JPH04246326A - Mr imaging method - Google Patents

Abstract

PURPOSE:To obtain the picture constitution image in which an artifact caused by phase disorder is reduced remarkably by executing a data collection which suppresses disorder of a phase of a resonance signal. CONSTITUTION:A pulse sequence is repeated, while varying the phase encoding quantity by varying magnitude of a pulse 57 of a phase encoding inclined magnetic field Gp. In the part in which magnitude of the phase encoding inclined magnetic field Gp is small, that is, the part in which the phase encoding quantity is small, the sequence is repeated twice by its same phase encoding quantity, and averaged, and data is obtained. A data train ((a) part) shown by a thick line is a data train obtained by collecting this datum two times and averaging them. In such a way, in the vertical direction of a raw data space, the influence of phase disorder of echo signal data in the center part is reduced.

Description

[Detailed description of the invention]

0001

[Industrial Field of Application] This invention relates to nuclear magnetic resonance (NMR).
) relates to an MR imaging method for performing imaging.

0002

2. Description of the Related Art In MR imaging, a specific slice plane of a subject is selectively excited, positional information in one axis direction within the slice plane is expressed as the frequency of an echo signal, and positional information in other axial directions is expressed as an echo signal. are encoded into the phase of
The received echo signal is subjected to two-dimensional Fourier transform to decode the position information in the two axial directions and obtain a tomographic image on the slice plane.

Conventionally, the amount of phase encoding is changed by the same amount, and a data string is obtained for each of them. That is, as shown in FIG. 4, the phase encoding gradient magnetic field Gp is changed by equal amounts, and the NMR generated at each change is
The signal is sampled at each time, and a data string is obtained for each phase encode amount.

0004

[Problem to be solved by the invention] However, MRI
Due to instability in the device's transmission/reception system and gradient magnetic field generation system, phase disturbances often occur in the resonance signals emitted from the subject, and this phase disturbance causes phase ghosts to appear in the reconstructed image, causing artifacts in the phase direction of the image. There's a problem.

In view of the above, the present invention provides an improved MR imaging method that suppresses artifacts in reconstructed images due to disturbances in the phase of resonance signals caused by instability of the transmission/reception system and gradient magnetic field generation system of an MRI apparatus. The purpose is to

[0006]

[Means for Solving the Problems] In order to achieve the above object, in the MR imaging method according to the present invention, the excitation/reception sequence is performed by changing the amount of phase encoding, but the sequence of excitation and reception is performed with the same amount of phase encoding in the portion where the amount of phase encoding is small. The feature is that data is collected multiple times for each phase encode amount. By averaging the data obtained in multiple sequences like this, the influence of phase disturbance of the resonance signal can be reduced. In other words, artifacts caused by phase disturbances in resonance signals are largely caused by phase disturbances that occur near the center of the raw data space of the collected resonance signal data, that is, in areas where the amount of phase encoding is small. By reducing the influence of , a reconstructed image with fewer artifacts can be obtained.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram showing an MRI apparatus that performs an MR imaging method according to an embodiment of the present invention. First, a transmitting coil 12 and a receiving coil 13 are attached to a subject 11, and these are connected to a main magnet 15 and a gradient coil. 14 and a gradient magnetic field superimposed thereon. The gradient coils 14 are configured to be able to independently generate gradient magnetic fields whose magnetic field strengths are gradient in each direction of three orthogonal axes. The gradient magnetic fields of the three orthogonal axes are a slice selection gradient Gs, a readout (frequency encoding) gradient Gr, and a phase encoding gradient Gp.
shall be. The gradient coil 14 has gradient magnetic fields Gs, Gr, Gp.
A current is supplied from each power source 21, 22, 23, and gradient magnetic fields in each direction are formed. The waveforms of currents supplied by the gradient magnetic field power supplies 21 to 23 are controlled by a gradient magnetic field controller 24 so that the gradient coil 14 forms gradient magnetic field pulses with predetermined waveforms.

On the other hand, the transmitting coil 12 is connected to a high frequency power source 3.
The RF pulse sent from 3 is supplied. This RF pulse is obtained by AM modulating the RF sine wave signal from the synthesizer 34 as a carrier signal in the frequency converter 32 with a sinc waveform from the RF waveform generator 31, and amplifying it by the high frequency power source 33.

NMR generated after the subject 11 is irradiated with an RF pulse from the transmitting coil 12 to excite its nuclear spins.
The signal is received by the receiving coil 13. It is also possible to use the transmitting coil 12 and the receiving coil 13 as both, and to switch between the high frequency power source 33 on the transmitting side and the preamplifier 35 on the receiving side using a signal switcher (not shown). This received NMR
After the signal is amplified by a preamplifier 35, it is detected by a quadrature phase detector 36, and then converted into digital data by an A/D converter 37 and taken into the host computer 41. This quadrature phase detector 36 is a PSD (Phase
This is a detection circuit of the Sensitive Detector type, which mixes the reference signal sent from the synthesizer 34 and the received signal, and outputs the difference in frequency between the two signals.

Under the control of the host computer 41, the sequence controller 42 provides waveform information and generation timing information of each gradient magnetic field pulse to the gradient magnetic field control device 24.
The F waveform generator 31 is given sinc waveform information and generation timing information of the RF pulse, and the synthesizer 3
4 to control the sampling timing of the A/D converter 37, etc.

The host computer 41 includes a console 4 having a display device and an input device such as a keyboard device.
3 is connected. The data taken into the host computer 41 is subjected to two-dimensional Fourier transform to reconstruct an image, and the resulting image is displayed on the display device of the console 43.

In the MR imaging method according to this embodiment, a pulse sequence as shown in FIG. 2 is performed as a pulse sequence for imaging. This pulse sequence is basically a spin echo method with modifications. First, when a 90° pulse 51 is applied to tilt the nuclear spins by 90°, a gradient magnetic field Gs pulse 53 for slice selection is simultaneously applied. This selectively excites only nuclear spins within a predetermined slice plane.

Next, a pulse 55 of the gradient magnetic field Gr for readout (frequency encoding) and a pulse 57 of the gradient magnetic field Gp for phase encoding are added to encode the positional information in one axis direction in the slice plane into a frequency. At the same time, position information in other axial directions within the slice plane is encoded into the phase.

Thereafter, a 180° pulse 52 is applied together with a gradient magnetic field pulse 54 for slice selection, and further thereafter,
A pulse 56 of a gradient magnetic field Gr for readout (frequency encoding) is applied to generate a spin echo signal 58.

This pulse sequence is then repeated while changing the magnitude of the pulse 57 of the phase encoding gradient magnetic field Gp and changing the amount of phase encoding. As shown enlarged in FIG. In a portion where the magnitude of the encoding gradient magnetic field Gp is small, that is, a portion where the amount of phase encoding is small, the same amount of phase encoding results in 2
Repeat the sequence twice and average to obtain the data. The data string (portion a) indicated by the thick line is the data string obtained by collecting and averaging the data twice. This makes it possible to reduce the influence of phase disturbance of the echo signal data in the vertical center of the raw data space.

In general, the phase disturbance in the resonance signal that has the greatest effect as a phase ghost on an image is the phase disturbance in the center of the raw data space in the phase direction, that is, in the portion where the amount of phase encoding is small. If a phase disturbance occurs in this part, an artifact in the phase direction will appear in the low frequency component (outline of the subject) on the image, since this part corresponds to a low frequency component on the image.

Therefore, as described above, by being able to obtain data in which the influence of phase disturbance is reduced in a portion where the amount of phase encoding is small, artifacts caused by phase disturbance on an image can be significantly reduced.

In the MR imaging method according to this embodiment, as shown in FIG. 3, in a portion where the amount of phase encoding is large, the sequence with the originally necessary amount of phase encoding is not performed, and the amount of phase encoding is thinned out. . The data to be collected using the thinned out phase encode amount is obtained as shown by the dotted line by interpolation from the data of the adjacent phase encode amount (shown by the solid line). Once all the data strings are prepared in this way, the image is reconstructed by two-dimensional Fourier transformation.

[0019] Since the phase encode amount is thinned out in the portion where the phase encode amount is large in this way, it is possible to avoid prolonging the entire imaging time due to repeating the sequence of the same phase encode amount in the portion where the phase encode amount is small. Can be done.

Although the spin echo method has been described in the above embodiment, it is of course applicable to other pulse sequences.

[0021]

Effects of the Invention As described in the embodiments above, according to the MR imaging method of the present invention, data can be collected while suppressing the phase disturbance of the resonance signal. A constituent image can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an MRI apparatus for performing an MR imaging method according to an embodiment of the present invention.

FIG. 2 is a time chart showing a pulse sequence of the MR imaging method according to the same example.

FIG. 3 is a conceptual diagram showing the relationship between phase encoding gradient magnetic field pulses and data strings according to the same embodiment.

FIG. 4 is a conceptual diagram showing the relationship between a phase encoding gradient magnetic field pulse and a data string according to a conventional example.

[Explanation of symbols]

11 Subject 12
Transmission coil 13
Receiving coil 14
Gradient coil 15 Main magnet 21 Gradient magnetic field power supply for slice selection 22 Gradient magnetic field power supply for readout 23
Gradient magnetic field power supply 24 for phase encoding
Gradient magnetic field control device 31
RF waveform generator 32
Frequency converter 33
High frequency power supply 34 Synthesizer 35 Preamplifier 36
Quadrature phase detector 37
A/D converter 41
host computer 42
Sequence controller 43
console 51
90° pulse 52
180° pulses 53, 54 Slice selection gradient magnetic field pulses 55, 56
Gradient magnetic field pulse for readout (frequency encoding)

Claims (1)

[Claims] Claim 1: Selectively excite a specific slice plane of the subject, and set the position information in one axis direction within the slice plane to the frequency of the echo signal, and the position information in the other axis direction to the phase of the echo signal, respectively. encode the received echo signal into 2
In the MR imaging method, which obtains a tomographic image on the slice plane by decoding the position information in the two axes through dimensional Fourier transformation, the excitation/reception sequence is performed by changing the amount of phase encoding. An MR imaging method characterized in that data is collected by performing data multiple times for one phase encode amount in a small portion.