JPH1119065A - MR imaging device - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To automatically set the conditions of an imaging sequence according to the size and position of a subject. SOLUTION: A computer 21 and a sequencer 22 control a waveform and a timing of an RF pulse emitted from an RF coil 13 and a gradient magnetic field pulse generated from a gradient magnetic field coil 12 to perform phase encoding of an imaging sequence of a main scan. The imaging sequence of applying the readout gradient magnetic field pulse in the original readout direction and the imaging sequence of applying the readout gradient magnetic field pulse in the phase encoding direction are performed as a prescan after the generation of the gradient magnetic field pulse, and the main scan is performed based on the information obtained therefrom. Automatically set conditions.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an MR imaging apparatus and, more particularly, to an MR imaging apparatus for performing a sequence for measuring a cross-sectional size of a subject.

[0002]

2. Description of the Related Art In an MR imaging apparatus, if a subject is deviated from the center of a field of view in a section to be imaged, adverse effects such as aliasing artifacts occur. Therefore, conventionally, an imaging sequence for positioning is performed prior to an imaging sequence (main scan) for obtaining a diagnostic image of an actual subject, and based on this, the subject is positioned at the center of the image field of view. Also, the field of view (FOV) is determined so as to match the size of the subject.

[0003]

However, the conventional positioning imaging sequence is the same as a normal imaging sequence, and there are cases where the imaging sequence must be performed many times without obtaining a desired image. There is a problem that the operability is poor. Further, in the case of a multi-slice method in which images in a plurality of slices are imaged in one imaging sequence, since conditions are set based on information on a cross section where a cross-sectional image of the subject is the largest,
For smaller cross-sectional images, more data is collected than the required data amount, and the burden of high-frequency irradiation on the subject is large.

In view of the above, the present invention performs a special pre-scan for measuring the size of a cross section of a subject prior to a main scan, and automatically sets optimum conditions based on the pre-scan, thereby improving operability. It is an object of the present invention to provide an MR imaging apparatus that has been made to operate.

[0005]

In order to achieve the above object, an MR imaging apparatus according to the present invention comprises:
RF transmitting means, gradient magnetic field pulse applying means for applying gradient magnetic field pulses in predetermined slice selection directions, phase encoding directions and frequency encoding directions, data collecting means for receiving NMR signals and collecting data,
By controlling the F transmitting means, the gradient magnetic field pulse applying means and the receiving means, and changing the amplitude of the phase encoding gradient magnetic field pulse, while performing the main scan imaging sequence of repeating RF excitation, signal generation, reception, and data collection, Prior to the imaging sequence of this main scan, an imaging sequence in which only the phase encoding gradient magnetic field pulse is not applied, and an imaging sequence in which the readout gradient magnetic field pulse is applied in the phase encoding direction instead of being applied in the frequency encoding direction, are performed in the frequency encoding direction. And control means for automatically setting the conditions of the main scan based on the information on the cross-sectional position and size of the subject and the cross-sectional position and size of the subject in the phase encoding direction. ing.

[0006] As a pre-scan prior to the main scan,
An imaging sequence in which only the phase encoding gradient magnetic field pulse is not applied and an imaging sequence in which the readout gradient magnetic field pulse is applied in the phase encoding direction instead of being applied in the frequency encoding direction are performed. In these imaging sequences, a gradient magnetic field pulse for phase encoding is not applied, and thus can be completed by one repetition. And
A signal in which the position information in the frequency encoding direction is encoded into the frequency information and a signal in which the position information in the phase encoding direction is encoded into the frequency information can be obtained, respectively. The position information can be reproduced, and the position and size of the subject cross section in each of these directions can be obtained. Since the conditions of the main scan are automatically set based on this, the photographing condition setting items of the operator can be reduced, and the operability is improved. In addition, by performing the pre-scan on each slice in the multi-slice method, optimal conditions can be set for each slice, and the burden of high frequency irradiation applied to the subject is reduced.

[0007]

Next, embodiments of the present invention will be described in detail with reference to the drawings. The MR imaging apparatus according to the present invention is configured as shown in FIG. In FIG. 1, a main magnet 11 generates a strong static magnetic field, and an object (not shown) is arranged in the static magnetic field. Further, the gradient magnetic field coil 12 has X,
Three sets are provided so as to generate three gradient magnetic fields Gx, Gy, Gz whose magnetic field strengths incline in the directions of the three orthogonal axes Y and Z are superimposed on the static magnetic field. An RF coil 13 for transmission and an RF coil 14 for receiving NMR signals are attached to the subject.

A host computer 21 controls the entire system, and a sequencer 22 acquires data for reconstructing an image of a desired section of a subject under the control of the host computer 21 (FIG. 2). Etc.) are sent to the transmission system, the reception system, and the gradient magnetic field generation system. As for the gradient magnetic field generation, a predetermined pulse waveform related to Gx, Gy, Gz is generated at a predetermined timing from the waveform generator 15 and sent to the gradient magnetic field power supply 16, and the gradient magnetic field coil 12 outputs Gx, Gy and Gz are generated.

A slice selection gradient magnetic field, a readout (frequency encoding) gradient magnetic field, and a phase encoding gradient magnetic field use any one of Gx, Gy, and Gz.
Alternatively, it can be made in any direction by being made by combining some of them. Here, Gz is used as a slice selection gradient magnetic field, Gy is used as a phase encoding gradient magnetic field, and Gx is used as a readout gradient magnetic field. Is used. That is, a predetermined area in the Z direction in the real space is selectively excited, the position information in the X direction is encoded into a frequency, and the position information in the Y direction is encoded into a phase.

The waveform generator 15 generates an RF pulse waveform at a predetermined timing under the control of the sequencer 22 and sends it to the amplitude modulator 24. The amplitude modulator 24 has an RF
The RF signal from the signal generator 23 is sent as a carrier, and the carrier is amplitude-modulated according to the waveform signal from the waveform generator 15. This RF signal generator 23
Is controlled by the host computer 21 so that the frequency of the RF signal generated from the RF signal coincides with the resonance frequency of the subject. The output of the amplitude modulator 24 is sent to the RF coil 13 via the RF power amplifier 25. Thus, the RF coil 1
By determining the waveform (envelope) and timing of the RF signal transmitted from 3 by the sequencer 22, a 90 ° pulse or a 180 ° pulse shown in FIG.

The NMR signal generated from the subject is received by the receiving RF coil 14 and sent to the phase detector 27 via the preamplifier 26. The phase detector 27 has a transmission RF
An RF signal having a frequency based on the RF signal serving as a pulse carrier is transmitted from the RF signal generator 23, and this signal is used as a reference signal to perform phase detection. The frequency of this reference signal is
1 is controlled. The A / D converter 28 samples the detection signal from the phase detector 27 according to the sampling pulse from the sampling pulse generator 29 whose timing and the like are controlled by the sequencer 22, and converts the sampling signal into digital data. This digital data is taken into the host computer 21 and subjected to Fourier transform processing by the image reconstruction device 33. The image thus reconstructed is displayed by the display device 32. The indicator 31 is a keyboard, a mouse, and the like for an operator or the like to give necessary instructions to the host computer 21.

In such an MR imaging apparatus, under the control of the host computer 21 and the sequencer 22, an imaging sequence (main scan) for obtaining an actual diagnostic image of a subject and an adjustment sequence (pre-scan) preceding it are performed. ). Here, as the main scan, an imaging sequence based on the spin echo method as shown in FIG. 2 is performed. In FIG. 2, first, a 90 ° pulse (nutation pulse) 41 is applied, and then a 180 ° pulse (refocus pulse) 42 is applied. Pulses 43 and 44
Add. And the gradient magnetic field G for phase encoding in the Y direction
A y-pulse 45 is applied, and a gradient magnetic field Gx pulse 4 for reading (and for frequency encoding) in the X direction is applied.
A spin echo signal 48 is generated while performing frequency encoding by adding 6 and 47, and sampling is performed at each timing of a sampling pulse to collect data.

That is, a Gx pulse 46 is applied between the RF pulses 41 and 42 for dephasing,
Gx pulse 4 for rephasing after pulse 42
7, the signal 48 is generated, and the signal 48 is generated during the application of the Gx pulse 47 to encode the position information in the X direction into a frequency.
The Gy pulse 45 encodes the position information in the Y direction into a phase, changes the amplitude little by little at each repetition time, and sets a series of lines arranged at different positions in the k space (raw data space). Ensure that data is obtained at each repetition time.

As a pre-scan prior to such a main scan, a pulse sequence as shown in FIGS. 3 and 4 is performed. These pre-scan pulse sequences basically exclude the application of the gradient magnetic field pulse for phase encoding in the pulse sequence of FIG.
That is, the pulse sequence of FIG. 3 is based on the spin echo method as in FIG. 2, but does not apply the gradient magnetic field pulse for phase encoding in the Y direction. In FIG. 4, instead of applying a phase encoding gradient magnetic field pulse in the Y direction, readout (and frequency encoding) gradient magnetic field pulses 46 and 47 are applied, and no X direction gradient magnetic field Gx is applied.

The signal 48 generated in FIG. 3 encodes the position information in the X direction as frequency information, and the signal 48 generated in FIG. 4 encodes the position information in the Y direction as frequency information. If the data collected by sampling / A / D conversion by the pulse 49 is Fourier-transformed to decode the frequency information, the position information in the X direction and the position information in the Y direction can be reproduced.

That is, the signal 48 generated in FIG.
The signal c is obtained by performing a Fourier transform on this, and the size Lx and the center position Px of the subject cross section 51 in the X direction can be obtained. Further, the signal 48 generated in FIG. 4 can be represented as shown in FIG. 5B.
And the size Ly and the center position Py of the subject cross section 51 in the Y direction can be obtained. Therefore, if the frequency of the reference signal of the phase detection in the main scan (FIG. 2) is automatically set based on the relationship between Px and Py and the center of the magnetic field, the center of the visual field in the X direction and the Y direction can be set to the cross section of the subject. By adjusting the amplitude of the Gx pulses 46 and 47, the gradient of Gx can be changed, so that the field size in the X direction can be adjusted to Lx, and the number of steps of the Gy pulse and one step By changing the amount of change in the hit, the number of phase encodes in the Y direction can be determined, and the field size in the Y direction can be adjusted to Ly.

In order to make the matrix of all images the same for the sake of image display, for example, the sampling pulse 4
9 are the same, and Gx pulse 4 for frequency encoding is used.
6 and 47 are changed according to Lx of each section, the number of steps of the phase encoding Gy pulse 45 is reduced according to Ly, and the frequency of the phase detection reference signal is also changed according to Px and Py. Let it. As a result, data having a smaller number of lines than the number of lines to be originally arranged in the raw data space can be obtained, and the data of the missing line is substituted by a predetermined value (a value such as 0 or -1000) to perform a matrix reduction method. To reconstruct the image. Thus, an image in which the cross-sectional image of the subject is located at the center of the visual field can be reconstructed. In this case, since the number of repetitions of the pulse sequence decreases, the number of RF irradiations on the subject decreases, and the burden on the subject decreases.

The above description relates to one embodiment, and it goes without saying that the present invention is not intended to be limited to this description. For example, the pulse sequence is not limited to the spin echo method as described above, and other pulse sequences such as a field echo method can be adopted. Further, the slice selection direction, the phase encoding direction, and the frequency encoding direction are not limited to the Z, Y, and X directions as described above, and may be any directions. Further, the specific configuration is not limited to the configuration shown in FIG. 1 and may be various configurations.

[0019]

As described above, according to the MR imaging method of the present invention, it is possible to automatically set optimal imaging parameters according to the size and position of a subject on a slice plane to be imaged. Operability is improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an MR imaging apparatus according to an embodiment of the present invention.

FIG. 2 is a time chart showing an imaging sequence of a main scan performed in the embodiment.

FIG. 3 is a time chart showing an imaging sequence of a pre-scan performed in the embodiment.

FIG. 4 is a time chart showing another imaging sequence of the pre-scan performed in the embodiment.

FIG. 5 is a schematic diagram showing a relationship between data obtained by pre-scanning and a cross section of a subject.

[Explanation of symbols]

Reference Signs List 11 Main magnet for generating static magnetic field 12 Gradient magnetic field coil 13 RF coil for transmitting 14 RF coil for receiving 15 Waveform generator 16 Gradient magnetic field power supply 21 Host computer 22 Sequencer 23 RF signal generator 24 Amplitude modulator 25 RF power amplifier 26 Preamplifier 27 Phase detector 28 A / D converter 29 Sampling pulse generator 31 Indicator 32 Display device 33 Image reconstruction device 41, 42 RF pulse 43, 44 Gradient magnetic field pulse for slice selection 45 Gradient magnetic field pulse for phase encoding 46, 47 Readout Gradient pulse (for frequency encoding) 48 Spin echo signal 49 Sampling pulse 51 Cross section of subject

Claims (1)

[Claims] 1. An RF transmitting means, a gradient magnetic field pulse applying means for applying a gradient magnetic field pulse in a predetermined slice selection direction, a phase encoding direction, and a frequency encoding direction, respectively, and a data collecting means for receiving an NMR signal and collecting data. By controlling the RF transmitting means, the gradient magnetic field pulse applying means, and the receiving means, the main scan imaging sequence in which RF excitation, signal generation, reception, and data collection are repeated while changing the amplitude of the phase encoding gradient magnetic field pulse is performed. At the same time, prior to the imaging sequence of the main scan, an imaging sequence in which only the phase encoding gradient magnetic field pulse is not applied, and an imaging sequence in which the readout gradient magnetic field pulse is applied in the phase encoding direction instead of being applied in the frequency encoding direction, are performed. Subject in encoding direction An MR imaging apparatus comprising: control means for obtaining information relating to a cross-sectional position / size and a cross-sectional position / size of a subject in a phase encoding direction and automatically setting the conditions of the main scan based on the information.