JP3847519B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus that obtains a tomographic image of a desired part of a subject by utilizing a nuclear magnetic resonance (hereinafter referred to as NMR) phenomenon, and in particular, in a minimum time required for depicting the running of a vascular system. The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) capable of ensuring a desired rendering range and image quality.
[0002]
[Prior art]
The MRI apparatus uses the NMR phenomenon to measure the density distribution, relaxation time distribution, etc. of nuclear spins (hereinafter simply referred to as spins) at a desired examination site in the subject, The cross section is displayed as an image.
[0003]
As an imaging function of this MRI apparatus, there is MR angiography (hereinafter abbreviated as MRA) for drawing a blood flow, and MRA includes a method using no contrast agent and a method using a contrast agent.
As a method using a contrast agent, a method of combining a T1 shortened contrast agent such as Gd-DTPA with a short TR sequence of a gradient echo system is generally used. In this method, since the blood flow spin containing the T1 shortening type contrast agent has a T1 shorter than that of the surrounding tissue, saturation is unlikely to occur when excitation by a high-frequency magnetic field is received for a short repetition time TR. A blood vessel filled with blood containing a contrast agent is depicted with high contrast with respect to other tissues by using a high signal emitted from other tissues. By measuring the volume data (specifically three-dimensional) including blood vessels while the contrast agent stays in the target blood vessel, by superimposing the obtained three-dimensional image and performing projection processing, Draw blood flow. Here, in general, a sequence based on a three-dimensional gradient echo method is used in order to obtain high-resolution information in a wide range.
[0004]
In order to obtain a good image in such a three-dimensional contrast MRA, (1) a contrast medium injection method and (2) an imaging time and timing are important. As for (1), the contrast medium must be injected so as to stably maintain a high concentration in the blood vessel to be imaged. For this reason, rapid injection using an automatic injector is generally used.
[0005]
With regard to (2), for example, in order to separate and selectively image only an 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. In particular, it is ideal that the contrast agent concentration reaches a peak in the central portion (low frequency region) of the k space that governs the contrast of the image, and the timing is set according to the data acquisition method of the pulse sequence to be used.
[0006]
In the data collection method, the sequential order is mainly measured from one end of the high frequency side of the k space to the other end of the high frequency side through the low frequency region, and the measurement is alternately performed from the low frequency region of the k space toward both ends of the high frequency. There is a centric order, and a centric order is generally used. In the centric order of three-dimensional measurement, one or both of the phase encoding loop and the slice encoding loop are controlled to be the outer loop and the other is controlled to the centric order.
[0007]
However, the centric order in this case is not a true centric order because the distance from the origin on the k space to the measurement point (sampling point) varies as shown in FIG. It was easy to receive, and there were cases where arteriovenous separation was insufficient.
[0008]
Elliptical centric ordering that takes into account the relative FOV in the ky-kz space as a method to solve such problems and controls the distance from the origin in the k space to gradually move away as the signal measurement progresses (Elliptical Centric Ordering) has been proposed (Fig. 1 (c)) ("Performance of an Ellipitical Centric View Order 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 ").
[0009]
[Problems to be solved by the invention]
In this data acquisition method, low-frequency data that determines the contrast of the image is measured at the beginning of the imaging time, so that an arteriovenous image can be selectively obtained by starting imaging when the contrast agent concentration of the target blood vessel increases. It became possible to get.
[0010]
However, although the centric order and elliptical centric ordering described above are effective in determining the contrast of an image early and selectively obtaining an arterial image, if the optimal imaging timing is deviated, the contrast agent concentration is reduced. Since the low frequency information is acquired only in a thin time zone, there is a problem that the image quality is likely to deteriorate. In particular, if the measurement starts too early, the data in the low frequency region will be sampled in a time zone where the blood vessel has a very low signal, while the data in the high frequency region will be sampled in a time zone where the blood vessel has a high signal. Since sampling is performed, ringing artifacts having no DC component are generated.
[0011]
On the other hand, in the sequential order, even if the measurement timing is slightly shifted, it is possible to obtain a stable image with no noticeable artifacts appearing in the image. However, as with the centric order described above, it is weak to the subject's body movement and has sufficient arteriovenous separation. There was a problem that was not done.
[0012]
Accordingly, an object of the present invention is to provide an MRI apparatus capable of rendering 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 optimum imaging timing. It is another object of the present invention to provide an MRI apparatus that is hardly affected by body movement and can separately depict arteries and veins in MRA. A further object of the present invention is to provide a data collection method suitable for MRA.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, in the present invention, the measurement points in the ky-kz space are measured by dividing them into two groups having a complex conjugate relationship with each other. Data collection method in which the distance from the origin to the measurement point is gradually increased in the second group that measures from the measurement point of the distance to the origin so that the distance from the origin gradually decreases. Is adopted.
[0014]
That is, the MRI apparatus of the present invention includes 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 gradient magnetic fields in the slice direction, the phase encoding direction, and the readout direction to the space. A transmission system that irradiates a high-frequency magnetic field to cause nuclear magnetic resonance in the nucleus of the biological tissue of the subject; a reception system that detects an echo signal emitted by the nuclear magnetic resonance; and the gradient magnetic field generation means, A control system for controlling the transmission system and the reception system, a signal processing system for performing image reconstruction calculation using an echo signal detected by the reception system, and a means for displaying the obtained image, the control system comprising: A three-dimensional sequence to which slice encoding and phase encoding are applied is executed. At this time, measurement points in k space defined by the slice encoding number and the phase encoding number are shared with the origin. 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. Are controlled so as to gradually increase in the order of measurement.
[0015]
When dividing the measurement points into two groups, it is preferable to divide adjacent measurement points so that they belong to different groups. In order to satisfy the condition that the measurement points of the two groups are in a complex conjugate relationship with each other, it is necessary that measurement points that are partially adjacent to each other in the vicinity of the origin belong to the same group. Therefore, in this specification, “dividing adjacent measurement points so as to belong to different groups” satisfies the condition that they are in a complex conjugate relationship and satisfies the maximum condition that adjacent measurement points belong to different groups. The state that has been done.
[0016]
In the three-dimensional image data acquisition method of the present invention, a step of selecting and exciting a predetermined region of a subject, applying a gradient magnetic field encoded in at least two directions, and measuring an echo signal generated from the region is performed by the gradient magnetic field. In a data collection method that collects three-dimensional image data by repeating multiple times while changing the intensity of the measurement point, the measurement point in the measurement space defined by the encode gradient magnetic field intensity in the two directions shares the origin and has a complex conjugate relationship with each other. In addition, the adjacent measurement points are divided into the first and second groups so as to belong to different groups, and the first and second groups are sequentially measured. At that time, in the first group measured first, In the second group, which is measured later, the distance from the origin of the measurement space to the measurement point gradually increases in the order of measurement. To measure to.
[0017]
According to the data collection method of the present invention, as shown in FIG. 1 (a), it is possible to perform measurement without variation in the distance from the origin, and when the lowest frequency component is measured and the signal intensity of the target blood vessel is The target blood vessel can be depicted with high contrast by matching the peak point with the contrast agent. Moreover, even if there is a slight difference between the time point when the low frequency component is measured and the peak of the signal intensity, the low frequency component can be reliably measured, and the image is not deteriorated. FIGS. 1B to 1D show changes in distance from the k-space origin in the conventional centric order, elliptical centric order, and sequential order.
[0018]
According to a preferred aspect of the data collection method of the present invention, the first group measures only some of the measurement points, and the second group measures all measurement points. And
Since the two groups are in a complex conjugate relationship, even if one group does not measure some measurement points, data that is not measured can be estimated from the measurement data of the other groups. Thereby, the measurement time of one group can be shortened. In particular, when measurement of the first group is performed between the time when the concentration of the contrast medium increases and the peak, unnecessary data with low signal intensity can be eliminated, and a good image can be obtained.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below 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 MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject, and includes a static magnetic field generating magnet 2, a magnetic field gradient generating system 3, a sequencer 4, a transmitting system 5, a receiving system 6, and a signal. A processing system 7 and a central processing unit (CPU) 8 are provided.
[0020]
The static magnetic field generating magnet 2 generates a uniform static magnetic field around the subject 1 in the direction of the body axis or in a direction perpendicular to the body axis, and is a permanent magnet in a space around the subject 1 A magnetic field generating means of a normal type, a normal conductive type or a superconductive type is arranged.
[0021]
The magnetic field gradient generating system 3 includes a gradient magnetic field coil 9 wound in three axes of X, Y, and Z, and a gradient magnetic field power source 10 for driving each gradient magnetic field coil. By driving the gradient magnetic field power supply 10 of each coil, gradient magnetic fields Gx, Gy, and Gz in the three-axis directions of X, Y, 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 can be defined. it can.
[0022]
The sequencer 4 operates under the control of the CPU 8 and sends various commands necessary for collecting tomographic image data of the subject 1 to the magnetic field gradient generation system 3, the transmission system 5, and the reception 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 in detail later.
[0023]
The transmission system 5 irradiates a high-frequency magnetic field in order to cause nuclear magnetic resonance to occur in the atomic nucleus constituting the living tissue of the subject 1 by the high-frequency pulse sent out from the sequencer 4, and includes a high-frequency oscillator 11, a modulator 12, It comprises a high frequency amplifier 13 and a high frequency coil 14a on the transmission side. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 in accordance with a command from the sequencer 4, and after the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13, the high-frequency pulse arranged close to the subject 1 By supplying the coil 14a, the subject 1 is irradiated with electromagnetic waves.
[0024]
The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nucleus of the biological tissue of the subject 1, and receives the high-frequency coil 14b, the amplifier 15, the quadrature detector 16 and the A / D converter 17. The response electromagnetic wave (NMR signal) of the subject 1 due to the electromagnetic wave irradiated from the transmission-side high-frequency coil 14a is detected by the high-frequency coil 14b disposed in the vicinity of the subject 1. The detected echo signal is input to the A / D converter 17 through the amplifier 15 and the quadrature detector 16 and converted into a digital quantity, and further sampled by the quadrature detector 16 at the timing according to the command from the sequencer 4. Two series of collected data are sent to the signal processing system 7.
[0025]
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, correction coefficient calculation image reconstruction, etc. The signal intensity distribution or a distribution obtained by performing an appropriate calculation on a plurality of signals is imaged and displayed on the display 20 as a tomographic image. In FIG. 2, the high-frequency coils 14 a and 14 b and the gradient magnetic field coil 9 on the transmission side and the reception side are installed in the magnetic field space of the static magnetic field generating magnet 2 arranged in the space around the subject 1.
[0026]
Next, the blood flow imaging function in the MRI apparatus of the present invention will be described.
As already described, the sequence 4 controls the operation timing of the magnetic field gradient generation system 3, the transmission system 5, and the reception system 6 according to a predetermined pulse sequence, here, a three-dimensional MRA sequence. This pulse sequence is pre-installed as a program in a memory provided in the CPU 8, and can be executed by the user as appropriate in accordance with the purpose of imaging, as with other pulse sequences. That is, when the MRA using the contrast agent is selected via the input device of the CPU 8, the sequence 4 is controlled by the CPU 8, and the three-dimensional MRA sequence is executed.
[0027]
This pulse sequence is a sequence based on the gradient echo method as shown in FIG. 3, for example, and is typical for a three-dimensional MRA sequence. That is, after exciting the region (slab) including the target blood vessel by applying the high-frequency magnetic field pulse RF simultaneously with the region selection gradient magnetic field Gs, applying the gradient magnetic field pulse Ge1 in the slice direction and the gradient magnetic field pulse Ge2 in the phase encoding direction, Next, the read gradient magnetic field Gr is applied and the polarity is reversed to measure the echo signal. The high-frequency magnetic field pulse RF to echo signal measurement are repeated at a predetermined repetition time TR while changing the magnetic field strength of the gradient magnetic field Ge1 in the slice direction and the gradient magnetic field Ge2 in the phase encoding direction, and three-dimensional data is obtained.
[0028]
The number of encodings in the slice direction and the phase encoding direction determines the image resolution in both directions, and is set in advance in consideration of the measurement time and the like. For example, the number of encodes in the phase encode direction is set to 128, 256, and the slice direction is set to 10-30. The k space (ky-kz) is defined by the number of encodes in the slice direction and the phase encode direction. That is, in the sequence of FIG. 3, signals measured when the gradient magnetic field strength value Ge1 (Gz) in the slice direction and the gradient magnetic field strength value Ge2 (Gy) in the phase encoding direction correspond to Gz and Gy. It is arranged at a lattice point in the k space, (ky, kz).
[0029]
For example, the three-dimensional MRA sequence itself shown in FIG. 3 is general in MRA, but in this sequence adopted by this embodiment, the data collection method is different from the conventional centric order or elliptical centric ordering. .
[0030]
In this method, grid points in a ky-kz space (hereinafter simply referred to as k space), that is, measurement points are divided into two groups, and these two groups are sequentially measured. When dividing the measurement points into two groups, 1) the two groups share the origin, the measurement points belonging to the two groups are in a complex conjugate relationship with each other, and 2) the adjacent measurement points are different in k-space. It is necessary to belong to a group.
[0031]
FIG. 4 shows an 8 * 8 matrix k-space in which the number of slice encodings is 8 and the number of phase encodings is 8, as a simplified example of the above-described data collection method. There are 64 grid points (measurement points) in this matrix, and these grid points are divided into a first group shown on the left side and a second group shown on the right side in the figure. The 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 relationship, adjacent lattice points belong to one group in the vicinity of the origin.
[0032]
In the first group to be measured first of these two groups, the measurement starts from a point with a large distance from the origin 0 on the k space, and then gradually changes the sampling point from the high frequency component to the low frequency so as to approach the origin 0. Sampling control is performed toward the component. In the second group, 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.
[0033]
In the figure, the numbers in circles indicate the data collection order. The measurement points with the same number have no order, indicating that measurement may be performed from any of them.
In the first group that starts measurement first, the measurement starts from the grid point (number 1) farthest from the origin (the grid point numbered 33), that is, the highest frequency component, and then the grid point number 2 Measure to the origin sequentially, such as 3 grid points. Subsequently, measurement of the second group is performed. Here, measurement is started from a lattice point (number 34) closest to the origin, that is, a low-frequency component, and measurement is sequentially performed in a sequence away from the origin.
[0034]
Next, an embodiment of contrast-enhanced MRA using the MRI apparatus employing such a data collection method will be described with reference to FIG.
First, a subject is placed in a measurement space in a static magnetic field magnet, an imaging region including a 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 contrast medium (about 1 to 2 ml) is test-injected to obtain a time-signal curve at the target site, as shown in FIG. This is a method of measuring the time t1 and determining the timing for performing the main imaging based on this result. As a timing imaging method, in addition to this test injection method, an ROI is set for a specific region within the monitor area regarding the arrival of the contrast agent, signal changes in the same region are captured, and when the set threshold is exceeded, automatic There are methods that start imaging, short-time imaging called fluoroscopy, and methods of observing the target blood vessel in real time by repeated display and starting imaging when an appropriate signal rise is obtained. Although it is possible to adopt, the test injection method is preferable because the timing can be accurately measured by using the contrast agent prior to the main imaging.
[0035]
After timing imaging, main imaging is performed as shown in FIG. Although the main imaging may be performed only after the contrast agent injection, preferably the images before and after the contrast agent injection are captured. Imaging before and after contrasting is continuously performed for the same slice or slab position under the same conditions.
The imaging sequence is a sequence based on the short TR three-dimensional gradient echo method as shown in FIG. In this case, since blood flow is an imaging target, a gradient magnetic field, that is, a gradient moment nulling (Gradient Moment Nulling) for rephasing the phase due to the flow may be added, but this is not essential, and TR / TE For shortening, it is preferable to use a simple gradient echo.
[0036]
Since the imaging time T is determined when the repetition time TR of the pulse sequence, the matrix size (number of slice encodes and phase encodes), and the number of additions are determined, the contrast agent arrival time t1 of the target blood vessel obtained by the above timing imaging is determined. In addition, the imaging start time t2 (time from the injection of the contrast agent to the start of imaging) so that the data measurement of the low frequency region of the ky-kz space is performed when the contrast agent reaches the target blood vessel Set. In imaging, first measurement of the first group is started, and then measurement of the second group is performed. In this case, the sequencer 4 controls the slice group gradient magnetic field pulse and the phase encode direction gradient magnetic field pulse to measure the high-frequency component to the low-frequency component in order in the first group to be measured first, and then measures the first group. In the second group, control is performed so as to measure in order from the low frequency component to the high frequency component. Thus, three-dimensional image data after contrast is obtained.
[0037]
If three-dimensional image data is obtained before contrasting in the same procedure, three-dimensional data only for blood vessels can be obtained by taking these differences. The difference processing is performed by, for example, performing a complex difference between slices having the same slice position in three dimensions. The difference may be an absolute difference. In this way, the method of removing tissue other than blood vessels by performing difference processing between images before and after contrast is a known technique called 3D MR-DSA (Digital Subtraction Angiography), which is not essential in the present invention. It is suitable for rendering thin blood vessels where it is difficult to obtain sufficient contrast with tissues other than blood vessels.
[0038]
Furthermore, the three-dimensional data after the difference processing can be stereoscopically observed by projecting it in an arbitrary direction such as a coronal section, a sagittal section, or an axis crossing. As a projection method, a known maximum value projection method or the like can be employed. It is also possible to perform pseudo three-dimensional display processing such as volume rendering, and it is also possible to perform simple all-slice integration processing because data after difference is used.
[0039]
As described above, according to the contrast-enhanced MRA of the present invention, the low-frequency component in the 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. Can be drawn with high contrast. In addition, as shown in FIG. 5, there is a time zone for measuring the low frequency component on both sides of the contrast agent arrival time t1, so that the contrast agents of the two may differ depending on slight differences in conditions between timing imaging and main imaging. Even when there is a shift in the arrival time t1, it is possible to obtain a high-quality image with almost no deterioration in image quality.
[0040]
In the above embodiment, the case of measuring all measurement points belonging to both the first group and the second group as the data collection method has been described. However, as shown in FIG. May omit the measurement of a predetermined high-frequency component, and may adopt a data collection method for measuring a low-frequency component in a short time.
[0041]
An example of such a data collection method is shown in FIG. FIG. 6 also illustrates an 8 * 8 matrix k-space with 8 slice encodings and 8 phase encodings. In this embodiment as well, the k-space is divided into two groups under the same conditions as in the embodiment shown in FIG. 4, but in this case, the first group to be measured first does not measure a predetermined high-frequency component, and the low frequency Measure only the components. In the illustrated embodiment, only the lattice points existing in the 4 * 4 matrix in the low frequency region among the lattice points in the k space are measured. First, the grid point having the longest distance from the origin (the grid point of No. 9) among the grid points in the low frequency region is used as a starting point, and the grid point of No. 2 and the grid point of No. 3 are measured in that order.
[0042]
In the second group, measurement is performed from the lattice point (number 10) adjacent to the origin as in the embodiment shown in FIG. 4, and the entire lattice points belonging to the second group are measured up to the highest frequency component in the order away from the origin.
[0043]
In this case, the data of the high frequency region that was not measured in the first group can be estimated based on the complex conjugates of the first group and the second group. As a method for estimating unmeasured data, a method based on a known half-Fourier reconstruction method can be employed. FIG. 7 schematically shows these processes. First, the measurement data is subjected to a one-dimensional Fourier transform in the frequency encoding direction (kx direction) to obtain actual measurement data in a three-dimensional hybrid space. Hybrid space data is obtained by obtaining three-dimensional estimation data from the actual measurement data and synthesizing the actual measurement data and the estimation data. Three-dimensional image data is obtained by two-dimensional Fourier transform of the hybrid space data.
[0044]
Thereby, even if the number of data points is reduced, the spatial resolution is not deteriorated.
In this embodiment as well, the low frequency component of the k-space is measured when the contrast medium reaches the target blood vessel and the signal intensity of the blood flowing through the target blood vessel becomes the highest as in the data collection method shown in FIG. Thus, an artery image can be drawn with high contrast.
[0045]
In general, as shown in FIG. 7, the contrast agent concentration (signal intensity) rises sharply after injection of the contrast agent, so that the time point at which the lowest frequency component is measured is aligned with the peak of the signal intensity, and is unnecessary before reaching the contrast agent. In order to avoid measurement of a large signal, this embodiment is preferable because the measurement before the peak is short.
[0046]
The results of simulating differences in arteriovenous separation due to differences in imaging methods (data collection methods) are shown in FIGS. This simulation uses simulated blood vessels of arteries and veins, in which a contrast medium is introduced at a flow rate of 40 cm / s, arterial venous return time of 7 seconds, and injection speed of 2 cc / s, and this is imaged by different imaging methods. . FIG. 8 shows the signal intensity under the above conditions. First, the peak of the signal from the artery is seen, and the peak of the signal from the vein appears later. FIG. 9A shows an image obtained by the imaging method of the present invention, and FIG. 9B shows an image obtained by an elliptical centric order.
[0047]
As can be seen from the figure, in the elliptical centric order, veins as well as arteries are imaged, and arteriovenous separation is not complete, whereas the imaging method of the present invention can draw only the arteries with high contrast.
[0048]
In the above embodiment, the gradient echo method is exemplified as the three-dimensional MRA sequence. However, an EPI (Echo Planer Imaging) method for measuring a plurality of echo signals by one excitation, a division type EPI, or the like is also employed. be able to.
[0049]
Furthermore, the data collection in the k-space is not limited to the rectangular matrix as shown in FIGS. 4 and 6, but it is also possible to collect data in a circle (ellipse) centered on the origin as shown in FIG. . In the first group indicated by a solid line in the figure, the measurement starts from a point where the distance from the origin 0 in the k space is large, and then the sampling point gradually moves from the high frequency component toward the low frequency component so as to approach the origin 0. Sampling control. In the second group, 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.
[0050]
Also in this case, the same effect as the embodiment shown in FIGS. 4 and 6 can be obtained. Also in this case, measurement of the first group of high frequency components can be omitted as necessary.
[0051]
【The invention's effect】
As described above, according to the MRI apparatus of the present invention, the measurement points in the ky-kz space are divided into two groups having a complex conjugate relationship with each other, and measurement is performed in the order of the first group and the second group. In the first group, the sampling point is controlled from the high frequency component to the low frequency component so that the distance from the origin in the k space gradually approaches, and in the second group, the distance gradually increases. In this way, sampling points are controlled from the low-frequency component to the high-frequency component in this way, so that the effects of imaging timing shifts are reduced, and artifacts that depend on the contrast timing, which has been a problem with conventional contrast MRA, are generated. In addition, it is possible to prevent overlapping of the arteries and veins and improve blood vessel rendering ability. Further, by reducing the number of measurement points of the first group to be measured first, unnecessary signal measurement and image degradation caused thereby can be eliminated, and the imaging time can be shortened.
[Brief description of the drawings]
FIG. 1 is a schematic diagram for explaining a data collection method employed by an MRI apparatus according to the present invention and a conventional data collection method. FIG. 2 is a block diagram showing an overall configuration of an MRI apparatus to which the present invention is applied. The figure which shows one Example of the pulse sequence of the contrast MRA measurement which the MRI apparatus of invention performs FIG. 4 The figure which shows typically one Example of the data collection order of k space by this invention FIG. Fig. 6 is a diagram for explaining MRA imaging by an MRI apparatus. Fig. 6 is a diagram schematically showing another embodiment of the k-space data collection order according to the present invention. Fig. 7 is an image reconstruction method to which the data collection method of Fig. 6 is applied. FIG. 8 is a diagram showing a simulation for evaluating MRA imaging by the MRI apparatus of the present invention. FIG. 9 is a diagram showing results of simulating MRA imaging by the MRI apparatus of the present invention and MRA imaging by a conventional method. FIG. 10 shows k-space data according to the present invention. Diagram showing another embodiment of the current sequence schematically EXPLANATION OF REFERENCE NUMERALS
DESCRIPTION OF SYMBOLS 1 ... Subject 2 ... Magnetic field generator 3 ... Magnetic field gradient generation system 4 ... Sequencer 5 ... Transmission system 6 ... Reception system 7 ... Signal processing system 8 ... CPU
9 ... Gradient magnetic field coil
14a, ... High-frequency coil on the transmission side
14b ... High frequency coil on the receiving side

Claims (6)

A static magnetic field generating means for generating a static magnetic field in a space in which the subject is placed; a gradient magnetic field generating means for applying gradient magnetic fields in the slice direction, phase encoding direction, and readout direction to the space; and a nucleus of the biological tissue of the subject A transmission system that irradiates a high-frequency magnetic field to cause nuclear magnetic resonance in the antenna, a reception system that detects an echo signal emitted by the nuclear magnetic resonance, and a control that controls the gradient magnetic field generating means, the transmission system, and the reception system In a magnetic resonance imaging apparatus comprising: a system; a signal processing system for performing image reconstruction calculation using an echo signal detected by a receiving system; and a means for displaying the obtained image
The control system assigns at least one of slice encoding and phase encoding to execute a two-dimensional or three-dimensional sequence, and at this time, k-space measurement points defined by at least one of the slice encoding number and the phase encoding number are set. , Share the origin, and divide into the first and second groups that are in a complex conjugate 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 measurement starts from the origin. A magnetic resonance imaging apparatus characterized by controlling at least one of the gradient magnetic field generating means in the slice direction and the phase encoding direction so that the distance to the point gradually increases in the order of measurement.   The magnetic resonance imaging apparatus according to claim 1, wherein the measurement points in the k space are divided so that adjacent measurement points belong to different groups.   The magnetic resonance imaging apparatus according to claim 1, wherein at least one of the two groups divided into two includes a point that is not measured.   4. The magnetic resonance imaging apparatus according to claim 1, wherein the sum of the two divided regions is a circle, an ellipse, a sphere, or an ellipsoid inscribed in the measurement space. 5. Means for selecting and exciting a predetermined region of the subject; gradient magnetic field applying means for applying gradient magnetic fields in at least two directions to the subject; measuring means for measuring an echo signal generated from the region; and the excitation means The step of controlling the gradient magnetic field applying means and the measuring means, applying the gradient magnetic field encoded in at least two directions, and measuring the echo signal generated from the region is repeated a plurality of times while changing the intensity of the gradient magnetic field, thereby obtaining a three-dimensional image. In a magnetic resonance imaging apparatus comprising a control means for collecting data,
The control means share the origin of the measurement points in the k-space defined by the encode gradient magnetic field strengths in the two directions, have a complex conjugate relationship with each other, and adjacent lattice points belong to different groups. Dividing into the second group, the first group and the second group are measured sequentially. At that time, in the first group measured first, the distance from the origin of the k-space to the measurement point gradually decreases in the order of measurement. In the second group to be measured later, the magnetic resonance imaging apparatus performs measurement so that the distance from the origin of the k space to the measurement point gradually increases in the measurement order.   7. The magnetism according to claim 6, wherein the control unit measures only some of the measurement points in the first group and measures all the measurement points in the second group. Resonance imaging device.

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