CN101287997A - Highly constrained reconstruction of motion encoded mr images - Google Patents

Abstract

A series of velocity encoded MR image frames are acquired. To increase the temporal resolution of the acquired image frames radial projections are acquired and each image frame is highly undersampled. The radial projections for each velocity encoding direction are interleaved throughout the scan and a composite phase image is reconstructed from these and used to reconstruct a velocity image for each image frame in a highly constrained backprojection method.

Description

The restructuring procedure that comprises the motion encoded MR image of highly constrained rear-projection

The cross reference of relevant application

The application requires the rights and interests of following two U.S. Provisional Patent Application: the application 60/719,445 that is entitled as " Highly Constrained Image Reconstruction Method " that on September 22nd, 2005 submitted to; And the application 60/780,788 that is entitled as " Highly Constrained Reconstruction Of velocityEncoded MR Images " of submission on March 9th, 2006.

Statement about federal funding research

The present invention has obtained the government-funded of fund project HL 072260 of NIH and LH 066488.U.S. government enjoys some rights and interests of this invention.

Background of invention

The field of the invention is Magnetic resonance imaging (" MRI ") method and system.More particularly, the present invention relates to representing that the gradient of moving gathers the process with the MR image of reconstruct tape pulse sequence.

When the material such as tissue is subjected to uniform magnetic field (polarization field B ₀) do the time spent, the magnetic moment of each spin in this tissue attempts to aim at this polarization field, but by its feature Larmor frequency with any order around its precession.If this material or tissue are subjected to being in the x-y plane and near magnetic field (the exciting field B of Larmor frequency ₁) effect, then aim at magnetic moment M only _zCan rotate or " inclination " thus produce clean laterally magnetic moment M in the x-y plane _tA kind of signal is sent in the spin of these excited target, and at pumping signal B ₁After the termination, form image thereby can receive and handle this signal.

When adopting these signals to produce image, can use magnetic field gradient (G _x, G _yAnd G _z).Usually, the zone that is treated as picture by a series of measuring periods is scanned, and above-mentioned these gradients change according to used specific portion method in these measuring periods.By using one of many known reconfiguration techniques, the NMR that a winding of gained is received is signal digitalized and it is handled with reconstructed image.

The common method that is used to gather NMR signal and reconstructed image has been used a kind of variant of well known Fourier transform (FT) imaging technique.In people's such as W.A.Edelstein the article that is entitled as " Spin-Warp NMR Imaging andApplications to Human Whole-Body Imaging ", this technology has been discussed and (has been seen for details Physics in Medicine and Biology, volume 25, the 751-756 page or leaf, 1980).Thereby it had used a kind of variable amplitude phase encoding magnetic field gradient pulse that the spatial information on this gradient direction is carried out phase encoding before gathering NMR spin-echoed signal.In two-dimentional implementation (2DFT), for example, by using phase encoding gradient (G along a direction _y), on this direction, spatial information is encoded, then, with a direction of this phase-encoding direction quadrature on have the magnetic field gradient (G that reads _x) situation under gather spin echo signal.At the readout gradient that exists during the spin-echo acquirement spatial information on the orthogonal directions is encoded.In typical 2DFT pulse train, in the view sequence of above-mentioned scan period collection, increase phase encoding gradient pulses G _yAmplitude (Δ G _y), to produce one group of NMR data, from this group NMR data, can reconstruct entire image.Make in this way, just with the scan mode shown in Fig. 2 A fourier space or " k-space " are sampled along cartesian coordinate system.

In order to increase the speed of images acquired, by gathering less phase encoding view or by using faster pulse train (must cause picture quality lower), picture quality may be sacrificed to some extent.Therefore, when using Fourier transformation method, for the acquisition rate of the number that realizes the view that desired images resolution and quality are gathered and the NMR data that are used for complete image this aspect two between, will inevitably weigh to some extent.

Developed and a kind of MR method, this method is encoded to spin motion in the phase place of the signal of being gathered, United States Patent (USP) Re.32, and 701 have disclosed this method.These common formation one classes are called as the technology of phase correlation (PC) method.At present, most of PC technology are gathered two kinds of images, have different sensitivity for each image of identical speed component.Then, by forming this, just can produce image to phase differential between the velocity encoded cine image or complex difference.In alleged usually phase correlation nuclear magnetic resonance (NMR) vessel visualization (PCMRA), above-mentioned this motion encoded method is used to the blood that flows is carried out imaging.

Phase contrast techniques also has been used for the quantitative measurment that convection cell carries out imaging and blood flow is provided.In the fluid imaging process, scan period, used motion encoded gradient was very sensitive to the speed component on the direction of two or three quadratures.From the velocity component images of gained, can produce total quantitative flow images.Yet in the time must utilizing different motion encoded gradients to gather four to six images of sampling fully, above-mentioned scanning becomes long.

As United States Patent (USP) 6,188,922 is described such, by with a series of staggered projection views being sampled in the k space, just can shorten the gatherer process of velocity encoded cine MR data.Projection view is sampled to the k space along radial trajectories, and has found to compare with the phase encoding view of being sampled in the k space along cartesian coordinate system, produces the required projection view of high quality graphic and will lack a lot.Fig. 2 B shows this radially sample mode.

At United States Patent (USP) 6,710, in 686, two kinds of methods that are used for reconstructing from one group of projection view gathering image have been described.In MRI, prevailing method is that the k space sample is radially fixed the cartesian grid position on the sample track again from it.Then, by the k space sample after fixing is again carried out two dimension or three-dimensional Fourier transform, reconstruct image.The second method that is used for reconstruct MR image is: by each projection view is carried out first Fourier transform, above-mentioned radially k space projection view is transformed to the Radon space.By filtering these signal projections and they being backprojected in the field of view (FOV), just from these signal projections, reconstruct an image.As known in the art, if the signal projection that collects is not enough to satisfy Nyquist (Nyquist) sampling rule at quantitative aspects, then in the image that reconstructs, can produce streak artifacts.

Fig. 3 shows employed standard backprojection method among the MRI.By come along the projection path shown in the arrow 16 projection each at the sample of signal 14 in the distribution 10 of conversion and make it pass FOV 12, each signal projection that collects distributes and 10 has just experienced Fourier transform and next be backprojected on the visual field 12.In the process that each sample of signal 14 is projected among the FOV 12, we not about just by the priori of the object of imaging, and the NMR signal among the supposition FOV 12 be homomorphism and sample of signal 14 should be distributed to equably in each pixel that projection path passes.For example, Fig. 3 shows projection path 8, and when it was passed in N pixel among the FOV 12, it was corresponding to distribute individual signals sample 14 in 10 of a signal projection through conversion.The signal value (P) of between this N pixel, cutting apart this sample of signal 14 equably:

S _n＝(P×1)/N (1)

Wherein: S _nIt is the signal value of in projection path 8, distributing to n pixel with N pixel.

Obviously, the signal of rear-projection is that this supposition of homomorphism is incorrect among the FOV 12.Yet, as known in the art, proofread and correct if each signal distributions 10 is carried out some, and the distribution of gathering sufficient amount with the projection angle of respective amount, then the caused mistake of this wrong supposition reaches minimum and image artifacts is inhibited.In typical, filtered rear projecting method about image reconstruction, for 256 * 256 pixel two dimensional images, need 400 projections, for 256 * 256 * 256 volume elements 3-D views, then need 103,000 projections.

Summary of the invention

The present invention is a kind of method that reconstructs image from the velocity encoded cine MR data that collect, and more particularly, the present invention is a kind of highly constrained rear projecting method that can reconstruct the velocity encoded cine image from the data set of highly owing to sample.The highly constrained rear projecting method of the application of the invention can come the picking rate coded image with few view, and can not produce tedious clinically pseudomorphism because of owing sampling.This has reduced acquisition time, and can come images acquired by different velocity encoded cines.

Discovery of the present invention is: if use the priori of the signal profile among the FOV 12 in restructuring procedure, then can produce high quality graphic with few projection signal's distribution 10.As the part of MRI scanning, gathered a composograph, and it by reconstruct to provide about just by the priori of the object of imaging.In the restructuring procedure of the velocity encoded cine image of highly owing to sample, use this composograph so that the distribution of backprojected views is weighted.With reference to Fig. 4, for example, the signal profile among the FOV 12 may comprise structure 18 and 20.In this case, when backprojection path 8 is passed these structures, this distribution is weighted, thereby sample of signal 14 is distributed in each pixel more accurately by known signal profile according to this pixel position.As a result, in the example of Fig. 4, the major part of sample of signal 14 will be distributed on and structure 18 and 20 those pixel places of intersecting.For the backprojection path 8 with N pixel, this highly constrained rear-projection can be expressed as:

$S_n=(P\×C_n)/\underset{n=1}{\overset{N}{\mathrm{\Σ}}}{C}_n---\left(2\right)$

Wherein: S _n=rear-projection the signal amplitude at pixel n place in just by the image of reconstruct;

The rear-projection sample of signal value of P=in the projection through conversion distributes; And

C _n=along the signal value of the priori composograph at n pixel place of backprojection path.

This composograph is to reconstruct the data that collect from scan period, and can comprise that and other view data that collects that is used to describe the structure of visual field that is used for this image of reconstruct.Molecule in the formula (2) utilizes in this composograph corresponding signal value to come each pixel is weighted, and denominator makes this value normalization, make all rear-projection sample of signal reflected picture frame projection and and not multiply by this composograph with.Should be noted that, although above-mentioned normalization after rear-projection, each pixel is carried out separately,, in many clinical practices, it is then easier before rear-projection projection P to be carried out normalization.In this case, passing by same view angle in the projection process of this composograph, by divided by corresponding value P _c, make above-mentioned projection P normalization.Normalized projection P/P _cBy rear-projection, then, the image of gained multiply by this composograph.

Fig. 5 shows a three-dimensional embodiment, corresponding to being the single 3 D projection view of feature with view angle theta and φ.This projection view be along axle 16 by rear-projection and be extended in the Radon plane 21 at distance r place along rear-projection axle 16.As substituting of filtered rear-projection (wherein projection signal's value is filtered and is distributed to equably in the continuous Radon plane),, utilize the information in the composograph that projection signal's value is distributed in the Radon plane 22 along axle 16.Composograph in the example of Fig. 5 comprises structure 18 and 20.Based on relevant position x in the composograph, y, the intensity at z place will leave the picture position x in the Radon plane 21 through the signal profile value of weighting in, y, z place.This is the simple multiplication processes of signal distributions value and corresponding composograph voxel values.Then, by making this product, make this product normalization divided by the distribution value in the respective image space distribution that from composograph, forms.The formula that is used for three-dimensionalreconstruction is:

I(x，y，z)＝∑(P(r，θ，φ)＊C(x，y，z) _{(r，θ，φ)}/P _c(r，θ，φ) (2a)

Wherein summation (∑) is to carry out on just by all projections in the picture frame of reconstruct, and the x in the specific Radon plane, y, and the z value is to use at the suitable r in this plane, θ, (r, θ φ) calculate the distribution value P at φ value place.P _c(r, θ φ) are corresponding distribution value from composograph, and C (x, y, z) _{R, θ, φ}Be (r, θ, the composograph value of φ) locating.

The objective of the invention is to shorten for acquisition phase contrast nuclear magnetic resonance (NMR) vessel radiography (PCMRA) image required sweep time.In the time will gathering a series of such image, the present invention can make in order to gather the required view number of PCMRA image a lot of minimizings.

Another object of the present invention is to shorten for picking rate or stream picture required sweep time, does not lose the quantitative measurment ability simultaneously again.Come one of problems that the reconstruct velocity image runs into to be with this highly constrained rear projecting method: the speed at any image pixel place according to the direction of the spin motion of this pixel position and may have on the occasion of or also may have negative value.As a result, when the acquired projections view, projection ray may pass the pixel with positive velocity amplitude and have the pixel of negative velocity value.Even possible, may add up along the general speed of any projection ray and to equal zero.For fear of these problems, this can show as according to content of the present invention, and signals all in highly constrained backprojection are all treated with its absolute value, then, recovers the symbol of treated signal again in the image that reconstructs.

Another object of the present invention is to utilize highly constrained rear projecting method to produce a kind of complex difference image.This is by following realization: utilize highly constrained rear projecting method to come reconstruct I component and Q component image individually; Then, altogether with the I component of gained and Q component image sets.

To see above and other objects of the present invention and advantage from the following description.In the following description, with reference to being used to constitute the accompanying drawing of this paper part, in these accompanying drawings, show better embodiment of the present invention by explaining.Yet this embodiment must not represented four corner of the present invention, therefore, also needs with reference to claims in order to explain scope of the present invention.

Description of drawings

Fig. 1 is the block diagram of the MRI system that uses in better embodiment of the present invention;

Fig. 2 A is the diagram of the mode of in the process of using MRI system acquisition Fourier or spin-warp image being sampled in the k space;

Fig. 2 B is the diagram of the mode of in the process of using the typical reconstruction from projection of MRI system acquisition image being sampled in the k space;

Fig. 3 is the diagram of the rear-projection step of routine in image reconstruction procedure;

Fig. 4 is to use the two-dimensional diagram of the same steps as of highly constrained rear-projection;

Fig. 5 is to use the diagram of three dimensional realization mode of the same steps as of highly constrained rear-projection;

Fig. 6 is in order to put into practice the preferable two-dimentional pulse train of the present invention by the MRI system use of Fig. 1;

Fig. 7 is the polar plot of a plurality of component of signals;

Fig. 8 is the diagram of the sampling in the k space that takes place when putting into practice better embodiment of the present invention;

Fig. 9 is in order to put into practice the process flow diagram of better embodiment of the present invention by many steps of the MRI system use of Fig. 1;

Figure 10 is the process flow diagram that is used for many steps of reconstructed image in the method for Fig. 9;

Figure 11 is the process flow diagram of the data structure that produces of the method according to Fig. 9; And

Figure 12 is the process flow diagram that is used to produce many steps of phase image.

Embodiment

With reference to Fig. 1, in the MRI system, used better embodiment of the present invention especially.This MRI system comprises workstation1 10, and workstation1 10 has display 112 and keyboard 114.Workstation1 10 comprises processor 116, and processor 116 is the commercial programmable machines that can move commercial operation system.Workstation1 10 provides operation interface, and the indication that will be imported in this MRI system can be scanned in this interface.

Workstation1 10 is coupled to four station servers: pulse sequence server 118; Data acquisition server 120; Data processing server 122; With data storage server 23.In better embodiment, data storage server 23 is to be realized with relevant disc drive interface circuit by workstation processor 116.Its excess-three station server 118,120 and 122 all is by being installed in the single chassis and with 64 backplane bus the different processor of its interconnection to be realized.Pulse sequence server 118 adopts commercial microprocessor and commercial four worker's communication controlers.Data acquisition server 120 all adopts identical commercial microprocessor with data processing server 122, and data processing server 122 also comprises one or more array processor based on the parallel vector processor of commercialization.

Workstation1 10 all is connected to serial communication network with each processor that is used for server 118,120 and 122.This serial network transmits the data that download to server 118,120 and 122 from workstation1 10, and it also is transmitted between each server and the label data that transmits between workstation and server.In addition, between data processing server 122 and workstation1 10, also provide high speed data link, so that image data transmission is arrived data storage server 23.

Pulse sequence server 118 is worked in response to the program element of downloading from workstation1 10, so that operation gradient system 24 and RF system 26.Generation is used to carry out the necessary gradient waveform of scanning of appointment, and they are applied to gradient system 24, the gradient coil in gradient system 24 Drive assemblies 28, thus produce the magnetic field gradient G that is used for position encoded NMR signal _X, G _YAnd G _Z Gradient coil assembly 28 constitutes the part of magnet assembly 30, and magnet assembly 30 also comprises polarized magnets 32 and monoblock type RF coil 34.

The RF excitation waveform is applied to RF coil 34 by RF system 26, thereby carries out the magnetic resonance pulse sequence of appointment.RF system 26 receives the NMR signal by the 34 detected responses of RF coil, under the commander of the order that pulse sequence server 118 is produced, to these signals amplify, demodulation, filtering and digitizing.RF system 26 comprises the RF transmitter, and this RF transmitter produces the multiple RF pulse that is used in the MR pulse train.This RF transmitter has the RF pulse of expected frequency, phase place and pulse amplitude waveform in response to scanning indication and commander from pulse sequence server 118 with generation.The RF pulse that is produced can be applied to monoblock type RF coil 34 or be applied to one or more local coils or coil array.

RF system 26 also comprises one or more RF receiver channels.Each RF receiver channel comprises: the RF amplifier is used to amplify by the received NMR signal of the coil that is attached thereto; And quadrature detector, be used to detect the I and the Q quadrature component of the NMR signal that receives and make their digitizings.So, by I and Q component square root sum square, can determine the amplitude of the NMR signal that receives in any sample point:

$M=\sqrt{I^2+Q^2},---\left(3\right)$

And the phase place of the NMR signal that receives also can be determined:

φ＝tan ^-1Q/I. (4)

Pulse sequence server 118 also randomly receives the patient data from physiology acquisition controller 36.Controller 36 receives the signal from a plurality of different sensors that link to each other with patient, for example, comes the ECG signal of self-electrode or from the breath signal of lung.Pulse sequence server 118 uses these class signals that the performance of scanning and experimenter's breathing or heartbeat is synchronous or carry out " gate " usually.

Pulse sequence server 118 is also connected to scan room interface circuit 38, and this circuit receives from various sensors, relevant with patient status signals and from the signal of magnet system.Patient positioning system 40 also receives various command by scan room interface circuit 38 just, thereby patient is moved to the position of expectation in scanning process.

Should be clearly, in scanning process, 118 pairs of MRI system elements of pulse sequence server are carried out control in real time.As a result, must operate its hardware element with the programmed instruction of carrying out in good time mode by the program of working time.The description composition that is used for scanning indication is that form is downloaded from workstation1 10 with the object.Pulse sequence server 118 comprises some programs like this, and they receive these objects and convert thereof into by the employed object of the program of working time.

The digitized NMR sample of signal that RF system 26 is produced is received by data acquisition server 120.Data acquisition server 120 is operated in response to the description composition of downloading from workstation1 10, so that receive real-time NMR data and buffer-stored is provided, makes and loses according to overload without any the data factor.In some scanning process, data acquisition server 120 is just given data processing server 122 with the NMR data transfer that collects.Yet, need from the NMR data that collect, obtain information so that in those scanning processes of other performance of gated sweep, data acquisition server 120 just is programmed to produce this category information and it is transferred to pulse sequence server 118.For example, in the process of prescan, gather the NMR data, and use it for calibration by the performed pulse train of pulse sequence server 118.Equally, in scanning process, can gather navigator signal and use it for and adjust RF or gradient system running parameter or be used to control the view order of being sampled in the K space.In addition, data acquisition server 120 can be used for handling the NMR signal, and these signals are used to detect the arrival of the contrast preparation in MRA scanning.In all these examples, data acquisition server 120 is gathered the NMR data and in real time it is handled, thereby produces the information that is used to control this scanning.

The NMR data that data processing server 122 receives from data acquisition server 120, and according to from the description composition of workstation1 10 downloads it being handled.This class is handled and can be comprised: produce bidimensional or 3-D view thereby original K space NMR data are carried out Fourier transform; Image applications filtering to reconstruct; The NMR data that collect are carried out back projected picture reconstruct; Computing function MR image; Calculate motion or stream picture etc.

The image of 122 reconstruct of data processing server back is transferred to workstation1 10 again, and stores.Realtime graphic is stored in the archival memory high-speed cache (not shown), and exports it to operating personnel's display 112 or display 42 from this high-speed cache, and this display is placed near the magnet assembly 30 so that doctor's use.The realtime graphic of batch mode image or selection is stored in the master data base on the disk storage device 44.When this class image by reconstruct and when being transferred to memory device, data processing server 122 is the data storage server 23 of notice on workstation1 10 just.Workstation1 10 can be used by the operator, so that archival image, produce film or send image by network to miscellaneous equipment.

Two embodiments of the present invention are described below, and they have used the MRI system of Fig. 1.First embodiment provides velocity image, and this velocity image has indicated total roll rate at each image pixel place quantitatively.Second embodiment produces a kind of PCMRA image, and motion encoded gradient provides and has been used for phase correlation mechanism that the blood that moves is carried out imaging in this image.

With reference to Fig. 6, the typical motion encoded pulse sequences of being carried out by pulse sequence server 118 is a kind of gradient-recurrence echo pulse sequence, wherein at G especially _zUse RF driving pulse 250 under the situation that sheet selects gradient 252 to exist, at G _xAnd G _yGather NMR echoed signal 254 under readout gradient 256 and 257 situations about existing.Before each readout gradient 256 and 257, phase shift gradient 258 and 259 are arranged respectively, the cross magnetization generation phase shift (dephase) that they are produced RF driving pulse 250.What readout gradient 256 and 257 made echo time TE place adjusts phase place from gyromagnetic ratio, so that produce peak value in NMR echoed signal 254.

Under the situation of not using any motion encoded gradient, this pulse train is repeated, and above-mentioned two readout gradients 256 step to different values with 257 amplitude so that gather NMR echoed signal 254 under the different projection angles.Fig. 8 shows this situation, and wherein each radial line has been represented the k that the NMR echoed signal 254 that collected by each is finished _x-k _yThe sampling in space.The amplitude of readout gradient 256 and 257 amplitude and corresponding phase shift gradient pulse 258 and 259 all stepping makes each continuous projection all rotate a θ angle by a plurality of values.

Referring again to Fig. 6, in order to produce motion encoded MR image, by bipolar motion encoded gradient G _MThe projection that each the is collected speed sensitization that becomes.As known in the art, velocity encoded cine gradient G _MComprise two equal and opposite in directions and opposite polarity gradient lobe 260 and 262.Motion encoded gradient G _MCan be applied on any direction, and it has just been decayed after RF driving pulse 250 produces cross magnetization and before collecting NMR echoed signal 254.Motion encoded gradient G _MOne phase shift is imposed on by just in gradient G _MDirection on the NMR signal that spin produced that moves, and the value of this phase shift is speed and motion encoded gradient G by the spin of just moving _MFirst magnetic moment decide.First magnetic moment (the M ₁) equal the area of gradient pulse 260 or 262 and the product in the time interval between them (t).The first magnetic moment M is set ₁So that significant phase shift to be provided, thereby but can not excessively cause phase place to be unrolled with very high roll rate.

Only because of spin motion causes, usually, under each projection angle, carry out reference collection in order to ensure the phase shift in the NMR signal 254 that collects.In preferred implementation, at having the first magnetic moment M ₁Motion encoded gradient G _MThe motion encoded projection view of each that collects is gathered second projection view, and second projection view has identical motion encoded gradient G _M, but this motion encoded gradient G _MHas the first negative magnetic moment-M ₁By making above-mentioned two G simply _MGradient lobe 260 and 262 polarity reversal have just realized above-mentioned this point.As hereinafter will explaining, when the signal subtraction of two gained, just from determining, above-mentioned speed removed and unprovoked spin motion and the phase shift that causes.Hereinafter, these phase shifts that do not expect to have are called as background phase φ _B

As mentioned above, motion encoded gradient G _MCan be applied to any direction.In preferred implementation,, use motion encoded gradient G individually along each gradient axes x, y and z _M, make to produce the image that is used to indicate total roll rate.That is, by in bipolar motion encoded gradient G _MBe added to G shown in Figure 6 _zImages acquired under the situation of gradient waveform has just produced the speed (v that is used to indicate along the z axle _z) image, in motion encoded gradient G _MAdd G to _xGather second speed image V under the situation of gradient waveform _x, in motion encoded gradient G _MBe added to G _yGather third speed image V under the situation of gradient waveform _yThen, by corresponding pixel value in above-mentioned three velocity images is combined, just produced the image that is used to indicate total roll rate.

$V_T=\sqrt{V_x^2+V_y^2+V_z^2}---\left(5\right)$

Although might gather motion encoded NMR echoed signal 254 being sampled fully in the k space by each projection angle (θ), in the present embodiment, different motion encoded direction is to gather by different staggered projection angles.Fig. 8 shows this situation, wherein G _MXThe expression projection of gathering, G along the motion encoded gradient of x axle orientation _MYThe expression projection of gathering, G along the motion encoded gradient of y axle orientation _MZThe expression projection of gathering along the motion encoded gradient of z axle orientation.For in above-mentioned three motion encoded direction each, gather m=10 different projection altogether, and these are spaced apart from each other by angle same 3 θ.The projection that each group collects is all staggered with the projection of gathering at other both direction, and consequently, all projection views that are used for a picture frame all are spaced apart from each other by angle same θ, thus by basically uniformly mode sampled in the k space.

With reference to Fig. 9, in first better embodiment of the present invention, use above-mentioned two-dimentional pulse train to gather a series of images frame especially, from these picture frames, can reconstruct corresponding velocity image.Shown in process frame 200, for each direction in x, y and the z direction, the MRI system carries out pulse train so that gather one group (m=10) motion encoded NMR signal.These projection views be all uniformly-spaced so that sampled in the k space as far as possible equably, and different motion encoded direction pictures is staggered as shown in Figure 8.Acquired image frames continuously and promptly by this way, up to collecting the picture frame (n) that specifies number, just as decision frame 202 determined.

In embodiments of the present invention, after the data acquisition phase of scanning is finished, ability carries out image restructuring procedure.This restructuring procedure can carry out on data processing server 122, discharges the MRI system thereby the data that perhaps collect can be discharged into independent workstation.Still with reference to Fig. 9, the first step in the image reconstruction procedure is: at each motion encoded direction, carry out each right ± M ₁The plural subtraction of projection, just as shown in the process frame 204.Also with reference to Figure 11, this is a kind of at+M ₁With-M ₁Each I of corresponding signal sample and the subtraction of Q component in the projection data set 303 and 305, with produce complex difference ( ) projection data set 307.Fig. 7 shows this complex difference vector The picture frames that all n collect are realized above-mentioned these operations, then, shown in process frame 206 like that, from each group complex difference projection Reconstruct picture frame in 307, to produce corresponding real space complex difference image 309.This is a kind of restructuring procedure of standard, and in preferred implementation, this restructuring procedure comprises ten

K space sample in the projection fixes in the cartesian coordinate system again, then, carries out two-dimentional plural inverse fourier transform.Yet, be appreciated that and inciting somebody to action with 1DFT

Projection is transformed into after the Radon space, also can come these images of reconstruct with the filtered backprojection method of routine.Which kind of situation no matter is because the k space highly owes to sample, so from clinical angle, these images will have the quality of non-constant.Yet they have kept complex difference really at each pixel place The direction of vector person's character, particularly this vector.From following discussion, will find out significantly, should " symbol " information will be restored to higher-quality absolute value images.

Shown in process frame 201, next step is: separate each

The I and the Q component at picture frame pixel place, independent to form With

Picture frame 311 and 313.Then, get in these picture frames

With

The absolute value of component, to form corresponding absolute image frames | I| and | Q|315 and 317.These picture frames can because they have preserved the phase shift that is produced by spin motion, but not preserved direction by the unified phase image that is considered as being used to indicate spin " speed ".In these absolute value images, direction or symbolic information have been lost.

Shown in process frame 203, next formed and be used for | the composograph 319 of I| component.This is by adding corresponding for all n picture frame 315 | and the I| pixel value is realized.As mentioned above, the projection view of gathering at n picture frame is interlaced with each other, consequently, | I| composograph 319 has than any absolute image frames | the quality that I|315 is much higher.Shown in process frame 205, use | Q| component image 317 repeats this process, to form | Q| composograph 321.These | I| and | Q| composograph 319 and 321 can be by the unified synthesis phase image that be considered as, because they have been preserved phase information and have therefore preserved speed.

Shown in process frame 207, next step is: at n | each of I| component image frame 315 produces one group | and I| component projection 323.This is the Radon conversion of standard, wherein is being used under the same view angle of acquired image frames, produces 10 projections (in this better embodiment).This is that the trade mark that utilizes Mathworks incorporated company to sell is realized for the Radon transformation tool in the business software of " MATLAB ".Thus, at complex difference | each in the n of the I| component picture frame 315, we have one group of 10 projection.

Then, shown in process frame 209, carry out a kind of highly constrained image reconstruction procedure, so that from these Radon space projections 323, produce n high-quality | I| component image 325.This reconstructing method has used | and I| component composograph 319 hereinafter can be described in more detail with reference to Figure 10.

Shown in process frame 211 and 213, next, at | the Q| component repeats above-mentioned a plurality of step.Utilize the Radon conversion, calculate one group at each picture frame 317 | Q| component projection 327, then, utilize highly constrained rear projecting method to come reconstruct n | Q| component image frame 329 is described this now.

Especially with reference to Figure 10, | I| and | each in the Q| component image frame 325 and 329 is all utilized it separately | I| and | Q| projection data set 323 and 327 and corresponding | I| or | Q| composograph 319 and 321 comes reconstruct.Above this highly constrained backprojection reconstruction has been described and Fig. 4 illustrates this in conjunction with equation (2).More particularly, shown in process frame 231, each | I| and | Q| component projection P is all by normalization.By making each component projection P divided by the projection P in the corresponding composograph under its same view angle _c, just make each component projection P normalization.Then, normalized projection P/P _cBe backprojected among the FOV.This is a kind of backprojection of standard, but is not with filtering.

Shown in process frame 233, the rear-projection value of gained just is added to by reconstruct | I| or | the Q| picture frame, and make at decision frame 235 places test with all projection views of being identified for the current images frame whether all by rear-projection.If not, then current | I| or | the next projection view in the Q| picture frame is by rear-projection, just as shown in the process frame 237.

When for | I| or | when all projection view was all by rear-projection and through summation for the Q| picture frame, the picture frame of total was with corresponding | I| or | Q| composograph 319 and 321 multiplies each other.This is a kind of matrix multiplication, wherein | I| or | the pixel value in the Q| picture frame and each | I| or | the value of respective pixel multiplies each other in the Q| composograph.Should be clearly, can also use other to be used to carry out the method for this highly constrained image frame reconstruction, the U.S. Patent application that awaits the reply jointly 11/482 that is entitled as " Highly Constrained Image Reconstruction Method " that on July 7th, 2006 submitted to, 372 are described this, and this application is quoted at this as a reference.

Producing at each picture frame | I| and | in the Q| component image, symbolic information has been lost and we do not know each image pixel place | I| and | the symbol (±) of Q| component.At process frame 215 places, as shown in Figure 9, by producing I and Q sign map 333, this symbolic information just is resumed.By checking the I in the complex difference image 309 of owing to sample and the symbol of Q component, I and Q sign map have just been produced.As mentioned above, these all are the second-rate images that causes because of owing to sample, but they are enough to indicate the symbol at each image pixel place.

Shown in process frame 217, next, this symbolic information be resumed to | I| and | Q| picture frame 325 and 329.This realizes by following operation: make | I| component image frame 325 multiply by its corresponding I sign map 333; And making | Q| component image frame 329 multiply by its corresponding Q sign map 333.Shown in process frame 219, then, be used for the signed I of each picture frame and Q component be combined with form complex difference image (

) 335.

For each motion encoded direction, repeat said process.In better embodiment, be used on three all gradient directions motion encoded, and producing at all directions | CD| image 331 Hes

After the image 335 (just as determining that frame 221 is determined), this system transfers to process frame 223 and sentences n velocity image of calculating.

For calculating spin speed, must calculating+M ₁Motion encoded image and-M ₁Phase difference between the motion encoded image _vThis figure 7 illustrates, wherein angle φ _BIt is the background phase that factor produced except that spin motion.

Cosine law is used to calculate the phase of each image pixel _v:

φ _v＝cos ^-1(-|CD| ²+|-M ₁| ²+|+M ₁| ²/2·|+M ₁|·|-M ₁) (6)

As shown in figure 12, as described above at | CD| image 331 calculated complex differences | CD|, still+M ₁With-M ₁Magnitude image must be calculated separately.This can realize by many modes, but in preferred implementation, be used for all of single direction staggered+M ₁It is single that projection view 303 all is used to reconstruct |+M ₁| image 350, and be used for all of single direction staggered-M ₁It is single that projection view 305 all is used to reconstruct |-M ₁| image 352.These are conventional backprojection reconstructions through filtering, because there is abundant view that high quality graphic is provided.According to the plural I and the Q value of gained, calculate each |+M ₁| and |-M ₁| the amplitude at image pixel place.

By using corresponding n | CD| image 331 and two magnitude image |+M ₁| 350 Hes |-M ₁| 352, use equation (6) to produce n phase image | φ _v| each in 354.Yet these phase images 354 do not comprise symbolic information, and this must be added.For this reason, the complex difference image from owing to sample

Producing sign map 356 in 309, it indicates symbol at its each pixel place phase differential and is+and 1 or-1.Absolute value phase image 354 multiply by its corresponding sign map 356, so that produce phase image φ _v358.

(x in the better embodiment, y and z) calculates phase image φ at each motion encoded direction _v358, and therefrom calculate speed component V _x, V _yAnd V _z, used formula is as follows:

V＝VENC*φ _v/π/2

Wherein: the VENC=roll rate, it uses the selected gradient first magnetic moment M ₁Produce size and be the phase shift φ of pi/2 _v

Next, in equation (5), these three speed components are combined as mentioned above, to produce n corresponding total velocity diagram picture frame.

Although in better embodiment, used along the velocity encoded cine of all three gradient axes, some clinical settings are like this arranged, wherein only just enough along the velocity encoded cine of one or two gradient axes.For coronary artery is measured, for example, can collect the two dimensional image in the sheet vertical with this fluid moving phase.Only a velocity axis is encoded.This has shortened steps such as collection and image reconstruction.In this case, velocity encoded cine gradient G _MBe one with the corresponding pitch angle of direction coronarius, and it is by two or three gradient axes G in the pulse train of Fig. 6 simultaneously _x, G _yOr G _zGenerate G _MGradient waveform produces.

The present invention also can be used to produce a series of phase correlation MRA images.In this application, can use two dimension or three-dimensional pulse train, and collect and a series ofly wherein only used motion encoded image along an axle.With+M ₁With-M ₁Motion encoded or+M ₁And M ₁=0 is motion encoded, gathers two kinds of images.In PCMRA, the phase at the image pixel place that each reconstructs _vAll directly shown, and calculating spin speed not, the result, said process can simplify more.If gathered 3-D view, then above be performed, just as described among Figure 10 with above-mentioned three-dimensional equation (2a) in conjunction with Figure 10 and the described highly constrained rear-projection of equation (2).

In above-mentioned better embodiment,, just formed composograph by the information combination that will from the projection that whole scan period collected, obtain.Although this provides maximum SNR in the picture frame that reconstructs, the velocity variations that scan period may occur may not can clearly illustrate in above-mentioned a series of n velocity image.Thus, when during dynamic scan, occur changing, the number that is used to form the projection of composograph can be reduced to one round current by the window of the collection of the picture frame of reconstruct.For example, one add before the current images frame by the current images frame and two picture frames afterwards in the window that constitutes of the projection of being gathered can be used to form composograph.This window moves at each processed picture frame, thus, forms different composographs at above-mentioned each picture frame in a series of.

In better embodiment, by deducting with opposite polarity bipolar gradient (or the first magnetic moment M ₁) two signals producing, detect the background phase that other factors caused except that spin motion.A kind ofly for the alternate ways that realizes identical result be: gather second projection view with identical pulse train, but need not be any motion encoded (be M ₁=0).The difference of two signals that collect of gained will disclose the background phase shifts of not expecting, but the SNR of the velocity image of gained has reduced.Present embodiment has jump, and promptly one, direction of motion that two or three are different coding can use single with reference to gathering.Thus, as substituting of six times in above-mentioned better embodiment collections, four of the following needs of each projection angle.In addition, also can use " 4-point balance/hammard " encoding scheme.

Another alternate embodiment of the present invention is used a kind of distinct methods that is used in image reconstruction procedure save symbol information.Reconstruct absolute value (being speed) image and the directional information that is comprised in they and the sign map merged as mentioned above no longer, independent positive velocity image and negative velocity image can and be combined to form velocity image by reconstruct.In this case, no longer form definitely at process frame 201 places | I| and | Q| component image frame has formed positive I and Q and negative I and Q component picture frame.After highly constrained rear-projection step, as mentioned above, all these coverlet reason of staying alone, and next be combined.

The present invention can be applied to motion encoded acquisitions especially, and wherein spin motion is reflected in the phase information, and must preserve correct phase information in highly constrained backprojection.Other application is also arranged, wherein must preserve phase information or phase information.The present invention also is applied to these situations.For example, there are some to use, wherein motion encoded pulse sequences and be not used in and gather above-mentioned two groups and deducted to form the projection views of complex difference data set 307.The present invention is used in as above-described on this complex difference data set, with the corresponding complex difference image of reconstruct.In addition, have some to use, wherein by using highly constrained reconstructing method, the one group of complex projection views that collects is used to reconstructed image, and phase information will be saved.In this application, said process is used on many group complex projection views, and corresponding composograph therefrom can be extracted phase information accurately by reconstruct.

Claims (19)

1. the method for an experimenter who is used for producing the field of view (FOV) that is positioned at nuclear magnetic resonance imaging system image, described method comprises the steps:

A) have under the situation of pulse train of the sensitization gradient of pointing to along first direction of moving in use, with one group of projection view that is arranged in the experimenter of FOV of MRI system acquisition, the sampled images data set is owed in this group projection view formation first;

B) have under the situation of pulse train of the different sensitization gradient of pointing to along first direction of moving in use, be arranged in second group of projection view of the experimenter of FOV with the MRI system acquisition, the sampled images data set is owed in this second group of projection view formation second;

C) repeatedly repeating step a) and b) to gather first and second groups of additional projection views, wherein the projection view in Fu Jia many groups projection view interlocks;

D) described first and second owe I in the respective projection view and Q component in the sampled images data set by deducting each, produce a plurality of complex difference projection data set;

E) from corresponding a plurality of complex difference projection data set, a plurality of complex difference image of owing to sample of reconstruct;

F) from a plurality of complex difference image, produce a plurality of I component projection data set;

G) in the data for projection from a plurality of described I component projection data set, produce the I component composograph;

H) from a plurality of complex difference image, produce a plurality of Q component projection data set;

I) in the data for projection from a plurality of described Q component projection data set, produce the Q component composograph;

J) reconstruct I image from the I component projection data set through the following steps:

J) i) projection view in the I component projection data set is backprojected among the FOV, and the numerical value that is backprojected in each I image pixel is weighted with the normalization numerical value of corresponding pixel in the I component composograph; With

J) ii) the rear-projection numerical value that is used for each I image pixel is sued for peace;

K) reconstruct Q image from the Q component projection data set through the following steps:

K) i) projection view in the Q component projection data set is backprojected among the FOV, and the numerical value that is backprojected in each Q image pixel is weighted with the normalization numerical value of corresponding pixel in the Q component composograph; With

K) ii) the rear-projection numerical value that is used for each Q image pixel is sued for peace; And

1) with the I image that reconstructs and the Q image sets that reconstructs altogether to form complex difference image.

2. the method for claim 1 is characterized in that, at step j) i) and k) i) in calculate each I and Q image pixel rear-projection numerical value S according to following formula _n:

$S_n=(P\×C_n)/\underset{n=1}{\overset{N}{\mathrm{\Σ}}}{C}_n$

Wherein: P=is by the I of rear-projection or Q component projection view numerical value;

C _nCorresponding pixel value in=I or the Q component composograph;

S _n=in by the I of reconstruct or Q image along the value of n pixel of backprojection path; And

N=is along the sum of the pixel of backprojection path.

3. the method for claim 1 is characterized in that, described FOV is three-dimensional, produces three-dimensional complex difference image, and at step j) and k) in by the I of reconstruct and Q image be:

I(x，y，z)＝∑(P(r，θ，φ)＊C(x，y，z) _{(r，θ，φ)}/P _c(r，θ，φ)

Q(x，y，z)＝∑(P(r，θ，φ)＊C(x，y，z) _{(r，θ，φ)}/P _c(r，θ，φ)

Wherein on all projection views that are used for reconstruct I or Q image, sue for peace (∑); I _{(x, y, z)}It is the image value at location of pixels x, y, z place in the I of reconstruct image; Q _{(x, y, z)}It is the image value at pixel x, y, z place in the Q of reconstruct image; P _{(r, θ, φ)}Be the numerical value that rear-projection goes out from the I that collects by view angle theta, φ or Q component projection view; C _{(x, y, z)}Be I or Q component composograph numerical value at location of pixels x, y, z place; And P _C(r, θ are from the projection distribution numerical value of I or Q component composograph when being θ, φ at the visual angle φ).

4. the method for claim 1 is characterized in that, by coming repeating step j with different I component projection data set and Q component projection data set), k) and l), produce additional complex difference image.

5. the method for claim 1 is characterized in that, also comprises: from complex difference image, produce phase image.

6. the method for claim 1 is characterized in that, also comprises: from complex difference image, produce velocity image.

7. the method for claim 1 is characterized in that, also comprises:

M) with step a), b) and c) in the motion encoded gradient of pointing to along second direction come repeating step a) to l), so that produce second complex difference image;

N) with step a), b) and c) in the motion encoded gradient of pointing to along third direction come repeating step a) to l), so that produce the 3rd complex difference image; And

O) produce velocity image with above-mentioned three kinds of complex difference image.

8. method as claimed in claim 5 is characterized in that, also comprises:

From the complex difference image of owing to sample, the compute sign mapping; And

Make phase image multiply by sign map.

9. the method for an experimenter who is used for producing the field of view (FOV) that is positioned at nuclear magnetic resonance imaging system image, described method comprises the steps:

A) use pulse train, with one group of projection view that is arranged in the experimenter of FOV of MRI system acquisition, this group projection view forms first and owes the sampled images data set;

B) use pulse train, be arranged in second group of projection view of the experimenter of FOV with the MRI system acquisition, second group of projection view forms second and owes the sampled images data set;

C) repeatedly repeating step a) and b) to gather first and second groups of additional projection views, wherein the projection view in Fu Jia many groups projection view interlocks;

D) described first and second owe I in the respective projection view and Q component in the sampled images data set by deducting each, produce a plurality of complex difference projection data set;

E) from corresponding a plurality of complex difference projection data set, a plurality of complex difference image of owing to sample of reconstruct;

F) from a plurality of complex difference image, produce a plurality of I component projection data set;

G) in the data for projection from a plurality of described I component projection data set, produce the I component composograph;

H) from a plurality of complex difference image, produce a plurality of Q component projection data set;

I) in the data for projection from a plurality of described Q component projection data set, produce the Q component composograph;

J) reconstruct I image from the I component projection data set through the following steps:

J) i) projection view in the I component projection data set is backprojected among the FOV, and the numerical value that is backprojected in each I image pixel is weighted with the normalization numerical value of corresponding pixel in the I component composograph; With

J) ii) the rear-projection numerical value that is used for each I image pixel is sued for peace;

K) reconstruct Q image from the Q component projection data set through the following steps:

K) i) projection view in the Q component projection data set is backprojected among the FOV, and the numerical value that is backprojected in each Q image pixel is weighted with the normalization numerical value of corresponding pixel in the Q component composograph; With

K) ii) the rear-projection numerical value that is used for each Q image pixel is sued for peace; And

L) with the I image that reconstructs and the Q image sets that reconstructs altogether to form complex difference image.

10. method as claimed in claim 9 is characterized in that, at step j) i) and k) i) in calculate each I and Q image pixel rear-projection numerical value S according to following formula _n:

$S_n=(P\×C_n)/\underset{n=1}{\overset{N}{\mathrm{\Σ}}}{C}_n$

Wherein: P=is by the I of rear-projection or Q component projection view numerical value;

C _nCorresponding pixel value in=I or the Q component composograph;

S _n=in by the I of reconstruct or Q image along the value of n pixel of backprojection path; And

N=is along the sum of the pixel of backprojection path.

11. method as claimed in claim 9 is characterized in that, described FOV is three-dimensional, produces three-dimensional complex difference image, and at step j) and k) in by the I of reconstruct and Q image be:

I(x，y，z)＝∑(P(r，θ，φ)＊C(x，y，z) _{(r，θ，φ)}/P _c(r，θ，φ)

Q(x，y，z)＝∑(P(r，θ，φ)＊C(x，y，z) _{(r，θ，φ)}/P _c(r，θ，φ)

Wherein on all projection views that are used for reconstruct I or Q image, sue for peace (∑); I _{(x, y, z)}It is the image value at location of pixels x, y, z place in the I of reconstruct image; Q _{(x, y, z)}It is the image value at pixel x, y, z place in the Q of reconstruct image; P _{(r, θ, φ)}Be the numerical value that rear-projection goes out from the I that collects by view angle theta, φ or Q component projection view; C _{(x, y, z)}Be I or Q component composograph numerical value at location of pixels x, y, z place; And P _C(r, θ are from the projection distribution numerical value of I or Q component composograph when being θ, φ at the visual angle φ).

12. method as claimed in claim 9 is characterized in that, by coming repeating step j with different I component projection data set and Q component projection data set), k) and l), produce additional complex difference image.

13. method as claimed in claim 9 is characterized in that, also comprises: from complex difference image, produce phase image.

14. method as claimed in claim 13 is characterized in that, also comprises:

From the complex difference image of owing to sample, the compute sign mapping; And

Make phase image multiply by sign map.

15. the method for an experimenter who is used for producing the field of view (FOV) that is positioned at nuclear magnetic resonance imaging system image, described method comprises the steps:

A) use pulse train, with one group of projection view that is arranged in the experimenter of FOV of MRI system acquisition, this group projection view forms first and owes the sampled images data set;

B) repeatedly repeating step is a) to gather first group of additional projection view, and wherein the projection view in Fu Jia many groups projection view interlocks;

C) from corresponding a plurality of projection data set, a plurality of images of owing to sample of reconstruct;

D) from a plurality of images of owing to sample, produce a plurality of I component projection data set;

E) in the data for projection from a plurality of described I component projection data set, produce the I component composograph;

F) from a plurality of images of owing to sample, produce a plurality of Q component projection data set;

G) in the data for projection from a plurality of described Q component projection data set, produce the Q component composograph;

H) reconstruct I image from the I component projection data set through the following steps:

H) i) projection view in the I component projection data set is backprojected among the FOV, and the numerical value that is backprojected in each I image pixel is weighted with the normalization numerical value of corresponding pixel in the I component composograph; With

H) ii) the rear-projection numerical value that is used for each I image pixel is sued for peace;

I) reconstruct Q image from the Q component projection data set through the following steps:

I) i) projection view in the Q component projection data set is backprojected among the FOV, and the numerical value that is backprojected in each Q image pixel is weighted with the normalization numerical value of corresponding pixel in the Q component composograph; With

I) ii) the rear-projection numerical value that is used for each Q image pixel is sued for peace; And

J) with the I image that reconstructs and the Q image sets that reconstructs altogether to form complex image.

16. method as claimed in claim 15 is characterized in that, at step h) i) and i) i) in calculate each I and Q image pixel rear-projection numerical value S according to following formula _n:

$S_n=(P\×C_n)/\underset{n=1}{\overset{N}{\mathrm{\Σ}}}{C}_n$

Wherein: P=is by the I of rear-projection or Q component projection view numerical value;

C _nCorresponding pixel value in=I or the Q component composograph;

S _n=in by the I of reconstruct or Q image along the value of n pixel of backprojection path; And

N=is along the sum of the pixel of backprojection path.

17. method as claimed in claim 15 is characterized in that, described FOV is three-dimensional, produces three-dimensional complex difference image, and at step h) and i) in by the I of reconstruct and Q image be:

I(x，y，z)＝∑(P(r，θ，φ)＊C(x，y，z) _{(r，θ，φ)}/P _c(r，θ，φ)

Q(x，y，z)＝∑(P(r，θ，φ)＊C(x，y，z) _{(r，θ，φ)}/P _c(r，θ，φ)

Wherein on all projection views that are used for reconstruct I or Q image, sue for peace (∑); I _{(x, y, z)}It is the image value at location of pixels x, y, z place in the I of reconstruct image; Q _{(x, y, z)}It is the image value at pixel x, y, z place in the Q of reconstruct image; P _{(r, θ, φ)}Be the numerical value that rear-projection goes out from the I that collects by view angle theta, φ or Q component projection view; C _{(x, y, z)}Be I or Q component composograph numerical value at location of pixels x, y, z place; And P _C(r, θ are from the projection distribution numerical value of I or Q component composograph when being θ, φ at the visual angle φ).

18. method as claimed in claim 15 is characterized in that, also comprises: from complex image, produce phase image.

19. method as claimed in claim 18 is characterized in that, also comprises:

From the image of owing to sample, the compute sign mapping; And

Make phase image multiply by sign map.

Images