WO2016178413A1 - 磁気共鳴イメージング装置 - Google Patents

磁気共鳴イメージング装置 Download PDF

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Publication number
WO2016178413A1
WO2016178413A1 PCT/JP2016/063421 JP2016063421W WO2016178413A1 WO 2016178413 A1 WO2016178413 A1 WO 2016178413A1 JP 2016063421 W JP2016063421 W JP 2016063421W WO 2016178413 A1 WO2016178413 A1 WO 2016178413A1
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Prior art keywords
magnetic field
gradient magnetic
shape
pulse
noise
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PCT/JP2016/063421
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English (en)
French (fr)
Japanese (ja)
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眞次 黒川
陽 谷口
由香里 山本
久晃 越智
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株式会社日立製作所
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves
    • A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
    • 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
    • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
    • 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

Definitions

  • the present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly to a technique for reducing noise during imaging.
  • an MRI apparatus magnetic resonance imaging apparatus
  • One of the problems in the MRI apparatus is the reduction of noise generated by switching the gradient magnetic field pulse at high speed.
  • noise reduction and image quality improvement (high-speed sequence adopted for that purpose), and it is important to reduce noise while suppressing deterioration in image quality as much as possible.
  • Patent Document 1 discloses internal repetition of a gradient magnetic field.
  • a technique is described that determines the slew rate and gradient magnetic field strength to achieve low noise with a fixed time (intra-repetition-interval).
  • Another approach is to determine an imaging sequence that can be realized within an acceptable noise level, as opposed to the above approach.
  • Patent Document 2 an acceptable noise level is set and a selected pulse is selected. It is disclosed that the noise level when the sequence is executed is calculated and the pulse sequence is changed when the sequence exceeds an allowable value.
  • Patent Document 3 proposes a technique related to a UI that calculates a noise level of a selected imaging sequence, displays the result, and allows an operator to change the imaging sequence according to an expected value of the noise level. ing.
  • Patent Document 4 discloses a technique for changing the shape of a gradient magnetic field pulse by providing a filter before the gradient magnetic field pulse generator.
  • Non-Patent Documents 1 and 2 propose changing the shape of the gradient magnetic field pulse to a shape typified by a sine wave instead of a trapezoid.
  • the gradient magnetic field pulse shape is based on a known model waveform such as a trapezoid or a sine wave, and the optimization is within the range of the model waveform. .
  • the conventional noise reduction approach is to optimize the imaging sequence when the noise level is kept within an allowable range, or to suppress the noise level under the conditions of the given imaging sequence. In doing so, it is necessary to determine the priority of noise suppression and image quality improvement in a trade-off relationship in consideration of the tolerance for the sound to be inspected and the image quality that is the purpose of the inspection.
  • An object of the present invention is to obtain a shape of a gradient magnetic field pulse having a noise suppressing effect, to provide an MRI apparatus that executes a pulse sequence including such a gradient magnetic field pulse, and further, an inspector has a degree of freedom.
  • An MRI apparatus that can determine the priority of noise suppression and image quality improvement is provided.
  • the subject of the present invention is further to present the relationship between the parameters such as the bandwidth and the noise level to the inspector in the MRI apparatus adopting the gradient magnetic field pulse whose waveform is not trapezoidal, and to set the imaging condition of the inspector. Including providing useful information.
  • the present inventor searches for a waveform of a gradient magnetic field pulse that can suppress the noise most under the condition of the shape considering the output of the MRI apparatus, and the waveform having a predetermined characteristic suppresses the noise most. I found out that I can do it.
  • the MRI apparatus of the present invention comprises a pulse sequence including a novel gradient magnetic field pulse having this specific waveform.
  • the MRI apparatus further includes means for providing an operator with imaging parameters related to noise in relation to noise when performing imaging with noise reduction. Further, the MRI apparatus of the present invention comprises means for allowing the operator to select the priority for noise reduction and image quality improvement.
  • the present invention by using a gradient magnetic field pulse having a specific waveform as a gradient magnetic field pulse applied for imaging, noise caused by application of the gradient magnetic field pulse can be minimized. Further, according to the present invention, it is possible to provide information that makes it easy for an operator to determine imaging conditions, and it is possible to simplify trial and error and complicated procedures for noise reduction.
  • FIG. 1 is a block diagram showing an overall outline of an MRI apparatus to which the present invention is applied.
  • Functional block diagram of control unit The figure which shows an example of the pulse sequence integrated in the MRI apparatus of this invention Diagram showing an example of gradient magnetic field pulse shape that minimizes noise Flow showing the procedure for obtaining the gradient magnetic field pulse shape of the first embodiment The figure explaining the step of FIG.
  • FPF frequency response function
  • required by the method of 2nd embodiment The figure which shows the shape after filtering the shape of FIG.
  • Flow showing procedure for obtaining gradient magnetic field pulse waveform according to third embodiment The figure explaining the procedure by 3rd embodiment.
  • Flow showing procedure of modified example of third embodiment The graph which shows the relationship (an example) between the imaging parameter used with the MRI apparatus of 4th embodiment, and a noise level
  • Graph showing another example of relationship between imaging parameters and noise level The figure which shows the other example of the display screen in the MRI apparatus of 4th embodiment.
  • the MRI apparatus of the present embodiment includes a magnetic field application unit that applies a high frequency magnetic field pulse and a gradient magnetic field pulse according to a pulse sequence that describes the application intensity and timing of the high frequency magnetic field pulse and the gradient magnetic field pulse, and a magnetic field applied by the magnetic field application unit. And a receiving unit that receives a nuclear magnetic resonance signal generated from the inspection object.
  • at least one type of gradient magnetic field pulse included in the pulse sequence used for controlling the magnetic field application unit satisfies the slew rate of the gradient magnetic field in the magnetic field application unit and is symmetric. It has the required shape that minimizes noise.
  • One aspect of the shape of the gradient magnetic field pulse that minimizes noise is a shape depicted by a curve having three or more inflection points.
  • the shape of the gradient magnetic field pulse that minimizes the noise is, for example, a shape formed by sequentially stacking a plurality of minute rectangles until a predetermined area is reached, and until the final gradient magnetic field pulse shape is formed by stacking the minute rectangles. Is a shape obtained by updating the provisional shape so as to be a shape that minimizes noise.
  • the initial value is a model combining a plurality of waveforms specified by the shape parameter, and the gradient magnetic field pulse shape obtained when the shape parameter is changed becomes a shape that minimizes noise. It is the shape obtained by repeating the update.
  • FIGS. 1 and 2 the configuration of the MRI apparatus of the present embodiment will be described in detail with reference to FIGS. 1 and 2.
  • those having the same function are denoted by the same reference numerals, and repeated explanation thereof is omitted.
  • An MRI apparatus 100 shown in FIG. 1 obtains a tomographic image of a subject 101 by using an NMR phenomenon, and a static magnetic field generation unit 120 that generates a static magnetic field and a subject 101 disposed in the static magnetic field.
  • the gradient magnetic field generation unit 130 and the transmission unit 150 correspond to the magnetic field application unit of the present invention.
  • the static magnetic field generator 120 generates a uniform static magnetic field in the space around the subject 101.
  • a static magnet type, normal conduction type, or superconducting type static magnetic field generation source arranged around the subject 101 is used. Prepare. There are a vertical magnetic field method and a horizontal magnetic field method depending on the direction of the generated static magnetic field. In the vertical magnetic field method, a static magnetic field is generated in a direction perpendicular to the body axis. In the horizontal magnetic field method, a static magnetic field is generated in the body axis direction.
  • the gradient magnetic field generation unit 130 is a gradient magnetic field coil 131 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (apparatus coordinate system) of the MRI apparatus 100, and a gradient magnetic field power source that drives each gradient magnetic field coil. 132.
  • the gradient magnetic field coil 131 drives the gradient magnetic field power supply 132 of each gradient magnetic field coil 131 in accordance with a command from the sequencer 140, thereby generating gradient magnetic field pulses Gx, Gy, Gz in the three axis directions of X, Y, and Z. Apply.
  • Each of the gradient magnetic field pulses Gx, Gy, and Gz is applied in a direction orthogonal to the slice plane (imaging cross section) at the time of imaging, and sets the slice plane for the subject 101 and is orthogonal to the set slice plane.
  • the MRI apparatus of this embodiment is characterized in that a pulse having a shape that minimizes noise is prepared as a pulse applied by the gradient coil 131.
  • the transmitter 150 irradiates the subject 101 with a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the biological tissue of the subject 101.
  • RF pulse high-frequency magnetic field pulse
  • the high frequency oscillator 152 generates an RF pulse and outputs it at a timing according to a command from the sequencer 140.
  • the modulator 153 amplitude-modulates the output RF pulse.
  • the high-frequency amplifier 154 amplifies the amplitude-modulated RF pulse and supplies the amplified RF pulse to the transmission coil 151 disposed close to the subject 101.
  • the transmission coil 151 irradiates the subject 101 with the supplied RF pulse.
  • the receiving unit 160 detects a nuclear magnetic resonance signal (echo signal, NMR signal) emitted by nuclear magnetic resonance of the nuclear spin constituting the living tissue of the subject 101, and receives a high-frequency coil (receiving coil) on the receiving side. 161, a signal amplifier 162, a quadrature detector 163, and an A / D converter 164.
  • the reception coil 161 is disposed in the vicinity of the subject 101 and detects an NMR signal of the response of the subject 101 induced by the electromagnetic wave irradiated from the transmission coil 151.
  • the detected NMR signal is amplified by the signal amplifier 162 and then divided into two orthogonal signals by the quadrature phase detector 163 at the timing according to the command from the sequencer 140, and each is digitally converted by the A / D converter 164. The amount is converted and sent to the controller 170.
  • the receiving coil 161 may be composed of a single coil, a multiple array coil (phased array coil) in which a plurality of small coils are combined is often used.
  • the sequencer 140 functions as an imaging control unit together with the control unit 170, operates in accordance with instructions from the control unit 170, sends various commands necessary for collecting tomographic image data of the subject 101, the transmission unit 150, and the gradient magnetic field.
  • the data is transmitted to the generation unit 130 and the reception unit 160.
  • the sequencer 140 repeatedly applies the RF pulse and the gradient magnetic field pulse according to a predetermined pulse sequence.
  • the pulse sequence describes (determines) the high-frequency magnetic field, gradient magnetic field, signal reception timing and intensity, and there are various pulse sequences (basic pulse sequences) that differ depending on the imaging method. Stored in the storage device 172 in advance.
  • the control unit 170 controls the entire MRI apparatus 100, performs operations such as various data processing, displays and stores processing results, and includes a CPU 171, a storage device 172, a display device 173, and an input device 174.
  • the storage device 172 includes an internal storage device such as a hard disk and an external storage device such as an external hard disk, an optical disk, and a magnetic disk.
  • the storage device 172 stores data necessary for the calculation of the CPU 171 and data in the middle of calculation or as a calculation result.
  • a gradient pulse shape that minimizes noise or a pulse sequence using the gradient pulse shape is stored together with the basic pulse sequence. As will be described in detail later, when the gradient magnetic field pulse shape that minimizes the noise is obtained under the conditions of a predetermined imaging parameter, the pulse shape is the imaging parameter used as the condition. Stored together.
  • the display device 173 is a display device such as a CRT or a liquid crystal.
  • the input device 174 is an interface for inputting various control information of the MRI apparatus 100 and control information of processing performed by the control unit 170, and includes, for example, a trackball or a mouse and a keyboard.
  • the input device 174 is disposed in the vicinity of the display device 173. The operator interactively inputs instructions and data necessary for various processes of the MRI apparatus 100 through the input device 174 while looking at the display device 173. Further, it is possible to input imaging parameters necessary for executing the pulse sequence according to the target imaging method.
  • the CPU 171 implements each process of the control unit 170 such as control of the operation of each unit of the MRI apparatus 100 and various data processing by executing a program stored in advance in the storage device 172 according to an instruction input by the operator. .
  • the CPU 171 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 101 as a result on the display device 173.
  • processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 101 as a result on the display device 173.
  • it is stored in the storage device 172.
  • the data from the receiving unit 160 is normally data arranged in a data space called k-space.
  • the coordinates (position) of the k space are determined by the application amount of the gradient magnetic field pulse given to the echo signal, and the points (k space matrix) to be acquired are determined according to the image size and the visual field.
  • the control unit 170 controls the pulse sequence so as to measure points to be acquired, obtains data, and performs predetermined image processing on the data set obtained by measurement to obtain an image.
  • the CPU 171 has a function for creating a pulse sequence that minimizes noise.
  • the CPU 171 can include a pulse sequence creation unit 180, a noise level calculation unit 181, and a noise level comparison unit 182.
  • a display control unit 183 is provided for causing the display device 173 to display information necessary when the operator creates or designs a pulse sequence. Some or all of these may be omitted depending on the embodiment of the present invention.
  • the pulse sequence creation unit 180 reads the basic pulse sequence and the imaging parameters stored in the storage device 172, and creates a pulse sequence (hereinafter also referred to as an imaging sequence to distinguish from the basic sequence) executed by the sequencer 140. At this time, if imaging that minimizes noise is selected by the operator, an imaging pulse sequence is created using the gradient magnetic field pulse shape stored in the storage device 172.
  • the noise level calculation unit 181 calculates the noise level of the created imaging pulse sequence using a frequency response function (FRF) unique to the apparatus.
  • FRF frequency response function
  • the noise level comparison unit 182 compares the noise levels calculated by the noise level calculation unit 181 for a plurality of imaging pulse sequences, and causes the display device 173 to display the comparison result.
  • each of these units corresponds to a program including a predetermined algorithm mounted on the CPU 171.
  • a part of this function may be replaced by hardware such as a well-known ASIC (Application Specific Integrated Circuit).
  • the pulse sequence creation unit 180 When imaging, the operator selects an arbitrary imaging method via the input device 174, so that the pulse sequence creation unit 180 reads a predetermined pulse sequence from the storage device 172 and stores the gradient magnetic field pulses stored in the storage device 172. A predetermined waveform is selected from the shape.
  • the pulse sequence creation unit 180 selects these pulse sequences, gradient magnetic field pulse shapes, and imaging parameters selected by the operator via the input device 174, for example, echo time (TE), repetition time (TR), inter-echo time (IET). ),
  • An imaging sequence executed by the sequencer 140 is created using an imaging field of view (FOV) or the like.
  • FOV imaging field of view
  • the gradient magnetic field pulses generally used for imaging include a slice selection gradient magnetic field pulse, a phase encode gradient magnetic field pulse and its rephase or dephase pulse, a frequency encode gradient magnetic field pulse and its rephase or dephase pulse, a spoiler pulse, etc.
  • the specific gradient magnetic field pulse can be used for any of these various gradient magnetic field pulses.
  • the frequency encoded gradient magnetic field pulse that is essential for the pulse sequence and has a large influence on noise. The case where it applies to is demonstrated.
  • the type of the basic pulse sequence is not particularly limited.
  • the FSE (Fast Spin Echo) sequence shown in FIG. 3 will be described.
  • the horizontal axis RF indicates the RF pulse application timing
  • Gs, Gp, and Gf indicate the slice gradient magnetic field, phase encode gradient magnetic field, and frequency encode gradient magnetic field, respectively.
  • the sampling time of the echo signal is set within the application time of the frequency encode pulse.
  • the frequency encode pulse 301 and its rephase pulse 302 are each trapezoidal as shown in FIG.
  • the gradient magnetic field pulse employed in this embodiment is a gradient magnetic field pulse that has a special waveform 400 as shown in FIG. 4 and minimizes noise under given conditions.
  • FSF frequency response characteristics
  • a method for obtaining a gradient magnetic field pulse shape that minimizes noise will be described.
  • a target waveform area is set, and a shape condition is determined in consideration of feasibility by the MRI apparatus.
  • the area of the waveform is determined by the encoding time and the gradient magnetic field strength (in other words, the reception bandwidth BW), but here, a provisional value may be set to determine the shape.
  • conditions relating to imaging parameters such as the number of echo trains and inter-echo time IET may be set. Since these imaging parameters affect the frequency spectrum of the gradient magnetic field, and the noise level generated changes as it changes, these imaging parameters may also be parameters for minimizing noise. Then, to make the explanation simple, it is fixed.
  • a slew rate of the gradient magnetic field (Slew Rate) is limited, and a shape condition that only one extreme value is included in a section in which the symmetric and positive / negative signs are the same is added.
  • the slew rate of the gradient magnetic field is a change amount per unit time of the gradient magnetic field strength T / m generated by the apparatus, and is determined by the performance of the gradient magnetic field amplifier that drives the gradient magnetic field coil.
  • the gradient is limited by the slew rate.
  • the condition of symmetry is intended to simplify the calculation, and the condition of having only one extreme value in the interval where the sign of the sign is the same is intended to prevent large fluctuations in the gradient magnetic field. This is because if the gradient magnetic field greatly fluctuates during reception, the substantial bandwidth (BWeff) at each sampling frequency fluctuates greatly, and it is considered that noise characteristics become unnatural and a desirable image quality is not obtained.
  • the substantial bandwidth is defined by the following equation (1).
  • BWeff ( ⁇ / 2 ⁇ ) ⁇ G ⁇ FOV (1)
  • ⁇ the magnetic rotation ratio
  • G the gradient magnetic field strength
  • FOV the imaging field of view.
  • noise is proportional to the square root of the bandwidth ( ⁇ (BW))
  • BWeff includes the influence of sampling density in BW.
  • a state in which no gradient magnetic field is applied is set as an initial state (S101), and a step of adding a small gradient magnetic field pulse shape (S102, S103) is repeated until the target area is reached.
  • Adding a small gradient magnetic field pulse shape The rectangle added in step S103 indicates that the gradient magnetic field pulse that is formed when it is added to the gradient magnetic field pulse shape that is being updated depends on the set condition (only one extreme value in the same symmetrical and positive / negative sign interval). (S102).
  • the condition of symmetry can be satisfied by, for example, arranging one rectangle in the center.
  • the small gradient magnetic field pulse shape is composed of various rectangles having the same area which is sufficiently small, for example, a size of 1 / 10,000 of the final area.
  • the slew rate may be limited by the maximum slew rate (T / m / s) of the line connecting the vertices of the superimposed rectangles.
  • the selection of a rectangle that minimizes noise is performed by multiplying the frequency spectrum of the waveform when the candidate rectangle is added to the gradient magnetic field pulse shape being updated by the frequency response function (FRF) of the sound pressure. This is done by calculating the level and selecting the rectangle that minimizes the noise level.
  • FPF frequency response function
  • the frequency response function As the frequency response function, a value obtained in advance as a value unique to the apparatus is used.
  • a weighted A-weighted characteristic that takes into account the human hearing that is the inspection target of the MRI apparatus is used.
  • FIG. 1 Using the FRF weighted with the A characteristic and the frequency spectrum of the gradient magnetic field waveform, the time-average noise level LA eq is calculated by the following equation.
  • G (f) is the frequency spectrum of the gradient magnetic field waveform
  • FRF A (f) is the FRF including the weighting of the A characteristic
  • P0 is the reference sound.
  • G (f) is, for example, Fourier transform of the gradient magnetic field pulse shape of Gs, Gp, and Gf in FIG. IET, rephase pulse time, encoding time, etc. will affect G (f).
  • the step S103 selects a rectangle that minimizes the noise level in this way, and obtains a waveform having a target area while repeating the accumulation of rectangles that satisfy the same condition on the rectangle.
  • a gradient magnetic field pulse waveform 410 as shown in FIG. 8 is finally obtained.
  • the waveform finally obtained has a high frequency. It is not a smooth curve with corresponding irregularities.
  • post-processing for example, filtering processing such as moving average filtering
  • filtering processing such as moving average filtering
  • the waveforms 400 and / or 410 may be obtained for each condition by changing conditions such as the rephase pulse time / echo time, the number of echo trains, and the inter echo time IET.
  • FIG. 9 shows gradient magnetic field pulse shapes obtained by varying the rephase pulse time and the echo encoding time.
  • the method of obtaining is the same as that shown in the flow of FIG. 5, but the obtained waveform has a shape in which a convex waveform is superimposed on a substantially trapezoidal waveform.
  • Each of the waveforms shown in FIGS. 8 and 9 has the following characteristics. First of all, it is clear from the premise of how to find out, but it has only one extreme value in the same interval where the sign is symmetric and has the same sign. In addition, there are a number of inflection points in both cases, considering fine irregularities. In particular, in FIG. 9, even if only the section in which the gradient magnetic field strength is positive is considered, the shape is such that two trapezoids overlap each other, and there are four inflection points even if fine irregularities are ignored.
  • the gradient magnetic field pulse shape calculation by the greedy method described above may be performed by a computer different from the MRI apparatus shown in FIG. 1, or may be performed by the pulse sequence creation unit 180 of the MRI apparatus.
  • the above-described calculation of the gradient magnetic field pulse shape employs a method of determining the brute force by the greedy method, and therefore is not suitable for calculation with an MRI apparatus every time imaging is performed. Therefore, in practice, the optimum gradient magnetic field pulse shape is calculated in advance under some conditions (for example, rephase pulse time, encode pulse time, reception bandwidth), and the optimum gradient for each condition is calculated. It is preferable to store the magnetic field pulse shape and select the optimum gradient magnetic field pulse shape according to the conditions set or selected during imaging.
  • the waveform obtained by another computer is stored in the storage device 172.
  • the pulse sequence creation unit 180 reads the gradient magnetic field pulse shape stored in the storage device 172 and the imaging parameter conditions for obtaining the gradient magnetic field pulse shape, and is read for the basic pulse sequence.
  • a pulse sequence to be actually executed is created by applying shapes and conditions.
  • the operation of the MRI apparatus of this embodiment is the same as that of the conventional MRI apparatus, and detailed description thereof is omitted.
  • the operator selects a basic pulse sequence and sets imaging when setting imaging conditions. Set the parameters.
  • an imaging for minimizing noise is selected, or selection of a gradient magnetic field pulse for minimizing noise is instructed.
  • the pulse sequence creation unit changes the basic pulse sequence using the gradient magnetic field pulse waveform stored in the storage device 172 and passes it to the sequencer 140.
  • the application of the RF pulse and the gradient magnetic field pulse and the measurement of the echo signal are repeatedly performed, and the tomographic image and spectrum image of the subject are reconstructed based on the measured echo signal.
  • the display on the display device is the same as that of the conventional MRI apparatus.
  • FIG. 10 shows a gradient magnetic field pulse that minimizes noise at each rephase pulse time by changing the rephase pulse time while keeping the total application time of the rephase pulse and the frequency encode pulse constant. It is a graph showing calculated values.
  • FIG. 10 shows noise levels (calculated values) when trapezoidal gradient magnetic field pulses are used with the same rephase pulse time (conventional method).
  • the gradient magnetic field pulse of this embodiment can obtain a noise reduction effect of about 10 dB compared to the conventional gradient magnetic field pulse at any rephase pulse time. That is, the sound pressure of noise can be reduced to about one third. It can also be seen that the noise reduction effect is greatest when the rephase pulse time is about 4 ms.
  • the gradient magnetic field pulse that minimizes noise is a frequency encode pulse combined with a rephase pulse, and the frequency encode pulse is obtained under the condition that it has only one extreme value in the same positive / negative sign interval.
  • the conditions for the shape are not limited to those described in the first embodiment, and can be added or changed as appropriate.
  • the present invention can also be applied to a phase pulse itself, a slice selective gradient magnetic field pulse other than a frequency encode pulse, a phase encode gradient magnetic field pulse, or a spoiler pulse.
  • the pulse sequence and the gradient magnetic field waveform are stored in the storage area.
  • the pulse sequence generation unit 180 may generate the pulse sequence every time imaging conditions are input.
  • ⁇ Modification 1> the condition that the frequency encode pulse shape has only one extreme value in the section where the signs of the positive and negative signs are the same is used. However, this modified example is characterized in that this condition is excluded. Other conditions are the same as in the first embodiment, and are symmetry, maximum slew rate, area, rephase pulse time, and encode pulse time.
  • the method for obtaining the gradient magnetic field pulse shape is the same as the procedure shown in FIG. 5 except that the initial setting conditions are different.
  • the rectangle added in step S102 is an axis passing through the center along the time axis of the frequency encoding pulse as shown in FIG. It becomes a set of rectangles arranged symmetrically. Since a set of rectangles may touch at the center, in this case, it is substantially the same as arranging one rectangle.
  • a noise level is calculated when such a rectangular group is arranged, and a rectangular group that minimizes the noise level is determined.
  • FIG. 12 shows the final gradient magnetic field pulse shape obtained by setting the rephase pulse time to 2.276 ms.
  • each has an independent convex shape at the center, and in the frequency encoding section. It has the feature of having three or more inflection points.
  • the present modified example has fewer conditions than the first embodiment, and the gradient magnetic field pulse shape obtained in the present modified example can be said to be a shape that prioritizes the noise reduction effect.
  • ⁇ Modification 2> This modification is characterized in that a condition for the maximum gradient magnetic field strength is further added to the condition for the frequency encoding pulse shape in the first embodiment. Other conditions are the same as in the first embodiment, and only one extreme value exists in the section where the positive and negative signs are the same, the symmetry, the maximum slew rate, the area, the rephase pulse time, and the encode pulse time.
  • the method for obtaining the gradient magnetic field pulse shape is the same as the procedure shown in FIG. 5 except that the initial setting conditions are different.
  • the rectangle to be added in step S102 it is a condition that the gradient magnetic field pulse formed by the addition of the rectangle does not exceed the maximum gradient magnetic field strength.
  • FIG. 13 shows the gradient magnetic field pulse shape obtained with the rephase pulse time of 2.276 ms in this modification.
  • the shape of FIG. 13 is compared with the gradient magnetic field pulse shape (FIG. 9) obtained at the same rephase pulse time in the first embodiment, each of them has a width smaller than that at the center of the trapezoid having the same width as the pulse width. It has a shape in which narrow convex portions overlap each other and has three or more inflection points including two inflection points at both ends of the narrow convex portion.
  • the height of the waveform is lower than the waveform of FIG.
  • a gradient magnetic field pulse having a specific shape that minimizes noise is used as a gradient magnetic field pulse not accompanied by a rephase pulse or a reverse polarity pulse.
  • the imaging sequence to be employed is not particularly limited.
  • the imaging sequence can be applied to the rephase pulse 302 and the slice selection gradient magnetic field pulse 305.
  • gradient magnetic field pulse shape conditions are set as the initial setting (S101).
  • the conditions are a rephase pulse time, a maximum slew rate, a symmetric and positive / negative sign having only one extreme value, and an area.
  • the initial value of the shape is the smallest rectangle whose side length is the rephase pulse time. After that, when a rectangle was added, a rectangle that satisfies the two conditions of having only one extreme value in the same slew rate and symmetric, positive and negative signs in the same interval was determined and added.
  • the shape is updated (S102, S103).
  • the determination as to whether or not the noise is minimized is made for the maximum slew rate and all the rectangles that can be added that satisfy the two conditions of having only one extreme value in the same symmetric and positive and negative signs in the same interval (1 ) To calculate the noise level and compare the noise levels.
  • FIG. 14 shows the rephase pulse obtained by such a method.
  • This waveform also has fine discontinuities due to the upper limit of the frequency in the FRF.
  • post-processing filtering
  • Incorporation is preferred.
  • the present embodiment is characterized in that a gradient magnetic field pulse shape that minimizes noise is obtained from a combination of a plurality of waveform models.
  • the use of the obtained gradient magnetic field pulse shape for the pulse sequence selected during imaging is the same as in the first embodiment.
  • the apparatus configuration is also the same as that of the first embodiment shown in FIGS. 1 and 2, and thus the description thereof will be omitted and different points will be described.
  • FIGS. 15 and 16 respectively show a case where there is no condition having only one extreme value in the same positive / negative sign interval (modified example 1 of the first embodiment) and a single extreme value in the same positive / negative sign interval.
  • FIG. 16 shows the change of the frequency spectrum and the result of multiplying it by FRF and A characteristics as the shape progresses.
  • a gradient magnetic field pulse shape that minimizes noise is obtained using a model in which a pulse having a reverse polarity in the same shape as a rephase pulse is added to a frequency encoding gradient magnetic field pulse.
  • FIG. 17 shows an example of a plurality of waveform models used in this embodiment.
  • a model is formed by superimposing five trapezoids.
  • the five trapezoids two are rephase pulses G1 and G1 ', and the central pulse is a normal frequency encoding pulse G3 included in the basic pulse sequence.
  • the pulses G2 and G2 'adjacent to the rephase pulses G1 and G1' have the same application time, rise time, and reverse polarity as the rephase pulses G1 and G1 ', respectively.
  • the noise reduction effect is the greatest.
  • the parameter search may be performed by using a known optimization technique or by brute force.
  • This shape is the same as the gradient magnetic field pulse shape (FIG. 8) obtained by the same IET, rephase pulse time, and encoding time in the first embodiment, and it can be seen that this modeling is appropriate.
  • the waveform (FIG. 18) obtained by modeling may be smoothed by filtering as in the first embodiment.
  • FIG. 19 shows a waveform obtained by applying a moving average filter of 0.6 ms to the waveform of FIG.
  • the noise level (calculated value) of this frequency encoding gradient magnetic field is ⁇ 51.9 dB, which is equivalent to the noise level ( ⁇ 52.4 dB) of the frequency encoding gradient magnetic field of the first embodiment obtained under the same conditions. It can be seen that the effect of the optimized shape can be obtained even with the filter alone.
  • FIG. 17 shows a model in which two pulses having the same shape as the rephase pulse and opposite polarity are added to the two rephase pulses and the frequency encode pulse, but the model is not limited to this.
  • the model is not limited to this.
  • the rise time of the rephase pulse G1, the rise time of the frequency encode pulse G3, the rise time of the frequency encode pulse G3, and the area ratio of one frequency encode pulse G3 to the entire frequency encode pulse are used as parameters. A set of parameters that can obtain a noise reduction effect is searched.
  • the shape of the pulse constituting the model is a trapezoid.
  • the shape is not limited to a trapezoid as long as the parameter can be searched analytically.
  • a sine wave from 0 to ⁇
  • a sine square is used. It is also possible to use waves, quadratic functions, etc.
  • calculation of the gradient magnetic field pulse shape using the model may be performed by a computer different from the MRI apparatus, or may be performed by the pulse sequence creation unit 180 of the MRI apparatus. According to the present embodiment, by using a model, it is possible to calculate the optimum gradient magnetic field pulse shape in a shorter time than in the first embodiment.
  • the gradient magnetic field pulse shape that minimizes the noise by fixing the imaging parameter is obtained, but in this embodiment, the imaging parameter itself is used as a condition for obtaining the gradient magnetic field pulse shape that minimizes the noise. It is a feature to add. Imaging parameters to be added as conditions are not particularly limited, and examples include repetition time (TR), echo interval (IET), rephase pulse time, and the like. Add as a condition.
  • the predetermined width is a range of imaging parameter values allowed in a general pulse sequence.
  • This embodiment is different from the first and second embodiments only in the method for obtaining the gradient magnetic field pulse shape, and the other configuration and imaging method are the same, so the description thereof will be omitted.
  • the gradient magnetic field pulse shape according to the present embodiment The procedure for obtaining is described with reference to FIG.
  • the initial value may be an arbitrary value such as a minimum value, a maximum value, or a median value of a predetermined width of each imaging parameter.
  • the same conditions as those of the first embodiment or the modified example 1 or 2 are provided for the shape (S101). That is, for example, the maximum slew rate, a symmetric and positive / negative sign in the same section, has only one extreme value or simply a symmetric shape, and an area of a waveform (pulse).
  • the shape of the gradient magnetic field pulse that minimizes the noise is obtained (S102 ').
  • This step S102 ' may employ either the above-described first embodiment (including a modified example) or the method employed in the second embodiment (step S102 in FIG. 5). If it is the method of 1st embodiment, the procedure of determining the rectangle which minimizes a noise when the rectangle of a unit area is piled up sequentially is repeated. At that time, if the shape condition includes only one extreme value with the same sign, the maximum gradient magnetic field strength does not include only one extreme value with the same sign. There may be cases where additional conditions are added.
  • a modeled waveform is used as an initial value, and a parameter that gives a shape that minimizes noise is searched from among parameters that define the model, and finally a waveform that minimizes noise.
  • a model a model combining trapezoids as shown in FIGS. 17 and 20 or a model combining shapes other than trapezoids can be adopted.
  • step S102 ′ when calculating the noise level when a rectangle of unit area is added, the noise level is taken into account when the imaging parameter is changed, that is, by taking into account the repeating pattern of the gradient magnetic field pulse. Is calculated, and the shape and imaging parameters that minimize the noise level are obtained. In the case of modeling, a combination of parameters that minimizes the noise level is obtained for all combinations of the shape parameter and the imaging parameter.
  • a noise level is calculated when a provisional gradient magnetic field pulse obtained by a combination of one imaging parameter is used as another imaging parameter.
  • a combination of imaging parameters that minimizes may be obtained.
  • the combination of imaging parameters being updated is provisional, and in the next iteration, the updated gradient magnetic field pulse is updated to a combination of imaging parameters determined to minimize noise.
  • step S103 the gradient magnetic field pulse shape obtained in step S102 'is set as a new gradient magnetic field pulse shape. That is, the gradient magnetic field pulse shape to be processed in step S102 'is updated. Steps S102 'and S103 are repeated until the updated gradient magnetic field pulse shape reaches the set gradient magnetic field pulse area (S104).
  • the combination of the gradient magnetic field pulse shape and the imaging parameter that ultimately minimizes the noise is finally determined by the above processing.
  • the gradient magnetic field pulse shape and the imaging parameters thus obtained are stored in the storage device 172.
  • the pulse sequence creation unit 180 reads the gradient magnetic field pulse shape and imaging parameters stored in the storage device 172, changes the basic pulse sequence, and is actually used for imaging. An imaging sequence is created and passed to the sequencer 140.
  • an imaging parameter can be set when an operator selects a pulse sequence.
  • the imaging parameters determined when the gradient magnetic field pulse shape that minimizes noise is obtained on the imaging parameter setting screen are as follows. The value may be displayed by default. Further, another imaging parameter may be set by the operator.
  • the pulse sequence creation unit 180 uses the gradient magnetic field pulse shape read from the storage device 172, and a new one set by the operator. An imaging sequence is created based on the basic pulse sequence using the imaging parameters. Also in this case, since the shape of the gradient magnetic field pulse is a form that reduces noise, noise can be greatly reduced compared to the case where a pulse sequence using a basic trapezoidal gradient magnetic field pulse is executed.
  • ⁇ Modification example of the third embodiment> when obtaining the gradient magnetic field pulse shape that minimizes noise, adding the imaging parameter width as a condition, and obtaining the optimum gradient magnetic field pulse shape within this width are the same as in the third embodiment. It is. However, in this modified example, the imaging parameter condition is handled in the same way as the shape condition as in step S102 ′ of the third embodiment, and the gradient magnetic field pulse shape is not searched but applied step by step.
  • imaging parameters are initially set, and a gradient magnetic field pulse shape that minimizes noise is determined based on the imaging parameters.
  • the imaging parameter may be one or a plurality of combinations.
  • the noise level at this time is calculated and recorded (S201).
  • change the imaging parameters or a combination thereof determine the gradient magnetic field pulse shape that minimizes the noise in the same way, calculate the noise level at this time, and compare it with the noise level in the conditions of the imaging parameters recorded before that. Then, it is updated to one having a lower noise level (S202).
  • Step S202 is repeated while changing the imaging parameters (S203), and finally the gradient magnetic field pulse shape when noise can be reduced most among all the imaging parameters or combinations is determined.
  • the update convergence condition in step S203 methods such as the steepest descent method and the bisection method can be applied. Or you may perform step S202 about all the imaging parameters by the brute force method. In the case of the round robin method, since all changes are examined in one step S202, step S203 is omitted.
  • the gradient magnetic field pulse for minimizing noise and the imaging parameter as the condition at that time may be calculated by the MRI apparatus (pulse sequence creation unit 180). What is calculated in advance by the computer may be stored in the storage device 172 of the MRI apparatus. In addition, when calculating in advance, gradient magnetic field pulses that minimize noise obtained under a plurality of conditions with different imaging parameters or combinations thereof are stored, and those closest to the imaging parameters selected by the operator at the time of imaging are stored. May be read from the storage device 172 and executed. Alternatively, some candidates may be presented to the operator via the display device 173 and selected. The GUI for causing the operator to make a selection will be described in detail in the next embodiment.
  • a low noise gradient magnetic field pulse that takes imaging parameters into consideration is used, so that the burden on the subject due to noise can be reduced.
  • the present embodiment is characterized in that it generates information for an operator to select an image that minimizes noise and an image that prioritizes image quality, and includes a GUI that presents the information to the operator.
  • the MRI apparatus according to the present embodiment includes a display control unit (183) that displays on the display device (173) the reception bandwidth that minimizes noise and the relationship between the reception bandwidth that minimizes noise and generated noise. ).
  • the MRI apparatus of the present embodiment is based on the display control unit (183) that displays a UI for selecting the priority for noise reduction and image quality improvement on the display device (173), and the priority selected via the UI.
  • An imaging control unit (control unit 170) that controls imaging parameters is further provided.
  • the pulse sequence creation unit uses a combination of gradient magnetic field pulse shape and imaging parameters that can minimize noise. Create an imaging pulse sequence that minimizes noise.
  • the operator can select whether or not to select an image that minimizes noise, but the image quality is slightly higher than that of an intermediate image, that is, an image that minimizes noise.
  • the image resolution, SNR, imaging time, and the like change monotonically with respect to the imaging parameters. Therefore, the operator can easily determine how to change the imaging parameters when trying to change them.
  • the noise level does not necessarily change monotonously with changes in the imaging parameters, so how much is changed to achieve the desired noise level while considering the trade-off with image quality. It is difficult to know what to do.
  • the MRI apparatus of the present embodiment includes means for providing the operator with the relationship between the imaging parameter and noise or information derived therefrom, thereby facilitating the setting of the imaging parameter by the operator.
  • the relationship between the noise and the imaging parameter is the noise level when the imaging parameter is changed in the process of obtaining the gradient magnetic field pulse shape that minimizes the noise with the predetermined imaging parameter by the method of the first to third embodiments. It can be derived by calculating the change. Alternatively, in the method of the modified example of the third embodiment, it is possible to derive the relationship between the noise level calculated when obtaining the gradient magnetic field pulse shape that minimizes the noise while changing the imaging parameter and the imaging parameter.
  • FIG. 24 shows the relationship between the average reception bandwidth BWave and the noise level as an example of the relationship between the imaging parameter and the noise level.
  • BWave is defined by the following equation (3) using the average gradient magnetic field strength Average (G) when the gradient magnetic field strength being received is not constant.
  • BWave ⁇ / (2 ⁇ ) ⁇ Average (G) ⁇ FOV (3)
  • the graph shown in FIG. 24 is obtained by plotting the noise level when the BWave is changed with respect to the pulse shape after obtaining the gradient magnetic field pulse shape that minimizes the noise with a predetermined BWave. The parameters are fixed.
  • the change in noise level is not monotonous with respect to the change in BWave.
  • the larger the BWave the better the image quality.
  • the change in the noise level is not known unless the BWave is changed. Therefore, the operator changes the BWave to grasp the tendency of the noise level change, for example, when the BWave is automatically determined by the MRI apparatus of the first embodiment and the gradient magnetic field pulse shape and the pulse sequence based on it are created. The operator needs to remember the BWave, then manually determine the BWave and start again, which complicates the procedure.
  • such a relationship between noise and imaging parameters is obtained in advance and stored in the storage device 172, and is presented when the imaging parameters are set by the operator.
  • the operator can determine how to change BWave, for example.
  • the information presented by the MRI apparatus can take various forms. For example, a graph as shown in FIG. 24 may be displayed on the imaging parameter setting screen of the display device 173, or as shown in FIG. A display block of the minimum noise BWave may be provided on such an imaging parameter setting screen, and the value of BWave that minimizes the noise may be displayed.
  • the BWave for minimizing noise is the minimum value of the graph shown in FIG. 24 and can be obtained by a normal optimization process.
  • the value of the minimum noise BWave when the operator wants to reduce the noise, it is only necessary to change the BWave so as to approach the BWave that minimizes the noise. It can also be seen that the displayed value is the BWave limit that minimizes noise.
  • FIG. 26 shows the relationship between BWk0 and the noise level. This relationship is also obtained by the same method as the relationship shown in FIG. 24, and it can be seen that the change in noise level is not monotonous with respect to the change in BWk0, that is, has a minimum value.
  • the graph and / or the minimum value of this relationship is displayed on the display device 173.
  • noise level As information displayed on the display device 173 for noise, in addition to the information based on the relationship between the noise and the imaging parameter described above, for example, the noise level itself, the noise level when the imaging parameter is varied (maximum value) , Minimum values, imaging parameters that take those values), and combinations of recommended imaging parameters.
  • Such information can be created by the functions of the noise level calculation unit 181 and the noise level comparison unit 182 of the CPU 171.
  • the imaging parameter for example, reception bandwidth
  • the imaging parameter that minimizes the noise and / or the relationship between the noise and the imaging parameter are displayed on the display device 173, so that the imaging parameter can be changed. It is possible to obtain information such as whether the noise level can be reduced by changing the imaging parameters, and taking appropriate images in consideration of the required image quality and subject noise tolerance Can do.
  • the noise level calculated by the noise level calculation unit 181 may be displayed by adding a display block for displaying the noise level, as shown in FIG.
  • an “undo” function and a “redo” function may be added as shown in FIG. 28 in order to set the parameters and the noise level before and after the change, as shown in FIG.
  • two parameter setting screens may be prepared, set while comparing the two, and finally, an imaging parameter may be determined by pressing a decision button.
  • an MRI apparatus with reduced noise is provided.
  • an MRI apparatus is provided that makes it easy for an operator to capture desired noise in consideration of the image quality for noise that has a trade-off relationship with image quality.
  • 120 Static magnetic field generation unit
  • 130 Gradient magnetic field generation unit
  • 140 Sequencer
  • 150 Transmission unit
  • 160 Reception unit
  • 170 Control unit (imaging control unit), 171 ... CPU, 172 ... storage device, 173 ... display device, 180 ... pulse sequence creation unit, 181 ... noise level calculation unit, 182 ... noise level comparison unit, 183 ... -Display control unit.

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