JPH0572814B2 - - Google Patents

Description

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

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

[Conventional technology]

The MR imaging device includes a means for irradiating a subject with an excitation high-frequency signal (a high-frequency transmitter and an RF coil serving as a transmitting antenna), a gradient magnetic field for slice selection,
A means for applying a gradient magnetic field for phase encoding and a gradient magnetic field for frequency encoding (a gradient magnetic field generation coil and a power supply that supplies current to it), and a means for receiving NMR signals from the subject and collecting data (which serves as a receiving antenna). RF coil and high frequency receiver)
and a means (computer) for reconstructing an image from the collected data. In a normal MR imaging device, when trying to obtain an N×N 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). The waveforms and timing of the excitation high-frequency signal and each gradient magnetic field are controlled so that Reducing the number of phase encodes will reduce the number of repetitions of the imaging sequence,
This 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 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 reconstruction A problem arises in which the spatial resolution of the image deteriorates. The present invention provides an improved MR imaging device that can perform an imaging sequence 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. The purpose is to

[Means to solve the problem]

In order to achieve the above object, MR according to this invention
In an imaging device, for a non-square imaging field of view in which the size in the phase encoding direction is arbitrarily determined to be smaller than the size in the frequency encoding direction,
The matrix in the phase encoding direction and the matrix in the frequency encoding direction are determined so that the pixels are isotropic, the number of phase encodes 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. is determined, and the imaging sequence is performed by controlling the gradient magnetic field generating means so that a gradient magnetic field for phase encoding and a gradient magnetic field for frequency encoding corresponding to this are applied.

[Effect]

The imaging field of view is made non-square by arbitrarily 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. A number of phase encodes corresponding to the matrix in this phase encode direction are performed. Thereby, 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. I can do it. 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. Figure 3 shows this invention.
This shows an MR imaging device that performs a pulse sequence as shown in FIG. 1. This pulse sequence is an application of the idea of this invention to the field echo method. First, the RF transmitter/receiver 34 shown in FIG.
A high frequency signal with an envelope as shown in FIG. 1A is sent to the coil 35 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. ) is applied. After that, a current having a waveform as shown in FIG. 1C and D is 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 (here, Gx) is sent to the gradient magnetic field generating coil 33.
and a phase encoding gradient magnetic field (here, Gy) is applied. These waveforms and timings are determined by the measurement control device 31. The NMR signal generated from the subject is received by the RF coil 35, and is taken into the computer 36 via the RF transmitting/receiving device 34 to collect data. The subject to be imaged is usually a human body, and in general, the horizontal and vertical sizes of the cross-sectional image 21 are not always the same in many parts, as shown in Fig. 2. Therefore, the horizontal size Lx is larger than the vertical size Ly. A non-square imaging field of view 22 with a larger size is often better. Therefore, the imaging field of view 22
It is not important that Lx=Ly, but rather the isotropy of the pixel itself is more important. Therefore, for this non-square imaging field of view 22, the horizontal direction (frequency encoding direction) matrix MATfreq and the vertical direction ( (phase encoding direction) matrix MATphase is 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 MATfreq−MATphase. This allows the overall imaging time to be shortened. Here, a parameter defined as Wmat=MATphase/MATfreq is introduced as the matrix weight (Wmat) that defines the above arbitrary matrix. The phase encode amount G _p each time shown by the shaded area in FIG. 1D is determined as follows. Since phase control equivalent to sampling at the Nyquist frequency is required, the phase encode waveform G _y needs to satisfy the relationship γLy∫Gydt=π (γ: gyromagnetic ratio). In imaging with an arbitrary matrix, the following equation is derived from the above three equations so as to include the Wmat parameter. ∫Gydt=(π/γLx)(1/Wmat) 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. 1 when using an arbitrary matrix.
It can be seen that the amount of phase encoding in the full encoding method can be multiplied by 1/Wmat. The range of Wmat is 0<Wmat≦1, and when Wmat=1, imaging is performed using the full encoding method. The sizes Lx and Ly of the imaging field of view 22 are determined corresponding to the cross-sectional image 21, and MATfreq and MATphase are determined.
Once determined, Wmat is calculated by the computer 36, and the encoded amount Gp is determined.
This Gp 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 thus obtained is subjected to two-dimensional Fourier transformation by the computer 36, and an 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, arbitrary matrix 70% (Wmat=0.7)
When compared to imaging using the full encoding method, the imaging time is 70% (MATphase = 0.7 ×
MATfreq), the field of view size in the phase encoding direction is 70 of the field 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. Furthermore, 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 two-dimensional directions.

【Effect of the invention】

According to the MR imaging device of the present invention, if the object shape allows the imaging field to be non-square, the imaging time can be shortened without compromising the spatial resolution of the reconstructed image. . By shortening the imaging time, 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 drawings]

Fig. 1 is a time chart showing a pulse sequence performed in an embodiment of the present invention, Fig. 2 is a diagram showing the relationship between a cross-sectional image of a subject and an imaging field of view, and Fig. 3 is an MR imaging according to an embodiment of the invention. FIG. 2 is a block diagram showing the device. 21... Cross-sectional image, 22... Imaging field of view, 31
...Measurement control device, 32...Gradient magnetic field power supply, 3...Gradient magnetic field generation coil, 34...RF transmitting/receiving device, 35
...RF coil, 36...computer.

Claims (1)

[Claims] 1. Means for irradiating a subject with an excitation high-frequency signal, gradient magnetic field generating means for applying a gradient magnetic field for slice selection, a gradient magnetic field for phase encoding, and a gradient magnetic field for frequency encoding, and data collection by receiving NMR signals from the subject. means for reconstructing an image from the collected data; and a means for reconstructing an image from the collected data; A matrix in the phase encoding direction and a matrix in the frequency encoding direction are determined, the number of phase encodes is made 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, and the phase is determined accordingly. An MR imaging apparatus comprising: measurement control means for performing an imaging sequence by controlling the gradient magnetic field generation means so that an encoding gradient magnetic field and a frequency encoding gradient magnetic field are applied.