JPH04371139A - Magnetic resonance imaging method - Google Patents

Abstract

PURPOSE:To shorten the time for picking up images without damaging artifact reducing effects by obtaining data on low frequency components with a small amount of phase encoding at the timing of a fixed phase of respiratory move ment, and obtaining data on high-frequency components at timing not related to the-respiratory movement. CONSTITUTION:NMR signals, which are generated after the exposure of a subject 11 to RF pulses from a transmitting coil 12 and the excitation of the nuclear spins of the pulses, are received by a receiving coil 13. The received NMR signals are amplified, detected, digitized, and sampled into a host computer 41. These data are subjected to two-dimensional Fourier transform so that images are reconfigured and the images are displayed in a console 43. In this case, spin echo method is adopted as a pulse sequence for the imaging. Data are selected as to only low-frequency components using respiration gate method while high-frequency components are directly adopted regardless of the level of a respiration detection signal.

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
By performing a two-dimensional Fourier transform on the data obtained from the received echo signal, the position information in the two-axis directions is decoded and a tomographic image on the slice plane is obtained.

Frequency encoding is performed by generating an echo signal while applying a gradient magnetic field whose magnetic field strength is gradient in one axis direction. Phase encoding is performed by sequentially repeating the excitation/reception sequence for each of the gradient magnetic fields whose magnetic field strengths are tilted in other axial directions so that the strengths thereof change by equal amounts. By repeating the excitation/reception sequence in this way, data with a different amount of phase encoding is obtained for each sequence.

[0004] In such an MR imaging method, when the subject moves, artifacts tend to appear in the image due to the movement. In particular, when imaging the abdomen, artifacts are noticeable due to movement caused by respiratory motion. Therefore, an artifact removal method called the breathing gate method has been used.

[0005] In this method, a detector for detecting respiratory movement is provided, a threshold level (respiratory gate level) for the detection signal is set in advance, and when the level of the detection signal is below the threshold level, This method uses only the data obtained in the sequence to reconstruct the image.

[0006]

[Problems to be Solved by the Invention] However, in the case of the breathing gate method, the lower the breathing gate level, which specifies the tolerance for movement due to respiratory movement, the more data can be collected in a state of no movement, and the artifacts due to movement can be reduced. However, the problem is that it takes a long time to capture the image.

In view of the above, an object of the present invention is to provide an improved MR imaging method that shortens the imaging time without impairing the effect of reducing artifacts caused by respiratory motion.

[0008]

[Means for Solving the Problems] In order to achieve the above object, in the MR imaging method according to the present invention, data of low frequency components with a small amount of phase encoding are obtained at a timing of a constant phase of respiratory motion, but the phase A feature of this system is that high-frequency component data, which requires a large amount of encoding, is obtained at timings that are not related to respiratory movements. In other words, only the low frequency components related to phase encoding are collected using the breathing gate method, and the high frequency components are collected without using the breathing gate method. Therefore, all the data is not collected by the respiratory gate method, so it is possible to eliminate the long data collection time. In addition, regarding phase encoding, the low frequency component represents a low spatial frequency in the phase encoding direction, and the high frequency component represents a high spatial frequency, so if only the low frequency component is collected without movement, large patterns can be detected. Since no artifacts occur, and artifacts occur only in small patterns, the artifact reduction effect does not deteriorate significantly.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows a system configuration for performing an MR imaging method according to an embodiment of the present invention. In this figure, a subject 11 is exposed to a static magnetic field formed by a main magnet 15 and a gradient coil 14 superimposed thereon. is placed within a gradient magnetic field formed by. A transmitter coil 12 and a receiver coil 13 are attached to the subject 11 .

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 respectively a slice selection gradient magnetic field Gs, a readout (frequency encoding) gradient magnetic field Gr, and a phase encoding gradient magnetic field Gp. Current is supplied to the gradient coil 14 from respective power supplies 21, 22, and 23 for gradient magnetic fields Gs, Gr, and Gp, 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.

[0011] The transmitting coil 12 is supplied with RF pulses sent from a high frequency power source 33. This RF pulse is
In the frequency converter 32, R from the synthesizer 34
The RF waveform generator 3 uses the F sine wave signal as a carrier signal.
The sinc waveform from 1 is AM-modulated and amplified by the high-frequency power supply 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. This received NMR signal is amplified by a preamplifier 35, then 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 S
This is a detection circuit based on an active detector method, and uses a circuit that mixes a reference signal sent from the synthesizer 34 and a 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. Further, a detection signal from a respiration detector 44 attached to the subject 11 is sent to the host computer 41 . The respiration detector 44 may be one that detects pressure fluctuations by wrapping an air bag around the abdomen of the subject 11, whose pressure fluctuates as the abdomen moves due to respiration.

As a pulse sequence for imaging, a normal spin echo method, saturation recovery method, inversion recovery method, etc. can be used. In these sequences, for example, specific slices perpendicular to the S axis are sequentially selectively excited by applying an RF pulse while applying a gradient magnetic field Gs in the S direction, and the position information in the R direction is converted to the frequency of the NMR signal using the gradient magnetic field Gr. At the same time, position information in the P direction is encoded into the phase of the NMR signal by the gradient magnetic field Gp.

Here, it is assumed that a pulse sequence using the spin echo method is performed as shown in FIG. First, a 90° pulse 51 is applied to tilt the nuclear spins by 90°, and at the same time a gradient magnetic field Gs pulse 53 for slice selection is 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 the uniaxial direction in the slice plane into a frequency. , encodes other axial position information within the slice plane into phase. Thereafter, a 180° pulse 52 is applied together with a pulse 54 of a gradient magnetic field Gs for slice selection, and then a pulse 56 of a gradient magnetic field Gr for readout (frequency encoding) is applied to generate a spin echo signal 58.

The pulse sequence shown in FIG. 2 is repeated at regular time intervals while gradually changing the magnitude of the pulse 57 of the phase encoding gradient magnetic field Gp. Each of them generates an echo signal 58, and the echo signal is sampled at a predetermined sampling rate to be used by the A/D.
Since one column (one line) of data is obtained by the conversion, a data string is obtained for each phase encode amount as shown in FIG.

Assuming that the waveform of the detection signal representing the respiratory movement detected by the respiratory detector 44 is as shown in FIG. 4, according to the respiratory gate method, this waveform becomes below the set respiratory gate level. Only data collected at the same time will be accepted. That is, time t
When data is obtained at 1, t2, t3, t4, t5, ..., only the data obtained at the timing sequence of t1, t4 where the respiration detection signal is below the respiration gate level are used for image reconstruction. The usual breathing gate method is to use the breathing gate method.

[0020] Here, data selection using the breathing gate method is not performed on all data, but only low frequency components, that is, data obtained from pulse sequences with a small amount of phase encoding, are selected using the breathing gate method to remove high frequency components, i.e., data obtained from pulse sequences with a small amount of phase encoding. Data obtained from a pulse sequence with a large amount of phase encoding is used as is, regardless of the level of the respiration detection signal. The range for data selection using this breathing gate method is set using the keyboard device of the console 43 or the like.

When the range is set to 50%, for example, and the phase encode amount is gradually decreased from the largest on the negative side until it reaches zero, it is gradually increased on the positive side. As shown in FIG. 5, for the first 25% (lower side in FIG. 5), all data are adopted regardless of the level of the respiration detection signal. In the subsequent 50% (center), data selection is performed using the breathing gate method, and only data obtained in sequences performed at the timing when the level of the breathing detection signal is below the breathing gate level is adopted. When the phase encode amount further increases to the positive side and reaches the 25% range (upper side), all data are employed again regardless of the level of the respiration detection signal.

In this case, data selection using the respiratory gate method is performed for the center portion of the phase encode amount, that is, 50% of the low frequency component, so that data that is not affected by respiratory motion can be obtained for this portion. On the other hand, data will be collected for 25% and 25% (total 50%) on the high frequency side, regardless of movement due to respiratory motion. Therefore, since data collection using the respiratory gate method is limited, the overall data collection time (imaging time) does not become very long. Furthermore, since data with low frequency components unaffected by respiratory motion can be obtained, artifacts associated with large patterns can be eliminated. Artifacts occur in high-frequency components due to the influence of respiratory motion, but since high-frequency components are small patterns and the signal is small, the image quality does not deteriorate much.

[0023] In the above embodiment, the excitation/reception pulse sequence is performed sequentially at fixed time intervals while changing the amount of phase encoding, and only the data obtained by the matching timing sequence by the breathing gate method is used. However, it is also possible to shift the start timing of the excitation/reception pulse sequence itself so that the pulse sequence is not performed at fixed time intervals, but data is obtained at matching timing using the breathing gate method.

[0024]

Effects of the Invention As described in the embodiments above, according to the MR imaging method of the present invention, by balancing the imaging time and the reduction of artifacts due to respiratory motion, artifacts can be reduced without significantly prolonging the imaging time. A reconstructed image can be obtained.

[Brief explanation of the drawing]

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

FIG. 2 is a time chart showing an example of a pulse sequence.

FIG. 3 is a schematic diagram showing the relationship between phase encode amount and data string.

FIG. 4 is a time chart showing a waveform example of a respiration detection signal.

FIG. 5 is a schematic diagram showing a data string.

[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 reading 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 44
Breathing detector 51
90° pulse 52
180° pulses 53, 54 Gradient magnetic field pulses for slice selection 55, 56 Gradient magnetic field pulses 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. In the MR imaging method, which obtains the tomographic image on the slice plane by decoding the position information in the two axes by performing two-dimensional Fourier transform on the data obtained from the received echo signal, the amount of phase encoding is The excitation/reception sequence is performed sequentially while changing the amount of phase encoding, and the data of low frequency components with a small amount of phase encoding are obtained at a timing of a constant phase of respiratory movement, and the data of high frequency components with a large amount of phase encoding is obtained at a timing unrelated to respiratory movement. An MR imaging method characterized by obtaining.