JP2010167143A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an MRI apparatus capable of highly precisely adjusting echo peak shift due to response delay or the like in a short period of time without a necessity of adjusting an echo peak detection delay for every sequence. <P>SOLUTION: A sequencer control section 77 actuates a sequencer 6 at a plurality of slew rates and gradient magnetic field intensities set by a slew rate/gradient magnetic field intensity setting section 74. A time deviation measurement section 71 measures a time deviation between an echo peak and an AD sampling timing, a delay time calculation section 72 calculates a delay time, a graph creation section 73 creates a graph indicating their relationship and a storage section 80 stores it. A delay time readout section 75 reads out the delay time corresponding to the slew rate and the gradient magnetic field intensity to be executed from the storage section 80, a compensation pulse calculation section 76 calculates a compensation pulse, and the sequencer control section 77 feeds a command signal to the sequencer 6 by taking account of the delay in the system response and a deviation due to an error between gradient magnetic field areas. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging apparatus using a nuclear magnetic resonance phenomenon.

  As is well known in imaging by a magnetic resonance imaging apparatus (MRI apparatus), a specific slice cross-section is excited by irradiating a radio frequency pulse (RF) at a predetermined timing while applying a gradient magnetic field in the slice direction. Echo signals obtained by two-dimensional frequency modulation while applying a gradient magnetic field in two directions of phase and frequency are collected by AD sampling at a predetermined timing, and an MR image is created by image reconstruction by Fourier transform To do.

  The RF irradiation must be performed at a timing when the time integral value (area) of the slice gradient magnetic field strength becomes a predetermined value, and AD sampling is performed so that the area of the gradient magnetic field (read gradient magnetic field) strength in the frequency direction becomes a predetermined value. It must be executed at the timing.

  However, in an actual system, it is often not executed at an ideal timing due to a time delay due to a system response or distortion of a gradient magnetic field waveform.

  For this reason, the difference in the gradient magnetic field distortion according to the time delay of the system response, the polarity of the gradient magnetic field, and the rise / fall is corrected with high accuracy (for example, described in Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2). Technology) is performed independently only for the sequence in which it is required.

JP 2003-135416 A Referenceless Interleaved Echo-Planar Imaging ”: Scott B. Reeder, Magnetic Resonance in Medicine 41: 87-94 (1999) Reduction of A New Nyquist Ghost in Oblique Echo Planar Imaging: Proc. ISMRM 4th Annual Meeting, New York, 1996, p. 1477.

  In the MRI apparatus, the number of sequences that need to be adjusted such as a time delay due to the system response is increasing as the SNR is improved and the functions are increased with the recent increase in the magnetic field.

  However, in the MRI apparatus preparation in the prior art, a technique for correcting the deviation of the echo peak due to the gradient magnetic field polarity, slew rate, and distortion according to the rise and fall with high accuracy and in a short time has been established. However, adjustments are made for each sequence if necessary.

  An object of the present invention is to realize an MRI apparatus capable of adjusting an echo peak shift due to a delay in system response or the like with high accuracy and in a short time.

  In order to achieve the above object, the present invention is configured as follows.

  The magnetic resonance imaging apparatus of the present invention copes with a time lag between a signal peak of a magnetic resonance signal measured in advance and a plurality of gradient magnetic field strengths and a plurality of slew rates, and a reception timing of the magnetic resonance signal of the receiving means. Storage means for storing the gradient data for each gradient magnetic field strength and slew rate, reading the time shift corresponding to the gradient magnetic field strength and slew rate used for imaging from the storage means, and based on the read time shift The gradient magnetic field application timing and the gradient magnetic field area are adjusted.

  It is possible to realize an MRI apparatus capable of adjusting an echo peak shift due to a delay in system response or the like with high accuracy and in a short time.

1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. It is a functional block diagram of CPU shown in FIG. 1, and is a diagram showing functions related to the main part of one embodiment of the present invention. It is a figure which shows the sequence for echo peak delay measurement in one Embodiment of this invention. It is a whole operation | movement flowchart of echo peak delay measurement. It is a flowchart about the echo peak delay measurement regarding an X-axis. It is a figure which shows the Delay table which memorize | stores each of an X-axis, a Y-axis, and a Z-axis about the delay time for every slew rate and gradient magnetic field intensity. It is a figure which shows the example of the correction pulse addition in the Gradient Echo sequence to which one Embodiment of this invention is applied. It is a figure which shows the example of the correction pulse addition in 2 echo Gradient Echo sequence to which one Embodiment of this invention is applied. It is a figure which shows the example of the correction pulse addition in the gradient echo sequence of 3 echoes or more to which one Embodiment of this invention is applied. It is a figure which shows the example of the correction pulse addition in two RF subpulse excitation sequences to which one Embodiment of this invention is applied. It is a figure which shows the example of the correction | amendment pulse in the 3 or more RF subpulse excitation sequence to which one Embodiment of this invention is applied. It is explanatory drawing of the shift | offset | difference of the echo center in a Multi Echo sequence. It is explanatory drawing of the shift | offset | difference of the area error echo center in a some RF subpulse excitation sequence.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  Prior to the description of the embodiment, the occurrence of a shift in the echo peak (magnetic resonance signal peak) due to the response delay of the system will be described.

  For example, FIG. 12 shows an example of a sequence in which a plurality of echoes are continuously collected while the polarity of the readout gradient magnetic field as represented by the echo planar method is positively inverted. As shown in FIG. 12 (a), the peak position of the first echo measured by the first readout gradient magnetic field depends on the time delay of the system response, and the sampling timing of the A / D converter (the reception timing of the receiving means). ) With respect to delay 1 may occur.

  Further, as shown in FIG. 12B, the peak position of the first echo measured by the first readout gradient magnetic field is a factor such as eddy current, limit of hardware control, and rounding error due to digital processing of software. Thus, when a mismatch occurs between the dephasing gradient magnetic field area A and the readout gradient magnetic field area B in the drawing, there may be a time lag of delay2 with respect to the AD sampling timing. That is, in the first echo, the timing of the echo peak is shifted in time due to the shift in the timing of applying the gradient magnetic field and the shift in area.

  Next, as shown in FIG. 12 (c), the second echo measured by the second readout gradient magnetic field has a peak position with respect to the AD timing due to the time delay of the system response, like the first echo. There may be a shift by delay1. As shown in FIG. 12 (d), the second echo measured with the second readout gradient magnetic field is the positive gradient magnetic field area of B and C in the figure as in the first echo. And the sum of the negative gradient magnetic field areas of A and D may not be the same as the AD sampling timing. That is, also in this case, if the time lag and the area lag of application of the gradient magnetic field are not adjusted, the peak position of the echo is shifted in time.

  Next, FIG. 13 shows an example of a technique in which RF excitation is performed simultaneously with a plurality of RF subpulses while the polarity of the slice gradient magnetic field represented by the water selective excitation method is forward / reverse. In this method, it is necessary that the echo generated by the first RF is dephased by the gradient magnetic field of C and rephased by the gradient magnetic field of D, so that the magnetization phase is refocused at the excitation timing of the second RF pulse. is there.

  For this purpose, the areas of C and D need to match, but if they do not match, the peak position of the echo generated by the first RF is shifted in time by delay 4 with respect to the excitation timing of the second RF pulse. Observed in the state. Since the difference between the RF and AD sampling timings is considered to be the same (or adjusted in advance), the areas C and D shown in FIG. 13 and the lead-out areas C and D shown in FIG. Can be considered the same.

  In addition, delays 1 to 4 shown in FIGS. 12 and 13 may change depending on the gradient magnetic field strength and the slew rate. Since these delays 1 to 4 do not change depending on the subject, system adjustment is performed by measurement using a phantom.

  The present invention measures the above delays 1 to 4 in advance using a phantom, stores data applicable to various sequences, and uses the stored data regardless of sequence changes. Measurement of the delay time (delays 1 to 4) for each sequence change is not necessary, and the entire imaging time of the subject can be shortened.

  Hereinafter, embodiments of the present invention will be specifically described.

  FIG. 1 is an overall schematic configuration diagram of an MRI apparatus to which an embodiment of the present invention is applied. As shown in FIG. 1, the MRI apparatus includes a static magnetic field generation magnetic circuit 1, a gradient magnetic field generation system 2, a transmission system 3, a reception system 4, a signal processing system 5, a sequencer 6, a central processing unit ( CPU) 7 and an operation unit 8.

  The static magnetic field generating magnetic circuit 1 generates a uniform static magnetic field around the subject 9 in the direction of the body axis or in a direction perpendicular to the body axis. The static magnetic field generating magnetic circuit 1 is permanent in a space having a certain extent around the subject 9. Magnetic, normal conducting, or superconducting magnetic field generating means are arranged. The gradient magnetic field generation system 2 includes a gradient magnetic field coil 10 wound in three axial directions of X, Y, and Z, and a gradient magnetic field power supply 11 that supplies electric power for driving each coil. Then, the gradient magnetic field power supply 11 of each coil is driven in accordance with a command from the sequencer 6 described later, thereby applying gradient magnetic fields Gs, Gp, and Gf in three axis directions of X, Y, and Z to the subject 9.

  By applying this gradient magnetic field, the slice plane for the subject 9 can be set. The transmission system 3 irradiates a high-frequency signal in order to cause nuclear magnetic resonance to occur in atomic nuclei constituting the living tissue of the subject 9 by a high-frequency magnetic field pulse sent from a sequencer 6 to be described later. A modulator 13, a high-frequency amplifier 14, and a high-frequency coil 15 on the transmission side.

  The high-frequency pulse output from the high-frequency oscillator 12 is amplified by the high-frequency amplifier 14 and then supplied to the reception-side high-frequency coil 16 disposed close to the subject 9, so that the subject 9 is irradiated with electromagnetic waves. Is done.

  The receiving system 4 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nucleus of the living tissue of the subject 9. The receiving system 4 receives a high-frequency coil 16 on the receiving side, an amplifier 17, and a quadrature detector 18. And an A / D converter 19. The response electromagnetic wave (NMR signal) of the subject 9 due to the electromagnetic wave irradiated from the high-frequency coil 15 on the transmission side is detected by the high-frequency coil 16 disposed close to the subject 9, and the amplifier 17 and the quadrature detector 18 are connected. To the A / D converter 19 and converted into a digital quantity. Then, two series of collected data sampled by the quadrature detector 18 at a timing according to a command from the sequencer 6, and the signal is sent to the signal processing system 5.

  The signal processing system 5 performs image reconstruction calculation using the echo signal detected by the reception system 4 and displays an image. The signal processing system 5 includes a CPU 7 that performs processing such as Fourier transform, correction coefficient calculation, image reconstruction, and control of the sequencer 6 on the echo signal, a program that performs image analysis processing and measurement over time, and its execution. A ROM (read only memory) 20 for storing invariant parameters to be used, a measurement parameter obtained in the previous measurement, an echo signal detected by the receiving system 4, and an image used for setting the region of interest are temporarily stored and the region of interest is stored. And a RAM (anytime read / write memory) 21 for storing parameters for setting and the like.

  Further, the signal processing system 5 receives the magneto-optical disk 22 and the magnetic disk 23 serving as a data storage unit for recording the image data reconstructed by the CPU 7 and the image data read from the magneto-optical disk 22 or the magnetic disk 23. And a display 24 serving as a display unit that visualizes and displays as a tomographic image.

  The sequencer 6 is a control unit that repeatedly applies a high-frequency magnetic field pulse that causes nuclear magnetic resonance to the nuclei constituting the living tissue of the subject 9 in a predetermined pulse sequence, and operates under the control of the CPU 7. Various commands necessary for collecting tomographic image data are sent to the transmission system 3, the gradient magnetic field generation system 2, and the reception system 4.

  The operation unit 8 is used to input control information for processing performed by the signal processing system 5, and includes a mouse 25 and a keyboard 26.

  FIG. 2 is a functional block diagram of the CPU 7 shown in FIG. 1, and is a diagram showing functions related to the main part of one embodiment of the present invention.

  In FIG. 2, the CPU 7 calculates the peak position of the echo signal and the sampling time of the A / D converter 19 from the signal waveform input from the A / D converter 19 and the sampling timing of the A / D converter 19. A time shift measuring unit 71 for measuring the shift, a delay time calculating unit 72 for calculating a delay time due to a system response delay and a delay time due to an area shift based on the time shift measured by the time shift measuring unit 71, Based on outputs from the slew rate / gradient magnetic field strength setting unit 74 for setting the slew rate and the gradient magnetic field strength, the delay time calculation unit 72 and the slew rate / gradient magnetic field strength setting unit 74, the delay time / slew rate is set. A delay time for creating a gradient magnetic field graph, a slew rate, and a gradient magnetic field strength graph creation unit 73.The graph created by the delay time / slew rate / gradient magnetic field strength graph creation unit 73 is stored in the storage unit 80 (RAM 21, magneto-optical disk 22, magnetic disk 24).

  Further, the CPU 7 reads the delay time based on the graph stored in the storage unit 80 and the delay time / slew rate / gradient magnetic field strength graph output unit 73 based on the slew rate and the gradient magnetic field strength. 75, a correction pulse calculation unit 76 for calculating a correction pulse based on the delay time from the delay time reading unit 75, and the correction pulse and the delay time output from the correction pulse calculation unit 76 and the delay time reading unit 75. And a sequencer control unit 77 for outputting the command signal thus followed to the sequencer 6.

  Next, an imaging sequence for measuring echo peak deviation (delay measurement) will be described with reference to FIG. The adjustment using this sequence may be executed only once, for example, using a NiCl aqueous solution phantom at the time of installing the MRI apparatus at the installation location.

  In FIG. 3, the first RF pulse 101 and the slice selective gradient magnetic field 102 are applied to the subject 9 to excite the proton magnetization, and the slice rephase gradient magnetic field 103 performs the rephasing in the slice direction. A directional dephase gradient magnetic field 104 is applied, and then a readout gradient magnetic field is launched at a predetermined slew rate and intensity, and echoes 106 are collected between the readout gradient magnetic fields 105.

  Here, a time shift 107 between the time when the peak of the collected echo is measured and the center 108 of the AD window (sampling time of the A / D converter) is measured as dT1. The time lag between the peak time of the echo and the AD window center 108 can be measured as a first-order gradient of the phase in real space by performing Fourier transform on the acquired echo signal.

  Next, following the readout gradient magnetic field 105, a readout gradient magnetic field 109 with the gradient magnetic field polarity reversed is applied, and the echo 110 is collected. Here, a time shift 111 between the peak time of the echo 110 and the AD window center 112 (sampling time of the A / D converter) is measured as dT2. As described above, the time lag between the echo peak position and the AD window center 112 can also be observed as a primary phase gradient in real space. After applying the readout gradient magnetic field, crusher gradient magnetic fields 113 and 114 are applied to disperse the magnetization phase.

  Next, while irradiating the second RF pulse 115, the slice selection gradient magnetic field 116 having the same slew rate and gradient magnetic field strength as the readout gradient magnetic field 105 is started up. A slice gradient magnetic field 117 having the same polarity and opposite polarity to that of the slice selection gradient magnetic field 116 is applied, and the echo 118 is collected. At the same time, AD conversion by the A / D converter 19 is executed, and a time shift 119 between the peak time of the echo 118 and the center 120 of the AD window is measured as dT3.

  In the example shown in FIG. 3, the slice selection gradient magnetic field 102 of the first sequence is applied to the Gx axis, the polarity reversal gradient magnetic fields 109 and 117 of the first and second sequences are applied to the Gy axis, and what is applied to the Gz axis. FIG. 3 also shows a sequence in which the relationship between these three axes is changed, and the axis for applying the polarity reversal gradient magnetic field in the first and second sequences is set to the Gy and Gz axes in addition to Gx. Do the same as the example.

  Further, in order to measure the change of the echo peak position depending on the strength of the polarity reversal gradient magnetic field and the slew rate, the strength and slew rate of the four gradient magnetic fields 105, 109, 116, and 117 shown in FIG. Measure echo peak while changing.

  The flow of the above process is shown in FIG. The processing flow is composed of three loops: an adjustment axis switching loop, a polarity reversal gradient magnetic field loop, and a slew rate loop.

  When the process is started in step S1 of FIG. 4, the sequencer control unit 77 sets the adjustment axis to the first axis Axis # 1 (for example, the X axis) and sets the polarity reversal gradient magnetic field to G # 1 (Gx ) (Steps S2 and S3). Then, the slew rate / gradient magnetic field strength setting unit 74 sets the slew rate to Slew # 1 (step S4).

  Then, the time lag measuring unit 71 measures the time lags dT1, dT2, and dT3 (step S5), and the delay time calculating unit 72 calculates the delay times delay1, delay2, and delay3 based on formulas described later (step S6). .

  Next, the sequencer control unit 77 determines whether or not the slew rate number is a predetermined value, and if not, the sequencer control unit 77 increments the slew rate number and returns to step S5 (steps S7 and S8).

  If it is determined in step S7 that the slew rate number is a predetermined value, the process proceeds to step S9, and the sequencer control unit 77 determines whether or not the current gradient magnetic field strength number is a predetermined value. If not, the gradient magnetic field strength number is incremented and the process returns to step S4 (steps S9 and S11).

  If the gradient magnetic field strength number is a predetermined value in step S9, the process proceeds to step S10, and the sequencer controller 77 determines whether or not the current adjustment axis is a predetermined adjustment axis number.

  If it is a predetermined adjustment axis number, the process is terminated, and if it is not a predetermined adjustment axis number, the sequence control unit 77 increments the adjustment axis number and returns to step S3 (steps S10 and S12).

  FIG. 5 is an operation flowchart when performing delay measurement with 10 patterns of slew rate (Slew 1 to Slew 10) and 10 patterns of polarity reversal gradient magnetic field strength (G1 to G10) with respect to the X axis.

First, the adjustment axis is set to the X axis, the gradient magnetic field strength is set to G1, the slew rate is set to Slew1, and the three time shifts (dT1-x-g1-s1, dT2-x-g1-s1, dT3) shown in FIG. -X-g1-s1) is measured (steps S20 to S23).
However, (-x-g1-s1) means that the adjustment axis is the X axis, the gradient magnetic field strength is g1, and the slew rate is s1.

  Thereafter, in order to separate the time lag of each echo into elements caused by the system response and the gradient magnetic field area error, in step S24, delay times Delay1-x-g1-s1, Delay2-x-g1-s1, Delay3-x- g1-s1 is calculated (the calculation method will be described later).

Subsequently, a polarity reversal gradient magnetic field having a gradient magnetic field strength G1 and a slew rate Slew2 is applied to the X axis, and three time shifts (dT1-x-g1-s2, dT2-x-g1-s2, dT3- x-g1-s2) is measured (steps S25 and S26).
However, (-x-g1-s2) means that the adjustment axis is the X axis, the gradient magnetic field strength is g1, and the slew rate is s2.

  Thereafter, in order to separate the time lag of each echo into elements due to the system response and the gradient magnetic field area error, in step S27, delay times Delay1-x-g1-s2, Delay2-x-g1-s2, Delay3-x- g1-s2 is calculated.

  In the same manner as described above, the slew rate is changed from Slew 1 to Slew 10 to measure the time lag (steps S28, S29, S30). G2 is also measured for each slew rate in the same manner.

  Then, by the processing of steps S21 to S41, the time lag is measured by changing the gradient magnetic field strength from G1 to G10, and the slew rate is changed from Slew1 to Slew10 to measure the time lag.

  When adjusting the Y-axis and the Z-axis in the same manner as the X-axis, the process further moves to the upper loop, and in step S20, the adjustment axis is switched between the Y-axis and the Z-axis to measure the time lag. . That is, in this case, a total of 300 patterns (10 · 10 · 3) are measured.

  The time required for measuring the time lag is determined by the sum of the TR (repetition time) of the first sequence and the second sequence shown in FIG. 3 and the number of repetition patterns, and the sum of the first and second sequences is several tens of ms. Therefore, even if several hundreds of patterns are repeated, the time required for the entire process is several seconds, and a long time is not required.

  Next, calculation is performed using the measured hundreds of patterns of dT1, dT2, and dT3 to separate into elements of time delay due to system response and time delay due to gradient magnetic field area error. This calculation is executed by the delay time / slew rate gradient magnetic field strength graph creation unit 73.

  As shown in FIG. 12, since the first echo is determined by delay 1 due to the time delay of the system and delay 2 due to the A and B area errors in FIG. 12, dT1 is expressed by the following equation (1) using delay 1 and delay 2. It appears like

dT1 = delay1 + delay2 (1)
Further, dT2 of the second echo is determined by the time delay delay1 of the system, the time delay delay2 due to the area error of A and B shown in FIG. 12, and the delay3 which is the time delay due to the area error of C and D shown in FIG. appear. However, since the second echo inverts AD, each delay is inverted in time and is expressed by the following equation (2).

dT2 = -delay1-delay2-delay3 (2)
Next, since the third echo generated by the second RF pulse is generated by the time delay due to the system response and the time delay due to the area error of C and D in FIG. 13, it can be expressed by the following equation (3).

dT3 = delay1 + delay3 (3)
Therefore, delay1, delay2, and delay3 can be calculated as the following equations (4), (5), and (6) by solving the simultaneous equations of the above equations (1), (2), and (3). . Note that delay1 delays the system response.

delay1 = dT1 + dT2 + dT3 (4)
delay2 = − (dT2 + dT3) (5)
delay3 = − (dT1 + dT2) (6)
From the above, delay1 to delay3, which are echo peak generating elements, can be obtained from each slew rate and gradient magnetic field strength, and from these, a delay table as shown in FIG. 6 can be created. The delay table is stored in the storage unit 80.

  The delay is a discrete value measured with a discrete gradient magnetic field strength and slew rate, but can be stored in a table after interpolating an intermediate value using linear processing or an approximate curve.

  By using the delay table stored in the storage unit 80, for all sequences, the time delay due to the system response, the error between the dephase gradient magnetic field area, the area on the rising side of the gradient magnetic field, and the rise of the gradient magnetic field It is possible to adjust the gradient magnetic field area error between falling and rising.

  That is, the delay time reading unit 75 reads the delay time stored in the storage unit 80 in accordance with the slew rate and the gradient magnetic field strength set by the slew rate / gradient magnetic field strength setting unit 74. Is supplied to the unit 77, and delay 2 and delay 3 are supplied to the correction pulse calculation unit 76. Here, the graph stored in the storage unit 80 can also be displayed on the display 23.

  The correction pulse calculation unit 76 calculates a correction pulse using the magnetic field intensity area, delay 2 and delay 3 and supplies the correction pulse to the sequencer control unit 77 as described later. The sequencer control unit 77 controls the operation of the sequencer 6 using Delay 1 supplied from the delay time reading unit 75 and the correction pulse supplied from the correction pulse calculation unit 76.

  Specific adjustment will be described with reference to FIG. 7 by taking, for example, a gradient echo sequence as an example.

  The time delay of the system response calculated by the above equation (4) is advanced in advance by the delay magnetic field application timing G1, Gy, and Gz.

  Since delays 2 and 3 are delays due to area errors, delays 2 and 3 and the gradient magnetic field strength G set by the gradient magnetic field strength setting unit 74 are used again to calculate the phase by using the following equations (7) and (8). An area error dS1 between the lead-out gradient magnetic field and the leading-edge gradient magnetic field area and an area error dS2 between the falling-side gradient magnetic field area and the rising-side gradient magnetic field area are calculated.

dS1 = delay2 · G (7)
dS2 = delay3 · G (8)
A compensation gradient magnetic field pulse 1 having an area of dS1 is applied so as to cancel the dephasing gradient magnetic field of the readout axis and the rising area error using the area error calculated by the above equations (7) and (8). .

  In order to correct the gradient echo sequence of the second echo, as shown in FIG. 8, the time delay of the system response calculated by the above equation (4) is advanced in advance by the gradient magnetic field application timing by delay1, Gx, Gy, and Gz. In addition, the compensation gradient magnetic field pulses 1 and 2 having the areas of dS1 and dS2 are applied so as to cancel the dephasing gradient magnetic field of the readout axis, the area error on the rising side, and the area error on the falling side and the rising side. To do.

  To correct a gradient echo sequence of 3 echoes or more, as in the case of 2 echoes, as shown in FIG. Compensating gradient magnetic field pulses 1 and 2 having areas of dS1 and dS2 are applied so as to cancel the phase gradient magnetic field, the area error on the rising side, and the area error on the falling side and the rising side.

  Further, in the sequence of RF excitation using two RF subpulses, as shown in FIG. 10, the time delay of the system response calculated by the above equation (4) is set to the gradient magnetic field application timing as delay1, Gx, Gy, After being advanced in time by Gz, the compensation gradient magnetic field pulse 2 having the area of dS2 is applied so as to cancel the area error on the rising side and falling side of the slice selection gradient magnetic field.

  A compensation gradient magnetic field pulse 1 having an area of dS1 is applied to the lead-out axis so as to cancel an error between the dephase gradient magnetic field and the area on the rising side of the lead-out gradient magnetic field.

  In the sequence of RF excitation using three or more RF sub-pulses, as shown in FIG. 11, the time delay of the system response calculated by the above equation (4) is preliminarily set to the gradient magnetic field application timing as delay1, Gx, Gy, Gz. Then, a plurality of compensation gradient magnetic field pulses 2 having an area of dS2 are applied so as to cancel the area errors on the rising and falling sides of the slice selection gradient magnetic field.

  A compensation gradient magnetic field pulse 1 having an area of dS1 is applied to the lead-out axis so as to cancel an error between the dephase gradient magnetic field and the area on the rising side of the lead-out gradient magnetic field.

  In the above example, the compensation gradient magnetic field pulse is applied in the physical coordinate system of X, Y, and Z, but can be applied in the logical coordinate system of slice, phase, and frequency. In the above-described correction, as shown in the above-described known example (Non-Patent Document 1), measurement in which slice, phase, and readout axes are distributed over a plurality of physical axes, such as oblique measurement and radial measurement. In FIG. 4, the compensation gradient magnetic field pulse can be applied by calculating the gradient magnetic field strength from the distribution rate.

  As long as the area error of the readout gradient magnetic field is limited, the area error can be uniquely calculated as a Trajectory shift in the k space. Since the delay of the first echo due to the area error of the readout gradient magnetic field is dT1-delay1, the trajectory shift amount dkx1 in the readout direction (X direction) is obtained from the following equation (9).

dkx1 = γ · G · (dT1-delay1) (9)
Further, the Trajectory shift amount dkx2 in the readout direction of the second echo is expressed by the following equation (10) because the delay of the second echo is dT2-delay1.

dkx2 = γ · G · (dT2-delay1) (10)
Here, γ is the rotational magnetic ratio, and G is the readout gradient magnetic field strength.

  Therefore, the k-space is shifted according to the Trajectory shift amount with respect to the acquired k-space data, and when the Matrix size is not sufficiently large with respect to the shift amount, an interpolation process is performed to It is also possible to correct the magnetic field area error.

  As described above, according to the present invention, the time delay between the echo peak and the AD sampling timing due to the slew rate, the gradient magnetic field strength, and the area deviation of the gradient magnetic field is measured in advance, and the measured result is obtained as a slew rate. Values and gradient magnetic field strength values are stored as data for each of the X-axis, Y-axis, and Z-axis, and the echo peak detection delay due to the system response delay is corrected based on the stored data. In order to correct the time delay due to the above, a correction pulse for the gradient magnetic field pulse is calculated and applied.

  Therefore, there is no need to adjust the echo peak detection delay for each sequence, and an MRI apparatus capable of adjusting echo peak deviation due to system response delay etc. with high accuracy and in a short time is realized. can do.

  In the above example, the delay times delays 1, 2, and 3 (data corresponding to the time shift) are calculated from the measured time shifts dT1, dT2, and dT3, and stored in the storage unit 80. The relationship between the lew rate and gradient magnetic field strength and the time shifts dT1, dT2, and dT3 (data corresponding to the time shift) is graphed and stored in the storage unit 80, and after reading dT1, dT2, and dT3, A configuration is also possible in which the delay times delays 1, 2, and 3 are calculated from the time shifts dT1, dT2, and dT3.

DESCRIPTION OF SYMBOLS 1 ... Static magnetic field generation magnetic circuit, 2 ... Gradient magnetic field generation system, 3 ... Transmission system, 4 ... Reception system, 5 ... Signal processing system, 6 ... Sequence, 7 ... CPU, 8 ... operation unit, 9 ... subject, 10 ... gradient magnetic field coil, 11 ... gradient magnetic field power supply, 12 ... high frequency oscillator, 13 ... modulator, 14 ... A high-frequency amplifier, 15... High-frequency irradiation coil, 16... A reception high-frequency irradiation coil, 17... (High-frequency) amplifier, 18. 20 ... ROM, 21 ... RAM, 22 ... magneto-optical disk, 23 ... display, 24 ... magnetic disk, 25 ... trackball or mouse, 26 ... keyboard, 71. ..Time lag measuring unit, 72 ... Delay time calculating unit, 73. Delay time, slew rate, gradient magnetic field strength graph creation unit, 74 ... slew rate / gradient magnetic field strength setting unit, 75 ... delay time reading unit, 76 ... correction pulse calculation unit, 77 ... Sequencer control unit, 80 ... storage unit, 101, 115 ... high frequency pulse (RF pulse), 102 ... slice selection gradient magnetic field, 103 ... slice direction rephase gradient magnetic field, 104 ... reading direction 105: Positive readout gradient magnetic field 106: First echo signal 107: Echo peak deviation dT1, 108, 112, 120 ... A / D window 109 ... Negative readout gradient magnetic field, 110 ... second echo signal, 111 ... echo peak deviation dT2, 113, 14,121 ... crusher gradient, 116 ... slice selection gradient magnetic field, 117 ... second slice selection gradient, 118 ... third echo signal, 119 ... shift of an echo peak dT3

Claims (5)

A static magnetic field generation means, a gradient magnetic field generation means, a high-frequency magnetic field pulse irradiation means, a reception means for receiving a magnetic resonance signal generated from a subject, a signal processing means for reconstructing an image based on the magnetic resonance signal, In a magnetic resonance imaging apparatus, comprising: the static magnetic field generating means, the gradient magnetic field generating means, the high-frequency magnetic field pulse irradiating means, the receiving means and the signal processing means to control the imaging sequence.
Pre-measured data corresponding to the time lag between the signal peak of the magnetic resonance signal at a plurality of gradient magnetic field strengths and a plurality of slew rates and the reception timing of the magnetic resonance signal of the receiving means, the gradient magnetic field strength, A storage means for storing each slew rate is provided.
The control means reads the time lag corresponding to the gradient magnetic field strength and slew rate used for imaging from the storage means, and adjusts the application timing of the gradient magnetic field and the gradient magnetic field area based on the read time lag. A magnetic resonance imaging apparatus.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the control means includes a first time lag between a magnetic resonance signal by a readout gradient magnetic field and a reception timing of the reception means in a first sequence of imaging, and the readout inclination. A second time lag between the magnetic resonance signal by the gradient magnetic field whose polarity is reversed with respect to the magnetic field and the reception timing of the receiving means, and the magnetic resonance signal by the gradient magnetic field whose polarity is reversed from the slice selection gradient magnetic field in the second sequence of imaging. A third time lag with respect to the reception timing of the receiving means is measured, and a first time lag due to a system response lag from the measured first time lag, second time lag, and third time lag. And a second time delay due to an area error between the dephase gradient magnetic field area and the readout gradient magnetic field area in the first sequence of imaging. And a third time delay due to an area error between the slice selective gradient magnetic field area in the second imaging sequence and the gradient magnetic field area in which the polarity of the slice selective gradient magnetic field area is inverted, and the calculated first time delay A magnetic resonance imaging apparatus characterized in that the second time lag and the third time lag are data corresponding to the time lag.   3. The magnetic resonance imaging apparatus according to claim 2, wherein the control unit reads out the first time lag, the second time lag, and the third time lag from the storage unit, and reads the second time lag and the third time lag read out. A correction pulse for correcting the gradient magnetic field area is calculated from the time delay, and the application timing of the gradient magnetic field is adjusted based on the first time delay, and the calculated correction pulse is added to the gradient magnetic field, whereby the area error is calculated. A magnetic resonance imaging apparatus characterized by correcting the above.   The magnetic resonance imaging apparatus according to claim 1, wherein the data corresponding to the time lag is a first time lag between a magnetic resonance signal due to a readout gradient magnetic field and a reception timing of the receiving means in a first sequence of imaging, Due to a second time lag between the magnetic resonance signal due to the gradient magnetic field whose polarity is reversed with respect to the readout gradient magnetic field and the reception timing of the receiving means, and due to the gradient magnetic field whose polarity is reversed with respect to the slice selection gradient magnetic field in the second sequence of imaging. A magnetic resonance imaging apparatus, characterized by a third time lag between the magnetic resonance signal and the reception timing of the receiving means.   5. The magnetic resonance imaging apparatus according to claim 4, wherein the control means detects a first time due to a system response delay from a first time lag, a second time lag, and a third time lag read from the storage means. The second time delay due to the delay, the area error between the dephase gradient magnetic field area and the readout gradient magnetic field area in the first sequence of imaging, the slice selective gradient magnetic field area and the slice selective gradient magnetic field surface in the second sequence of imaging A third time delay due to an area error with the gradient magnetic field area in which the aggressiveness is reversed is calculated, a correction pulse for correcting the gradient magnetic field area is calculated from the calculated second time delay and the third time delay, Based on the time delay of 1, the gradient magnetic field application timing is adjusted, and the area error is compensated by applying the calculated correction pulse to the gradient magnetic field. Magnetic resonance imaging apparatus characterized by.

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