CN101515028B - Three-dimensional imaging method and system for alleviating magnetic susceptibility artifact - Google Patents
Three-dimensional imaging method and system for alleviating magnetic susceptibility artifact Download PDFInfo
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- CN101515028B CN101515028B CN2008100072201A CN200810007220A CN101515028B CN 101515028 B CN101515028 B CN 101515028B CN 2008100072201 A CN2008100072201 A CN 2008100072201A CN 200810007220 A CN200810007220 A CN 200810007220A CN 101515028 B CN101515028 B CN 101515028B
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Abstract
The invention discloses an imaging method for alleviating magnetic susceptibility artifact, which comprises the following steps: determining the number of three-dimensional sub-images required by synthesizing a three-dimensional image, and the frequency of using a radio-frequency pulse excited checked body required by reestablishing each three-dimensional sub-image; for each three-dimensional sub-image, using the frequency of using the radio-frequency pulse excited checked body, applying read-out gradient to receive echoed signals, and then reestablishing the three-dimensional sub-image according to all the echoed signals of the three-dimensional sub-image; and using all reestablished three-dimensional sub-images to synthesize the three-dimensional image. The invention also discloses a system for alleviating the magnetic susceptibility artifact. The method and the system can obviously alleviate the magnetic susceptibility artifact in three-dimensional magnetic resonance imaging based on high imaging efficiency.
Description
Technical field
The present invention relates to magnetic resonance imaging (MRI, Magnetic Resonance Imaging) technology, particularly a kind of three-D imaging method and system that alleviates magnetic susceptibility artifact.
Background technology
Magnetic resonance imaging is to utilize the magnetic resonance phenomenon to be carried out to a kind of technology of picture.The principle of magnetic resonance phenomenon mainly comprises: comprise the atomic nucleus of odd number proton, and the hydrogen nuclei that for example extensively exists in the human body, its proton has spin motion; Just as a small magnet, and the spin axis of these small magnets do not have certain rules, if apply exterior magnetic field; The spin axis of these small magnets will be arranged by the magnetic line of force of exterior magnetic field again; In this state, radio frequency (RF, the Radio Frequency) pulse with CF excites; These small magnets absorb certain quantity of energy and resonate, and have just produced the magnetic resonance phenomenon.After stopping to launch radio-frequency pulse, the small magnet that is excited progressively discharges the energy that absorbs with form of electromagnetic wave, the state before its phase place and energy level all return to and excite.The electromagnetic wave of small magnet emission is converted into frequency-region signal, gets final product reconstructed image through further processing such as space encodings again.
In mr imaging technique; The exterior magnetic field that applies comprises two kinds of main field and gradient magnetics; Wherein gradient magnetic is positioned at imaging region; Its effect is the gradient magnetic of the three dimensions linear change of an additional X, Y, Z quadrature in main field, makes the atomic nucleus of diverse location have different resonant frequency, is the precondition of imaging choosing layer and space encoding.
With the two-dimensional scan of axle position is example, applies slice selective gradient Gz in above-mentioned Z direction, applies phase encoding gradient Gy in the Y direction; Applying the frequency coding gradient at directions X is readout gradient Gx, and Fig. 1 shows the gradient that applies on X in the two-dimensional scan, Y, the Z direction, wherein 101 represents radio-frequency pulse; With the RF sign, first is 90 ° of pulses, and second is 180 ° of pulses; To nuclear excitation one time the time, use such set of pulses; 102 representative choosing layer (SS, Slice Selection) direction identify with SS; 103 represent phase encoding (PE, Phase Encode) direction, identify with PE; (RO, Read Out) direction is read in 104 representatives, and with the RO sign, 105 represent echoed signal.Comprise during imaging that following three steps apply gradient magnetic: 1) after choosing layer direction 102 applies slice selective gradient Gz 11; The frequency of the proton of diverse location is inequality on the Z axle; Utilize this moment the radio-frequency pulse of a certain frequency just optionally to excite the proton of a certain frequency; Make the proton of this frequency and the proton of other frequencies make a distinction, the proton of a certain frequency that is excited is called an aspect, the slice thickness that is excited is determined by the bandwidth of gradient field intensity and radio-frequency pulse jointly; When the radio-frequency pulse bandwidth is narrow more, when the gradient field intensity is big more; Aspect is thin more, and then the image resolution ratio of this aspect is high more, slice selective gradient Gz 11 apply position and radio-frequency pulse excite the position corresponding; 2) in the aspect of being excited, apply phase encoding gradient Gy 12; There is phase differential in the proton of diverse location on the Y direction; Phase encoding is exactly to utilize this phase differential to decide the locus of proton, and the position that applies of phase encoding gradient Gy 12 is positioned at after the slice selective gradient Gz 11; 3) another direction in the aspect of being excited applies readout gradient Gx 13; The residing magnetic field intensity of the proton of diverse location is inequality on the directions X; The signal that forms can be decayed with different frequencies, readout gradient Gx 13 apply position and phase encoding gradient Gy 12 to apply the position identical.State 2 on the implementation)-3) after the step, can obtain the echoed signal 15 that atomic nucleus sends, be recorded in the k space; Represent frequency coding with transverse axis, represent phase encoding, form a data matrix with the longitudinal axis; Columns wherein is a number of sampling, and line number is the phase encoding step number.1 row that per 1 echoed signal is filled the k space after the scanning completion obtains complete data matrix, carries out two-dimension fourier transform, rebuilds the image of subject.
Compare with above-mentioned two-dimensional scan, the 3-D scanning in the magnetic resonance three-dimensional imaging is applying aspect the gradient differently, and Fig. 2 shows the gradient that applies on X in the 3-D scanning, Y, the Z direction; Wherein 201 represent radio-frequency pulse; With the RF sign, first is 90 ° of pulses, and second is 180 ° of pulses; 202 representative choosing layer direction identify with SS; 203 represent phase-encoding direction, identify with PE; Direction is read in 204 representatives, identifies with RO; 205 represent echoed signal.Can find out on choosing layer direction 202; Making the slice selective gradient Gz21 that dots is the optional gradient that applies; Promptly on choosing layer direction 202, can not apply slice selective gradient, but will inevitably apply another kind of three-dimensional gradient 22 on the choosing layer direction 202, the position of this three-dimensional gradient 22 and phase encoding gradient 23 to apply the position identical; But amplitude is not necessarily identical with phase encoding gradient 23, and the position that applies of readout gradient 24 is positioned at after the radio-frequency pulse.
Magnetic susceptibility is one of base attribute of material; After the magnetic susceptibility of certain material was meant that this material gets into exterior magnetic field, the ratio of the magnetization and exterior magnetic field was when magnetic resonance imaging; Pseudo-shadow to occur in the bigger organizational interface of two kinds of difference in magnetic susceptibility, be called magnetic susceptibility artifact.Be imaged as example with human body; When in the seized tissue in the human body metal implant being arranged; Just there are bigger difference in magnetic susceptibility in this seized tissue and this metal implant, promptly have the uneven problem of magnetic susceptibility, and this will cause that local magnetic field is inhomogeneous; Causing the image local signal obviously to weaken or strengthening is that picture signal is inhomogeneous, anamorphose etc.For alleviating the magnetic susceptibility artifact in the imaging as far as possible; In number of applications, find to adopt spin echo (SE; Spin Echo) type rf pulse sequence excimer daughter nucleus can reduce magnetic susceptibility artifact in two-dimensional imaging, therefore a series of improvement occurred to the SE sequence; To realize better eliminating the purpose of magnetic susceptibility artifact, mainly comprise following two kinds of improving one's methods in the prior art two-dimensional imaging to the SE sequence:
1. this method can be called projection angle inclination (VAT, View angle tilting) method, mainly is on two-dimension fourier imaging sequence basis, to make amendment, and is specially when applying readout gradient, on the slice selective gradient direction, applies extra gradient.
Fig. 3 shows the sequential structure that uses in this method, and wherein 301 radio-frequency pulses for SE sequence composition identify with RF, and first is 90 ° of pulses, and second is 180 ° of pulses; 302 representative choosing layer direction with the SS sign, comprise slice selective gradient 31 on this direction, and the extra slice selective gradient 32 of arrow indication; 303 represent phase-encoding direction, with the PE sign, on this direction, comprise the phase encoding gradient 33 that applies; Direction is read in 304 representatives, with the RO sign, on this direction, comprises the readout gradient 34 that applies.Can find out when applying readout gradient 34 and on choosing layer direction 302, apply extra slice selective gradient 32.After applying extra slice selective gradient on the choosing layer direction, can make the projection angle run-off the straight of subject, to alleviate the influence of chemical shift to imaging.Shown in Fig. 4 a is that choosing layer direction do not apply the imaging when having the pseudo-shadow of chemical shift before the extra slice selective gradient; Wherein 41 is seized tissue; Wherein there is chemical shift between the different tissues; 42 for imaging, can find out that corresponding seized tissue 41 exists the position of chemical shift to have anamorphose on 42, identifies respectively with A and B.Shown in Fig. 4 b is that choosing layer direction applies the imaging when eliminating the pseudo-shadow of chemical shift after the extra slice selective gradient, and θ wherein is the angle of inclination, and 43 is seized tissue; Identical with Fig. 4 a, wherein also there is chemical shift between the different tissues, 44 are imaging; Can find out because the existence of θ; Corresponding seized tissue 43 exists the position of chemical shift not have anamorphose in the imaging, and therefore after choosing layer direction applied extra slice selective gradient, the pseudo-shadow of chemical shift had obtained obvious inhibition to the influence of imaging.Facts have proved that this method can be used for being reduced in the picture because the magnetic susceptibility artifact that metal implant etc. cause equally.
But there is certain defective in this method, i.e. the inclination of projected angle can cause soft edge, in order to overcome the problem of soft edge; There is the scheme of reading bandwidth that strengthens at present; Read bandwidth for gathering the inverse of each echoed signal time, strengthening and reading bandwidth is exactly to reduce to receive the time of echoed signal at k space padding data, but this way will reduce signal noise ratio (snr) of image; Thereby reduce the quality of image, some detail sections in the image are buried in oblivion.The experiment proof; In order to reduce the influence of magnetic susceptibility artifact to imaging, need be when using this method than reducing the bigger pitch angle of the pseudo-shadow of chemical shift, this will aggravate the fog-level of image border; If adopt the bigger bandwidth of reading, will make the further variation of signal noise ratio (snr) of image.
2. this method also can be called the z-Shim method, behind the slice selective gradient of ordinary two dimensional Fourier imaging sequence, applies a series of gradients that are called shimming (shimming).Fig. 5 shows the sequential structure that uses in this method, 501 radio-frequency pulses formed for the SE echo sequence wherein, and first is 90 ° of pulses, second is 180 ° of pulses; 502 representative choosing layer direction apply slice selective gradient 51, the shimming gradient of arrow indication 52 for applying on this direction; 503 represent phase-encoding direction, the phase encoding gradient 53 that on this direction, applies; 504 represent the readout gradient direction, the readout gradient 54 that on this direction, applies.
The theoretical foundation of this method is in each voxel of hypothesis subject; Because the inhomogeneous space distribution on choosing layer direction in the inhomogeneous magnetic field that causes of magnetic susceptibility is for linear; Through on choosing layer direction, applying one and the inhomogeneous magnetic field gradient in the opposite direction in this magnetic field, reach the purpose that the reunion phase place overcomes magnetic susceptibility artifact.In when imaging, whenever apply a shimming gradient and become piece image, apply several shimming gradients after; The some width of cloth images that obtain; According to the imaging effect of these several images, select a shimming gradient that compensation effect is best, as the shimming gradient of final imaging.
Facts have proved that this method can overcome the inhomogeneous picture signal problem of non-uniform that causes of magnetic susceptibility preferably, but find the complicated processes of a manual search of shimming gradient needs of final imaging, sweep time is longer, and the efficient that promptly forms images is relatively poor.
In sum, present in two-dimensional imaging, being directed against alleviates the method that magnetic susceptibility artifact is taked, and also can't obtain effect preferably based on high imaging efficiency, the also unspecial method that alleviates magnetic susceptibility artifact to three-dimensional imaging.
Summary of the invention
In view of this, a fundamental purpose of the present invention is to provide a kind of three-D imaging method that alleviates magnetic susceptibility artifact, and this method can significantly alleviate the magnetic susceptibility artifact in the three-dimensional magnetic resonance imaging based on high imaging efficiency.
Another fundamental purpose of the present invention is to provide a kind of 3-D imaging system that alleviates magnetic susceptibility artifact, and this system can significantly alleviate the magnetic susceptibility artifact in the three-dimensional magnetic resonance imaging based on high imaging efficiency.
Technical scheme of the present invention is achieved in that
A kind of three-D imaging method that alleviates magnetic susceptibility artifact, this method comprises:
Confirm the quantity of the synthetic required three-dimensional subimage of a width of cloth 3-D view and rebuild the three-dimensional subimage of each width of cloth and need use radio-frequency pulse to excite the number of times of subject;
For the three-dimensional subimage of each width of cloth, use radio-frequency pulse to excite the said number of times of subject, and apply readout gradient reception echoed signal, all echoed signals according to this three-dimensional subimage reconstruct this three-dimensional subimage then;
All three-dimensional subimages that use reconstructs, a synthetic width of cloth 3-D view.
The quantity of said definite three-dimensional subimage is: confirm the bed thickness of the three-dimensional subimage of a width of cloth, use the bed thickness of the bed thickness of a width of cloth 3-D view divided by the three-dimensional subimage of a said width of cloth, obtain the quantity of said three-dimensional subimage.
The bed thickness of determined three-dimensional subimage when said subject makes the main field skew big makes the bed thickness of hour determined three-dimensional subimage of main field skew less than said subject.
Said radio-frequency pulse is: variable flip angle fast acquisition interleaved spin echo, or spin-echo sequence, or fast acquisition interleaved spin echo.
This method further comprises: when applying readout gradient, on choosing layer direction, apply an extra slice selective gradient, make the projection angle run-off the straight of subject.
This method further increases when receiving echoed signal reads bandwidth.
Said synthetic step is to adopt maximum Gray Projection method or self-adaptation synthetic method to realize.
A kind of 3-D imaging system that alleviates magnetic susceptibility artifact comprises that a gradient magnetic applies module, an excitation module, a receiver module and an image-forming module, and this system further comprises a parameter determination module and a synthesis module, wherein:
Said parameter determination module is used for confirming the quantity of the synthetic required three-dimensional subimage of a width of cloth 3-D view and rebuilds the three-dimensional subimage of each width of cloth and need use radio-frequency pulse to excite the number of times of subject;
Said gradient magnetic applies module, is used for applying slice selective gradient in choosing layer direction, applies phase encoding gradient at phase-encoding direction, and applies readout gradient in the readout gradient direction;
Said excitation module is used for to subject emission radio-frequency pulse, to excite the said number of times of subject;
Said receiver module is used for when gradient magnetic applies module and applies readout gradient, receiving the echoed signal that subject sends;
Said image-forming module is used to utilize all echoed signals of the same three-dimensional subimage that said receiver module receives to reconstruct the three-dimensional subimage of a width of cloth;
Said synthesis module is used to use all three-dimensional subimages that reconstruct, a synthetic width of cloth 3-D view.
Said gradient magnetic applies module and is further used for when the readout gradient direction applies readout gradient, applies an extra slice selective gradient in choosing layer direction, makes the projection angle run-off the straight of subject.
Based on technique scheme, the present invention alleviates the three-D imaging method and the system of magnetic susceptibility artifact, and the echoed signal of utilizing the subject atomic nucleus to send is rebuild the three-dimensional subimage of some width of cloth, again all three-dimensional subimages that obtain is synthesized a width of cloth 3-D view.Because in the three-dimensional imaging; Choosing layer direction has three-dimensional gradient; Employed different three-dimensional gradients when therefore synthetic 3-D view has comprised each width of cloth three-dimensional subimage scanning; When being equivalent to directly form this 3-D view, apply the result of different z-shim gradient scanning, promptly alleviated the magnetic susceptibility artifact in the imaging.But when directly forming 3-D view; Form image once when needing z-shim gradient of every use; And, therefore use the method for above-mentioned compositing 3 d images to remove the process that goes up manual search from by the process of manual search The optimal compensation z-shim gradient, improved imaging efficiency.
Further, in the technical scheme of the present invention, can also on choosing layer direction, apply extra slice selective gradient, make and, can improve the projection angle run-off the straight because the problem of the inhomogeneous anamorphose that causes of magnetic susceptibility based on the projection angle of this inclination; And when rebuilding the three-dimensional subimage of above-mentioned each width of cloth, can increase and read bandwidth and apply the soft edge that is caused behind the extra slice selective gradient to overcome on choosing layer direction; But reading bandwidth, above-mentioned increase can't reduce signal noise ratio (snr) of image; This be because: the three-dimensional subimage of each width of cloth of compositing 3 d images; Use the data in the k space to rebuild respectively; Because the data in the k space all are to fill according to the echoed signal that atomic nucleus sends, when therefore using this synthetic method, are equivalent to the echoed signal that atomic nucleus that the three-dimensional subimage with each width of cloth uses sends and have carried out on average; According to the characteristic that noise occurs at random, above-mentionedly on average make final 3-D view to have increased when rebuilding the three-dimensional subimage of each width of cloth to read bandwidth and the defective that signal to noise ratio (S/N ratio) descends occurs.
Description of drawings
The gradient synoptic diagram of Fig. 1 for applying in the gradient magnetic direction in the prior art two-dimensional scan;
The gradient synoptic diagram of Fig. 2 for applying in the gradient magnetic direction in the prior art 3-D scanning;
The sequential structure synoptic diagram of Fig. 3 for using in the projection angle method of tilting in the prior art;
Fig. 4 a is in the prior art in the projection angle method of tilting, and choosing layer direction do not apply the imaging synoptic diagram when having the pseudo-shadow of chemical shift before the extra slice selective gradient;
Fig. 4 b is in the prior art in the projection angle method of tilting, and choosing layer direction applies the imaging synoptic diagram after the extra slice selective gradient;
The sequential structure synoptic diagram of Fig. 5 for using in the z-shim method in the prior art;
The sequential structure synoptic diagram that Fig. 6 uses in the three-D imaging method of magnetic susceptibility artifact for the present invention alleviates;
Fig. 7 alleviates the process flow diagram of the three-D imaging method of magnetic susceptibility artifact for the present invention;
Fig. 8 alleviates the 3-D imaging system structural representation of magnetic susceptibility artifact for the present invention.
Embodiment
For making objects and advantages of the present invention clearer, below in conjunction with accompanying drawing and embodiment the present invention is done further detailed explanation, these explanations are nonrestrictive.
At first, introduce the method that the present invention alleviates magnetic susceptibility artifact.
The present invention alleviates in the three-D imaging method of magnetic susceptibility artifact, confirms the quantity of the synthetic required three-dimensional subimage of a width of cloth 3-D view and rebuilds the three-dimensional subimage of each width of cloth and need use radio-frequency pulse to excite the nuclear number of times of subject; For the three-dimensional subimage of each width of cloth, the number of times that uses radio-frequency pulse excimer daughter nucleus to confirm, and apply readout gradient and receive echoed signal reconstructs this three-dimensional subimage according to all echoed signals of this three-dimensional subimage; All three-dimensional subimages that use reconstructs, a synthetic width of cloth 3-D view.
The present invention alleviates in the three-D imaging method of magnetic susceptibility artifact, and gradient magnetic can comprise: at the slice selective gradient that choosing layer direction applies, the extra slice selective gradient that applies in the time of the corresponding readout gradient of choosing layer direction; With the phase encoding gradient that applies at phase-encoding direction; With reading the readout gradient that direction applies.
In the description hereinafter, behind the number of times that every use radio-frequency pulse excimer daughter nucleus is definite, the three-dimensional subimage of reconstruction is called three-dimensional thin layer image, with using the synthetic 3-D view of three-dimensional subimage to be called three-dimensional thick-layer image.
The present invention alleviates in the three-D imaging method of magnetic susceptibility artifact; Radio-frequency pulse can be selected variable flip angle 3-dimensional fast spin echo sequence, and this is that the sequence used in a kind of existing three-dimensional imaging and the difference of SE sequence are; This sequence is made up of 90 ° of pulses and a plurality of other angle pulses of confirming according to actual parameter; Can produce a plurality of echoed signals, be called echo train, the echo number in each echo train is exactly an echo train length.This sequence has increased the quantity that receives echoed signal in the unit interval, and the filling speed in k space is accelerated, and has improved scan efficiency.Certainly; Except above-mentioned variable flip angle 3-dimensional fast spin echo sequence; Radio-frequency pulse in the inventive method can also adopt other sequences such as SE sequence, and when adopting the SE sequence, scan efficiency is lower than uses above-mentioned variable flip angle 3-dimensional fast spin echo sequence.
Fig. 6 shows the sequential structure that uses in the inventive method; Wherein 601 is rf pulse sequence; The variable flip angle 3-dimensional fast spin echo sequence of promptly selecting for use, first is that 90 ° of pulses, second are 180 ° of pulses, the 3rd to the 5th is the pulse of predetermined angle; 602 representative choosing layer direction; Apply slice selective gradient 61 and three-dimensional gradient 62 on this direction; Wherein the arrow indication is the extra slice selective gradient that applies 63, and wherein slice selective gradient 61 is optional gradients that apply, and this point has been described in detail in the three-dimensional imaging of background technology; 603 represent phase-encoding direction, apply phase encoding gradient 64 on this direction; Direction is read in 604 representatives, on this direction, applies readout gradient 65.Can find out that the readout gradient 65 that applies in the extra slice selective gradient that applies 63 and 604 in 602 has simultaneity; Promptly when applying readout gradient 65; Apply extra slice selective gradient 63 in choosing layer direction; The amplitude of this extra slice selective gradient that applies 63 can be identical with the slice selective gradient 61 that normally applies on the choosing layer direction, also can be set to other values, specifically can be provided with arbitrarily according to actual needs; The width of this extra slice selective gradient that applies 63 is time span just, and is identical with readout gradient 65.
The situation of carrying out two-dimensional imaging with use SE sequence in the prior art is identical; When stating variable flip angle 3-dimensional fast spin echo sequence in the use and carrying out three-dimensional imaging; When applying readout gradient, on choosing layer direction, apply extra slice selective gradient, can make the projection angle run-off the straight equally; Based on the projection angle of this inclination, can improve the anamorphose problem that causes by magnetic susceptibility artifact.
Alleviate in the three-D imaging method of magnetic susceptibility artifact in the present invention,, be divided into several thin layer images and scan respectively the three-dimensional thick-layer image that ultimate demand obtains; The foundation of layering is when being in the exterior magnetic field owing to subject; Can make exterior magnetic field that skew to a certain degree takes place, when this skew is big, can select the bed thickness of three-dimensional thin layer image thinner; And should skew hour, can select the bed thickness of three-dimensional thin layer image thicker.If for example the three-dimensional thick-layer image that obtains of ultimate demand is 3 millimeters; Can be that the three-dimensional thin layer image of 6 width of cloth scans respectively with should three-dimensional thick-layer image being divided into; The three-dimensional thin layer image of every width of cloth is 0.5 millimeter, and the bed thickness of image can be realized through the bandwidth of control radio-frequency pulse.Use the bed thickness of the bed thickness of thick-layer image, just can obtain the quantity of three-dimensional thin layer image divided by the above-mentioned three-dimensional thin layer image of determining.When rebuilding each width of cloth three-dimensional thin layer image; Possibly need to use radio-frequency pulse excimer daughter nucleus several times; This number of times need be confirmed according to parameters such as conceivable image resolution ratio, scanning sequence echo train lengths; Under different application scenes, determine different number of times, concrete definite method is identical with the method for definite excimer daughter nucleus number of times in the common three-dimensional image reconstruction method.Behind the number of times that uses radio-frequency pulse excimer daughter nucleus to determine; The echoed signal of utilizing atomic nucleus to send; Use common three-dimensional image reconstruction method to obtain the three-dimensional thin layer image of a width of cloth; After the three-dimensional thin layer image of quantification has all been rebuild, with the three-dimensional thick-layer image of the synthetic width of cloth of all three-dimensional thin layer images that reconstruct.
In practical application; The mode of Multi Slice Mode is always adopted in magnetic resonance imaging, need to suppose to m the three-dimensional thick-layer image of the synthetic m width of cloth of selected aspect, and the required three-dimensional thin layer image of the three-dimensional thick-layer image of synthetic this m width of cloth is the n width of cloth; Above-mentioned m and n are positive integer, and value is at least 2.The three-dimensional thin layer image of this n width of cloth is divided into the m group, comprises the three-dimensional thin layer image of the F width of cloth in each group, the three-dimensional thin layer image of the F width of cloth of above-mentioned each group just can be to the three-dimensional thick-layer image of the synthetic width of cloth of a selected aspect.
Above-mentioned use thin layer image synthesizes the thick-layer image; Can adopt very ripe at present maximum Gray Projection (MIP; Maximum Intensity Projection) method; In the method, each pixel on the three-dimensional thick-layer image that finally obtains, synthetic by the maximal value of the corresponding pixel points on the three-dimensional thin layer image of each synthetic width of cloth.Perhaps also can adopt other ripe synthetic technologys, for example self-adaptation synthetic method (Adapted Combine) technology is synthesized and is obtained final three-dimensional thick-layer image.
Fig. 7 alleviates the process flow diagram of the formation method of magnetic susceptibility artifact for the present invention, thereby this flow process is an example to the three-dimensional thick-layer image of the synthetic width of cloth of selected aspect of scanning.
This flow process shown in Figure 7 comprises:
Step 701: confirm the quantity of the required three-dimensional thin layer image of the three-dimensional thick-layer image of a synthetic width of cloth and rebuild the three-dimensional thin layer image of each width of cloth and need use radio-frequency pulse to excite the nuclear number of times of subject.
Step 702: for the three-dimensional thin layer image of each width of cloth, use radio-frequency pulse to excite the said definite number of times of subject atomic nucleus, and apply the echoed signal that readout gradient receives the transmission of subject atomic nucleus.The radio-frequency pulse that uses in this step can be selected multiple sequence for use, for example variable flip angle fast acquisition interleaved spin echo or SE sequence.
Step 703: according to all echoed signals of the three-dimensional thin layer image of a width of cloth, reconstruction should three-dimensional thin layer image.
Step 704: judge whether to accomplish the reconstruction of all three-dimensional thin layer images, if execution in step 705, otherwise return execution in step 702.In the present invention, three-dimensional thin layer image is behind the number of times that every use radio-frequency pulse excimer daughter nucleus is confirmed, to rebuild to obtain.When confirming the quantity of three-dimensional thin layer image, can confirm the bed thickness of three-dimensional thin layer image earlier, use the bed thickness of the bed thickness of three-dimensional thick-layer image, determine the quantity of three-dimensional thin layer image divided by three-dimensional thin layer image; The quantity of three-dimensional thin layer image also can directly be set.
Step 705: the three-dimensional thin layer image of all that will rebuild, the three-dimensional thick-layer image of a synthetic width of cloth.
Secondly, introduce the system that the present invention alleviates magnetic susceptibility artifact.
Fig. 8 alleviates the system architecture synoptic diagram of magnetic susceptibility artifact for the present invention, and this system comprises: parameter determination module, gradient magnetic apply module, excitation module, receiver module, image-forming module and synthesis module.
The parameter determination module is used for confirming the quantity of the synthetic required three-dimensional subimage of a width of cloth 3-D view and rebuilds the number of times that the three-dimensional subimage of each width of cloth need use radio-frequency pulse excimer daughter nucleus, and to other module these parameters is provided.
Gradient magnetic applies module and is used to apply gradient magnetic, specifically comprises: apply slice selective gradient in choosing layer direction, apply phase encoding gradient at phase-encoding direction, apply readout gradient in the readout gradient direction.
Excitation module is used for the number of times that excites determined according to the parameter determination module, to subject atomic nucleus emission radio-frequency pulse, to excite the above-mentioned number of times of subject atomic nucleus.
Receiver module is used for applying module at gradient magnetic and is reading direction when applying readout gradient, receives the echoed signal that the subject atomic nucleus sends.
Image-forming module is used to utilize all echoed signals of the same three-dimensional subimage that receiver module receives to reconstruct a width of cloth 3-D view, and the three-dimensional subimage of parameter determination module institute quantification is offered synthesis module.
Synthesis module is used to use all three-dimensional subimages that reconstruct, a synthetic width of cloth 3-D view.
The present invention alleviates in the system of magnetic susceptibility artifact, and gradient magnetic applies module and can also be further used for when the readout gradient direction applies readout gradient, applies an extra slice selective gradient in choosing layer direction, makes the projection angle run-off the straight of subject.
Gradient magnetic applies module and can comprise:
The slice selective gradient applying unit; Be used for also being further used for when the readout gradient applying unit applies readout gradient, in the readout gradient position that the corresponding readout gradient applying unit of choosing layer direction applies at choosing layer direction time slice selective gradient; Apply extra slice selective gradient; The width of the slice selective gradient that this is extra is time span just, and is identical with readout gradient, and amplitude can preestablish as required;
The phase encoding gradient applying unit is used for applying phase encoding gradient at phase-encoding direction;
The readout gradient applying unit is used for when said receiver module receives the echoed signal of atomic nucleus transmission, applying readout gradient in the readout gradient direction.
Alleviate in the system of magnetic susceptibility artifact in the invention described above; Gradient magnetic applies module and can use gradient magnetic field coil to realize, for example slice selective gradient applying unit, phase encoding gradient applying unit and readout gradient time quantum can use three gradient magnetic field coils to realize respectively; Excitation module can use radio-frequency sending coil and radio-frequency power amplifier to realize jointly; Receiver module can use RF receiving coil and signal gathering unit to realize; The realization that can use a computer of image-forming module and synthesis module, the echoed signal reconstruction of three-dimensional subimage that utilizes receiver module to provide uses the three-dimensional subimage compositing 3 d images of rebuilding.
The present invention alleviates the system of magnetic susceptibility artifact, and its principle is identical with the method that the present invention alleviates magnetic susceptibility artifact, and the detailed principle of this system and the advantage that obtains here therefore repeat no more.
In sum, more than being merely preferred embodiment of the present invention, is not to be used to limit protection scope of the present invention.All within spirit of the present invention and principle, any modification of being done, be equal to replacement, improvement etc., all should be included within protection scope of the present invention.
Claims (8)
1. three-D imaging method that alleviates magnetic susceptibility artifact, this method comprises:
Confirm the bed thickness of the three-dimensional subimage of a width of cloth; Use the bed thickness of the bed thickness of a width of cloth 3-D view divided by the three-dimensional subimage of a said width of cloth; Obtain the quantity of the synthetic required three-dimensional subimage of a width of cloth 3-D view, confirm to rebuild the three-dimensional subimage of each width of cloth and need use radio-frequency pulse to excite the number of times of subject;
For the three-dimensional subimage of each width of cloth, use radio-frequency pulse to excite the said number of times of subject, and apply readout gradient reception echoed signal, all echoed signals according to this three-dimensional subimage reconstruct this three-dimensional subimage then;
All three-dimensional subimages that use reconstructs, a synthetic width of cloth 3-D view.
2. the method for claim 1 is characterized in that, the bed thickness of determined three-dimensional subimage when said subject makes the main field skew big makes the bed thickness of hour determined three-dimensional subimage of main field skew less than said subject.
3. the method for claim 1 is characterized in that, said radio-frequency pulse is: variable flip angle fast acquisition interleaved spin echo, or spin-echo sequence, or fast acquisition interleaved spin echo.
4. the method for claim 1 is characterized in that, when applying readout gradient, on choosing layer direction, applies an extra slice selective gradient, makes the projection angle run-off the straight of subject.
5. method as claimed in claim 4 is characterized in that, this method further increases when receiving echoed signal reads bandwidth.
6. the method for claim 1 is characterized in that, said synthetic step is to adopt maximum Gray Projection method or self-adaptation synthetic method to realize.
7. 3-D imaging system that alleviates magnetic susceptibility artifact; Comprise that a gradient magnetic applies module, an excitation module, a receiver module and an image-forming module; It is characterized in that this system further comprises a parameter determination module and a synthesis module, wherein:
Said parameter determination module is used for confirming the quantity of the synthetic required three-dimensional subimage of a width of cloth 3-D view, and rebuilds the three-dimensional subimage of each width of cloth and need use radio-frequency pulse to excite the number of times of subject;
Said gradient magnetic applies module, is used for applying slice selective gradient in choosing layer direction, applies phase encoding gradient at phase-encoding direction, and applies readout gradient in the readout gradient direction;
Said excitation module is used for to subject emission radio-frequency pulse, to excite the said number of times of subject;
Said receiver module is used for when gradient magnetic applies module and applies readout gradient, receiving the echoed signal that subject sends;
Said image-forming module is used to utilize all echoed signals of the same three-dimensional subimage that said receiver module receives to reconstruct the three-dimensional subimage of a width of cloth;
Said synthesis module is used to use all three-dimensional subimages that reconstruct, a synthetic width of cloth 3-D view.
8. system as claimed in claim 7 is characterized in that, said gradient magnetic applies module and is further used for when the readout gradient direction applies readout gradient, applies an extra slice selective gradient in choosing layer direction, makes the projection angle run-off the straight of subject.
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| US20050024055A1 (en) * | 2003-07-30 | 2005-02-03 | Cavaluzzi Michael J. | Polymeric NMR sample tubes & method for using |
| US20050165470A1 (en) * | 2004-01-22 | 2005-07-28 | Jan Weber | Medical devices |
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| US20050024055A1 (en) * | 2003-07-30 | 2005-02-03 | Cavaluzzi Michael J. | Polymeric NMR sample tubes & method for using |
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