WO2014112235A1 - 磁気共鳴イメージング装置及びそのタイミングずれ検出方法 - Google Patents
磁気共鳴イメージング装置及びそのタイミングずれ検出方法 Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/385—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using gradient magnetic field coils
- G01R33/3852—Gradient amplifiers; means for controlling the application of a gradient magnetic field to the sample, e.g. a gradient signal synthesizer
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
- G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
- G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
- G01R33/56572—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of a gradient magnetic field, e.g. non-linearity of a gradient magnetic field
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/58—Calibration of imaging systems, e.g. using test probes, Phantoms; Calibration objects or fiducial markers such as active or passive RF coils surrounding an MR active material
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
- G01R33/546—Interface between the MR system and the user, e.g. for controlling the operation of the MR system or for the design of pulse sequences
Definitions
- the present invention relates to magnetic resonance imaging technology.
- the present invention relates to a method for detecting and adjusting a timing shift of a power source that drives a gradient coil.
- a magnetic resonance imaging (MRI) apparatus is a medical image diagnostic apparatus that applies a high-frequency magnetic field and a gradient magnetic field to a subject placed in a static magnetic field, measures a signal generated from the subject by nuclear magnetic resonance, and forms an image. .
- the gradient magnetic field generates a one-dimensional magnetic field strength distribution in the space where the subject is placed, and is used to add position information to the signal.
- a gradient magnetic field generator usually comprises a coil and a power source for driving it. Since it is necessary to give three-dimensional position information, three sets of coils and a power source are prepared so that gradient magnetic fields are applied independently to the three orthogonal axes (x, y, z).
- the problem is that the output timing of each power supply is shifted.
- the main causes of the timing shift are a variation in characteristics of each power source and a difference in impedance of the subcoil. If there is such a timing shift, artifacts and distortion occur in the captured image. Therefore, it is necessary to adjust so that each power supply can be completely synchronized.
- Artifacts occur in the captured image due to a shift in driving timing between the positive subcoil and the negative subcoil that form a pair.
- the size of the artifact due to this timing shift mainly depends on the sampling interval of the readout. For example, if the time lag is about 5 samples, the image phase is distorted, and if it is about 10 samples, the brightness also changes. Since the shortest sampling interval in MRI is about several microseconds, it is necessary to suppress the timing deviation to about 10 microseconds or less.
- the present invention has been made in view of the above circumstances, and when the gradient magnetic field coil is divided into sub-coils and the parallel drive is performed using the separate power sources in the positive side sub-coil and the negative side sub-coil, the individual power sources described above are provided. It is an object of the present invention to provide a technique for accurately detecting the output timing deviation without requiring an additional measuring device. It is another object of the present invention to provide a technique that eliminates the output timing shift of the individual power source and eliminates the phase distortion of the image due to the timing shift without using an additional measuring device. And
- the time shift is detected using the projection images on the positive side and the negative side of the gradient magnetic field.
- a time lag When there is a time lag, a change occurs in the phases of the positive and negative projection images.
- a time lag is detected from the phase difference.
- a specific timing shift detection pulse sequence having a slice gradient magnetic field pulse and a readout gradient magnetic field in the same direction as the gradient magnetic field in question is used.
- each measurement is performed with a pulse sequence in which the excitation frequency is positively shifted from the Larmor frequency corresponding to the static magnetic field intensity and a pulse sequence in which the excitation frequency is negatively shifted, and thus, in the two slices of the positive side position and the negative side position of the gradient magnetic field.
- Two projection images reflecting each of the spins are obtained.
- the position between the two projected images is matched to obtain the phase difference, and the inclination with respect to the position of the phase difference is obtained.
- imaging by the detection pulse sequence is performed a plurality of times by changing the drive timing of one of the positive side subcoil and the negative side subcoil by a predetermined time width.
- the calculation means obtains an inclination with respect to the position of the phase difference between the two projection images from each of the plurality of imaging results, and calculates a time shift corresponding to the inclination of the phase difference being zero from the values of the inclinations.
- the value of the inclination of the phase difference between the two projection images obtained from the imaging result of each time in the two-dimensional space of the change in the driving timing of the one sub-coil and the inclination of the phase difference between the two projection images Is plotted and linear function fitting is performed to estimate the amount of change in drive timing corresponding to a phase difference slope value of zero. That is, the drive timing deviation between the positive and negative subcoils before adjustment is calculated.
- the drive timing deviation of the positive side subcoil and the negative side subcoil is a deviation from the design of the rising waveform and falling waveform of the gradient magnetic field.
- a deviation from the design of the rising waveform of the readout gradient magnetic field pulse causes a shift in the echo appearance time.
- this shift appears as a first-order phase change with respect to the projected position.
- the difference in sampling timing of echo signal reception also appears as a primary phase change with respect to the position in the echo projection image.
- the primary phase change caused by the deviation of the positive and negative drive timings of the gradient magnetic field is caused by a projection image reflecting the positive slice of the gradient magnetic field and a projection image reflecting the negative slice.
- the polarity is reversed. Therefore, as described above, a projection image reflecting the positive slice of the gradient magnetic field and a projection image reflecting the negative slice are obtained by the timing shift detection pulse sequence, and the two projected images are obtained.
- the phase around the phase caused by the shift of the signal sampling timing mixed in the projected image of the spin echo is canceled, and the phase corresponding to the positive and negative drive timing shift of the gradient magnetic field is cancelled.
- the circumference is extracted, and the amount of positive and negative drive timing deviation of the gradient magnetic field is derived.
- not only the primary phase change with respect to the position but also the phase distribution peculiar to the probe and the phase distribution caused by the static magnetic field inhomogeneity are mixed in each projected image. Has been found to hinder extraction around the phase corresponding to the deviation of the positive and negative drive timings of the gradient magnetic field.
- a pulse sequence that combines a negative slice gradient magnetic field and a negatively shifted excitation frequency and a pulse sequence that combines a positive slice gradient magnetic field and a positively shifted excitation frequency, respectively.
- a projection image reflecting the spin in the slice at the positive side position is acquired, and a difference (first difference) between the projection images is calculated.
- a pulse sequence that combines a positive slice gradient magnetic field and a negatively shifted excitation frequency and a pulse sequence that combines a negative slice gradient magnetic field and a positively shifted excitation frequency reflect the spins in the negative slices.
- An image is acquired, and a difference (second difference) between the projected images is calculated.
- the two projection images for calculating the first difference are projection images reflecting the spins of slices at the same positive position. Therefore, by performing the difference calculation between the two, the phase distribution due to the probe-specific phase distribution and the static magnetic field inhomogeneity is canceled out. Similarly, since the two projection images for calculating the second difference are projection images of slices at the same negative position, the phase calculation due to the probe-specific phase distribution and static magnetic field inhomogeneity is canceled by the difference calculation. .
- the phase difference calculation method for obtaining the phase difference by combining the slice positions of the first difference and the second difference is adopted, and as in the above representative example, the positive side subcoil and the negative side subcoil
- the first difference between the projected images of the slices at the positive position and the negative position in each result of the multiple times of imaging are performed by changing one of the drive timings by a predetermined time width.
- Each of the slopes relative to the second difference between the projected images of the slices of the slices is obtained, and a timing deviation corresponding to zero of the phase difference is obtained from these slope values, that is, positive side before adjustment, negative side
- the drive timing deviation between the side subcoils is calculated.
- the timing shift is detected by capturing the projection image by executing the pulse sequence and processing the projection image, no additional measuring device is required, and the timing shift can be detected with high accuracy.
- FIG. 1 is a block diagram showing a schematic configuration of the MRI apparatus 100 of the present embodiment.
- the MRI apparatus 100 irradiates a high-frequency magnetic field and a nuclear magnetism with a magnet 101 that generates a static magnetic field, a gradient coil 102 that generates a gradient magnetic field, a sequencer 104, a gradient magnetic field power source 105, a high-frequency magnetic field generator 106, and the like.
- a probe 107 for detecting a resonance signal, a receiver 108, a calculator 109, a display 110, and a storage medium 111 are provided.
- a subject (for example, a living body) 103 is placed on a bed (table) in a static magnetic field space generated by a magnet 101.
- the sequencer 104 sends commands to the gradient magnetic field power source 105 and the high frequency magnetic field generator 106 to generate a gradient magnetic field and a high frequency magnetic field, respectively.
- the high frequency magnetic field is applied to the subject 103 through the probe 107.
- the nuclear magnetic resonance signal generated from the subject 103 is received by the probe 107 and detected by the receiver 108.
- the sequencer 104 sets a nuclear magnetic resonance frequency (detection reference frequency f0) as a reference for detection.
- the detected signal is sent to the computer 109, where signal processing such as image reconstruction is performed.
- the result is displayed on the display 110.
- the detected signal and measurement conditions can be stored in the storage medium 111 as necessary.
- a command (gradient magnetic field pulse waveform) from the sequencer 104 described later is input to the gradient magnetic field power source 105 through the time adjustment circuit 112, and the gradient magnetic field coil 102 is driven.
- the gradient magnetic field coil is composed of coils wound in three directions of the x axis (102-1), the y axis (102-2), and the z axis (102-3), and the gradient magnetic fields Gx, Gy, and Gz in the three axis directions are provided. Generated.
- the gradient magnetic field coils for each axis are composed of coils arranged on the positive side (102-11 and 102-12, 102-21 and 102-22, 102-31 and 102-32), and a subcoil arranged on the negative side (102 -13 and 102-14, 102-23 and 102-24, and 102-33 and 102-34). Examples of gradient magnetic field coils are shown in FIGS.
- FIG. 3 is an example of a gradient magnetic field coil in the x-axis direction of the MRI apparatus of the present embodiment.
- the MRI apparatus of this embodiment is a vertical magnetic field type, and the z axis perpendicular to the horizontal plane coincides with the direction of the static magnetic field.
- the subcoils 102-11 and 102-12 are arranged on the positive side of the x-axis and are driven by the same power source 105-11 as shown in FIG.
- the direction of the current when the gradient magnetic field has a positive polarity is as shown by the arrow.
- the subcoils 102-13 and 102-14 are disposed on the negative side of the x-axis and are driven by the same power source 105-13.
- FIG. 4 shows the distribution of the gradient magnetic field generated by the gradient magnetic field coil in the x-axis direction in FIG. 3 on the x-axis.
- a magnetic field in a direction that enhances the static magnetic field is generated from the positive side subcoils 102-11 and 102-12.
- the intensity distribution of the magnetic field is indicated by 121, and the intensity becomes maximum at a position deviated in the plus x direction from the gradient magnetic field origin (0, 0, 0). From the sub coils 102-13 and 102-14 on the negative side, a direction to attenuate the static magnetic field, that is, a magnetic field opposite to the static magnetic field is generated.
- the intensity distribution of the magnetic field is indicated by 122, and the intensity becomes maximum at a position deviated in the minus x direction from the gradient magnetic field origin.
- the gradient magnetic field coil 102-2 in the y-axis direction and the gradient magnetic field coil 102-3 in the z-axis direction are almost the same as the above-described gradient magnetic field coil in the x-axis direction although the shapes thereof are different. That is, the positive side sub-coil or sub-coil group and the negative side sub-coil or sub-coil group generate magnetic fields that are opposite to each other in the direction of increasing the static magnetic field and the direction of decreasing or decreasing the static magnetic field. Are different. Thus, when the positive side subcoil or subcoil group and the negative side subcoil or subcoil group are driven simultaneously, a gradient magnetic field having a gradient of magnetic field strength along a desired axis is generated.
- the z-axis which is the direction of the static magnetic field, is horizontal.
- the subcoils 102-21 and 102-22 are arranged on the positive side of the y axis, and the subcoils 102-23 and 102-24 are arranged on the negative side of the y axis. These subcoils are driven by a current indicated by an arrow to generate a gradient magnetic field (y-direction gradient magnetic field) that gives a gradient along the y-axis to the strength of the static magnetic field.
- the sequencer 104 normally performs control so that each device operates at a timing and intensity programmed in advance.
- a program that particularly describes a high-frequency magnetic field, a gradient magnetic field, and timing and intensity of signal reception is called a pulse sequence.
- the gradient magnetic field on / off control signals according to this pulse sequence are supplied to the gradient magnetic field power supplies 105-11 and 105 provided in each of the three axial directions and separately for the positive side subcoil group and the negative side subcoil group. -12, 105-21, 105-22, 105-31 and 105-32.
- Time adjustment circuits 112-11, 112-12, 112-21, 112-22, 112-31, and 112-32 are inserted in control signal paths from the sequencer 104 to the respective gradient magnetic field power supplies. With this configuration, it is possible to adjust the deviation of the effective drive timing between the positive side subcoil and the negative side subcoil that generate the gradient magnetic fields in the respective axial directions by synthesis.
- the deviation in the drive timing between the positive side and negative side of the gradient magnetic field is caused by variations in characteristics at various points such as between the positive side sub-coil and the negative side sub-coil, between power sources, and between signal paths.
- the deviation of the drive timing on the positive side and the negative side is a modification from the design of the rising waveform and falling waveform of the gradient magnetic field.
- the deformation from the design of the rising waveform of the readout gradient magnetic field pulse causes a shift in the echo appearance time. In the projection image obtained by inverse Fourier transform of the echo measurement signal, this timing shift appears as a primary phase change with respect to the position where the slice image is projected.
- the MRI apparatus 100 of the present embodiment includes a pulse sequence for detecting a drive timing shift between the positive side subcoil and the negative side subcoil of each axis.
- the computer 109 instructs the sequencer 104 to measure a nuclear magnetic resonance signal (echo) according to a timing shift detection pulse sequence, and creates a projection image by performing inverse Fourier transform on the measured echo.
- a measurement unit and a timing shift detection processing unit that detects a time shift from the projected image are provided. Each of these functions is realized when the CPU of the computer 109 loads the program stored in the storage medium 111 to the memory and executes it.
- a pulse sequence for measuring the timing deviation of each axis of the gradient magnetic field with the MRI apparatus 100 of the present embodiment will be described.
- FIG. 6 is a pulse sequence for measuring the drive timing shift of the gradient magnetic field in the x-axis direction using a gradient echo.
- RF is a high-frequency magnetic field waveform
- Gx is a gradient magnetic field waveform of an axis (here, x-axis) on which timing deviation is measured.
- the gradient magnetic fields in the y-axis direction and the z-axis direction are not generated as indicated by Gy and Gz.
- a slice gradient magnetic field pulse 201 in the axial direction (x direction) to be measured is applied and a high-frequency magnetic field (RF) pulse 202 having a proton resonance frequency fh is irradiated, and a predetermined slice in the target object is irradiated.
- RF high-frequency magnetic field
- the slice rephase gradient magnetic field pulse 203 and the dephase readout gradient magnetic field 204 for adding position information in the readout direction (x direction) Excites protons. Then, after applying the slice rephase gradient magnetic field pulse 203 and the dephase readout gradient magnetic field 204 for adding position information in the readout direction (x direction), the A / D conversion is performed (period 207), and a magnetic resonance signal is measured.
- the gradient magnetic field in the direction of the axis to be measured here, the x axis
- FIG. 7 is an example thereof, and is a pulse sequence for measuring the drive timing shift of the gradient magnetic field on the x axis, as in FIG.
- a high-frequency magnetic field (RF) pulse 202 having a proton resonance frequency fh is irradiated along with the application of the slice gradient magnetic field pulse 201 in the x direction to excite protons in a predetermined slice in the target object.
- RF magnetic field
- a 180-degree pulse 205 is irradiated.
- a / D conversion is performed while applying the readout gradient magnetic field pulse 206 (period 207), and one echo is measured.
- the projection image measurement unit repeats the pulse sequence of FIG. 6 or FIG. 4 four times to measure four echoes, and performs inverse Fourier transform on each echo to create four projection images.
- the slice gradient magnetic field, the excitation frequency, and the readout gradient magnetic field are changed as shown in FIG. That is, the measurement of the echo number 1 is performed at a frequency in which the polarity of the slice gradient magnetic field 201 in the x direction is negative and the frequency of the excitation RF pulse 202 is shifted to the negative with respect to the proton resonance frequency in the static magnetic field. Therefore, the excited slice position is shifted by a distance x1 in the plus x direction from the origin of the gradient magnetic field.
- the polarity of the readout gradient magnetic field 206 is positive. This corresponds to a sequence in which the waveform of the gradient magnetic field Gx is represented by a thin line in the sequence of FIG. 6 or FIG.
- the measurement of echo number 2 is performed with the polarity of the slice gradient magnetic field pulse 201 being positive and the frequency of the excitation RF pulse 202 being negatively shifted. Therefore, the excited slice position is shifted by x1 in the minus x direction from the origin of the gradient magnetic field.
- the lead-out gradient magnetic field 206 becomes negative, and this corresponds to the sequence indicated by the thick line of the gradient magnetic field Gx in FIG. 6 or FIG.
- the measurement of the echo number 3 is performed with the polarity of the slice gradient magnetic field pulse 201 being positive and the frequency of the excitation RF pulse 202 being shifted in the plus direction. Therefore, the slice position shifts in the plus x direction (+ x1), and the polarity of the readout gradient magnetic field 206 is negative.
- the measurement of echo number 4 is performed with the polarity of the slice gradient magnetic field pulse 201 being negative and the frequency of the excitation RF pulse 202 being shifted in the plus direction. Therefore, the slice position is shifted in the minus x direction ( ⁇ x1), and the readout gradient magnetic field 206 is positive. In this way, by changing the polarity of the slice gradient magnetic field and the excitation frequency, the measurement was performed twice by exciting the slice at the positive position on the x axis + x1 and the slice at the negative position on the x axis-x1 was excited. Is executed twice. The order of measurement may be arbitrary. However, when the same position is excited continuously, a waiting time is required until the magnetization is in an equilibrium state.
- the waiting time can be halved, thereby shortening the measurement time. It is possible.
- Other main imaging parameters are a visual field of 400 mm, a slice position of ⁇ 100 mm, a slice thickness of 20 mm, TR / TE of 1000/10 mm, and a sampling point of 512.
- the subject to be photographed is a water phantom 230 having a spherical shape with a diameter of 300 mm, for example, and is installed near the origin of the gradient magnetic field.
- the phase of the projected image is used to detect the drive timing shift of the positive and negative subcoils for generating the gradient magnetic field, as will be described later. For this reason, even if the signal intensity is different between the positive side and the negative side, it does not matter, so the installation accuracy of the object to be imaged is not important.
- the positions ⁇ x1 of slices excited by the timing shift detection pulse sequence are ⁇ 100 mm, and are shown as 231 and 232 in FIG.
- the imaging target is not limited to a uniform proton distribution such as the water phantom described above.
- the shape need not be spherical, and may be any imaging target in which sufficient protons exist in the range of the selected positive and negative slice positions.
- the gradient magnetic field pulse waveform rises and falls, and the echo shifts in time.
- the effect appears as a first-order phase change with respect to the position of the coordinates on which the projected image of the echo is projected.
- the positive side and the negative side of both the slice gradient magnetic field pulse 202 and the readout gradient magnetic field 206 are detected.
- the drive timing shift affects the phase of the echo projection image.
- phase change of the projected image due to the drive timing shift of the slice gradient magnetic field pulse is ⁇ S +, ⁇ and the phase change of the projection image due to the drive timing shift of the readout gradient magnetic field pulse is ⁇ R +, ⁇ , respectively, ).
- both ⁇ S +, ⁇ and ⁇ R +, ⁇ are proportional to the effective drive timing shift ⁇ t GC +, ⁇ between the positive and negative subcoils that generate the gradient magnetic field in the x direction by synthesis.
- the phase around due to the time lag is reversed.
- the intensity ratio between the magnetic field from the positive side subcoil and the magnetic field from the negative side subcoil is just reversed.
- this is used to eliminate the factors other than the drive timing shift of the positive side subcoil and the negative side subcoil, such as static magnetic field inhomogeneity and phase distribution of the receiving coil, and the phase and time shift. Clarify the relationship.
- ⁇ + x1 + Gr (x) is the phase at the position x of the projected image measured with the positive readout gradient magnetic field by exciting the slice position + x1.
- the phase of each projection image is the probe-specific phase distribution ⁇ RF (x), phase distribution ⁇ B (x) due to non-uniform static magnetic field, phase rotation ⁇ AD due to A / D period shift, positive and negative gradient magnetic fields This is the sum or difference of the phase shifts ⁇ + and ⁇ ⁇ due to the sub-coil drive timing delay and the position-independent phase offset c.
- ⁇ + x1 (x) indicates the phase of the difference between the projection images of echo numbers 1 and 3 reflecting the spin of the positive side (+ x1) slice
- ⁇ ⁇ x1 (x) is the negative side
- ⁇ The phase of the difference between the projection images of echo numbers 4 and 2 reflecting the spin of the slice of x1) is shown.
- the phase distribution inherent to the probe and the phase distribution due to the static magnetic field inhomogeneity cancel each other.
- the mutual phase difference between the two differences of Equation (3) is obtained.
- the slice position of the difference between the negative slices shown in the second equation of the equation (3) -x1 is translated by + 2x1, that is, adjusted to the slice position + x1 of the first difference in the expression (3).
- the phase difference between the two changes linearly with respect to the position x, and the slope thereof is the difference ⁇ t GC + between the delay time (delay) of the positive side subcoil and the delay time (delay) of the negative side subcoil. -Proportional to ⁇ t GC- . Therefore, in order to obtain the drive timing deviation between the positive side and the negative side, the slope of the phase difference is obtained while changing the delay time on one side, and the change width of the delay time at which the slope of the phase difference becomes zero is obtained.
- FIG. 10 shows an echo, a projected image, and a phase difference (result of Expression (5)) when the positive delay is set to 0 ⁇ s, 10 ⁇ s, and 20 ⁇ s.
- the echo and the projected image are displayed in a superimposed manner, and the echoes at the same slice position overlap, and the projected images are superimposed with the readout gradient magnetic field changed to positive and negative. Looks like it is displayed. It can be seen that the slope of the phase differences 241 to 243 near + x1 increases as the positive delay time increases.
- FIG. 11 is a graph in which this inclination is plotted against delay. FIG. 11 shows that the slope of the phase difference is proportional to delay.
- FIG. 12 shows the result of measuring the slope of the phase difference three times in total when the positive delay is changed by ⁇ 10 ⁇ s when the positive delay is 30 ⁇ s.
- the slope of the phase difference changes in proportion to the change in delay.
- the value of the intersection with the x-axis where the gradient of the phase difference becomes zero was ⁇ 29.92 ⁇ s. That is, if the positive delay is changed by ⁇ 29.92 ⁇ s, the difference between the positive delay and the negative delay can be eliminated.
- the time shift correction value -29.92 ⁇ s thus obtained is set in the time adjustment circuit 112-11. Thereby, the time of the gradient magnetic field pulse waveform generated from the sequencer 104 is adjusted, and the gradient magnetic field is generated at the same timing on the positive side and the negative side.
- the x-axis has been described as an example, but the time lag can be measured and set similarly for the y-axis and the z-axis.
- FIG. 13 shows an example of an operation screen 300 when adjusting the time lag of the gradient magnetic field as described above on the apparatus.
- the operation screen is similarly configured for each of the three axes x, y, and z. Each axis can be switched by tabs 301 to 303.
- An adjustment start button 304 and an adjustment result screen 305 are displayed on the operation screen for each axis.
- the adjustment start button is pressed, the slope of the phase difference is measured for the delay change value set in advance according to the above-described method, and linear function fitting is performed to obtain an adjustment value.
- the result is displayed on the adjustment result screen 305.
- a measurement point 306 of the phase difference gradient, a linear function fitting result 307, and an adjustment value 308 are displayed.
- the adjustment value is automatically set in the time adjustment circuit.
- the delay is changed and the slope of the phase difference is measured a plurality of times.
- the time lag can be directly calculated from Equation (6) obtained by transforming Equation (5).
- each slice affects the accuracy of time shift detection. Since the contribution to the magnetic field by the sub coil on the opposite side is as large as the origin, the contribution to the magnetic field by the positive and negative sub coils varies depending on the position when the slice thickness is increased and the origin is approached. For this reason, when the slice thickness is increased, the detection accuracy of the time shift is lowered. In addition, if the slice thickness is reduced, the signal-to-noise ratio (SN ratio) is lowered, so that the accuracy of time shift detection is lowered. In this embodiment, the thickness is set to 20 mm that provides a sufficient SN ratio. However, if the SN ratio is insufficient, the thickness may be increased to about 40 mm.
- the drive timing shift when the positive-side and negative-side gradient magnetic field subcoils are driven in parallel by different power sources changes in the phases of the positive and negative projection images. It detects from the difference of. As a result, it is possible to accurately detect the drive timing shift by removing the influence of the shift in the A / D period. Further, by using the phase difference of the projected image measured by reversing the sign of the gradient magnetic field pulse, the influence of the phase distribution of the probe and the static magnetic field inhomogeneity can be removed and the drive timing deviation can be detected with high accuracy. In addition, since a drive timing shift is detected by taking a projected image by executing a pulse sequence and processing the projected image, no additional measuring device is required.
- echoes are measured with four types of timing shift detection pulse sequences, and the phase distribution due to the probe-specific phase distribution and the static magnetic field inhomogeneity mixed in the phase of each projection image and reception are received. Processing to sequentially cancel the phase shift due to the difference in the A / D period of the signal was performed. If the former probe-specific phase distribution and static magnetic field inhomogeneity are small and the influence on the detection of the deviation of the driving timing of the gradient magnetic field subcoil is small, a simpler measurement method is used in which processing for canceling these is omitted. It is possible.
- the pulse sequences for measurement of echo number 1 and measurement of echo number 4 are used among the four types of pulse sequences indicated by echo numbers 1 to 4 in FIG. That is, the echo of the echo number 1 reflecting the spin in the slice at the positive position + x1 of the gradient magnetic field by a pulse sequence having a negative polarity of the slice gradient magnetic field and a frequency shifted negatively from the Larmor frequency in the static magnetic field strength. Measure. Further, the echo of the echo number 4 reflecting the spin in the slice at the negative side position ⁇ x1 of the gradient magnetic field is measured by a pulse sequence having a negative polarity of the slice gradient magnetic field and a frequency that is positively shifted from the Larmor frequency. Note that the time shift detection pulse sequence used here may be a sequence in which the gradient echo shown in FIG. 6 is generated and measured, or a sequence in which the spin echo shown in FIG. 7 is generated and measured.
- the phases of the projection images obtained by performing the inverse Fourier transform on the respective measurement signals are respectively as ⁇ + x1 + Gr (x) and ⁇ ⁇ x1 + Gr (x) in equation (2). Since the slice positions of both projection images are shifted by 2 ⁇ x1, the projection image of the echo number 4 is moved by 2 ⁇ x1 in the x-axis direction to match the slice positions.
- the phase of the two projected images after movement is as shown in equation (8).
- measurement is performed a plurality of times by changing the delay time of one of the positive side subcoil and the negative side subcoil by a predetermined time width, and the measurement of the plurality of times From each result, the slopes of the phase difference between the projection image of the echo number 1 and the projection image of the echo number 4 are obtained, respectively, and the slope of the phase difference is zero from the values of the slopes.
- a corresponding delay time shift that is, a drive timing shift between the positive and negative subcoils before adjustment is calculated.
- the pulse sequences for the measurement of the echo number 1 and the measurement of the echo number 4 are used among the four types of pulse sequences indicated by the echo numbers 1 to 4 in FIG.
- the deviation of the drive timing between the positive and negative subcoils is calculated using only the pulse sequence for measurement of echo number 1 and the measurement of echo number 2. That is, the polarity of the slice gradient magnetic field is negative, and the excitation frequency is a pulse sequence having a frequency that is negatively shifted from the Larmor frequency in the static magnetic field strength. The echo number in FIG. 1 echo is measured.
- the echo of echo number 2 reflecting the spin in the slice at the negative side position -x1 of the gradient magnetic field is measured by a pulse sequence in which only the polarity of the slice gradient magnetic field is changed to positive.
- the detection pulse sequence to be applied may be either the sequence shown in FIG. 6 or the sequence shown in FIG.
- the projection image of the echo of echo number 2 is moved by 2 ⁇ x1 in the x-axis direction, and the slice position is adjusted.
- the phases ⁇ + x1 + Gr (x) and ⁇ -x1 -Gr (x-2x1) of the two projection images obtained as a result can be expressed by Expression (10).
- measurement is performed a plurality of times by changing the drive timing of one of the positive side subcoil and the negative side subcoil by a predetermined time width, and the measurement of the plurality of times From each result, the inclination of the addition result of the projection image of the echo number 1 and that obtained by moving the position of the projection image of the echo number 2 is obtained with respect to the phase position, and the phase inclination corresponds to zero from the value of the inclination.
- Time difference that is, the drive timing difference between the positive and negative subcoils before adjustment is calculated.
- the influence of the deviation of the A / D period is removed by using a simpler procedure without using an additional measurement device, and the gradient magnetic field subcoils on the positive side and the negative side are removed.
- a drive timing shift can be detected, and the drive timing shift can be adjusted.
- the drive timing of the MRI apparatus that drives the gradient magnetic fields in parallel can be adjusted appropriately and accurately, and the image quality of the MRI apparatus can be improved and a high image quality level can be maintained. Expected to be done.
- 101 Magnet for generating a static magnetic field
- 102 Gradient magnetic field coil
- 103 Subject
- 104 Sequencer
- 105 Gradient magnetic field power source
- 106 High-frequency magnetic field generator
- 107 Probe
- 108 Receiver
- 109 Computer
- 110 Display
- 111 Storage medium
- 112 Time adjustment circuit
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Abstract
Description
以下、本発明を適用する第1の実施形態について説明する。以下、本発明の実施形態を説明するための全図において、同一機能を有するものは同一符号を付し、その繰り返しの説明は省略する。
上述の例では±100 mmでも十分な精度が得られている。
上述の第一の実施態様では、4種類のタイミングずれ検出用パルスシーケンスでそれぞれエコーを計測し、それぞれの投影像の位相に混入するプローブ固有の位相分布及び静磁場不均一による位相分布、並びに受信信号のA/D期間のずれによる位相回りを順次相殺する処理を行っていた。前者のプローブ固有の位相分布及び静磁場不均一が小さく、傾斜磁場サブコイルの駆動タイミングのずれの検出に与える影響が小さい場合には、これらを相殺する処理を省略した、より簡単な計測法を用いることが可能である。
Claims (10)
- 所定の検査空間に所定方向の静磁場を発生する手段と、
前記検査空間の磁場強度に直交3軸それぞれに沿った勾配をつけるための3方向の傾斜磁場を発生する傾斜磁場発生手段と、
前記検査空間に置かれた被検体に高周波磁場を印加する手段と、
前記3方向の傾斜磁場それぞれの発生、前記対象への高周波磁場の印加、及びそれらにより前記被検体から発生する磁気共鳴信号の受信を制御するシーケンサと、
検出した磁気共鳴信号を処理する演算手段とを備える磁気共鳴イメージング装置であって、
前記傾斜磁場発生手段は、前記3方向の傾斜磁場の各々を合成により発生する正側のサブコイル及び負側のサブコイル、並びに、前記正側のサブコイルと前記負側のサブコイルのそれぞれに電流を供給する電源を有し、
前記シーケンサは、前記3方向のうちの第1の方向の傾斜磁場を合成により発生する正側のサブコイルと負側のサブコイルとの駆動タイミングのずれを検出するための複数のパルスシーケンスであり、前記第1の方向のリードアウト傾斜磁場パルスの下で傾斜磁場原点から前記正側のサブコイル寄りにシフトした位置の第1スライスのエコー、及び前記負側のサブコイル寄りにシフトした第2スライスのエコーのいずれかをそれぞれ計測する複数のパルスシーケンスを実行し、
前記演算手段では、前記複数のパルスシーケンスの各々で計測されるエコー信号をそれぞれ逆フーリエ変換して前記第1スライスの投影像、前記第2スライスの投影像を導出し、前記第1スライスの投影像と第2スライスの投影像の間の位相差を求め、該位相差の前記第1の方向に沿った位置に対する傾きをゼロにするために必要な前記正側のサブコイル、負側のサブコイルのいずれか一方の駆動タイミングの変更幅を算出して前記正側のサブコイルと負側のサブコイルとの駆動タイミングのずれとする
ことを特徴とする磁気共鳴イメージング装置。 - 請求項1に記載の磁気共鳴イメージング装置において、
前記シーケンサは、前記第1の方向のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からマイナスシフトした中心周波数の励起パルスとの同時印加で前記第1スライスのスピンを励起し、該スピンのエコーを前記リードアウト傾斜磁場パルスの下で計測する第1のパルスシーケンスと、該第1のパルスシーケンスに対して励起パルスの中心周波数を前記ラーモア周波数からプラスシフトした中心周波数を持つものに代え、もって前記第2スライスのスピンを励起し、該スピンのエコーを前記リードアウト傾斜磁場パルスの下で計測する第2のパルスシーケンスと実行し、
前記演算手段は、前記第1、第2のパルスシーケンスで得られたエコー信号をそれぞれ逆フーリエ変換して前記第1、第2の投影像を導出し、前記投影像間の演算として、該第1、第2の投影像のスライス位置を合わせた後に両投影像の差分処理の演算を行う
ことを特徴とする磁気共鳴イメージング装置。 - 請求項1に記載の磁気共鳴イメージング装置において、
前記シーケンサは、前記第1の方向のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からシフトした中心周波数の励起パルスとの同時印加で前記第1スライスのスピンを励起し、該スピンのエコーを前記リードアウト傾斜磁場パルスの下で計測する第1のパルスシーケンスと、該第1のパルスシーケンスに対してスライス傾斜磁場パルスの極性およびリードアウト傾斜磁場パルスの極性を反転し、もって前記第2スライスのスピンを励起し、該スピンのエコーを反転したリードアウト傾斜磁場パルスの下で計測する第2のパルスシーケンスと実行し、
前記演算手段は、前記第1、第2のパルスシーケンスで得られたエコー信号をそれぞれ逆フーリエ変換して前記第1、第2の投影像を導出し、前記投影像間の演算として、該第1、第2の投影像のスライス位置を合わせた後に両投影像の加算処理の演算を行うことを特徴とする磁気共鳴イメージング装置。 - 請求項1に記載の磁気共鳴イメージング装置において、
前記シーケンサは、
前記第1の方向で負極性のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からマイナスシフトした中心周波数の励起パルスとの同時印加で前記第1スライスのスピンを励起し、該スピンのエコーを正極性のリードアウト傾斜磁場の下で計測する第1のパルスシーケンスと、 前記第1の方向で正極性のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からマイナスシフトした中心周波数の励起パルスとの同時印加で前記第2スライスのスピンを励起し、該スピンのエコーを負極性のリードアウト傾斜磁場の下で計測する第2のパルスシーケンスと、
前記第1の方向で正極性のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からプラスシフトした中心周波数の励起パルスとの同時印加で前記第1スライスのスピンを励起し、該スピンのエコーを負極性のリードアウト傾斜磁場の下で計測する第3のパルスシーケンスと、
前記第1の方向で負極性のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からプラスシフトした中心周波数の励起パルスとの同時印加で前記第2スライスのスピンを励起し、該スピンのエコーを正極性のリードアウト傾斜磁場の下で計測する第4のパルスシーケンスとを実行し、
前記演算手段は、前記第1のパルスシーケンスで得たエコーの投影像と前記第3のパルスシーケンスで得たエコーの投影像との第1の差分処理により、静磁場不均一に起因する位相回りを除去した前記第1のスライスの投影像を導出し、前記第4のパルスシーケンスで得たエコーの投影像と前記第2のパルスシーケンスで得たエコーの投影像との第2の差分処理により、静磁場不均一に起因する位相回りを除去した前記第2のスライスの投影像を導出し、前記第1の差分処理の結果と、前記第2の差分処理の結果におけるスライス位置を合わせた後に相互の差分処理を行って前記第1スライスの投影像と第2スライスの投影像の間の位相差を求める
ことを特徴とする磁気共鳴イメージング装置。 - 請求項1に記載の磁気共鳴イメージング装置において、
前記正側のサブコイルの電源、及び負側のサブコイルの電源に前記シーケンサからの制御信号をそれぞれ伝達する経路の各々に、各サブコイルの駆動波形に対する遅延時間を調整する時間調整手段を備え、
前記シーケンサは、前記正側のサブコイル、負側のサブコイルの一方の駆動波形の遅延時間を複数とおり変更して前記正側のサブコイルと負側のサブコイルとの駆動タイミングのずれを検出するための複数のパルスシーケンスの実行を繰り返し、
前記演算手段は、各遅延時間の値に対する前記第1スライスの投影像と第2スライスの投影像の間の位相差の傾きの関係を求め、前記傾きがゼロになる遅延時間を前記正側のサブコイルと負側のサブコイルとの駆動タイミングのずれとする
ことを特徴とする磁気共鳴イメージング装置。 - 請求項5に記載の磁気共鳴イメージング装置において、前記演算手段が導出した前記正側のサブコイルと負側のサブコイルとの駆動タイミングのずれを前記時間調整手段に設定する手段をさらに備える
ことを特徴とする磁気共鳴イメージング装置 - 所定の検査空間に所定方向の静磁場を発生する手段と、
前記検査空間の磁場強度に直交3軸それぞれに沿った勾配をつける3方向の傾斜磁場を、それぞれ正側のサブコイル及び負側のサブコイルの発生磁場の合成により発生する傾斜磁場発生手段と、
前記検査空間におかれた被検体に高周波磁場を印加する手段と、
前記3方向の傾斜磁場それぞれの発生、前記対象への高周波磁場の印加、及びそれらにより前記被検体から発生する磁気共鳴信号の受信を制御するシーケンサと、
検出した磁気共鳴信号を処理する演算手段とを備える磁気共鳴イメージング装置における前記正側のサブコイルと負側のサブコイルとの駆動タイミングのずれを検出する方法であって、
検出の対象となる前記正側のサブコイルと負側のサブコイルとで発生する第1の方向のリードアウト傾斜磁場パルスの下で、傾斜磁場原点から前記正側のサブコイル寄りにシフトした位置の第1スライス、及び前記負側のサブコイル寄りにシフトした第2スライスのエコーのいずれかをそれぞれ計測する複数のパルスシーケンスを実行し、
前記演算手段で、前記複数のパルスシーケンスの各々で計測されるエコー信号をそれぞれ逆フーリエ変換して前記第1スライスの投影像、前記第2スライスの投影像を導出し、
前記投影像の相互の演算により、前記第1スライスの投影像と第2スライスの投影像の間の位相差を求め、
前記位相差の前記第1の方向に沿った位置に対する傾きをゼロにするために必要な前記正側のサブコイル、負側のサブコイルのいずれか一方の駆動タイミングの変更幅を算出し、前記正側のサブコイルと負側のサブコイルとの駆動タイミングのずれとする
ことを特徴とする磁気共鳴イメージング装置のタイミングずれの検出方法。 - 前記複数のパルスシーケンスは、前記第1の方向のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からマイナスシフトした中心周波数の励起パルスとの同時印加で前記第1スライスのスピンを励起し、該スピンのエコーを前記リードアウト傾斜磁場パルスの下で計測する第1のパルスシーケンスと、該第1のパルスシーケンスに対して励起パルスの中心周波数を前記ラーモア周波数からプラスシフトした中心周波数を持つものに代え、もって前記第2スライスのスピンを励起し、該スピンのエコーを前記リードアウト傾斜磁場パルスの下で計測する第2のパルスシーケンスと含み、
前記演算手段は、前記第1、第2のパルスシーケンスで得られたエコー信号をそれぞれ逆フーリエ変換して前記第1、第2の投影像を導出し、前記投影像間の演算として、該第1、第2の投影像のスライス位置を合わせた後に両投影像の差分処理の演算を行う
ことを特徴とする請求項7記載の磁気共鳴イメージング装置のタイミングずれの検出方法。 - 前記複数のパルスシーケンスは、前記第1の方向のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からシフトした中心周波数の励起パルスとの同時印加で前記第1スライスのスピンを励起し、該スピンのエコーを前記リードアウト傾斜磁場パルスの下で計測する第1のパルスシーケンスと、該第1のパルスシーケンスに対してスライス傾斜磁場パルスの極性およびリードアウト傾斜磁場パルスの極性を反転し、もって前記第2スライスのスピンを励起し、該スピンのエコーを反転したリードアウト傾斜磁場パルスの下で計測する第2のパルスシーケンスとを含み、
前記演算手段は、前記第1、第2のパルスシーケンスで得られたエコー信号をそれぞれ逆フーリエ変換して前記第1、第2の投影像を導出し、前記投影像間の演算として、該第1、第2の投影像のスライス位置を合わせた後に両投影像の加算処理の演算を行うことを特徴とする請求項7記載の磁気共鳴イメージング装置のタイミングずれの検出方法。 - 前記複数のパルスシーケンスは、
前記第1の方向で負極性のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からマイナスシフトした中心周波数の励起パルスとの同時印加で前記第1スライスのスピンを励起し、該スピンのエコーを正極性のリードアウト傾斜磁場の下で計測する第1のパルスシーケンスと、
前記第1の方向で正極性のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からプラスシフトした中心周波数の励起パルスとの同時印加で前記第2スライスのスピンを励起し、該スピンのエコーを負極性のリードアウト傾斜磁場の下で計測する第2のパルスシーケンスと、
前記第1の方向で正極性のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からマイナスシフトした中心周波数の励起パルスとの同時印加で前記第1スライスのスピンを励起し、該スピンのエコーを負極性のリードアウト傾斜磁場の下で計測する第3のパルスシーケンスと、
前記第1の方向で負極性のスライス傾斜磁場パルスと静磁場強度に対応するラーモア周波数からプラスシフトした中心周波数の励起パルスとの同時印加で前記第2スライスのスピンを励起し、該スピンのエコーを正極性のリードアウト傾斜磁場の下で計測する第4のパルスシーケンスとを含み、
前記演算手段は、前記第1のパルスシーケンスで得たエコーの投影像と前記第3のパルスシーケンスで得たエコーの投影像との第1の差分処理により、静磁場不均一に起因する位相回りを除去した前記第1のスライスの投影像を導出し、前記第4のパルスシーケンスで得たエコーの投影像と前記第2のパルスシーケンスで得たエコーの投影像との第2の差分処理により、静磁場不均一に起因する位相回りを除去した前記第2のスライスの投影像を導出し、前記第1の差分処理の結果と、前記第2の差分処理の結果におけるスライス位置を合わせた後に相互の差分処理を行って前記第1スライスの投影像と第2スライスの投影像の間の位相の差分を求める
ことを特徴とする請求項7記載の磁気共鳴イメージング装置のタイミングずれの検出方法。
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US10429461B2 (en) | 2019-10-01 |
JPWO2014112235A1 (ja) | 2017-01-19 |
US20160025824A1 (en) | 2016-01-28 |
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