JP5000963B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and in particular, eliminates the influence of artifacts that occur when performing echo signal measurement in a spin echo signal (SE) sequence, and enables improvement in image quality. It relates to the MRI device.

  The MRI apparatus uses the nuclear magnetic resonance (NMR) phenomenon to measure the density distribution, relaxation time distribution, etc. of nuclear spins (hereinafter referred to as spins) in the cross section corresponding to the desired examination site in the subject. An image of the examination site of the subject is displayed from the measurement data.

  As an imaging method using an MRI apparatus, there is an echo signal measurement using an SE sequence. In this SE sequence, transverse magnetization is excited by a π / 2 pulse, and this transverse magnetization is refocused by a π pulse, and an echo signal is measured.

  However, a portion of the longitudinal magnetization component that remains unaffected by the π / 2 pulse is rotated to transverse magnetization by imperfection of the applied π pulse for echo signal measurement. This unnecessarily generated FID signal due to transverse magnetization is mixed in an echo signal formed by the π pulse, and causes artifacts on the image. Since the FID signal is not applied with phase encoding, it appears as a single linear artifact in the phase encoding direction on the image.

This linear artifact can be measured with the frequency seen in the phase encoding direction as the highest frequency by changing the applied phase of π / 2 pulse by π for each excitation and measuring the echo signal. The echo signal measured as can be arranged at the extreme end of the image (for example, Patent Document 1). As a result, it is possible to prevent the linear artifact from becoming an obstacle to diagnostic imaging.
Japanese Patent Laid-Open No. 02-95346

  However, even if such a method is used, in the case of off-center FOV measurement, linear artifacts cannot be removed from the image. Off-center FOV measurement is a measurement in which the center of the static magnetic field does not coincide with the center of interest (measurement region), and the image initially obtained by this measurement is shifted because the region of interest does not exist at the center of the image. However, an image in which the region of interest is arranged at the center of the image is obtained by performing shift processing by image processing. In this case, the linear artifact arranged at the end of the image is also shifted in the same manner, and therefore mixed in the region of interest.

  The present invention can move artifacts mixed in an image to an arbitrary position in the image based on the longitudinal magnetization component that remains unaffected by the π / 2 pulse in the measurement by such SE sequence. An object of the present invention is to provide an MRI apparatus.

  In order to achieve the above object, the MRI apparatus of the present invention is configured as follows. That is, a static magnetic field generating means for generating a static magnetic field, a transmitting means for applying a high-frequency electromagnetic wave to a subject placed in the static magnetic field, a gradient magnetic field generating means for giving a magnetic field gradient to the static magnetic field, and generated from the subject Receiving means for measuring nuclear magnetic resonance signals, control means for controlling these transmitting means, gradient magnetic field generating means and receiving means in accordance with a spin echo pulse sequence, and image processing means for performing image processing based on the measured magnetic resonance co-signals The control means includes means for adjusting the phase of the high-frequency magnetic field and the measurement phase of the nuclear magnetic resonance signal in accordance with the amount of deviation of the center of the measurement region from the center of the static magnetic field.

  According to the MRI apparatus of the present invention, by providing means for adjusting the phase of the excitation pulse based on the image shift amount in the off-center FOV measurement, the SE sequence is generated in the image due to the π / 2 pulse. Linear artifacts can be placed at any position in the phase encoding direction. As a result, even when the subject performs measurement at a position deviated from the center of the magnetic field in the phase encoding direction, the linear artifact can always be arranged at the extreme end of the image so that the adverse effect is not exerted on the subject image. Measurement is possible. As a result, an SE sequence image with higher image quality can be acquired.

Hereinafter, an embodiment of the MRI apparatus of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram of an overall configuration showing an MRI apparatus to which the present invention is applied. This MRI apparatus mainly includes a central processing unit (CPU) 1, a sequencer 2, a transmission system 3, a static magnetic field generating magnet 4, a reception system 5, and a signal processing system 6.

  The central processing unit (CPU) 1 controls each of the sequencer 2, the transmission system 3, the reception system 5, and the signal processing system 6 in accordance with a program based on the present invention. The sequencer 2 operates based on a control command from the central processing unit 1, and sends various commands necessary for collecting tomographic image data of the subject 7 to the transmission system 3 and the gradient magnetic field generation system (12 13) and the receiving system 6.

  The transmission system 3 has a high-frequency transmitter 8, a modulator 9, and an irradiation coil 11 as a high-frequency coil, and the amplitude modulation of the high-frequency pulse from the high-frequency transmitter 8 is performed by the modulator 9 according to a command from the sequencer 2. The high frequency pulse thus amplified is amplified through the high frequency amplifier 10 and supplied to the irradiation coil 11, so that the subject 7 is irradiated with a predetermined pulsed electromagnetic wave. The application phase of the high frequency pulse is controlled by the central processing unit 1.

The static magnetic field generating magnet 4 is for generating a uniform static magnetic field around the subject 7 in an arbitrary direction. Inside the static magnetic field generating magnet 4, in addition to the irradiation coil 11, a gradient magnetic field coil 13 for generating a gradient magnetic field and a reception coil 14 of the reception system 5 are installed.
The gradient magnetic field generation system includes a gradient magnetic field coil 13 that applies a gradient magnetic field independently in Cartesian coordinate axis directions orthogonal to each other, and a gradient magnetic field power supply 12 that supplies a current to the gradient magnetic field coil 13. Controlled by the sequencer 2 as described above.

  The receiving system 5 includes a receiving coil 14 as a high frequency coil, an amplifier 15 connected to the receiving coil 14, a quadrature phase detector 16, and an A / D converter 17, and receives an NMR signal from the subject 7 as a receiving coil. When the signal is detected, the signal is converted into a digital quantity via the amplifier 16, the quadrature detector 16, and the A / D converter 17, and the signal sampled by the quadrature detector 16 at the timing according to the command from the sequencer 2. The data is converted into series of collected data and sent to the central processing unit 1. The phase of this signal measurement is controlled by the central processing unit 1 in the same manner as the application phase of the high frequency pulse.

  The signal processing system 6 includes an external storage device such as a magnetic disk 20 and an optical disk 19 and a display 18 made up of a CRT or the like. When data from the receiving system 5 is input to the central processing device 1, the central processing device 1 performs processing such as signal processing and image reconstruction, and displays a desired cross-sectional image of the subject 7 as a result on the display 18 and records it on the magnetic disk 20 of the external storage device.

  FIG. 2 is a diagram showing a control procedure incorporated as a program in the central processing unit 1 having such a configuration, and shows a case where a spin echo signal is measured by an SE sequence. The SE sequence is incorporated in the sequencer 2 as a sequence table as shown in FIG. 3, and the application timing and intensity of the high-frequency pulse and the gradient magnetic field pulse and the signal measurement timing are determined based on this sequence.

That is, first, the excitation pulse 24 is applied together with the gradient magnetic field 27 for selecting the slice of the subject, the phase encode gradient magnetic field 30 and the readout gradient magnetic field 31 are applied, and the π pulse 25 is applied together with the slice gradient magnetic field 28. The excitation pulse is usually a π / 2 pulse. The echo signal 37 is measured while applying the read gradient magnetic fields 32 and 33 after the echo signal time TE from the excitation pulse 24 application. The area of the readout gradient magnetic field 33 applied after the echo time (the product of the application time and the intensity) is set to be at least the area of the readout gradient magnetic field 32 applied before that.
Such a sequence is repeated a predetermined number of phase encodings, and an image is reconstructed from the measured echo signals.

  The control of the present invention includes a step of adjusting the phase of the excitation pulse and signal measurement in such an SE sequence based on the shift amount of the image center in the off-center FOV measurement.

Next, spin echo signal measurement by the MRI apparatus of the present invention will be described with reference to FIGS.
First, a scout image is measured prior to spin echo signal measurement, and the amount of shift in the phase encoding direction of the measurement region with respect to the center of the static magnetic field is obtained based on the scout image (step 201). This deviation amount is automatically obtained by the central processing unit 1 by indicating the center of the region of interest on the scout image, for example. In MRI measurement, since the relationship between the image and the received measurement data is a relationship between two-dimensional Fourier transform and inverse transform, the image position shift corresponds to the rotation of the phase of the received signal. Therefore, the central processing unit 1 calculates the phase adjustment amount α by the following equation based on the deviation amount (step 202).
α = (2 × π × P) / FOV (1)
Here, P is the amount of position shift in the phase encoding direction, and FOV is the size of the imaging region in the phase encoding direction. Also, the initial phase, that is, the excitation pulse and signal measurement phase φ in the first iteration is set (step 203).

  Next, the phase of the excitation pulse for each phase encoding is obtained (step 204). If the phase of the excitation pulse in the first iteration (that is, the initial phase) is φ, the phase of the excitation pulse is π + φ + α in the second iteration, and the phase of the excitation pulse is 2π + φ + 2α in the third iteration. Hereinafter, in the nth iteration, the phase of the excitation pulse is 2nπ + φ + (n−1) α.

  Similarly, a measurement phase for each phase encoding is obtained (step 205). If the measurement phase (that is, the initial phase) in the first iteration is φ, the measurement phase is π + φ−α in the second iteration, and the measurement phase is 2π + φ−2α in the third iteration. Hereinafter, in the n-th iteration, the measurement phase is 2nπ + φ− (n−1) α.

  Next, the SE sequence shown in FIG. 3 is executed to perform spin echo signal measurement (step 206). FIG. 4 (a) shows the behavior of the magnetization vector in this case (behavior in a plane viewed from the direction of the static magnetic field). FIG. 2B shows the behavior of the magnetization vector in the conventional method for comparison. Both are examples in which φ = 0. The measurement phase is indicated by AD.

In the first time, a magnetization vector 41 is generated in the XY plane by a π / 2 pulse, and this magnetization vector 41 is inverted by a π pulse to become a magnetization vector 42. At this time, unnecessary transverse magnetization 43 is generated in the XY plane. At this time, the measurement phase is 0.
In the second measurement with the phase encoding amount changed (middle stage in FIG. 4 (a)), the excitation phase of the π / 2 pulse is inverted by π, and the phase adjustment amount is shifted by α as described above. Then, in the XY plane, the magnetization vector 46 is generated with a phase 76 that is further shifted by α on the axis opposite to that of the first time. Subsequently, a π pulse is applied with the same phase as the first time. Then, the magnetization vector 46 generated by the first π / 2 pulse becomes the magnetization vector 47, which is opposite to the first time and is shifted by α. The unnecessary transverse magnetization 48 generated by the π pulse becomes a magnetization vector in the same direction as the first time. At this time, the measurement phase is the same as the magnetization vector 47. Therefore, the phase of the FID signal due to unnecessary transverse magnetization 48 is π-α.

  Further, in the third measurement with the phase encoding amount changed (the lower part of FIG. 4 (a)), the excitation phase of the π / 2 pulse is reversed by π, and is applied with a phase shift of 2α. Then, in the XY plane, a magnetization vector 51 is generated with a phase 77 further shifted by 2α on the same axis as the first time. Subsequently, the π pulse is applied with the same phase as the first time. Then, the magnetization vector 51 generated by the first π / 2 pulse becomes the magnetization vector 52, which is the same as the first time and is shifted by 2α. The unnecessary transverse magnetization 48 generated by the π pulse becomes a magnetization vector in the same direction as the first time. At this time, the measurement phase is the same as the magnetization vector 52. Therefore, the phase of the FID signal due to unnecessary transverse magnetization 48 is 2π-2α.

When the echo signal is measured, the echo signal is measured by rotating the phase in the same manner as the phase rotation of the π / 2 pulse. As a result, the phase of the echo signal cancels the phase due to the phase rotation of the π / 2 pulse and becomes 0 (zero) phase, whereas the phase of the FID signal due to unwanted transverse magnetization that causes linear artifacts The phase is rotated in the same manner as the phase rotation of the measurement phase. Table 1 shows the phase of the echo signal and the phase of the FID signal measured under these conditions.

As can be seen from Table 1, the phase of the spin echo signal does not change and when viewed in the phase encoding direction, it becomes a direct current component, whereas the phase of the FID signal changes by π and further changes with the angular velocity α. To do. Therefore, the FID signal is digitally processed and imaged as a frequency shifted by a frequency corresponding to α from the highest frequency.
Compared to the conventional method shown in FIG. 4 (b), the unnecessary transverse magnetization vector 41 generated by the first π / 2 pulse in the conventional method is only reversed in direction every time the echo signal is measured. Become.

  Finally, an image is reconstructed from the signal obtained by the measurement as described above (step 206). FIG. 5 shows an example of an image obtained by image reconstruction. However, FIG. 5 shows the phase of the FID signal for each phase encoding and the image signal profile along the frequency encoding axis. FIG. 5 (a) shows an example of the image obtained by image reconstruction from the phase of the FID signal obtained by the conventional method of inverting the phase of the π / 2 pulse and the signal obtained by such measurement. . FIG. 5B shows an example in which an off-center process is further applied to FIG. FIG. 5 (c) shows the phase of the FID signal obtained by adding the phase rotation amount according to the offset amount to the phase of the π / 2 pulse and the measurement phase of the present invention, and the signal obtained by such measurement. An example of an image obtained by image reconstruction from FIG.

  As shown in FIG. 5 (a), when a component having phase information (501) that has only changed by π for each phase encoding is Fourier transformed, an image is formed on the highest frequency in the phase encoding direction of the image (502). On the other hand, as shown in FIG. 5 (b), when off-center measurement is performed, the imaging object may also form an image on the highest frequency (503). At this time, the FID signal is imaged on the highest frequency in the phase encoding direction of the image (504) as in FIG. 5 (a). When off-center reconstruction is performed, the imaging object is subjected to off-center processing and moves to the center of the image (505). Similarly, since the off-center processing is performed on the FID signal, the FID signal, that is, the linear artifact is mixed in the diagnosis area (506).

  On the other hand, the FID signal (507) having the phase information of FIG. 5 (c) already has the same phase information as the off-center reconstruction in addition to the phase information of FIG. Then, an image is formed at a position shifted from the highest frequency by an off-center frequency (508). Thereafter, since off-center reconstruction is performed, the imaging target is always moved to the center, and the linear artifact is always moved to the end of the image (509). Thus, since the shift α from the highest frequency of the FID signal corresponds to the shift amount of the FOV in the phase encoding direction, the linear artifact is shifted and arranged in the phase encoding direction by the shift amount. On the other hand, the imaged subject is also shifted by the same amount. Therefore, in the image finally obtained by performing the shift process, the linear artifact is arranged at the extreme end in the phase encoding direction of the image and does not affect the subject image.

  The above has specifically described one embodiment of the present invention in the case where the initial phase φ = 0, but when φ is not 0 (zero), the same is true if all the above specific examples are rotated by φ. Therefore, even when φ is not 0 (zero), the present invention can be implemented in the same manner as when φ = 0.

1 is a block diagram showing an entire MRI apparatus to which the present invention is applied. The figure which shows the control flow by the control apparatus (CPU) of the MRI apparatus of this invention. Explanatory drawing which shows the pulse sequence of SE sequence which the MRI apparatus of this invention employ | adopts, and the echo signal which generate | occur | produces. It is a figure explaining the behavior of the magnetization vector in spin echo signal measurement, (a) shows the method by the MRI apparatus of this invention, (b) shows the conventional method. The figure which shows the FID phase and image at the time of carrying out off-center FOV measurement with the MRI apparatus by this invention.

Explanation of symbols

1 Central processing unit (control means)
2 sequencer 3 transmission system 4 static magnetic field generating magnet 5 reception system 6 signal processing system (image processing means)
7 Subject 12, 13 Gradient magnetic field generation system

Claims (3)

Static magnetic field generating means for generating a static magnetic field, transmitting means for applying a high-frequency electromagnetic wave to a subject placed in the static magnetic field, gradient magnetic field generating means for giving a magnetic field gradient to the static magnetic field, generated from the subject Receiving means for measuring the nuclear magnetic resonance signal, control means for controlling these transmitting means, gradient magnetic field generating means and receiving means in accordance with a predetermined pulse sequence, and image processing for performing image processing based on the measured nuclear magnetic resonance co-signal With means ,
The predetermined pulse sequence is a pulse sequence that changes the phase of the excitation pulse and the measurement phase of the nuclear magnetic resonance signal by π when measuring the nuclear magnetic resonance signal by applying an excitation pulse and an inversion pulse as a high-frequency magnetic field. And
The control means obtains a phase control amount (α) corresponding to a shift amount of the center of the measurement region with respect to the center of the static magnetic field, and controls the phase of the excitation pulse and the measurement phase of the nuclear magnetic resonance signal. In the imaging device,
The control means is such that a part of the longitudinal magnetization component that remains without being influenced by the excitation pulse is rotated into transverse magnetization by the inversion pulse, and the FID signal generated by mixing in the first nuclear magnetic resonance signal. In order to shift the generated image artifact, the phase of the excitation pulse is increased by α each time the excitation pulse is applied or phase encoded, and the measurement phase of the first nuclear magnetic resonance signal is decreased by α. Magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 1.
The magnetic resonance imaging apparatus, wherein the control means makes the phase of the inversion pulse constant. The magnetic resonance imaging apparatus according to claim 1 or 2,
The image artifact is a linear artifact,
The control means changes the phase of the excitation pulse and the measurement phase of the first nuclear magnetic resonance signal so as to shift the center of the measurement region to the center of the image and shift the linear artifact to the edge of the image. A magnetic resonance imaging apparatus characterized by controlling.

Images