JP2002301044A - Magnetic resonance image diagnosis apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the deterioration of an image due to the movement of a patient in a sensitivity encoding method of MRI. SOLUTION: In the examee performing nearly periodical movement, sensitivity distribution information per coil of coil array per time phase of the periodical movement is obtained, and turn-back removal computation using the inherent sensitivity distribution information per time phase is performed. Consequently, it is possible to obtain an image without turn-back artifact of even the moving examee in the sensitivity encode method of MRI.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus (MRI), and more particularly to a magnetic resonance imaging apparatus that reduces the influence of the movement of a subject.

[0002]

2. Description of the Related Art In recent years, MRI has become an important diagnostic means for diseases along with X-ray CT as an image diagnostic apparatus having excellent tissue delineation ability. One of the weak points of MRI is that the imaging time is long. To solve this problem, 1) a method of improving the performance of the gradient magnetic field, 2) a burst-like excitation pulse or α-degree (α
(≠ 90, 180) Method using high-order echo by pulse, 3)
There is a method for reducing the number of measurement repetitions by thinning out the phase encoding. The first method includes EPI and spiral scan, and measures the signal by encoding all two-dimensional spatial information in one excitation of magnetization. Although it is fast because an image can be obtained with only one excitation, it is difficult to achieve a higher speed than the current situation because high-speed response characteristics and large amplitude characteristics of the gradient magnetic field are required. The second method is to replace the RF pulse with a burst pulse train and generate multiple echoes.
Acquire all two-dimensional spatial information with one excitation. Alternatively, different spatial information is encoded in the higher-order echo, and all two-dimensional spatial information is acquired by one excitation. The mechanical requirements for the gradient magnetic field are relaxed, but the efficiency of magnetization utilization is low.
There is a disadvantage that the SN ratio of the image is low. The third method is key
By sharing high-frequency components in the k-space that do not significantly contribute to image contrast between multiple images using the hole method, etc., the phase encoding step is reduced and the shooting time is reduced. Although there is no limitation in terms of hardware, artifacts occur when the subject moves greatly.

In recent years, a method for reducing the phase encoding has been proposed, which is a method for avoiding the aliasing of an image which occurs when the phase encoding is reduced by an arithmetic processing using the sensitivity distribution information of the detection coil. This includes SMASH (Sodickson DK, Manning WJ, “Sim
ultaneous acquisition of spatial harmonics (SMASH):
ultra-fast imaging with radiofrequency coil array
s ”, Magn Reson Med., vol. 38, 591-603 (1997) and SENSE performed in real space (Klaas P. Pruessmann, Markus Weiger, Ma.
rkus B. Scheidegger, and Peter Boesiger, “SENSE: s
ensitivity encoding for fast MRI ”, Magn Reson Me
d., vol. 42, 952-962 (1999)), and are collectively referred to as sensitivity encoding or parallel MRI. In each case, MR signals are simultaneously detected using a plurality of coil arrays. In the Fourier reconstruction, when the phase encoding in the k space is thinned out and the encoding interval (Δky) is increased, the field of view is reduced (FOVy = 2π / Δky). Therefore, when the subject is larger than the visual field, the subject is turned back. An image cannot be used for diagnosis as it is, but if the sensitivity information of the detection coil is known, aliasing can be removed by performing arithmetic processing (see the aforementioned Pruessmann paper). In the sensitivity encoding method, phase encoding can be reduced by the number of coils.
For example, with 4 coils, the number of phase encodes can be reduced to 1/4,
The shooting time can be reduced to 1/4 of the conventional one. As a trade-off with time reduction, the S / N ratio of the image is reduced. However, the advantages of compensating for the reduction of the S / N ratio are expected in cardiac imaging under breath holding and reduction of echo train in diffusion EPI.

[0004] In the sensitivity encoding method, sensitivity distribution information of the detection coil becomes important. This is because an error in the sensitivity distribution information causes a false image due to incomplete aliasing removal as well as image shading. In order to obtain the sensitivity distribution, there are a method of performing preliminary photographing of the phantom and a method of performing processing such as a low-pass filter on the image of the subject. Also, in order to obtain the sensitivity distribution with high accuracy, a method has been proposed in which local polynomial fitting is performed instead of a simple low-pass filter (Klaas P. Pruessmann,
Markus Weiger, Markus B. Scheidegger, Peter Boesig
er, “Coil sensitivity maps for sensitivity encodin
g and intensity correction ”, Proceedings of the I
SMRM 6th Annual Meeting, Sydney, 1998, p2087).

[0005]

The sensitivity distribution of the detection coil also depends on the relative arrangement of the coil and the subject. Therefore, when the subject moves or deforms in the coil array, the sensitivity distribution also slightly changes. Such examples include respiratory movements, joint flexion movements, and the like. In the heart, when extracting the sensitivity distribution from the images taken for each heartbeat phase, there is an example of processing in which the sensitivity distributions for all phases are added and the average is taken (M
arkus Weiger, Klaas P. Pruessmann and Peter Boesige
r, “Cardiac real-time imaging usins SENSE”, Magn R
eson Med., vol. 43, 177-184 (2000)). This is the sensitivity distribution S
In order to improve / N, information on the time phase is lost, and a precise sensitivity distribution cannot be obtained. Further, in the case of a motion in which the region where the subject exists moves largely in the field of view, such as a bending motion of a joint, an accurate sensitivity distribution cannot be obtained even by the averaging process. As described above, when the aliasing removal calculation of the sensitivity encoding method is performed using the same sensitivity distribution information or the averaged sensitivity distribution information before and after movement or mutation, an arithmetic error occurs, and aliasing remains in the final image, which is difficult to diagnose. There is a problem of causing obstacles.

[0006]

According to the present invention, in order to solve the above-mentioned problems, in a sensitivity encoding method, sensitivity distribution information for each coil of an array coil is obtained for each time phase of a periodic motion of a subject which performs a substantially periodic motion. Then, an aliasing removal calculation is performed using the unique sensitivity distribution information for each time phase. The coil sensitivity distribution is acquired at a desired time phase while generating a gate signal linked to the movement of the subject during the preliminary imaging. For detecting the movement of the subject, MRI may be used in addition to various sensors such as a respiration sensor and a pulse wave sensor. In the latter, a navigation echo is measured for each magnetization excitation, and the echo signal is measured.
The displacement may be detected from a comparison operation between the one-dimensional Fourier transform and the one-dimensional Fourier transform of the separately measured reference navigation echo signal, and the phase of the k-space data of the navigation echo and the reference navigation echo may be compared to determine the object. Information regarding the displacement may be extracted. The correspondence between the displacement and the time phase of the movement is determined in advance by preliminary imaging, tabulated and saved. The coil sensitivity distribution can also be extracted by performing a calculation such as a low-pass filter process on a series of images for each phase acquired in the main photographing.

As described above, since the aliasing removal processing is performed using the coil sensitivity sensitivity distribution in accordance with the time phase of the movement of the subject, even if the subject has a motion in the sensitivity encoding method, an image without aliasing artifacts can be obtained. Obtainable.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present method will be described in detail using embodiments. FIG. 4 is a schematic configuration diagram of a nuclear magnetic resonance diagnostic apparatus to which the present invention is applied. In the figure, reference numeral 402 denotes a static magnetic field generating magnetic circuit for generating a uniform static magnetic field B0 inside the subject, 401 denotes the subject, 414a denotes a transmitting coil for generating a high-frequency magnetic field, and 414b denotes nuclear magnetic resonance generated from the subject. 40 detection coils arranged in an array to detect signals
Reference numeral 9 denotes a gradient magnetic field coil that generates gradient magnetic fields Gx, Gy, and Gz whose intensities linearly change in three orthogonal directions x, y, and z. Reference numeral 410 denotes a gradient magnetic field power supply for supplying a current to the gradient magnetic field.
Reference numeral 408 denotes a computer, 406 denotes a signal processing and recording device, and 421 denotes an operation unit. Reference numerals 424 and 425 denote memories for storing data being calculated or final data. The gradient magnetic field generation system 403, the transmission system 404, and the detection system 405 are all sequencers 407.
The sequencer 407 is controlled by a computer 4
Controlled by 08. The computer 408 is controlled by a command from the operation unit 421. Further, the sensor 401 for detecting body movement (periodic movement such as respiration) is attached to the subject 401, but any sensor that can detect movement without directly attaching to the subject 401 is used. Good.

Next, an outline of the operation of the present apparatus will be described. The high frequency generated by the synthesizer 411 is amplitude or phase-modulated by the modulator 412 and amplified by the power amplifier 413.
By supplying it to 14a, a high-frequency magnetic field is generated inside the subject 401 to excite nuclear spins. Usually, 1H is targeted, but other nuclei with nuclear spin, such as 31P and 12C, may be targeted.

[0010] With the relaxation of the energy of the excited nuclei, the subject 401
The nuclear magnetic resonance signal emitted from the detector is received by a detection coil 414b, amplified by an amplifier 415 for each coil, and
At 16, quadrature phase detection is performed, A / D converted at A / D converter 417, and input to computer 408. After the signal processing, the computer 408 displays on the display 428 an image to which a contrast is given by the nuclear spin density distribution, relaxation time constant, diffusion coefficient, flow velocity, spectrum distribution and the like.

An embodiment of the present invention in such an apparatus will be described in detail below. Here, the abdomen 33 performing a respiratory movement is taken as an example of the subject. Figure 3
As shown at 32, six array coils 32 are arranged above and below the abdomen, and signals from each coil are simultaneously detected. The phase direction is set in the y direction, the upper and lower signals are combined, and the encoding for the main shooting is reduced to half. The array coil is held by a flexible support and is wrapped around the abdomen, and moves with respiratory movement. Since the biological load on each coil changes, the sensitivity distribution changes. The processing procedure in the first embodiment will be described with reference to FIG. First, the time phase at which the motion cycle is photographed and the number L of the time phases are determined. The following is performed as preliminary photography. According to the trigger signal (processing
11) Capture an image of the temporal phase ni (12). After photographing all desired time phases, the sensitivity distribution of each time phase ni (i = 1, 2,... L) is calculated and stored in the memory. For the preliminary photographing, any photographing sequence may be used as long as an image for each phase can be obtained. In the preliminary photographing, an image without aliasing is obtained without reducing the phase encoding.

In order to specify the time phase, a respiratory motion is measured by a mechanical respiration sensor such as a bellows, and a photographing trigger is generated for a specific time phase of the motion. That is, a signal is obtained by the sensor at the time point p0 when the displacement is maximum (31 in FIG. 3a),
By shifting the delay time from this time, each phase p1, p2, ...
Generate a shooting trigger.

As another method, motion information may be extracted from an MRI image by a navigation echo or the like, and the time phase may be made to correspond to the retrospective. The navigation echo is an echo obtained in the presence of only the readout gradient magnetic field without applying phase encoding, and this Fourier transform is a projection image of the subject on the readout axis.

There are the following methods for extracting a displacement from a navigation echo. y direction (here the direction of displacement is y
1D Fourier transform of navigation echo signal fn (t) obtained by applying readout gradient magnetic field
fn '(y) gives the projection of the subject on the y-axis. Therefore,
By comparing the one-dimensional Fourier transform f0 '(y) and fn' (y) of the reference navigation echo signal f0 (t) measured separately,
The displacement amount of the subject in the y direction can be extracted. For a rigid displacement, the correlation coefficient may be calculated, and for a non-rigid displacement, the displacement can be measured by comparing the edges of the subject. FIG. 8 shows an example of an EPI sequence with a navigation echo. After measuring the main echo 89, the navigation echo 801 is measured. The navigation echo performs a one-dimensional Fourier transform in the y direction to reconstruct a projection image of the subject on the y axis. FIG. 5 schematically shows the displacement Δy in the normal direction of the abdominal wall. By comparing the projected image (53 in FIG. 5A) with the projected image of the reference navigation echo (52 in FIG. 5A), the displacement of the subject in the y direction is calculated. In the case of rigid translation (FIG. 5a), the shift amount Δ at which the correlation coefficient between the projected images becomes maximum.
Find y. In the expansion-contraction movement, the edges of the projected images are compared (FIG. 5b). A displacement-time phase correspondence table is prepared in advance, and the time phase of the captured EPI image is determined.

As another displacement detection method, information on the displacement of the subject can be extracted from a comparison operation process between the phase of the k-space data of the navigation echo signal and the phase of the k-space data of the reference navigation echo signal. . Here, the parallel movement of the object in the real space utilizes the generation of a first-order rotation in the signal phase in the k-space according to the Fourier shift theory. In this case, since the measurement data is used, Fourier transform is not required, and the processing can be speeded up.
However, when the movement of the subject cannot be approximated by the parallel movement of the rigid body, the phase change is not simple, and the movement in the real space cannot be estimated.

As another MRI method, the measurement cycle of the navigation echo 94 by the low flip angle excitation 91 as shown in FIG. 9 is repeated with a short TR, the reconstruction of the projected image and the displacement detection are performed in real time, and the time phase is changed. Preliminary photography or actual photography may be performed at a desired time phase while identifying. However, this requires real-time processing of the hardware.

In order to obtain the sensitivity distribution of the coil from the image, the image of the subject may be subjected to low-pass filter processing.
Alternatively, a polynomial fitting may be performed. The latter is described in the aforementioned Pruessmann paper. By normalizing the array coil image with the body coil image, tissue information can be removed, the sensitivity distribution of the array coil can be extracted, and polynomial fitting can be performed. Although a highly accurate sensitivity distribution excluding the influence of the signal defect area can be obtained, the processing takes time. For this reason, real-time processing, which will be described later, is difficult, but there is no problem when sensitivity distribution measurement is performed separately from the actual imaging as in the embodiment of FIG. The sensitivity distribution is obtained only in the region where the subject exists. For this reason, if there is a difference between the subjects in the same time phase between the preliminary imaging and the main imaging, sensitivity information is lost. In order to avoid this, a process of extrapolating the sensitivity distribution to the outside of the subject is performed.

After the preliminary photographing, the main photographing is performed to obtain a trigger signal (process 11), and an image of the time phase ni is photographed in accordance with this (12). After photographing all desired time phases, an image is reconstructed for each coil at each time phase. In this imaging, since the phase encoding is thinned out, aliasing occurs in the phase direction. The operation of aliasing removal is described in a paper by Pruessmann et al. (Klaas P. Pruessmann, Markus Wei
ger, Markus B. Scheidegger, Peter Boesiger, “Coil se
nsitivity encoding for fast MRI ”, Proceedings of
the ISMRM 6th Annual Meeting, Sydney, 1998, p579).
For the sake of simplicity, a case where the number of encodes in the phase direction is reduced to half with two coils will be described. In this case, the image is turned twice, and the images p1, p2
Is represented by Equation 1.

[0019]

(Equation 1) In Equation 1, ci is the complex sensitivity distribution of the coil i, and s is the signal value of the pixel when there is no aliasing. Δy represents the return in the phase encoding direction. Equation 1 can be represented by a vector and a matrix as in Equation 2.

[0020]

(Equation 2) The pixel signal value without aliasing is given by Equation 3.

[0021]

(Equation 3) Here, C-1 represents the inverse matrix of C. Inverse matrix C-1 of sensitivity distribution
Is obtained by preliminary measurement, an image without aliasing is obtained from the aliased image of the actual shooting by Equation 3.

A second embodiment will be described with reference to FIG. Here, the sensitivity distribution of the time phase ni is extracted from the image of the time phase ni in the main photographing (process 23). Also, reconstruction and display are performed in real time in a temporal loop. Preliminary photographing is unnecessary, but high-speed matrix calculation is required. In the extraction of the sensitivity distribution, for example, the following steps are performed. The subject portion in the image is cut out, and the tissue structure of the detail is deleted by low-pass filter processing of the image, and the signal interpolation and the noise removal at the signal defect site are performed. Note that sensitivity distribution information outside the subject is not required.

The abdominal radiography has been described above as an example. Next, another application example will be described. FIG. 6A shows an example of cardiac imaging using four coils. Here, the heart repeatedly expands and contracts at a cycle of about 1 second, and the bioelectrical impedance changes due to a large change in blood flow, so that the sensitivity distribution of the coil also slightly changes. The heart is covered with four array coils 61, and the phase encoding direction is set to the direction of Gp in the figure. An electrocardiograph obtains a trigger signal 64, and measures images I1, I2,... Of each time phase with a delay time from the trigger. FIG. 7 shows an example of knee imaging using three coils, in which a patient is voluntarily performing a roughly periodic bending and stretching movement. Bending and stretching movements are commonly performed in the diagnosis of knee joint damage and the like. As shown in FIG. 7, an array coil 73 is arranged, and signals from each coil are simultaneously detected. The sensitivity distribution changes because the biological load on each coil changes with the movement of the knee.
In addition, since the position of the subject in the visual field changes greatly depending on the time phase due to the bending and stretching motion, it is necessary to obtain the sensitivity distribution for each time phase in order to obtain the sensitivity distribution from the image. The navigator echo may be used to identify the time phase, or an optical displacement sensor such as a laser displacement meter may be used.

The two-dimensional imaging has been described above as an example. However, even in a three-dimensional SENSE measurement, probe sensitivity distribution information for each phase is effective for a moving or deformed subject. In addition to the SENSE method, in the SMASH method and other MRI imaging methods in which spatial information is encoded using coil sensitivity distribution information, time-dependent probe sensitivity distribution information is similarly effective.

[0025]

As described above, according to the present invention, the MR
Even with moving subjects in the sensitivity encoding method of I,
An image without aliasing artifacts can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a typical processing procedure of the present invention.

FIG. 2 is a diagram showing another processing procedure of the present invention.

FIG. 3 is a diagram showing displacement due to respiratory movement of the abdomen.

FIG. 4 is a diagram illustrating an overall configuration of a magnetic resonance imaging diagnostic apparatus.

FIG. 5 is a diagram showing displacement of an abdominal wall and its projection.

FIG. 6 is a diagram illustrating the application of an array coil to the heart.

FIG. 7 is a diagram illustrating an example of application of an array coil to a knee.

FIG. 8 is a diagram showing a navigation echo.

FIG. 9 is a diagram showing another navigation echo.

[Explanation of symbols]

31 Displacement time curve, 32 Abdominal array coil, 33.
Subject, 34 bed, 401 Subject, 413 RF amplifier, 41
4a transmission coil, 414b detection coil, 415 preamplifier, 417 AD converter, 51 abdominal wall, 52 reference time projected image, 53 different time projected image, 61 heart array coil, 62 heart, 71 knee, 72 knee array coil . 85 phase encoding gradient magnetic field pulse, 86 readout gradient magnetic field pulse for navigation echo, 87 warp pulse,
88 Readout gradient pulse, 89 echoes, 801
Navigation echo, 93 Readout gradient magnetic field pulse for navigation echo, 94 Navigation echo

Claims (1)

[Claims] 1. A static magnetic field generating means for generating a static magnetic field for a subject, a gradient magnetic field generating means for generating a gradient magnetic field on the subject, a high frequency magnetic field generating means for irradiating the subject with a high frequency magnetic field, A plurality of signal detecting means for detecting each magnetic resonance signal from the subject, a reconstructing means for reconstructing an image using the detection signal, and a display means for displaying the reconstructed image; In a magnetic resonance imaging diagnostic apparatus including a control unit for controlling the operation of each of the units,
Comprising a detecting means for detecting the periodic movement of the subject,
The apparatus according to claim 1, wherein the control unit acquires sensitivity distribution information of each of the signal detection units at each time phase of the periodic motion.