figure l;
Figures 3, ~ and 5 show diagrams with reference to which the principle of the present invention will be discussed for a general instance;
and Figure 6 shows a block diagram of an embodiment of the present in-vention.

73~LZ3 In the configuration shown in Fig. 1, a source for producing a substantially flat, out-fanning beam of penetrating radiation, such as X-rays, is designated by 1. A capture screen of a detector device is des-ignated by 2. A subject 3 is placed between this capture screen and the source of radiation on a support (not shown). Without the invention being limited thereto, in the configuration shown in Fig. 1 it is assumed for simplicity that an assembly including the source of radiation and the detec- ;
tor device can perform a rotational movement about an axis O (normal to the plane of the drawing) extending through the subject, the source of radiation ~ !
moving along a circular path having radius R. Self-evidently~ it is also possible to fixedly mount the assembly and rotate the subject about the axis O. In both instances it is achieved that the subject can be irradiated from a series of positions on the circular path having radius R. The circular path having radius r is determined by the minimum distance required for an unobstructed rotation of the assembly, including collimators that may be necessary, about the subject under examinationJ or vice versa.
As will be explained later on, it is an advantage to bound the beam of radiation on one side by a flat plane defined by the axis of rotation extending through the centre of rotation O and the source of radiation itself.
Such a boundaryJ which has been proposed in Dutch patent application 76JO5687J
in the name of the present applicant and laid open to public inspection on November 29J 1977J has the advantages that the angular field of the capture screen as required for the detector device is considerably smaller than the angular field required in the event of an unbounded beam of radiationJ and ;
that the spreading of the radiation intensity is less due to the smaller beam ~idth. To avoid image distortion at the edges of the capture screen, it is an advantage to give this screen an arcuate shapeJ the centre of curvature coinciding with the position of the source of radiation and the radius of cur-vature being R + r. It will be clear that when the capture screen is to con-currently receive a plurality of laminal images of the subjectJ this capture screen must have a spherical surface. When, in the configuration described , - ;

~L0731Z3 above, the subject is irradiated by the out-fanning beam from the source of radiation, at a given position angle of the source-detector assembly relative to the subject a "transmission image" will be projected on the capture screen, which image corresponds with a disc-shaped slice of the subject, the ~hickness of this slice being determined by the height of the flat beam of radiation and the "length" of the strip-like image being defined by the angular width (angle ~ ) of the beam.
The detector device, which may be of a known per se type, prefer-; ably e.g. a detector device in the form of an X-ray image intensifier, is operative to convert a continuous radiation image in analogue form, e.g. an X-ray image, formed on the capture screen, which image is indicative of the radiation absorption in the respective lamina of the subject from the direc-tion in question (position angle a)~ to a corresponding, intensified electric image signal. After passing a thus-obtained electric image signal through a logarithmic intensifier, a signal is obtained that is representative of the density pattern associated with the lamina of the subject applying to the ;
respective positio~ angle ~. It will be clear that for a series of position ; angles ~ such as ~1' ~2' a3~ ~4, etc., a corresponding series of such electric signals can be achieved. After passing through the entire circle having radius R, a group of electric signals is obtained from which an image can be reconstructed that represents the complete density variation within the respective lamina of the subject, provided the angular width (angle ~) of the beam is selected so that each element of the lamina of the subject under examination is irradiated from each position angle d. It is observed in this connection that the electric signals (signal profiles) produced by the detector device are the result of using an out-fanning beam of radiation.
This means that when during the further processing of such signal profiles it is tried to eliminate the point spread function, no exact correction is possible for the full image field, which means a degradation ~f the ultimately achieved tomogram.
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This drawback can be eli~inated by recording the signal profiles provided by the detector device, prior to their further processing, in a separate memory device in a specific manner. Particularly, a signal profile corresponding with each position ~ngle of the source of radiation relative to the subject under examination is recorded as a continuous analogue signal located in accordance with a predetermined path. :[n order to be able to clearly indicate the direct relationship between the manner of recording and the instantaneous position of the device, an example o:f such a path is shown ~i in Fig. 1 by the path sections 0, 1, 2, 3 and 4, such a recorded signal pro- ;
file n corresponding with each position angle d of the source of radiation.
In the configuration shown in Fig. 1, these signal profiles are recorded in `~;
analogue form in accordance with arc-shaped paths having their centre of curvature on the circle having radius R, each of these recorded signal pro-files beginning in centre 0 of this circle. Consequently, signal profile 1 corresponding with position angle dl is achieved by describing a circular arc from centre of curvature ~ 1 at radius R, which arc begins in centre 0.
Each of the other signal profiles are recorded in an identical manner, re-sulting in a bundle of recorded signal profiles beginning in centre 0, so that a collestion of vane-like primary signal profiles is obtained.
It appears possible to scan such a collection in analogue form of primary signal profiles in accordance with another collection of paths, in such a manner that by such a scanning a collection of secondary signal profiles can be constructed, in which each of these secondary signal profiles can be taken as being obtained from an imaginary beam of parallel incident rays. This may be appreciated for the primary signal profiles 1-4 shown in Fig. l by a closer consideration of this Fig. 1. To this end, this Figure shows a beam of parallel incident rays corresponding with the position angle ~4, the primary signal profile fragments corresponding with these parallel incident rays being drawn as the points of intersection of the respective ray and the associated primary signal profile. In other words, the points of intersection between the ray originating from ~1 and signal profile 1~ the 1~731;~3 ray originating from ~2 and signal profile 2, the ray originating from c~3 and signal profile 3 and the ray originating from c~l and signal profile 4.
It appears that the thus-obtained points of intersection lie on an arc-shaped path b going through the centre 0 and having a radius of curvature R and a centre of curvature ~'4 that it diametrically opposite point d4 that is determinative of the direction of incidence of the respective beam of paral-lel rays, in other words the centre of curvature ~'~ is defined by ~ + ~.
Consequently, the secondary signal profile corresponding with such an arcuate scanning may be regarded as a signal profile derived from a beam of parallel rays. Therefore, each position angle ~1 corresponds with a position angle d 'i related thereto and determinative of the arcuate scanning from which a secondary signal profile corresponding with an imaginary beam of parallel rays incident at the angle ~i can be reconstructed. In other words, by such a scanning it is possible to reconstruct from the collection of primary sig-nal profiles the collection of secondary signal profiles, which secondary sig-~ nal proiles can readily be subjected to a standardized exact correction ; with respect to the point spread function.
Such scannings should lag the recording of the primary signal pro-files by a given angle, e.g. 30, as all the primary signal profiles to be scanned must have been actually recorded. After scanning the respective group of primary signal profiles, this group may be erased and different informa-tion may be recorded in the erased portion of the record medium.
In principle, it is also possible to use a beam of radiation hav-ing an angular width twice that shown in Fig. 1. In such a situation, the second half of each primary signal profile should be recorded as schemati-cally indicated by the chain-dotted arc 4', which is possible as the subject under examination is radiation-isotropic, which means that the absorption of the subject along a line depends only on the direction of this line and not on its sense Self-evidently, in this situation the assembly including the source of radiation and the detector device need only rotate through an angle of ~ radians relative to the subject in order to develop a complete tomogram.

1~73~l~Z3 The reconstruction of the primary signal profiles thus-recorded by means of a beam of double angular width can be obtained either by scanning through 2 radians in accordance with "half" arcs b, or by scanning through ~ radians along S-shaped paths, in other words in accordance with mirror~symmetrical arcuate paths b, b'.
The recording of the "half" primary signal profiles in the fnrm of circular arcs going through the origin 0, as discussed with reference to Fig. 1, has the drawback that around this origin the lines of the pattern are difficult to separate, as a result whereof and depending on the manner of scanning to obtain the secondary signal profiles, inaccuracies may occur around this origin. In fact, in the reconstruction of secondary signal pro-files corresponding with beams of parallel rays, as discussed above, the scanning is performed practically tangentially to the respective primary sig-nal profiles especially in the vicinity of this origin 0, which may lead to an unacceptably high inaccuracy.
To eliminate these drawbacks, consequently it is preferred to draw the continuous primary signal profiles in analogue form, instead of as vanè-s ; originating from the origin 0 as shown in Fig. 1, as arcuate spokes origin-ating from a rim as schematically shown in Fig. 2. In this Fig. 2, the circle having radius Rl is the pa~h corresponding with a relative movement of the source of radiation with respect to the centre of rotation 0. Just as in Fig. 1, the circle having radius r in Fig. 2 is determined by the minimum distance required for an unobstructed rotation about the subject. The dis-tance R3 is the distance through which each time a position angle point ~ i is shifted along the tangent to the circle Rl in that point to come to the position angle point ~ 'i on the circle R2 determinative of the respective primary "half" signal profile Pli, which is recorded as an arc-shaped spoke from a rim having radius R3.
Thus in a manner similar to that employed in the configuration shown in Fig. 1, by means of a series of position angle points, such as _ 9 ' :

1~73~Z3 ~ '0- ~'4, a collection of associated primary "half" signal profiles, such as p'0-p'4, is recorded. Without limiting the scope of invention, the dis-tance R3 is selected so that the locus of the extremities of the primary "half" signal profiles, which originally were on the circle having radius r, are now on a circle having radius R4. In other words, point o of the orig-inal primary signal profile p becomes the point o' of the primary signal profile P'O etc. In Fig. 2, the circle having radius R2 indicates the path of the source points shifted along the tangents through the distance R3.
This radius R2 is defined by R2 = ~ R12 -~ R32. In the case of a recording as achieved by means of a configuration as shown in Fig. 2, it appears that the reconstruction of the secondary "half" signal profiles corresponding with beams of parallel incident rays can be obtained by means of arc-shaped scann-ing paths having radius of curvature R2 and each time describecl from the same centre of curvature M corresponding with the point of intersection of the circle about the point of rotation 0 and having the radius Rl on the one hand and the ray of the parallel beam going through this point 0 on the other hand.
In such a configuration, the points of intersection between the primary sig-nal profiles and the scanning paths for achieving the secondary signal pro-files are defined in a better manner. This is schematically shown in Fig. 2 for the position angle point ~4 + ~ and the circular arc determined thereby, which has a radius of curvature R2. The respective points of intersection are indicated by 0', 1', 2', 3' and 4'. These points correspond with a beam of parallel rays incident at an angle d~. It will be clear from the above that as R3 approaches 0, the configuration discussed with reference to Fig. 2 changes to the one discussed with reference to Fig. 1. Consequently, self-evidently the arrangement can be made so that each time two "half" profiles are concurrently recorded, as a result whereof a rotation of only ~ radians is required.
Also in the configuration shown in Fig. 2 the scanning operation is delayed with respect to the recording of the primary signal profiles. The surface area within circle R3 is not initially used for recording in the ~073:~23 configuration shown in Fig. 2. If the thickness of the annular surface area situated between the circles having radii Rl and R3 can be selected suffic-iently small, in which connection the resolution of the selected record med-ium forms the limit, the collections of primary signal profiles can be re-corded in concentric annular areas within one another for a plurality of simultaneous tomograms. It can readily be seen ~see in this respect the left-hand part of Fig. 2) that if, instead of drawing the collection of primary signal profiles for a series of discrete position angle values ~i~ these primary profiles are continuously drawn upon a continuous, preferably uniform, relative rotation of the source about the subject, as will be preferred in practiceJ the path of the primary profiles drawn will not be determined by circular arcs (Pa, Pb, ...... , Pe) but by involutes (A, B, .... , E). It can further readily be seen that, nevertheless, by the described scanning in accordance with arc-shaped paths (J, ~1, ..... , E), again the secondary pro-ile associated with a parallel beam is obtained.
In principle, the invention is not limited to the embodiments ad-apted to the configurations shown in Figs. 1 and 2; the general principle of ; the invention, in which a collection of continuous primary signal profiles as derived by means of a substantially flatJ out-fanning beam of radiation is recorded in analogue form in accordance with a given path pattern and a thus-recorded collection is subsequently scanned in accordance with another col-lection of scanning paths having a pattern so that the secondary signal pro-files developed from this scanning may be regarded as to be derived by means of a beam of parallel rays, may be illustrated by the basic configuration ~ -shown in Fig. 6, in which various variants of the invention may be realized by an appropriate choice of the control functions determining the recording and scanning. In the configuration shown in Figs. 1 and 2, these control functions are such that the recording of the primary signal profiles is per-formed in accordance with arc-shaped paths (discontinuous rotation) or in accordance with convolutes such as e.g. hypercycloids (continuous rotation), .

~ ~73:~L23 while the scanning thereof to form the secondary signal profiles is performed in accordance with arc-shaped paths. In general, these control functions may be selected so that both the primary signal profiles and the paths along which ' these profiles are scanned to produce the secondary signal profiles, form a simple line pattern. For example, there are the following possibilities:
the primary signal profiles are recorded in analogue form in accordance with essentially linear paths either in juxtaposed or in superposed relationship, while the scanning is performed in accordance with arcuate paths intersecting the pattern of primary signal profiles; the primary signal profiles are re-corded in analogue form in accordance with similar, juxtaposed or superimposed -~ arcuate paths, while the scanning for the reconstruction of the secondary signal profiles is performed along linear paths intersecting the pattern of primary signal profiles; and the primary signal profiles are recorded in analogue form in accordance with essentially linear paths either in juxta-posed or in superimposed relationship, while the scanning is performed in accordance with essentially linear paths intersecting the pattern of primary signal profiles.
In the situation in which the subject under examination and the ~`
assembly including the source of radiation and the detector device are ro-tated relative to each other, a series of primary signal profiles is ob-tained which each are a function of the position angle of the source of radi-ation. The relationship between the primary and secondary signal proiles for this situation will be derived with reference to Fig. 3. In the config-uration shown in Fig. 3 it is assumed that the source of radiation moves along an arc-shaped path having radius R about the centre of rotation M from the position PO to the position P~ etc. By 1 is indicated a ray from the source of radiation when the latter is in position PO' this ray 1 enclosing an angle ~ O with the line PoM. By 2 is indicated a ray parallel to ray 1 and emanating from the source of radiation when the latter is in the position P~ , i.e. after a rotation about centre of rotation M through an angle ~ .

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~733L;23 The condition that this ray 2 is parallel with the ray 1 is that khe sum of ; the angle ~ enclosed between ray 2 and line P~ M and the angle q is equal to ~O. In other words, the secondary signal profile associated with direc-tion ~O is obtained by measuring the transmission of the ray enclosing an angle L~= yO- d with the connecting line P ~ M upon each angular rotation q of the source of radiation. The distance from this ray to the centre of ; rotation M is R.sin~ = R.sin ( y - ~). The radiation transmission values as measured when an out-fanning beam of radiation is used, e.g. a beam of X-rays, may or may not be logarithmatized, and recorded, for example so that the primary signal profiles derived from this out-fanning beam as a function of the above angle ~, sine ~ , tangent ~ , or in general f( y ), are paral-lel, straight lines, the spacing of which is proportional to the position angle ~ of the X-ray sour~e. The secondary signal profile corresponding with direction ~ O can then be found as a line through point ~ = 0, ~ = yO, and is further characterized by ~ ~ ~ = yO.
It will be clear that the path along which the scanning is per-formed for obtaining the secondary signal profile, is a straight line if the primary signal profile derived by measuring by means of an out-fanning beam is recorded as a function of the angle y, whereas this scanning path follows f( ~ ), if the primary signal profile is recorded as f(C~ ). This is illus-trated in Fig. 4.
The secondary signal profiles should be available as a function of the irradiation angle ~, particularly as a function of the distance R.sin to the centre of rotation M, which implies that, if the scanning is performed in accordance with the right ~ and, for example, the time is selected to be proportional to this distance, starting from ~ = 0, the scanning will have to be performed at increasing rate so as to maintain the first derivative with respect to time of the function sin L~at a constant value.
The shape of the primary profile as obtained as a function of C~
depends on the geometry of the source of radiation and the detector device, . ~

~L073~23 while this shape can be affected further by the optic and optoelectronic dis-tortions.
In fact, as appears from Fig. 5, in the event of a flat detector dr the primary profile follows tangent y , whereas in the event of a curved detector and the source of radiation disposed in a centra of curvature the primary profile follows sine ~. The secondary profiles may, for example, be derived by an appropriate scanning in the image intensifier tube bb re-ceiving the image developed by the detector device through a lens system ok.
In order to obtain an optimally uniform secondary profile, it may be advantageous to record the primary profiles so that the paths have such a width that they are contiguous. An optimalization with respect to the signal~
to-noise ratio or the resolution can be obtained by varying the width of the path followed during the scanning of the primary profiles.
In the block diagram of a general embodiment of the invention, as shown in Fig. 6, 1 is a recording device in which the electric signals derived from the detector device can be recorded. Such a recording device may include, for example an electronic image converter tube, a cathode ray tube having a persistent phosphor screen, or a continuo*s two-dimensional magnetic sheet memory. The electric signals derived from the detector device are supplied to input I. A scanner 2 is adapted to coact with the recording device to scan the information recorded in this device, the electric signals obtained as a result of the scanning being developed at output U. The paths along which the information supplied to input I is recorded are determined by a record control device 3. This record control depends on position information Pl con-cerning the position or direction from which the subject under examination is :, irradiated by the source of radiation. In the configuration shown in Fig. 1, ; the series of posîtions from which the irradiation is performed is indicated by a circular path about the axis of rotation extending through the subject under examination. In this configuration the above position information con-sists of position angles such as ~ 1~ ~2 etc. Assuming that the size of ::

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111373~Z3 radius R is also known, the control information required for writing-in the primary signal profiles, which information is to be processed by the record control device 3, is thus complete. Consequently, in response to the position angle information supplied, which information is converted by translator 4 ;~
to corresponding position signals, the record control device 3 supplies con-trol signals to the recording device 1, by means of which control signals the electric input signals supplied to recording device 1 are recorded, for example in accordance with the arc-shaped paths shown in Fig. 1. As stated earlier, by choosing different position information it can be achieved that, by means of the control signals for the recording device 1 as supplied by , ~:
the record control device 3 as a result thereof, the patterns of recorded primary signal profiles are selectively modified. The position signals pro- i duced by translator 4 are supplied to a function converter 5, as a result whereof it is achieved that the control signals intended for the scan control device 6 are related in accordance with a given function to the signals by means whereof the control of device 3 is determined. In the configuration -shown in Fig. 1, this relationship is represented by an angular difference ~ , -~
so that each position angle d determinative o~ the control performed by de-vice 3 is associated with a position angle ~ + ~ determinative of the con-trol performed by device 6. By means of the control action exerted by this device 6 on the scanning device 2 it is achieved that the recorded primary ;
signal profiles are scanned in accordance with the desired paths. In the con-figuration shown in Fig. 1, these paths are the arc-shaped paths each having a centre of curvature determined by d ~ ~ and a radius of curvature R. The control signals required for recording the primary signal profiles, as well as the control signals required for the scanning thereof, may be produced in a simple manner, for example, by means of electro-mechanical means serving for the transmission of angular information, which means are coupled to the drive mechanism producing the relative rotation of the subject relative to the assembly including the source of radiation and the detector device. The ~ .

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1~73~23 output signals obtained from such a scanning as developed at the output U of the scanning device are representative of the secondary signal profiles, which profiles may be regarded as signal profiles produced by the irradiation by means of an imaginary beam of parallel rays. As explained in the above, the scanning action performed by the scanning device 2 should lag a given period of time ~ith respect to the recording of the primary signal profiles. To this end, a delay device 7 is connected between the function converter 5 and the scan control device 6. If desired, this delay device may be a variable ; one in order to permit selective adjustment of the delay and a-laptation there-of to the conditions of operation.
Output signal U produced by the scanning device 2 is readily suit-ed for further processing. As an exact correction with respect to the point spread function has been performed previously, a high resolution tomogram can readily be obtained therefrom by superposition, in spite of the fact that a source of radiation producing an out-fanning beam is used. Although, primarily, the practical realization of the present apparatus will involve tomographic image reconstruction in analog manner, of course it is readily possible to digitalize the outpu~ signals U and to perform the image recon-struction point-by-point by means of a computer. The latter procedure may prove advantageous if not the entire cross-section but only specific parts thereof are considered oi i4terest.

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Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of pre-processing radiological information by means of electronic analogue techniques for use in a process for reconstructing an image of a desired laminal plane of a subject, said method comprising: ex-posing a subject to a substantially flat, out-fanning beam of penetrating radiation; recording a plurality of continuous image sections of the subject on a record medium in analogue form while altering the relative position of the beam, the subject and the record medium in accordance with a collection of recording paths each having a predetermined trajectory and spatial orien-tation related to an associated portion of exposure; scanning and reading-out the image information as recorded in accordance with another collection of scanning paths each having a predetermined trajectory intersecting various recording paths and also having a spatial orientation related to the associ-ated position of exposure; and recombining the read-out information accord-ing to said process for reconstructing an image of a selected lamina within the subject.

2. An apparatus for forming an image of a lamina of a subject under examination by means of penetrating radiation, such as X-rays, comprising a source for producing a substantially flat, out-fanning beam of said pene-trating radiation, a support for supporting the subject under examination so that said subject is irradiated by said beam, and a detector device for producing and processing continuous signal profiles in analogue form which are indicative of the radiation absorption in the lamina of the subject as determined by said beam and are the result of a relative movement between said subject and an assembly including said source of radiation and said detector device, characterized by first means for recording in an analogue mode and in dependence upon a series of first positions successively taken by said source of radiation relative to said subject, a collection of primary continuous signal profiles in analogue form corresponding therewith; and second means for scanning, in dependence upon a series of second positions related to said first positions, said collection of primary signal profiles in accordance with a collection of paths corresponding with said series of second positions, in such a manner that the signal profile fragments each corresponding with such a path together form a secondary signal profile that is representative of the radiation absorption of said subject as a result of an imaginary beam of parallel rays incident from a direction correspond-ing with one of said first positions.

3. An apparatus according to claim 2, characterized in that said first means for recording said primary signal profiles include record control means arranged so that each of said primary signal profiles is recorded in accord-ance with an arc-shaped path having a radius R1, the centre of which, in unique relationship with the respective one of said first positions, is on a circle having radius R2, in which R2 ? R1 round the point 0 representing the axis of rotation of the source of radiation with respect to the subject, the beginning of each primary signal profile being on a circle round said point 0 having radius R3, in which R3 ? 0 but may be < R2, and is deter-mined by the absorption, as caused by the subject, of the ray of the beam emanating from the respective first position that coincides with the axis of rotation; and that said second means for scanning the primary signal profiles include scan control means arranged so that said collection of primary, arc-shaped signal profiles is scanned in accordance with likewise arc-shaped paths each having a radius R2 and the respective centre of curvature M of which is in a point on said circle having radius R1 and the centre of curvat-ure of which corresponds with that end of the diameter indicating the direc-tion of the parallel beam that is remote from the curvature of the respective primary signal profiles scanned.

4. An apparatus according to claim 2 characterized by radiation bounding means for bounding the spreading of the out-fanning beam so that each primary signal profile has such a length that one of the two parts of the re-spective lamina of the subject, which parts are bounded by said axis of rota-tion, is complete irradiated.

5. An apparatus according to claim 3 or 4, characterized in that said record control means and said scan control means derive their control sig-nals from electro-mechanical means serving for the transfer of angle infor-mation and coupled to the drive mechanism producing the relative movement of the subject with respect to the assembly including said source of radiation and said detector device.

6. An apparatus in accordance with any of claims 2-4, characterized in that said first means are adapted to record a plurality of collections of primary signal profiles that are located within concentric, annular areas;
and that said second means are adapted to scan the respective one of said collections of primary signal profiles in accordance with a corresponding plurality of collections of paths.

7. An apparatus according to claim 2, characterized in that said first means are adapted to record said primary signal profiles in accordance with essentially linear paths; and that said second means are adapted to scan the pattern of primary signal profiles in accordance with arcuate paths inter-secting said pattern of primary signal profiles.

8. An apparatus according to claim 2, characterized in that said first means are adapted to record said primary signal profiles in accordance with arcuate paths; and that said second means are adapted to scan the pat-tern of primary signal profiles in accordance with essentially linear paths intersecting said pattern of primary signal profiles.

9. An apparatus according to claim 2, characterized in that said first means are adapted to record said primary signal profiles in accordance with linear paths; and that said second means are adapted to scan the thus-recorded primary signal profiles in accordance with likewise linear, parallel paths intersecting the respective primary profiles at a fixed angle.

10. An apparatus according to any one of claims 2-4, characterized in that said first means are adapted to record said primary signal profiles in accordance with paths that are longitudinally contiguous with one another or overlap one another; and that said second means include facilities for selectively adjusting the width of the paths along which the respective primary signal profiles are scanned.

11. An apparatus according to claim 2, characterized by delay means for delaying the operation of said second means relative to that of said first means.

12. An apparatus according to claim 11, characterized in that said delay means include setting means for setting the delay to be introduced at a selected value.

13. An apparatus according to any one of claims 2-4, characterized in that said first and second means are united in a memory device in the form of an electronic image converter tube, in which tube the respective collec-tion of primary signal profiles is recorded on a target by means of an elec-tron beam, after which the respective signal profiles are scanned by the same or by another electron beam.

14. An apparatus according to any one of claims 2-4, characterized in that said first means include a memory device in the form of a cathode ray tube having a persistent phosphor screen, and that said second means include an image scanning tube, the collection of primary signal profiles recorded on the screen of said cathode ray tube being displayed by optical means on the entrance window of said image scanning tube.

15. An apparatus according to any one of claims 2-4, characterized in that said first means include a memory device in the form of continuous two-dimensional magnetic sheet memory, the recording of the primary signal pro-files and the scanning of the secondary signal profiles being performed by means of magnetic heads.