JPH02224737A - Mr imaging method - Google Patents

Abstract

PURPOSE:To shorten the photographing time without deterioration of the resolution for image by narrowing the imaging view field in the direction of phase encoding. CONSTITUTION:From an RF transmitting/receiving device 34 a high frequency signal is sent to an RF coil 35 to excite a spin in an object to be photographed. From an inclined magnetic field power supply 32, current is sent to an inclined magnetic field generating coil 33 under the control of a measuring control device 31, and an inclined magnetic field for selection of slice is impressed, and also the inclined magnetic field for frequency encoding and inclined magnetic field for phase encoding are impressed. NMR signals generated from the object are received by an RF coil 35 and taken into a computer 36 to serve collecting the data. The sideways size Lx of the profile image 21 from the object shall preferably be greater than its longitudinal size Ly, which yields a non-square imaging view field 22. The sideways matrix MATfreq shall be related to the longitudinal matrix MATphase so that they meet the equation Lx/MATfreq=Ly/MATphase, so as not to impair the isotropic property of picture element. The number of phase encodings shall be the same as MATphase, and the number of repetitions in pulse sequence is decreased by MATfreq-MATphase, so as to shorten the photographing time.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an MR imaging method that performs imaging using nuclear magnetic resonance (NMR).

[Conventional technology]

In the normal MR imaging method, when trying to obtain an NXN matrix image, the number of encodes in the phase encoding direction is set to N, and data is collected by repeating the imaging sequence N times (this is called the full encoding method). ). Reducing the number of phase encodes will reduce the number of repetitions of the imaging sequence, which will shorten the overall imaging time. Therefore, in the past, the area around the phase encoding in the full encoding method described above is regarded as a noise level and data is not collected, and the area where the data is missing is filled with zero data to generate a square raw data matrix. Image reconstruction is performed by performing a two-dimensional Fourier transform on the image (this is called the encode reduction method).

[Problem to be solved by the invention]

However, although the encode reduction method certainly shortens the overall imaging time, since data is collected while keeping the imaging field of view square, the reconstructed pixels become anisotropic and the reconstructed image The problem arises that the spatial resolution of the image is degraded. An object of the present invention is to provide an MR imaging method that can obtain exactly the same spatial resolution as the full encoding method and shorten the overall imaging time as well as the encode reduction method.

[Means to solve the problem]

In order to achieve the above object, in the MR imaging method according to the present invention, pixels are arranged isotropically with respect to a non-square imaging field in which the size in the phase encoding direction is determined to be smaller than the size in the frequency encoding direction according to the shape of the subject. The matrix in the phase encoding direction and the matrix in the frequency encoding direction are determined so that the matrix in the phase encoding direction is the same as the matrix in the phase encoding direction, and the amount of phase encoding each time is determined by the ratio of the matrices in both directions. ing.

[For production]

The imaging field of view is made non-square by setting its size in the phase encoding direction to be smaller than its size in the frequency encoding direction. Then, for this non-square visual field, a matrix in the phase encoding direction and a matrix in the frequency encoding direction are determined so that the pixels are isotropic. The number of phase encodes corresponding to the matrix in the phase encode direction is performed, whereby the number of phase encodes can be reduced by the amount that the size in the phase encode direction is smaller than the size in the frequency encode direction, and the imaging time can be shortened. Furthermore, since the amount of phase encoding each time is determined by the ratio of the matrix in the phase encoding direction and the matrix in the frequency encoding direction, the isotropy of the pixel itself is maintained and the spatial resolution of the reconstructed image is prevented from deteriorating. Can be done. That is, by narrowing the imaging field of view in the phase encoding direction, it is possible to shorten the imaging time without causing deterioration in image resolution.

【Example】

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a pulse sequence of an embodiment in which the MR imaging method according to the present invention is applied to a field echo method, and this is performed, for example, by a system as shown in FIG. That is, the RF transmitter/receiver 34 sends the first
A high-frequency signal with an envelope as shown in Figure A is sent to excite the spins of a subject (not shown) to be imaged. At this time, under the control of the measurement control device 31, a current having a waveform as shown in FIG. Gz) is applied. Thereafter, currents with waveforms as shown in FIG. 1C and D are sent from the gradient magnetic field power supply 32 to the gradient magnetic field generating coil 33, and a gradient magnetic field for frequency encoding (herein referred to as Gx) and a gradient magnetic field for phase encoding ( Here, Gy) is applied. These waveforms and timings are determined by the measurement control device 31. The NMR signal generated from the object is sent to the R and F coils 3.
5, and is taken into the computer 36 via the RF transmitter/receiver 34, where the data is collected. The subject to be imaged is usually a human body, and generally, as shown in FIG. 2, the horizontal and vertical sizes of the cross-sectional image 21 are not always the same in many parts, so the horizontal size Lx is larger than the vertical size Ly. Therefore, it is not important that the imaging field 22 has Lx=Ly, but rather the isotropy of the pixel itself is more important. Therefore, for this non-square imaging field of view 22, L x / MAT freq = L y / MAT so as not to impair pixel isotropy.
A horizontal (frequency encode direction) matrix MAT freq and a vertical direction (phase encode direction) matrix MAT rhase that satisfy the relational expression of phase are determined. The number of phase encodes is set to be the same as MATphase, and the number of repetitions of the pulse sequence shown in FIG. 1 is decreased by MAT freq-MATphase. This allows the overall imaging time to be shortened. Here, as the matrix weight (VV+++at) that defines the above arbitrary matrix, W mat=MAT
Introduce parameters defined in phase/MAT freq. The phase encode amount ep each time indicated by the shaded area in the first diagram is determined as follows. Since phase control equivalent to sampling at the Nyquist frequency is required, the phase encode waveform Gy needs to satisfy the rIA coefficient of γLyiGydt=π (γ: gyromagnetic ratio). In imaging at the time of arbitrary matrix, the following equation is derived from the above three equations so as to include the W mat parameter. SGy dt= (π/γLx) (1/W mat
) Since π/γLx on the right side of this equation indicates the encoding amount in the normal full encoding method, the one-time phase encoding amount Gp in FIG. It can be seen that it is sufficient to multiply the value by mat. The range of this W mat is Q(W mat ≦ 1, and W mat = 1 means imaging by the full encoding method. The size L of the imaging field of view 22 corresponds to the cross-sectional image 21.
Define x, Ly, MAT freq, MATphas
Once e is determined, W mat is calculated in the computer 36, and the encoded amount Gp is determined. This G
p is sent to the measurement control device 31, and the amount of one encode of the phase encoding gradient magnetic field Gy is controlled accordingly. The raw data obtained in this way is
After dimensional Fourier transformation, the image is reconstructed in a non-square imaging field of view 22 as shown in FIG. In this reconstructed image, the isotropy of pixels is maintained, and the spatial resolution of the image is not degraded. For example, when the arbitrary matrix is set to 70% (W mat = 0.7), the imaging time is 70% (MATphase = 0, 7
MAT freq), the field of view size in the phase encoding direction is 70% of the field of view size in the frequency encoding direction.
(Ly=0.7Lx). Although an embodiment in which the present invention is applied to the field echo method has been described above, it is of course applicable to other imaging methods such as the spin echo method as long as phase encoding is performed. Moreover, it can be easily applied not only to two-dimensional imaging but also to three-dimensional imaging. Furthermore, it can also be used for encoding in three-dimensional directions.

【Effect of the invention】

According to the MR imaging method of the present invention, if the object shape allows the imaging field to be non-square, the imaging time can be shortened without any loss in the spatial resolution of the reconstructed image. By shortening the imaging time in this way, it is possible not only to shorten the patient's restraint time and improve throughput, but also to suppress the probability of contamination by artifact components due to subject movement.

[Brief explanation of the drawing]

FIG. 1 is a time chart showing a pulse sequence of an embodiment in which the present invention is applied to a field echo method, and FIG.
The figure is a diagram showing the relationship between a cross-sectional image of a subject and an imaging field of view, and FIG. 3 is a block diagram showing a system configuration according to the above embodiment. 21... Cross-sectional image, 22... Imaging field of view, 31
・、. Measurement control device, 32... Gradient magnetic field power supply, 33... Gradient magnetic field generation coil, 34... RF transmitting/receiving device, 35
...RF coil, 36...computer.

Claims (1)

[Claims] (1) For a non-square imaging field of view where the size in the phase encoding direction is set smaller than the size in the frequency encoding direction according to the shape of the subject, the matrix in the phase encoding direction and the frequency encoding are set so that the pixels are isotropic. An MR imaging method in which a direction matrix is determined, the number of phase encodes is the same as the phase encode direction matrix, and the amount of phase encode each time is determined by the ratio of the matrices in both directions.