WO2001058352A1 - Magnetic resonance imaging device - Google Patents

Abstract

While contrast MRA measurement is being carried out, the concentration of the contrast agent is made to reach its peak when the low-frequency component is measured by controlling the measurement order of the k-space, considering the distance from the origin. The measurement points in the k-space are divided into two groups. When the concentration of the contrast agent in a target vessel becomes high, the measurement of the first group is started. For the first group, the sample points are sampling-controlled from the high-frequency component toward the low-frequency component in such a way that the distance from the origin gradually decreases. For the other group to be measured consecutively, the sample points on the opposite side of the origin are sampling-controlled from the low-frequency component toward the high-frequency component in such a way that the distance from the origin gradually increases. The influence of the error of the imaging timing in the contrast MRA measurement is reduced, and the whole vessel is drawn in high contrast. The image of an artery can be selectively drawn.

Description

Magnetic resonance imaging equipment

 The present invention relates to a magnetic resonance imaging apparatus (hereinafter, referred to as an MRI apparatus) for obtaining a tomographic image of a desired part of a subject using a nuclear magnetic resonance (hereinafter, abbreviated as “MRJ”) phenomenon, and particularly to a vascular system. The present invention relates to an MRI apparatus capable of securing a desired rendering range and image quality in a minimum time required for rendering a travel.

 An MRI device measures the density distribution, relaxation time distribution, etc. of nuclear spins (hereinafter simply referred to as spins) at a desired examination site in a subject using NMR phenomena, and uses the measured data to determine the Are displayed as images.

 One of the imaging functions of this MRI apparatus is MR angiography (hereinafter abbreviated as “MRA”) that draws blood flow. MM has a method that does not use a contrast agent and a method that uses a contrast agent.

'A common method of using a contrast agent is to combine a T1 shortened contrast agent such as M-DTPA with a short TR sequence in the gradient echo system. This method includes a T1 shortened contrast agent Since the blood flow spin has a T1 shorter than that of the surrounding tissue, saturation is unlikely to occur when a high-frequency magnetic field excitation is applied for a short repetition time TR, and a relatively high signal is emitted from other tissues. This method is used to visualize blood vessels filled with blood containing a contrast agent with high contrast to other tissues Measurement of Volume data including blood vessels while the contrast agent stays in the target blood vessel (Specifically, three-dimensional measurement) is performed, and the obtained three-dimensional images are superimposed to perform a projection process to draw a blood flow. Three-dimensional to get information Based on the three-dimensional Daladier cement echo method to get over data Is used.

 In order to obtain a good X-ray image in such a three-dimensional contrast MRA, (1) injection method of a contrast agent, and (2) imaging time and timing are important. Regarding (1), the contrast agent must be injected into the blood vessel to be imaged so as to maintain a stable and high concentration. For this reason, rapid injection using an automatic injector is generally used.

 Regarding (2), for example, in order to separate and selectively image the artery, it is necessary to set the imaging timing so that the concentration of the contrast agent in the artery becomes high during data collection. Ideally, the contrast agent concentration peaks in the center of k-space (low frequency region), which controls the contrast of the image, and the timing is set according to the data collection method used and the sequence of the sequence. I do.

 The data collection method mainly consists of a sequential order in which measurement is performed from one high-frequency side of k-space to the other high-frequency side through a low-frequency region, and measurement is alternately performed from the low-frequency region of k-space toward both high-frequency ends. There is a centric order, and centric trick order is generally used. Three-dimensional measurement (^ In the centric order, one of the phase-encoding-donorap and the slice-en code loop is the outer loop, the other is the inner loop, and either or both are controlled by centric, order.

 However, the centric order in this case is not a true centric order because the distance from the origin in k-space to the measurement point (sampling point) fluctuates as shown in Fig. 1 (b), In some cases, and arteriovenous separation was insufficient. '.

 As a method of solving such a problem, the relative F0V in the ky-kz space is also considered, and the elliptic force centric ordering is controlled so that the distance from the origin in the k space gradually increases as the signal measurement progresses ( Elliptical Centric Ordering) has been proposed (Fig. 1 (c)) ("Performance of an Elliptical

Centric View Oraer for Signal Enhancement and Motion Artifact Suppression in Breath-hold Three-Dimensional Gradient Echo Imaging. Alan, et al. Magnetic Resonance in Medicine 38: 793-802, 1997. In this data acquisition method, since the frequency data for determining the contrast of the image is measured at the beginning of the imaging time, the imaging is selectively started by starting the imaging when the concentration of the contrast agent in the target blood vessel increases. It became possible to obtain an image.

 However, although the above-mentioned centric order eccentricity bidding is effective for determining the contrast of the image at an early stage and effectively obtaining an arterial image, the optimal imaging timing force is shifted by S ^^ There is a problem that the image quality deteriorates due to the acquisition of low-frequency information when the agent is thin. In particular, if the measurement time is too early, the data in the low-frequency region will be sampled during the time when the signal from the blood vessel is extremely low, while the data in the high-frequency region will have a high signal from the blood vessel. Since the signal is sampled during the time zone, a ringing artifact without a DC component is generated. In addition, there is a problem that the imaging time becomes longer because the measurement is performed in the high-frequency region in the vertical and horizontal directions of the k space with respect to the origin.

 On the other hand, in the sequential order, even if the measurement timing is slightly shifted, a remarkable artifact does not appear in the image, and a stable image can be acquired. Was not performed sufficiently.

Therefore, an object of the present invention is to provide an MRI apparatus capable of delineating the entire target blood vessel with high contrast in a short time while reducing the influence on the image quality due to the shift of the optimal imaging timing. It is another object of the present invention to provide an MRI apparatus which is hardly affected by body motion and can separate and depict arteries and veins with MRA. It is another object of the present invention to provide a data collection method suitable for MRA. Disclosure of the invention

 In order to achieve the above object, in the present invention, the measurement points in the k space are divided into two groups, and in the first group to be measured first, the distance from the origin in the k space gradually approaches U In the other group of measurements, the measurement order of the measurement points is changed from the low-frequency component so that the distance gradually increases. Adopt a data collection method that controls toward high frequency components.

 That is, the MRI apparatus of the present invention comprises: a static magnetic field generating means for generating a static magnetic field in a space where a subject is placed; and a gradient magnetic field generating means for applying a gradient magnetic field in the Rice, phase encoding, and reading directions to the space. A transmission system for irradiating a high-frequency magnetic field to cause nuclear magnetic resonance to an atomic nucleus of the biological tissue of the subject; a receiving system for detecting an echo signal emitted by the nuclear magnetic resonance; A gradient magnetic field generation means, a control system for controlling the transmission system and the reception system, a signal processing system for performing an image rain synthesis operation using an echo signal detected by the reception system, and a means for displaying the obtained image. The control system executes a three-dimensional sequence for giving a slice code and a phase encode, and at this time, a total of k space defined by the number of slice encodes and the number of vertical encodes is provided. The points are divided into two groups.In the first group, the distance from the origin in k-space to the measurement point gradually decreases in the order of measurement.In the second group, the distance from the origin in k-space to the measurement point is measured. The gradient magnetic field generating means is controlled in the slice direction and the phase encoder direction so that the values gradually increase in the order of measurement.

 The method of dividing the measurement points in k-space into two groups is that at least one group contains the measurement points from the low-frequency region to the high-frequency region, and the measurement point of the other group contains at least the measurement points in the low-frequency region. I just need. Also, the number of measurement points actually measured among the measurement points belonging to the two groups may be the same or different. That is, one of the forces of the two groups may include a non-measurement point (a point that is not measured).

Specifically, as one aspect, the measurement points are divided into two groups according to the k-space region. According to this aspect, the control system attaches the slice encode and the vertical encode. The k-space defined by the number of slice encodes and the number of phase encodes is divided into two, and the distance from the origin in k-space to the measurement point in k-space is The gradient magnetic field generating means in the slice direction and the phase encode direction is controlled so that the distance gradually decreases in the order of measurement, and in the other region, the distance from the origin on the k-space gradually increases in the order of measurement.

 As another mode, the measurement points in the k space are divided into two groups having a complex conjugate relationship with each other. According to this aspect, the control system executes the three-dimensional sequence for applying the slice encoding and the phase encoding, and at this time, sets the measurement point of the lc space defined by the slice encoding number and the phase encoding number to the origin. Divided into first and second groups that are shared and have a complex relationship with each other.In the first group, the distance from the origin to the measurement point gradually decreases in the order of measurement, and in the second group, the distance from the origin to the measurement point The gradient magnetic field generating means in the slice direction and the cross-encode direction are controlled so as to gradually increase in the order of measurement.

 In this case, it is preferable to divide the adjacent measurement points so that they belong to different groups. In order to satisfy the condition that the measurement points of the two groups have a complex conjugate relationship with each other, the measurement points that are partially adjacent to each other near the origin must belong to the same group. Therefore, in this specification, “division of adjacent measurement points so that they belong to different groups” means that the condition that the complex conjugate relationship is satisfied and the condition that adjacent measurement points belong to different groups is maximal. A state that is satisfied.

 In another aspect of the device of the present invention, the control system does not measure all of the measurement points in one of the two divided areas but measures a small number of measurement points compared to the other area. The control for measuring is performed. Or, in the measurement of one group, instead of measuring all of the measurement points, control is performed to measure a smaller number of measurement points than in the other group.

In the three-dimensional image data acquisition method of the present invention, a predetermined region of a subject is selected and excited, and a gradient magnetic field that encodes at least two directions is applied. The step of measuring one signal is repeated a plurality of times while changing the strength of the gradient magnetic field.When three-dimensional image data is collected, the measurement space defined by the two-direction encoder gradient magnetic field strength is divided into two, Measurement is performed sequentially on the two divided areas, and at that time, in the first measurement area, the measurement is performed so that the distance from the origin of the measurement space to the measurement point gradually decreases in the measurement order, and the area measured later In, measurement is performed such that the distance from the origin of the measurement space to the measurement point gradually increases in the order of measurement. '

 In the data collection method of the present invention, preferably, when there are several points having the same distance from the origin, the point closest to the k-space from the previous measurement point is measured as the next measurement point. '

 Further, the three-dimensional image data acquisition method of the present invention includes a step of selecting and exciting a predetermined region of the subject, applying a gradient magnetic field that encodes in at least two directions, and measuring an echo signal generated from the region. When collecting the three-dimensional image data by repeating the gradient magnetic field intensity a plurality of times while changing the gradient magnetic field intensity, the measurement points in the measurement space defined by the two-direction encode gradient magnetic field intensities are shared with the origin and are mutually complex. Adjacent measurement points in a conjugate relationship are divided into first and second groups so that they belong to different groups, and measurements are sequentially performed on the first and second groups. In the first group, the distance from the origin of the measurement space to the measurement ^ is measured so as to gradually decrease in the order of measurement, and in the second group measured later, the distance from the origin of the measurement space to the measurement point is measured. So that it gradually increases in the order of measurement To measure.

According to Ryoaki Motoaki's data collection method, as shown in Fig. 1 (a), measurement without distance fluctuation from the origin can be performed, and the time when the lowest frequency component is measured and the target blood By matching the point at which the signal intensity of the tube peaks with the contrast agent, the target blood vessel can be drawn with high contrast. In addition, even if there is a slight deviation between the point at which the low-frequency component is measured and the peak of the signal strength, the low-frequency component can be reliably measured, and there is no image deterioration. Figures 1 (b) to 1 (d) show the conventional Centric orderer, elliptic force centric order, and sequential channel order. Indicates the change in distance from the k-space origin. -According to a preferred aspect of the data collection method of the present invention, in the first group, of all the measurement points, only the measurement points of the minus part are measured, and in the second group, all the measurement points are measured.

 Since the two groups are in a complex conjugate relationship, one group can collect the data that was not measured without measuring some of the measurement points. In particular, when the first group is measured before the peak of the contrast medium increases, the measurement of unnecessary data having a low signal intensity can be eliminated, and a good image can be obtained. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram illustrating a data collection method adopted by an MRI apparatus according to the present invention and a conventional data collection method. FIG. 2 is a block diagram illustrating an overall configuration of the MRI apparatus to which the present invention is applied. FIG. 4 is a diagram showing an embodiment of a pulse sequence of a contrast MRA measurement performed by the MRI apparatus of the present invention. FIG. 4 is a diagram schematically showing an embodiment of a k-space data collection order according to the present invention. FIG. 5 is a view for explaining MRA imaging by the MRI apparatus of the present invention, FIG. 6 is a view showing a result of simulation of MRA imaging by the MRI apparatus of the present invention and MRA imaging by the conventional method, and FIG. FIG. 8 is a diagram schematically illustrating another embodiment of the k-space data collection order according to the present invention. FIG. 8 is a diagram schematically illustrating another embodiment of the k-space data collection order according to the present invention. Is a diagram schematically illustrating an example of an order of data collection in k-space according to the embodiment. FIG. 10 is a diagram for explaining MRA imaging by the MRI apparatus of the present invention, FIG. 11 is a diagram schematically showing another embodiment of the k-space data collection order according to the present invention, and FIG. FIG. 10 illustrates an image reconstruction method to which the data acquisition method of FIG. 10 is applied, FIG. 13 is a diagram illustrating a simulation for evaluating MRA imaging by the MRI apparatus of the present invention, and FIG. The figure which shows the result of having simulated the MRA imaging by the MRI apparatus of this invention, and the MRA imaging by the conventional method. FIG. 15 is a figure which shows typically the other example of the data collection order of k space by this invention. BEST MODE FOR CARRYING OUT THE INVENTION

 Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

 FIG. 2 is a block diagram showing the overall configuration of the MRI apparatus according to the present invention. This MKI device obtains a tomographic image of the subject using the 丽 R phenomenon, and includes a static magnetic field generating magnet 2, a magnetic field gradient generating system 3, a sequencer 4, a transmitting system 5 ', and a receiving system 6. , A signal processing system 7, and a central processing unit (CPU) 8.

 The static magnetic field generating magnet 2 generates a uniform static magnetic field around the subject 1 in the direction of its body axis or in a direction orthogonal to the body axis, and has a certain space around the subject 1. A permanent magnet type, normal conduction type or superconducting type magnetic field generating means is provided.

 The magnetic field gradient generating system 3 includes a gradient magnetic field coil 9 wound in three directions of X, Υ, and Z, a gradient magnetic field IT source 10 for driving each gradient magnetic field coil, and a force. By driving the gradient magnetic field power supplies 10 of the respective coils in accordance with the instructions from 4, the gradient magnetic fields Gx, Gy, Gz in the three axial directions of X, Υ, and Z are applied to the subject 1. By applying this gradient magnetic field, a specific slice or slab of the subject 1 can be selectively excited, and the position of the measurement point (sampling point) in the measurement space (k space) and the measurement order must be specified. Can be.

 The sequencer 4 operates under the control of the CPU 8 and sends various commands necessary for data collection of tomographic images of the subject 1 to the magnetic field gradient generating system 3, the transmitting system 5, and the receiving system 6. The operation timing of the magnetic field gradient generation system 3, the transmission system 5, and the reception system 6 controlled by the sequencer 4 is called a pulse sequence. Here, a sequence for three-dimensional blood flow imaging is adopted as one of the pulse sequences. The control of the sequencer 4 will be described later in detail.

The transmission system 5 irradiates a high-frequency magnetic field to cause nuclear magnetic resonance in nuclei of atoms constituting the biological tissue of the subject 1 by high-frequency pulses sent from the sequencer 4, and modulates with the high-frequency oscillator 11. 12 and high-frequency amplifier 13 and high frequency on the transmitting side And a wave coil 14a. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 according to the instruction of the sequencer 4, and the high-frequency pulse subjected to the amplitude modulation is amplified by the high-frequency amplifier 13 and then placed close to the subject 1. The electromagnetic wave is applied to the subject 1 by supplying it to the high-frequency coil 14a.

 The receiving system 6 detects the echo signal (fan R signal) emitted by nuclear magnetic resonance of the nucleus of the living body a ^ of the subject 1, and includes a high-frequency coil 14b and an amplifier 15 on the receiving side and a quadrature phase detector. It consists of a container 16, A / D conversion ^ ¾ 17, and power. The electromagnetic (response signal) of the response of the subject 1 due to the electromagnetic wave radiated from the high-frequency coil 14a on the transmitting side is detected by the high-frequency coil 14b arranged close to the subject 1. The detected echo signal is input to the A / D converter 17 via the amplifier 15 and the quadrature detector 16 and is converted into a digital value. Further, the quadrature detector is output at a timing according to an instruction from the sequencer 4. The collected data is sampled into two series and is sent to the signal processing system 7.

 The signal processing system 7 includes a CPU 8, a recording device such as a magnetic disk 18 and a magnetic tape 19, and a display 20 such as a CRT. The CPU 8 performs processing such as Fourier transform and correction coefficient calculation image reconstruction on the echo signal (digitized data) sent from the receiving system 6 and performs an appropriate operation on the signal intensity distribution of an arbitrary cross section or a plurality of signals. The obtained distribution is imaged and displayed on the display 20 as a tomographic image. In FIG. 2, the high-frequency coils 14a and 14b on the transmitting side and the receiving side and the gradient magnetic field coil 9 are installed in the magnetic field space of the static magnetic field generating magnet 2 arranged in the space around the subject 1. .

 Next, a first embodiment of the blood flow imaging function in the MRI apparatus of the present invention will be described.

As described above, the sequencer 4 operates according to a predetermined pulse sequence, here a three-dimensional MM sequence, according to the operation timing of the magnetic field gradient generation system 3, the transmission system 5, and the reception system 6. Control the ringing. This pulse sequence is pre-installed as a program in the memo provided in the CPU 8), and can be executed by the user selecting as appropriate according to the purpose of photographing, similarly to other pulse sequences. That is, the sequence 4 in which the MRA using the contrast agent is selected via the input device of the CPU 8 is controlled by the CPU 8 to execute a three-dimensional MRA sequence.

 This panless sequence is a sequence based on the gradient echo method, as shown in FIG. 3, for example, and is common to three-dimensional MRA sequences. That is, the high-frequency magnetic field panelless RF is simultaneously applied with the region selection gradient magnetic field Gs to excite the region (slab) including the target blood vessel, and then the gradient magnetic field pulse Gel in the slice direction and the gradient in the directional code direction. The magnetic field pulse Ge2 is applied, then the readout gradient magnetic field Gr is applied, and the polarity is inverted to measure the echo signal. From the high-frequency magnetic field pulse RF to the echo signal measurement, the magnetic field strength of the gradient magnetic field Gel in the slice direction and the gradient magnetic field Ge2 in the phase encode direction are changed at a predetermined repetition time TR to obtain three-dimensional data.

 The number of encoders in the slice direction and the phase encoder direction determines the image resolution in the direction A, and is set in advance in consideration of the measurement time and the like. For example, the number of encodes in the phase encoding direction is set to 128, 256, and the slice direction is set to 10 to 30. The k-space (ky-kz space, where the slice direction is the z direction and the phase encoding direction is the y direction) is defined by the number of encoders in the slice direction and the phase encode direction. That is, in the sequence of FIG. 3, the signals measured when the gradient magnetic field intensity in the slice direction is a certain value Gel (Gz) and the gradient magnetic field intensity in the phase encoder direction is a certain value Ge2 (Gy) are Gz and Gy. Is located at the grid point (ky, kz) in k-space corresponding to.

For example, the three-dimensional MRA sequence itself shown in FIG. 3 is general in MRA, but in this sequence adopted in the present embodiment, the data collection method uses the conventional centric orderer or elliptic force sensor. This is different from trick ordering. In the method of-, the ky-kz space is divided into two along the ky axis or the kz axis, and in one area where measurement is first started, the area is determined from the point at a large distance from the origin 0 in the k space. Start the measurement, and then perform sampling control from the high-frequency component to the low-frequency component so that the sampling point gradually approaches the origin 0. On the other hand, in the other area, the sampling point is controlled from the low frequency component to the high frequency component so that the distance from the origin 0 gradually increases from or near the origin 0.

 In this case, if there are several points at the same distance from the stool point, the point with the closest distance in k-space from the previous sampling point is measured as the next sampling point. That is, this data collection method is as follows: 1) One of the two divided areas starts from the point farthest from the origin, and thereafter, determines the subsequent sampling points so that the distance from the origin gradually decreases. In the other area, start from the origin or the point closest to the origin, and then determine the subsequent sampling points so that the distance from the origin gradually increases. 2) Make the distance between sampling points adjacent in time the shortest. Is defined by the AND of the two conditions. .

 In the data collection method of the present invention, the condition of 2) is not essential, but by making sampling points adjacent to each other as close as possible, it is possible to reduce the artifat. For this purpose, the condition in 2) may be to select not the distance between the two sampling points but the force with the same ky value, for example. The optimum sampling point may be determined in consideration of not only the relationship between two sampling points but also the relationship between a plurality of sampling points in the next measurement and further subsequent measurements.

Fig. 4 shows a simplified example of the data collection method described above, showing the data collection order in a 5 * 9 Matritus k-space where the number of slice codes is 5 and the number of phase codes is 9. In the figure, circled numbers indicate the data collection order. This divided into two k-space in kz axis, toward the origin (No. 25) a point in a high frequency region indicated with the lower region (E → C region) _"S tart" from (No. 1) in numerical order Measure In the upper 简 area (C → E area), measurement is performed from the origin to the high frequency area in the numerical order up to number 45. Note that the distance Δz between two adjacent points on the coordinates of the k space is 1 ZFOVz.

 Next, an embodiment of a contrast-enhanced MRA using the above-described MRI apparatus employing such a data acquisition method will be described with reference to FIG.

 First, the subject is placed in the measurement space inside the static magnetic field magnet, the imaging region including the target blood vessel is determined, and timing imaging is performed. Timing imaging is performed by the test injection method. In this method, a small amount of a contrast medium (about 1 to 2 ml) is first injected for test injection to obtain a time signal curve at the target site as shown in FIG. The arrival time tl of the contrast agent is measured from this curve, and the timing for performing the main imaging is determined based on the result. In addition to this test injection method, the timing imaging method sets R0I at a specific site in the monitor area for the arrival of the contrast agent, captures the signal change of the same site, and exceeds the set threshold. There is a method of automatically starting imaging at a point in time, or a method called fluoroscopy, in which a target blood vessel is observed in real time by repeating short-time imaging and display, and imaging is started when an appropriate signal rise is obtained. It is also possible to employ these methods. However, the test injection method is preferable because the timing can be accurately measured by using a contrast agent prior to the main imaging.

 After the timing imaging, the main imaging is performed as shown in FIG. 5 (b). The main imaging may be performed only after the injection of the contrast agent, but preferably, images before and after the injection of the contrast agent are taken. The imaging before and after the imaging is performed continuously for the same slice or slab position under the same conditions.

'The imaging sequence is a sequence based on the short TR 3D gradient echo method as shown in Fig. 3. In this case, since the blood flow is to be imaged, a gradient magnetic field for rephasing the dephasing due to the flow, that is, Gradient Moment Nulling may be added, but this is not essential. Instead, it is preferable to use a rather simple gradient echo for shortening TR / TE. '·

 The imaging time T is determined when the pulse sequence repetition time TR, matrix size (slice chain code number and phase end number), and addition number are determined, so that the target blood vessel obtained by the above timing imaging reaches the contrast agent of interest. Based on time tl, imaging start time t2 (imaging is started after injection of contrast agent so that data measurement in the low-frequency region of the ky-kz space is performed when the contrast agent reaches the target blood vessel. Set the time up to).

 In the imaging, measurement of the E → C area shown in Fig. 4 is started first, and then measurement of the C → E area is performed. In this case, the sequencer 4 uses the gradient magnetic field in the slice direction in the region to be measured first (for example, the E → C region). And the gradient magnetic field noise in the direction of the phase encoder are controlled so that high-frequency components and low-frequency components are measured in order, and in the subsequent measurement area (for example, C → E area), low-frequency components and high-frequency components are measured. Control to measure the thigh. In this case, as described above, control is performed so that the distance from the origin to the sampling point gradually decreases in the first region, and the distance from the origin gradually increases in the subsequent region.

 As a result, when the contrast agent reaches the target blood vessel and the signal of the blood flowing through the target blood vessel reaches the highest intensity, the low-frequency component in the k-space is measured, and the image of the artery is displayed with high contrast. I can draw. Moreover, as shown in Fig. 5 (b), there is a time zone for measuring low-frequency components on both sides of the contrast agent arrival time tl, so that the slight difference in the conditions between timing imaging and main imaging Even when the contrast agent arrival time tl is deviated due to the above, a high-quality image with almost no deterioration in image quality can be obtained.

Figure 6 shows the results of simulating the difference in arteriovenous separation due to the different data collection methods. In this simulation, FOV: 320, TR: 10 ms, number of phase encodes: 160, number of slice encodes: 16, image matrix: 256 * 16, slice: 5 、 The arteriovenous separation is expressed as the ratio of the signal strength of the artery to the signal strength of the vein.

 As can be seen, the data collection method of the present invention has an increased arteriovenous signal ratio as compared to sequential ordering and elliptical centric ordering. Therefore, even if there is a vein near the artery that is confusing, it is possible to draw only the artery with high contrast.

 Thus, after obtaining the three-dimensional image data before and after the contrast, respectively, by taking the difference between the three-dimensional image data, the three-dimensional data of only the blood vessel can be obtained. The difference processing is performed, for example, by performing a complex difference between slices at the same slice position in three dimensions. The difference may be a difference between absolute values. The method of removing tissue other than blood vessels by performing differential processing between images before and after contrast in this manner is called 3D MR-DSA (Digital Subtraction Angiography) and is a known method, and is not essential in the present invention. In particular, it is suitable for delineating thin blood vessels in which it is difficult to obtain sufficient contrast with tissues other than blood vessels.

 Furthermore, the three-dimensional data after the difference processing can be viewed three-dimensionally by projecting it in any direction, such as coronal section, sagittal section, and transverse axis. As a projection method, a known maximum intensity projection method or the like can be employed.

 As described above, the first embodiment of the present invention has been described based on the examples shown in FIGS. 4 and 5, but various modifications are possible. For example, in the above embodiment, a sequence based on the gradient echo method is exemplified as a three-dimensional MRA sequence, but an EPI (Echo Planer Imaging) method for measuring a plurality of echo signals with one excitation, a split-type EPI, and the like are also employed. can do.

 Further, in the above embodiment, the k space may be divided by the force ky axis which indicates the case where the k space is divided by the kz axis. Although the case where data is collected symmetrically for the divided areas has been described, the areas may be asymmetric.

An embodiment in the case of asymmetry will be described with reference to FIG. Again the matrix is 5 * 9 and the case where the k space is divided by the kz axis will be described as an example. In this asymmetric data collection, force measurement is started not in the high-frequency region but in the medium-frequency region (No. 1) in the region to be measured first, for example, in the E → C region, and continues until the low-frequency region (No. 15). Measure in order. After that, measurement is performed for the entire C → E region from the low frequency region to the high frequency region as in the case of Fig. 4. Or, conversely, it is possible to measure the whole area for the area to be measured first, and to measure from the low frequency area to the medium frequency area for the latter area. For example, referring to FIG. 7, the measurement may be started from number 35, and thereafter, the measurement may be performed up to number 1 in the order of decreasing numbers.

 By making the number of measurement points of one of the two divided areas smaller than that of the other, the imaging time as a whole can be reduced. In other words, as shown in Figs. 5 (c) and (d), the number of measurement points in the area to be measured first is reduced, or the number of measurement points in the area to be measured later is reduced, so that the imaging time is shorter than in Fig. 5 (b). Become. Furthermore, the measurement method shown in Fig. 5 (d) is effective when the distance between the target blood vessel and the nearby vein is short, and the time when the contrast agent reaches the target blood vessel after reaching the target blood vessel is short. By measuring the low frequency component to the middle frequency component in the second half region, it is possible to prevent the signal from the nearby vein (the signal strength curve shown by the dotted line in the figure) from being mixed. ,

 In this case, the data in the area with a small number of measurement points may be estimated from the data in the area where the unmeasured data (the data in the shaded area in Fig. 7) is measured, or May be. Also in this embodiment, in addition to the above-described effects, it is possible to selectively obtain a high-contrast artery image while reducing the influence of the shift in the three-dimensional measurement timing.

Furthermore, data collection in k-space is not limited to rectangular matrices as shown in Figs. 4 and 7, and data within a circle (ellipse) centered on the origin (No. 15) as shown in Fig. 8 It is also possible. In the embodiment shown in FIG. 8, the number of slice encodes is 5, and the number of phase encodes is 9. Here, both ky and kz are high-frequency components. The area (outside the circle) is collected, but data concentrically arranged around the origin (No. 15) is collected. As shown by the circled numbers in the figure, the order of data collection is as follows: in the lower half area, which is measured first, from the center to the center in order from the side farther from the center, and in the upper half area, the measurement is in the direction away from the center. I do.

 In this case, the same effect as that of the embodiment shown in FIG. 4 can be obtained. Also in this case, data interpolation can be performed as needed.

 As described above, as the first embodiment of the present invention, the embodiment in which the region of the k space is divided and the measurement points are divided into two groups according to the region has been described. It is also possible to divide them into groups.

 Next, as a second embodiment of the present invention, a case will be described in which measurement points in k-space are divided such that those having a complex relationship with each other belong to different groups.

 In this embodiment, when dividing grid points in k space, for example, ky-kz space, that is, measurement points into two groups, 1) the two groups share the origin, and the measurement points belonging to the two groups are complex with each other. 2) Divide so that the adjacent measurement points in k-space belong to different groups. Then, the two groups thus divided are sequentially measured.

 FIG. 9 shows a k-space of an 8 * 8 matrix in which the number of slice encodes is 8 and the number of phase encodes is 8, as an example of the data collection method according to the second embodiment, which is zero-purified. There are 64 grid points (measurement points) in the rice matrix, and these grid points are divided into a first group on the left and a second group on the right. Lattice points belonging to these two groups have a complex conjugate relationship with each other, and adjacent lattice points belong to different groups. However, in order to satisfy the complex conjugate relation, adjacent lattice points belong to one group near the origin. '

The first of these two groups, which measures first, starts the measurement from the point in k-space where the distance from the origin 0 force is large, and then gradually increases the sampling point from the high-frequency component so as to approach the origin 0 gradually. Sampling control is performed toward low frequency components. Also the second In the group, on the contrary, the sampling point is controlled from the low-frequency component to the high-frequency component such that the distance from the origin 0 gradually increases from or near the origin 0.

In the figure, circled numbers indicate the data collection order. No hierarchy is the measurement point of the same number, indicating that it may be measured from either of them _ώ

 In the first group, which starts measurement first, the measurement starts from the grid point (number 1) farthest from the origin (the grid point with number 33), that is, the highest frequency component, and then the grid point of number 2 Measurements are sequentially performed up to the origin, such as the grid point of number 3. Next, measurement of the second group is performed. Here, measurement is started from the lattice point (No. 34) closest to the origin, that is, the low-frequency component, and measurement is performed in order from the origin in order.

 Next, an embodiment of a contrast-enhanced MRA using the MRI apparatus adopting such a data collection method will be described with reference to FIG.

 First, similarly to the first embodiment, the subject is placed in the measurement space in the static magnetic field magnet, an imaging region including a target blood vessel is determined, and timing imaging is performed. Also in this case, the timing imaging is performed by, for example, a test injection method. That is, a small amount of a contrast medium (about 1 to 2 ml) was test-injected, and as shown in Fig. 10, the time at the target site "^ sign curve was obtained, from which the arrival time of the syrup (signal intensity) Measure the tl and determine the timing to perform the main imaging based on the result. 'Timing imaging, perform the main imaging as shown in Fig. 10 (b). May perform only imaging after the injection of the contrast medium, but preferably images before and after the injection of the contrast medium.Imaging before and after the imaging is performed under the same conditions under the same slice or slab position. Is performed continuously.

The imaging sequence is a sequence based on the short TR 3D gradient echo method as shown in Fig. 3. In this case, since the blood flow is to be imaged, a gradient magnetic field, that is, Gradient Moment Nulling may be added for rephasing the phase due to the flow, but this is not essential. Instead, it is preferable to use a simple gradient echo for shortening TR / TE.

 Once the pulse sequence repetition time TR and matrix size (number of slice codes and number of phase codes) and the number of additions are determined, the imaging time T is determined. Based on tl, imaging start time t2 (from the injection of the contrast agent to the start of imaging, so that data measurement in the low-frequency region of the ky-kz space is performed when the contrast agent reaches the target blood vessel. Time). In imaging, measurement of the first group is first started, and then measurement of the second group is performed. In this case, the sequencer 4 controls both the gradient magnetic field pulse in the slice direction and the gradient magnetic field in the phase encode code in the first group to be measured first so that the high frequency component and the low frequency component are sequentially measured, and then the measurement is performed thereafter. In the second group, control is performed so that measurement is performed in order from low-frequency components to high-frequency components. In this way, three-dimensional image data after contrast is obtained.

 In Fig. 9 and Fig. 10 (b), the data collection method shows a case where all the measurement points belonging to both the first group and the second group are measured, as shown in Fig. 10 (c). As described above, the first group may employ a data collection method in which measurement of a predetermined high-frequency component is omitted and low-frequency components are measured in a short time. An example of such a data collection method is shown in FIG.囪 11 also illustrates an 8 * 8 matrix k-space in which the number of slice switches is 8, and the number of phase switches is 8. In this embodiment, the k-space is divided into two groups under the same conditions as in the embodiment shown in Fig. 9, but here the measurement is performed first. Measure only the components. In the illustrated embodiment, only grid points present in 4 * 4 madridas in the low frequency region are measured among grid points in the k space. First, among the grid points in these low-frequency regions, the grid point with the greatest distance of the origin (grid number 9) is used as the starting point, and the grid point of number 2 and the grid point of number 3 are measured up to the origin.

In the second group, as in the embodiment shown in FIG. 9, the grid point (number 10) adjacent to the origin From the origin, and measure all grid points belonging to the second group up to the highest frequency component in the order away from the origin.

 In this case, the data of the high frequency region that is not measured in the first group can be estimated based on the complex characteristics of the first group and the second group. As a method of estimating unmeasured data, a method based on a known half Fourier reconstruction method can be employed. FIG. 12 schematically shows these processes. First, the measured data is subjected to one-dimensional Fourier transform in the frequency encoder direction (k x direction) to obtain the actual measured data in the three-dimensional hybrid space. The three-dimensional estimated data is obtained from the actual measurement data, and the hybrid spatial data is obtained by combining the actual measurement data and the estimation data. Three-dimensional image data is obtained by subjecting the hybrid space data to two-dimensional Fourier transform. Thus, even if the number of data points is reduced, the spatial resolution is not degraded. In this embodiment, as in the data collection method shown in FIG. 9, the low-frequency component in k-space is measured when the contrast agent reaches the target blood vessel and the signal intensity of the blood flowing through the target blood vessel becomes the highest. This means that artery images can be drawn with high contrast.

 In general, as shown in Fig. 12, after the injection of the contrast agent, the concentration of the contrast agent (signal intensity) rises sharply. By implementing the imaging method of the present embodiment, it is possible to avoid measurement of an unnecessary signal before reaching the contrast agent.

Figures 13 and 14 show the results of simulating differences in arteriovenous separation due to differences in imaging methods (data collection methods). In this simulation, a simulated artery and vein was used, and a contrast medium was flowed into it at a flow rate of 40 cm, an arterial venous return time of 7 seconds, and an injection speed of 2 cc / s. FIG. 13 shows the signal strength under the above conditions. As shown in the figure, the peak of the signal from the artery is seen first, and the peak of the signal from the vein appears later. FIG. 14 (a) is an image obtained by the imaging method of the present invention, and FIG. 14 (b) is an image obtained by elibutanore centric ordering. As can be seen from the figure, in the elliptical centric ordering, veins as well as arteries are imaged, and arteriovenous separation is not complete, whereas the imaging method of the present invention allows only arteries to be drawn with bulk contrast.

 Note that, in the second embodiment described above, a sequence using the Daradent echo method has been exemplified as the three-dimensional MM sequence, but an EPI (Echo Planer Imaging) method in which a plurality of echo signals are measured by one excitation or a split type EPI can also be adopted.

 Furthermore, data collection in k-space is not limited to the rectangular matrix shown in Fig. 9 and Fig. 11, and data in a circle (ellipse) centered on the origin as shown in Fig. 15 can also be collected. It is possible. In the first group shown by the solid line in the figure, measurement starts at a point in k-space that is farther from the origin 0, and then gradually moves the sampling point from the high-frequency component to the low-frequency component so as to approach the origin 0 gradually. Sampling control. On the other hand, in the second group, the sampling point is controlled from the low-frequency component to the high-frequency component such that the distance from the origin 0 gradually increases from or near the origin 0.

 In this case, the same effects as those of the embodiment shown in FIGS. 9 and 11 can be obtained. Also in this case, the measurement of the first group of high-frequency components can be omitted as necessary. Industrial applicability

As described above, according to the MRI apparatus of the present invention, the grid points (measurement points) in the ky-kz space are divided into two groups, and one of the groups measured first has a distance from the origin in the k space. The sampling point is controlled from the high-frequency component to the low-frequency component so that the sampling point gradually approaches, and in the other group, the sampling point is shifted from the low-frequency component to the high-frequency component so that the distance gradually increases. Sampling control is performed for each component, so that the effect of high contrast can be achieved while reducing the effect of the imaging timing shift. A vein-separated surface image can be obtained. Also, the imaging time can be reduced by reducing the number of measurement points in one of the two divided areas.

Claims

The scope of the claims 1. a static magnetic field generating means for generating a static magnetic field in a space where a subject is placed; a gradient magnetic field generating means for applying a gradient magnetic field in each of a slice direction, a phase encoding direction, and a readout direction to the space; A transmitting system that irradiates a high-frequency magnetic field to cause nuclear magnetic resonance in an atomic nucleus of a living body of a specimen; a receiving system that detects an echo signal emitted by the nuclear magnetic resonance; a gradient magnetic field generating unit; a transmitting system; A magnetic resonance imaging apparatus comprising: a control system that controls the system; a signal processing system that performs an image reconstruction operation using the echo signal detected by the reception system; and a unit that displays an obtained image. The control system executes a three-dimensional sequence that provides slice encoding and erroneous phase encoding. At this time, the k-space defined by the number of slice encodings and the number of phase encodings is used. The measurement points are divided into two groups.The measurement of the first group gradually reduces the distance from the origin in k-space to the measurement point in the order of measurement.The measurement of the second group measures the distance from the origin in k-space to the measurement point. A magnetic resonance imaging apparatus characterized in that the gradient magnetic field generation means in the slice direction and the phase encode direction is controlled so that the distance gradually increases in the order of measurement. 2. The two groups belong to two regions that are obtained by dividing the k-space into two:! 2. The magnetic resonance imaging apparatus according to claim 1, wherein:  3. The magnetic resonance imaging apparatus according to claim 1, wherein the two groups share an origin and are in a complex conjugate relationship with each other. 4. The magnetic resonance imaging apparatus according to claim 3, wherein the measurement points in the k-space are divided so that adjacent measurement points belong to different groups.  5. The magnetic resonance imaging apparatus according to claim 1, wherein at least one of the two groups includes a non-measurement point.  6. The magnetic resonance imaging apparatus according to claim 1, wherein the union of the two groups is within a circular region inscribed in the k-space. 7. Select and excite a given area of the subject and encode in at least two directions A data acquisition method for acquiring three-dimensional image data by repeating a step of applying a gradient magnetic field and measuring an echo signal generated from the region a plurality of times while changing the intensity of the gradient magnetic field.  Dividing the measurement points of the measurement space defined by the two-way encoder gradient magnetic field strength into the first and second groups,  The first and second groups were measured sequentially,  At that time, in the first group that is measured first, the distance from the origin of the measurement space to the measurement point is measured so as to gradually decrease in the order of measurement, and in the second group that is measured later, the origin of the measurement space is measured. A data collection method characterized in that the distance from a measurement point to a measurement point gradually increases in the order of measurement.  8. A plurality of steps of selecting and exciting a predetermined region of the subject, applying a gradient magnetic field that encodes in at least two directions, and measuring an echo signal generated from the region while changing the intensity of the gradient magnetic field. Data collection method to collect 3D image data  The measurement space defined by the two-direction encoder gradient magnetic field strength is divided into two, and measurements are sequentially performed on the two divided regions.  At that time, in the area to be measured first, measurement is performed so that the distance from the origin of the measurement space to the measurement point gradually decreases in the order of measurement, and in the area to be measured later, the measurement point is measured from the origin of the measurement space to the measurement point. A data collection method characterized in that measurements are taken so that the distance to the target gradually increases in the order of measurement.  9. Select a predetermined region of the subject, excite it, apply a gradient magnetic field that encodes in at least two directions, and measure the echo signal generated from the region. In a data collection method that collects 3D image data by repeating multiple times while changing The measurement points in the measurement space defined by the two-direction encoding gradient magnetic field intensities share the origin, and the lattice points adjacent to each other have a complex conjugate relationship and belong to different groups. Divided into the first and second groups,  The first and second groups were measured sequentially,  At that time, in the first group that is measured first, the distance from the origin of the measurement space to the measurement point is measured so as to gradually decrease in the order of measurement, and in the second group that is measured later, the origin of the measurement space is measured. A data collection method characterized in that the distance to a measurement point is gradually increased in the order of measurement.  10. The method according to claim 7, wherein one of the two groups measures only a part of the measurement points in one of the two groups and measures all the measurement points in the other group. Data collection method. 1 1. A three-dimensional blood flow drawing method using a contrast agent,  Measuring the time it takes for the contrast agent to reach the target blood vessel after administration of the contrast agent,  Carrying out the data collection method according to any one of claims 7 to 9,  A three-dimensional blood flow drawing method, wherein the measurement start time of the first group is controlled so that the end of the measurement of the first group coincides with the arrival time of a contrast agent.