JP2704156B2 - Magnetic resonance imaging - Google Patents

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance imaging apparatus for acquiring image data in a subject at high speed. 2. Description of the Related Art As is well known, magnetic resonance imaging rotates at a specific frequency when a group of nuclei having a unique magnetic moment is placed in a uniform static magnetic field. This is a technique for visualizing chemical and physical microscopic information of a substance by utilizing the phenomenon of resonantly absorbing the energy of a high-frequency magnetic field. [0003] In this magnetic resonance imaging method, data acquisition time is much longer than in other medical image diagnostic apparatuses such as an ultrasonic diagnostic apparatus and an X-ray CT. Therefore, there is a problem that an artifact is caused by a movement of the subject such as respiration, and it is difficult to visualize a moving heart or a vascular system. Also,
Since the imaging time is long, the pain given to the subject is great. Therefore, as a method of reconstructing an image at high speed in a magnetic resonance imaging method, an echo planar method by Mansfield, a fast Fourier method by Hutcison et al., And the like have been proposed. FIG. 3 shows a pulse sequence for acquiring image data by the echo planar method, in which a 90 ° high frequency pulse for selective excitation is applied as a high frequency magnetic field RF, and at the same time, a slice gradient magnetic field Gs is applied. After selectively exciting the magnetization in the slice plane by 180
° After applying the high-frequency pulse, the readout gradient magnetic field Gr is switched and applied a plurality of times at high speed in a direction parallel to the slice plane, and at the same time, parallel to the slice gradient magnetic field Gs and orthogonal to the readout gradient magnetic field Gr. The gradient magnetic field for phase encoding Ge is statically applied in the direction in which the phase encoding is performed. On the other hand, the fast Fourier method differs from the echo planar method in that the gradient magnetic field Ge for phase encoding is applied in a pulsed manner every time the gradient magnetic field Gr for reading is inverted. According to these methods, the readout gradient magnetic field is switched at a high speed within a time period in which the magnetization in the slice plane excited by the 90 ° high-frequency pulse is relaxed by the relaxation phenomenon of the transverse magnetization. , An echo signal (multi-echo) is generated, and image data of a slice plane can be collected, thereby enabling high-speed imaging. In order to perform correct image reconstruction in such high-speed imaging, the peak positions of the echo signals must be equally spaced. If the intervals are not equal, the echo signal cannot be sampled correctly and collected,
This is because a correct reconstructed image cannot be obtained. In the peak of the echo signal, the phase of the signal is zero, that is, the area Sia of the positive waveform and the area Si-1b (1 = 1, 2,... N) of the negative waveform of the read gradient magnetic field Gr in FIG. Since it occurs at time, if the switching waveform of the read gradient magnetic field Gr is an ideal rectangular wave as shown by a solid line,
The peak position of the echo signal is an intermediate position between positive and negative periods like Tp1, Tp2,... Tpn, and the interval is constant. However, in practice, the switching waveform of the read gradient magnetic field Gr is an ideal rectangular wave due to the inductance of the gradient magnetic field generating coil and the eddy current induced in the metal body near the gradient magnetic field generating coil. However, the waveform becomes a rising and falling as shown by a broken line in FIG. Further, the read gradient magnetic field Gr is a positive / negative amplitude value G due to the drive power supply of the gradient magnetic field generating coil.
_+, G _- is the difference or, even offset, or the like. Due to the rounding of the waveform of the read gradient magnetic field Gr, the positive / negative amplitude difference, the offset, and the like, the peak position of the echo signal becomes a regular position Tp1, Δτ2,.
Tp2,... Tpn, resulting in non-uniform spacing. Alternatively, the multi-echo disappears. Therefore, conventionally, the switching timing of the read gradient magnetic field, the positive / negative amplitude, the offset, and the like have been adjusted so that the peak position of the echo signal is at a constant interval. Requires a lot of work. As described above, according to the conventional high-speed imaging method, the waveform of the readout gradient magnetic field is rounded,
In order to perform correct image reconstruction with respect to the positive / negative amplitude difference, offset, and the like, there is a problem that complicated system adjustment is required to make the interval between the peak positions of the echo signals constant. An object of the present invention is to provide a magnetic resonance imaging apparatus capable of obtaining a good reconstructed image without requiring complicated system adjustment in high-speed imaging. According to the present invention, a readout is performed after a predetermined slice plane is excited by applying a high-frequency magnetic field and a gradient magnetic field for slicing to a subject placed in a uniform static magnetic field. A plurality of first echo signals based on magnetic resonance necessary for image reconstruction of a slice plane are collected by applying a gradient magnetic field for phase encoding and applying a gradient magnetic field for phase encoding in a direction orthogonal to the read gradient magnetic field. In the magnetic resonance imaging apparatus for performing image reconstruction based on the sampled data, information on the peak positions of a plurality of second echo signals obtained without applying a phase encoding gradient magnetic field is detected. The readout gradient magnetic field is controlled so that the interval between the peak positions of adjacent first echo signals is substantially constant based on the information on the peak positions. It was done. When a sequence for high-speed imaging is performed without applying a phase encoding gradient magnetic field,
Since a plurality of second echo signals having similar waveforms are obtained each time the switching is performed by switching the read gradient magnetic field, this second echo signal is obtained.
The information on the peak position of the eco-signal is easily detected.
Based on the detected information on the peak position of the second echo signal, feedback is applied to, for example, the drive source of the gradient magnetic field generating coil, and the switching timing of the read gradient magnetic field, positive / negative amplitude value, offset, etc. are automatically determined. If adjusted, the interval between the peaks of the first echo signal will be substantially constant. FIG. 1 is a diagram showing a configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention. In FIG. 1, a static magnetic field magnet 1 and a gradient magnetic field generating coil 3 are driven by an excitation power supply 2 and a drive circuit 4 controlled by a system controller 10, respectively. Such a static magnetic field and a gradient magnetic field whose magnetic field intensity changes in two orthogonal directions x and y in a desired cross section (slice plane) of interest and in a z direction perpendicular thereto are applied. In the present embodiment, the gradient magnetic field applied in the z direction is hereinafter referred to as a slice gradient magnetic field Gs,
The gradient magnetic field applied in the X direction is read gradient magnetic field Gr, y
The gradient magnetic field applied in the direction is a gradient magnetic field G for phase encoding.
This will be described as e. Under the control of the system controller 10, a high-frequency magnetic field generated from the probe 7 by a high-frequency signal from the transmission unit 8 is applied to the subject 5. In the present embodiment, the probe 7 is commonly used as a transmission coil for generating a high-frequency magnetic field and a reception coil for receiving magnetic resonance signals related to various nuclei in the subject 5. They may be provided separately. The magnetic resonance signal (echo signal) received by the probe 7 is amplified and detected by the receiving unit 9 and then sent to the data collecting unit 11 under the control of the system controller 10. The data collection unit 11 collects the magnetic resonance signals taken out via the reception unit 9 under the control of the system controller 10, samples them by an A / D converter, digitizes them, and sends them to the computer 12. . The computer 12 is controlled by the console 13 and performs image reconstruction processing by Fourier transform on the sampling data of the echo signal input from the data collection unit 11 to obtain image data. The computer 12 also controls the system controller 10. The image data obtained by the electronic computer 12 is supplied to the image display 14 and displayed as an image. A pulse sequence for acquiring image data of a slice plane in the subject 5 according to the present invention is shown in FIG.
The echo planar method or the fast Fourier method shown in FIG. This pulse sequence is controlled by the system controller 10. In this embodiment, before the pulse sequence of FIG. 4 is executed, the high-frequency magnetic field RF, the slice gradient magnetic field Gs, and the read gradient magnetic field Gr are applied as shown in FIG. A pulse sequence without applying the gradient magnetic field Ge is performed. In this case, as the echo signal, Sig. As shown by ′, similar signals whose amplitudes attenuate with the time constant of T ₂ ^* are obtained. Note that T ₂ ^* is a lateral relaxation time in consideration of the non-uniformity of the static magnetic field strength. Then, information on the peak positions (positions on the time axis) of these echo signals is detected. The detection of the peak position information may be performed by software processing in the computer 12 or may be performed by hardware in the data collection unit 11 or the reception unit 9. Since the waveform of the echo signal obtained in the state where the phase encoding gradient magnetic field Ge is not applied has one peak, it is easy to detect the information of the peak position. Information on the peak position of the echo signal detected in this way is stored in a memory array or the like in the computer 12. Next, when the slice plane is actually imaged using the pulse sequence shown in FIG. 3, sampling data of the echo signal is taken more than the number of data necessary for image reconstruction (converted to time width). Then, of these sampling data, image reconstruction is performed using sampling data of a period of ± Tb (Tb <Ta) with the peak position of the stored echo signal as a center. In this way, the readout gradient magnetic field Gr
Even if the peak position of the echo signal is displaced due to the rounding of the waveform or the like, correct image reconstruction can always be performed using the sampling data of the echo signal centered on the peak position. In this case, complicated system adjustment for the readout gradient magnetic field Gr required conventionally is not required. On the other hand, even if the above-mentioned method is used, the deviation of the peak position of the echo signal from the normal position is so large that data necessary for image reconstruction cannot be secured with extra sampling data, If it is necessary to reduce the number of extra sampling data due to time restrictions, the peak position itself of the echo signal is corrected using feedback control by a hardware method according to the present invention. That is, before executing the pulse sequence of FIG. 3, the gradient magnetic field Ge for phase encoding in FIG.
FIG. 2 shows a pulse sequence in which Si is not applied.
g. 'Are obtained, and information on the peak positions of these echo signals is detected. In FIG. 2, the deviation of the peak position of the detected echo signal from the normal position is ΔTi (1
= 1, 2,... N). Assume that the positive amplitude of the read gradient magnetic field Gr is G ₊ , the negative amplitude is G _−, and the inversion timing accompanying the switching is τi. As described above, the position where the peak of the echo signal occurs is determined by the area Sia of the positive waveform of the read gradient magnetic field Gr and the area Sia of the negative waveform.
This is the time when i-1b (i = 1, 2,... n) becomes equal. To satisfy the condition of Sia = Si-1b, τi or G ₊ ,
G _- or offset may be adjusted. As a specific example, G ₊ and G _- are all the same for each echo signal,
A case where only the switching timing τi is adjusted will be described. The peak of the i-th echo signal has a shift of ΔTi, and it is assumed that correction is performed by setting this to (τi + Δti). Assuming that the peak of the i-th echo signal has a positive amplitude, −Δτi · G ₊ −Δti · G ₊ = Δti · G ₋ + Δτi−
Than 1 · G, Δti = one _{(Δτi · G + + Δτi-} 1 · G -) / (G +
+ G _-) to become. Therefore, the value of Δti may be calculated by the computer 12 and the operation timing of the pulse sequencer in the system controller 10 for controlling the readout gradient magnetic field Gr may be controlled. Ideally, the deviation of Δτi can be eliminated by one control, but if Δτi is large, the same control may be repeated several times. The present invention can be variously modified in addition to the above embodiment. In the above embodiment, the switching timing of the read gradient magnetic field Gr is controlled. However, the positive and negative amplitudes G ₊ , G ₋ , offset, and the like may be controlled, and two or more of these three parameters are simultaneously controlled. Is also good. The pulse sequence used in the present invention is not limited to the echo planar method, but may be any pulse sequence for high-speed imaging in which a read gradient magnetic field is switched and applied. According to the present invention, information on the peak position of an echo signal is detected by performing a sequence for high-speed imaging without applying a gradient magnetic field for phase encoding, and the peak of the detected echo signal is detected. By controlling the switching timing of the readout gradient magnetic field, the positive / negative amplitude value, the offset, and the like based on the position to keep the interval between the peaks of the echo signal constant, high-speed imaging does not require complicated system adjustment. Thus, a correct reconstructed image can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention. FIG. 2 is obtained by a high-speed imaging sequence without applying a gradient magnetic field for phase encoding in the present invention. FIG. 3 is a diagram showing a relationship between a read echo signal and a read gradient magnetic field. FIG. 3 is a diagram showing a pulse sequence of an echo planar method which is one of high-speed imaging techniques used in the present invention. 2 Excitation power supply 3 Gradient magnetic field generating coil 4 Drive circuit 5 Subject 6 Bed 7 Probe 8 Transmitter 9 Receiver 10 System controller 11 Data collector 12 Computer 13 Console 14 … Image display

Claims (1)

(57) [Claims] A high-frequency magnetic field and a gradient magnetic field for slicing are applied to a subject placed in a uniform static magnetic field to excite a predetermined slice plane, and then a gradient magnetic field for reading is applied, and a direction perpendicular to the gradient magnetic field for reading is applied. Data collecting means for collecting and sampling a plurality of first echo signals based on magnetic resonance necessary for image reconstruction of a slice plane by applying a gradient magnetic field for phase encoding to the slice data, and sampling data obtained by the means. Image reconstruction means for performing image reconstruction on the basis of: a means for detecting information on the peak positions of a plurality of second echo signals obtained without applying the phase-encoding gradient magnetic field; A control means for controlling the readout gradient break based on the information on the detected peak position so that the interval between adjacent peak positions is substantially constant. Magnetic resonance imaging apparatus characterized by comprising and. 2. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit changes at least one of a switching timing of the read gradient magnetic field, a positive / negative amplitude value, and an offset.