WO2013002233A1 - 磁気共鳴イメージング装置および傾斜磁場波形推定方法 - Google Patents
磁気共鳴イメージング装置および傾斜磁場波形推定方法 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
- G01R33/44—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
- G01R33/56518—Correction of image distortions, e.g. due to magnetic field inhomogeneities due to eddy currents, e.g. caused by switching of the 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
- G01R33/54—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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- 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
Definitions
- the present invention measures a nuclear magnetic resonance signal (hereinafter referred to as an NMR signal) from hydrogen, phosphorus, etc. in a subject and images a nuclear density distribution, relaxation time distribution, etc. (hereinafter referred to as MRI).
- NMR signal nuclear magnetic resonance signal
- MRI nuclear density distribution, relaxation time distribution, etc.
- the present invention relates to a technique for compensating for image quality deterioration caused by gradient magnetic field distortion.
- the MRI device measures NMR signals generated by the spins of the subject, especially the tissues of the human body, and visualizes the form and function of the head, abdomen, limbs, etc. in two or three dimensions Device.
- a subject is placed in a static magnetic field (polarization magnetic field B0), a high-frequency magnetic field pulse is applied together with a slice selective gradient magnetic field pulse to selectively excite a specific region, and then a phase encoding gradient magnetic field pulse is applied.
- polarization magnetic field B0 polarization magnetic field B0
- a high-frequency magnetic field pulse is applied together with a slice selective gradient magnetic field pulse to selectively excite a specific region, and then a phase encoding gradient magnetic field pulse is applied.
- a readout gradient magnetic field pulse the excitation range is encoded and position information is given.
- Gradient magnetic field pulses provide a desired gradient magnetic field by supplying a pulsed current (referred to as an input gradient magnetic field waveform) from a gradient magnetic field power source to a plurality of coils that generate linear gradient magnetic fields in three orthogonal axes.
- a pulse (referred to as an output gradient magnetic field waveform) is output.
- the input gradient magnetic field waveform and the gradient magnetic field waveform generated in the MRI apparatus should coincide with each other, but errors occur due to various factors.
- One of the causes of errors is the effect of eddy currents generated in magnetic parts or electric circuits constituting the MRI apparatus due to a sudden change in magnetic field due to a gradient magnetic field pulse.
- the eddy current gives a change in the magnetic field in the opposite direction to the change in the magnetic field due to the gradient magnetic field pulse depending on time and space, and distorts the output gradient magnetic field waveform.
- the output gradient magnetic field waveform of the gradient coil includes distortion such as response delay depending on the characteristic (Q value) of the gradient coil.
- a circuit that performs feedback control or the like to compensate for this includes a gradient magnetic field. It is often included in a gradient magnetic field generation system including a power source. Such a control circuit also serves to compensate for eddy currents, but can also contribute to generating an output gradient magnetic field waveform different from the input gradient magnetic field waveform.
- the distortion of the output gradient magnetic field waveform caused by the eddy current and the influence of the control circuit of the gradient magnetic field power supply varies depending on the shape of the input gradient magnetic field waveform, and causes various problems depending on the application axis of the gradient magnetic field.
- distortion of the slice selective gradient magnetic field pulse causes an error in the excitation profile and the excitation position.
- the influence is large in the case of VERSE (variable-rate-selective-excitation) method in which a high-frequency magnetic field pulse is applied while changing the application intensity of the gradient magnetic field pulse.
- the distortion of the readout gradient magnetic field pulse causes artifacts such as distortion and ghost in the image.
- EPI Echo Planar Imaging
- spiral measurement that scans the k-space spirally
- ultra-short TE measurement that starts scanning from the center of the k-space, etc.
- the influence of distortion is significant.
- One is to apply a magnetic field that compensates for eddy currents using shim coils or gradient magnetic field coils, etc., measuring the magnetic field generated by eddy currents after applying a gradient magnetic field pulse temporally and spatially, A waveform of a gradient magnetic field pulse that cancels the output characteristics of the eddy current obtained from the measurement result is output to compensate for distortion of the output gradient magnetic field waveform (Patent Documents 1 and 2).
- Patent Document 3 a method of changing the irradiation timing of high-frequency magnetic field pulses according to the output gradient magnetic field waveform.
- the distortion of the gradient magnetic field pulse is measured, the delay time is calculated from the center of gravity of the pulse area, and the timing of the high frequency magnetic field pulse and the gradient magnetic field pulse is changed based on the delay time, thereby reducing the excitation profile. Suppress.
- a method has been proposed in which a change in the output gradient magnetic field waveform corresponding to the input gradient magnetic field waveform is eliminated by using a fixed input gradient magnetic field waveform (Non-Patent Document 1). In this method, when changing the excitation width of the high frequency magnetic field pulse, the amplitude of the high frequency magnetic field pulse is adjusted instead of changing the input gradient magnetic field waveform.
- Non-Patent Document 2 A method for calculating the position and compensating for image distortion has been proposed (Non-Patent Document 2). In this method, the coefficient of the modeled mathematical formula is determined based on the appearance of the image. Also, instead of modeling the above method, a method has been proposed in which the coordinate position of the NMR signal in k-space is calculated from the result of actually measuring the gradient magnetic field waveform after the imaging is completed, and the distortion of the image is compensated (Non-Patent Document 3). ).
- each of the above-described compensation techniques for the output gradient magnetic field waveform has the following problems.
- Non-Patent Document 3 In the method of changing the irradiation timing of the high-frequency magnetic field pulse according to the output gradient magnetic field waveform (Patent Document 3), only the irradiation timing of the high-frequency magnetic field pulse is changed. Not compensated.
- adjusting the amplitude of the high-frequency magnetic field pulse using only the specific input gradient magnetic field waveform is the SAR (Specific Absorption Rate). There is a risk of increasing the reference value indicating the accumulated amount.
- Non-patent Document 2 if the system response does not fit the model formula by the RLC circuit, the distortion of the output gradient magnetic field waveform cannot be fully expressed, and the image distortion, ghost, etc. Will leave the artifact. This can occur when a complicated command such as feedback control is given by the gradient magnetic field power source. Further, since each coefficient of the model formula is determined based on the appearance of the image, there is a problem that the approximation accuracy of the model formula varies depending on the operator who determines the coefficient.
- Non-patent Document 3 In the method of measuring the gradient magnetic field waveform after imaging and using it for reconstruction (Non-patent Document 3), fluctuations due to noise are superimposed because the measured gradient magnetic field waveform is used to calculate k-space coordinates. When processing such as a filter is performed to reduce noise, distortion of the gradient magnetic field waveform due to the filter processing becomes a problem.
- an object of the present invention is to accurately calculate the distortion of the output gradient magnetic field waveform that changes in accordance with the input gradient magnetic field waveform and to perform various corrections using the calculated output gradient magnetic field waveform.
- the present invention for solving the above problems calculates an input gradient magnetic field waveform and an output gradient magnetic field waveform corresponding to the input gradient magnetic field waveform, and uses the input gradient magnetic field waveform and the output gradient magnetic field waveform to generate the output gradient magnetic field waveform.
- Various corrections are performed using the calculated value of the output gradient magnetic field waveform.
- the high-frequency magnetic field pulse set in the imaging sequence is corrected and / or the k-space coordinates of the nuclear magnetic resonance signal obtained in the imaging sequence are corrected.
- a response function is calculated by taking into account a plurality of elements that affect the output waveform of the gradient magnetic field, for example, the response functions of the control circuit of the eddy current and the gradient magnetic field power supply, and thereby the output of the gradient magnetic field. Since the waveform is calculated, not only eddy current but also the gradient magnetic field distortion caused by the control circuit can be compensated, and it could not be removed by the prior art without using a high-cost compensation gradient coil etc. Artifacts such as image distortion and ghosting can be suppressed.
- FIG. 1 is a block diagram showing an overall configuration of an embodiment of an MRI apparatus according to the present invention.
- This MRI apparatus uses an NMR phenomenon to obtain a tomographic image of a subject, and as shown in FIG. 1, a static magnetic field generation system 2, a gradient magnetic field generation system 3, a transmission system 5, and a reception system 6, a signal processing system 7, and a sequencer 4.
- the static magnetic field generation system 2 includes a static magnetic field generation source such as a permanent magnet, a normal conducting magnet, or a superconducting magnet, and generates a uniform static magnetic field in a space where the subject 1 is placed.
- a static magnetic field generation source such as a permanent magnet, a normal conducting magnet, or a superconducting magnet
- a uniform static magnetic field in a space where the subject 1 is placed there are a vertical magnetic field method, a horizontal magnetic field method, etc. If the vertical magnetic field method is used, the body axis is used in the direction perpendicular to the body axis in the space around the subject 1 and if the horizontal magnetic field method is used. Generate a uniform static magnetic field in the direction.
- the gradient magnetic field generation system 3 includes a gradient magnetic field coil 9 wound in the three axes of X, Y, and Z, which is a coordinate system (static coordinate system) of the MRI apparatus, and a gradient magnetic field power source 10 that drives each gradient magnetic field coil.
- the gradient magnetic fields Gx, Gy, and Gz are applied in the three axis directions of X, Y, and Z by driving the gradient magnetic field power supply 10 of each coil in accordance with a command from the sequencer 4 to be described later.
- a slice direction gradient magnetic field pulse is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and in the remaining two directions orthogonal to the slice plane and orthogonal to each other
- a phase encoding direction gradient magnetic field pulse and a frequency encoding direction gradient magnetic field pulse are applied, and position information in each direction is encoded in the echo signal.
- the sequencer 4 is a control means that repeatedly applies a high-frequency magnetic field pulse and a gradient magnetic field pulse in a predetermined pulse sequence, operates under the control of the signal processing system 7 (digital signal processing device 8), and tomographic image data of the subject 1 Various commands necessary for collection are sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6. There are various pulse sequences depending on the imaging method, and these are stored in advance on the magnetic disk 18 as a program.
- the transmission system 5 irradiates the subject 1 with a high frequency magnetic field pulse in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the biological tissue of the subject 1, and the high frequency oscillator 11, the modulator 12, and the high frequency It comprises an amplifier 13 and a high frequency coil (transmission coil) 14a on the transmission side.
- the high-frequency magnetic field pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the sequencer 4, and the amplitude-modulated high-frequency magnetic field pulse is amplified by the high-frequency amplifier 13 and then close to the subject 1.
- the subject 1 is irradiated with a high frequency magnetic field pulse.
- the receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1, and receives a high-frequency coil (receiving coil) 14b on the receiving side and a signal amplifier 15 And a quadrature phase detector 16 and an A / D converter 17.
- the quadrature phase detector 16 divides the signal into two orthogonal signals at the timing according to the command from the sequencer 4, and each signal is converted into a digital quantity by the A / D converter 17 and sent to the signal processing system 7.
- the signal processing system 7 performs various data processing and display and storage of processing results.
- the digital signal processing device 8, an external storage device such as a magnetic disk 18 and an optical disk 19, a display 20, and a digital signal processing device 8 has internal memories such as ROM21 and RAM22 for storing the processing results of 8 and data used by the digital signal processing device 8, and when the data from the receiving system 6 is input to the digital signal processing device 8, the digital signal processing The apparatus 8 performs processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result on the display 20 and records it on the magnetic disk 18 of the external storage device.
- the digital signal processing device 8 performs calculation related to device characteristics using pre-measurement data, and calculation of parameters necessary for execution of the pulse sequence stored on the magnetic disk 18. Control the operation of sequencer 4.
- the operation unit 25 inputs various control information of the MRI apparatus and control information of processing performed by the signal processing system 7 and includes a trackball or mouse 23 and a keyboard 24.
- the operation unit 25 is disposed close to the display 20, and the operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.
- the high-frequency coil 14a and the gradient magnetic field coil 9 on the transmission side face the subject 1 in the static magnetic field space of the static magnetic field generation system 2 into which the subject 1 is inserted, in the case of the vertical magnetic field method. If the horizontal magnetic field method is used, the subject 1 is installed so as to surround it.
- the high-frequency coil 14b on the receiving side is installed so as to face or surround the subject 1.
- the MRI apparatus of the present embodiment further includes, as a function of the signal processing system 7, an input waveform (input gradient magnetic field waveform) to the gradient magnetic field coil 9 and a gradient magnetic field waveform (output gradient magnetic field waveform) generated thereby in the static magnetic field space. And a gradient magnetic field waveform calculating means for calculating an output gradient magnetic field waveform using the response function.
- the details of the digital signal processing apparatus 8 provided with these means are shown in FIG.
- the digital signal processing device 8 includes a response function calculation unit 81 that calculates a response function of the gradient magnetic field, and an output gradient magnetic field waveform of the gradient magnetic field used in the actual imaging sequence using the calculated response function.
- Output gradient magnetic field waveform calculating unit 82, gradient magnetic field waveform calculating unit 82, sequence calculating unit 83 for correcting high-frequency pulses and the like used in the imaging sequence using the output gradient magnetic field waveform calculated by gradient magnetic field waveform calculating unit 82, and gradient magnetic field waveform calculating unit 81 Using the calculated output gradient magnetic field waveform, the k-space coordinate calculation unit 84 corrects the coordinate position of the k-space where the NMR signal is arranged, and performs image reconstruction calculation using the NMR signal to generate image data.
- a component 85 is provided.
- the processing performed by the digital signal processing device 8 of the MRI apparatus of this embodiment is roughly divided into four processing.
- the relationship between the four processes will be described with reference to the block diagram of FIG.
- the first process 101 is a process performed by the response function calculation unit 81 as a pre-measurement, and is a process for calculating the response function 1112 of the control circuit of the eddy current and gradient magnetic field power supply.
- Processes 102 to 104 are processes performed in actual imaging.
- the process 102 is a process for estimating each output gradient magnetic field waveform using the response function 1112 obtained in the process 101 and the input gradient magnetic field waveforms 1113 and 1114 set for each imaging sequence. 82 does.
- the process 103 is a process of recalculating the imaging sequence high-frequency magnetic field pulse using the result of the process 102, and is performed by the high-frequency magnetic field pulse calculating unit 831 of the sequence calculating unit 83.
- the process 104 is a process for recalculating k-space coordinates corresponding to each sampling point of the measured NMR signal 1115 using the result of the process 102, and is performed by the k-space coordinate calculation unit 84.
- each process will be described in detail.
- step 301 the control parameters of the gradient magnetic field power supply are invalidated, and operations such as feedback control and feedforward control are set so that the gradient magnetic field power supply does not perform.
- step 302 the output gradient magnetic field waveform is measured for each space.
- the measurement of the output gradient magnetic field can be performed using a known technique (for example, the technique described in Patent Document 3). For example, as shown in FIG. 4, each space is measured at intervals of 50 mm in the X, Y, and Z axis directions.
- the output gradient magnetic field waveform 1111 and the input gradient magnetic field waveform 1110 used at the time of measurement are stored in the memory RAM 22 (FIG. 1) of the signal processing system.
- step 303 the input gradient magnetic field waveform and the measured output gradient magnetic field waveform stored in the RAM 22 are read, and the response function of the eddy current is calculated. A method for calculating the response function of the eddy current will be described later.
- step 304 the control parameters of the gradient magnetic field power supply are validated to bring the feedback control, the feedforward control, and the like into operation. In this state, the output gradient magnetic field waveform is measured (step 305). This measurement result is also stored in the RAM 22.
- step 306 the response function of the control circuit of the gradient magnetic field power supply is calculated. A method for calculating the response function of the control circuit of the gradient magnetic field power supply will also be described later. The above processing is performed for each gradient magnetic field coil arranged in the three-axis directions of X, Y, and Z.
- the magnetic field generated by the eddy current distorts the output gradient magnetic field waveform 502 with respect to the input gradient magnetic field waveform 501 as shown in FIG.
- the relationship between the input gradient magnetic field waveform and the output gradient magnetic field waveform can be expressed by the following equation, for example.
- s is a Laplace variable
- Gout (s) is a Laplace transform type of the output gradient magnetic field waveform
- Gin (s) is a Laplace transform type of the input gradient magnetic field waveform
- H (s) is an eddy current transfer characteristic.
- the output gradient magnetic field waveform is individual data measured for each space as shown in FIG. Formula (1) is an example, and can be replaced with another expression such as the following formula.
- the expression (1) is used. Since the input gradient magnetic field waveform Gin (s) and the output gradient magnetic field waveform Gout (s) are known, the transfer characteristic H (s) of eddy current can be theoretically calculated from the following equation. The response function of eddy current can be obtained by inverse Laplace transform of equation (3.1).
- ILT [] represents the inverse Laplace transform.
- the response function of eddy current is applied to the model formula, and the coefficient in the model formula is searched for and obtained.
- An example of a model equation for the response function of eddy current is shown in the following equation (4).
- the response function of Equation (4) expresses the transient response of the magnetic field generated by eddy current.
- t is time
- ⁇ 1 to ⁇ n are amplitude gains
- ⁇ 1 to ⁇ n are time constants.
- the number n of exponential functions in the equation (4) is determined by the type of time constant possessed by the parts constituting the MRI apparatus. According to the inventors' experience, if n is 10, a transient response of a magnetic field generated by an eddy current can be expressed sufficiently.
- step 601 initial values of amplitude gain and time constant are set.
- the initial value all the coefficients of the amplitude gain ⁇ are set to zero, and a positive numerical value other than zero is set to the time constant ⁇ .
- step 603 only ⁇ i is changed, and the optimum value of ⁇ i is searched. At this time, the optimum value is a value that minimizes the evaluation amount e1 of the following equation.
- Equation (4) is the response function shown in Equation (4).
- Expression (5) is a numerical value indicating how similar the result of convolution of the response function with the output gradient magnetic field waveform and the input gradient magnetic field waveform is. Accordingly, there is essentially no change even if an evaluation formula other than the formula (5) is used.
- -1 ⁇ i ⁇ a 1 range to calculate the e1 by changing the alpha i in steps 0.001, and optimum value alpha i which becomes a value e1 is minimized.
- a search algorithm typified by the golden section method may be used.
- step 604 After the searched optimum value ⁇ i is set to h (t), in step 604, only ⁇ i is changed, and the value of ⁇ i that minimizes the evaluation amount e1 is searched.
- the search method is the same as ⁇ i .
- step 605 it is determined whether the values of ⁇ i and ⁇ i have converged as a result of continuously searching for the optimum values of ⁇ i and ⁇ i and updating the values.
- the convergence condition of ⁇ i and ⁇ i can be defined as follows, for example.
- ⁇ i _prev and ⁇ i _prev are optimum values of ⁇ i and ⁇ i detected during the previous search. In other words, at least two repeated searches are required.
- ⁇ i and ⁇ i are searched again. If the convergence condition is satisfied, if i is less than n, i is incremented, and the next combination of ⁇ i and ⁇ i is searched. If i is greater than or equal to n, the process ends.
- ⁇ can take not only positive values but also negative values. It is correct from the principle of eddy current generation that ⁇ is only a positive value, but ⁇ 1 to ⁇ n are obtained in order, and the search is performed by ignoring the components of ⁇ and ⁇ to be obtained later. Can occur. The response function of eddy current is obtained by the above procedure.
- the control circuit for the gradient magnetic field power supply causes the output gradient magnetic field waveform 702 to include behavior such as overshoot or undershoot with respect to the input gradient magnetic field waveform 701.
- the gradient magnetic field pulse output from the gradient magnetic field power source is obtained by adding the component from the control circuit to the input waveform and can be expressed as the following equation.
- gamp (t) is a gradient magnetic field pulse output from the gradient magnetic field power source
- gin (t) is an input gradient magnetic field waveform
- u (t) is a control component of the gradient magnetic field power source.
- This control component is also defined by a model equation as in the case of the eddy current.
- the model equation of the control component can be appropriately defined according to the control method adopted by the control circuit.
- intelligent control such as PID control, simpler P control, PD control, PI control, or more complicated neural network can be assumed, and it is installed in the MRI apparatus to which the present invention is applied.
- the control theory may be selected according to the specifications of the gradient magnetic field power supply. In the present embodiment, assuming that the control circuit of the gradient magnetic field power supply performs PID control, u (t) is defined by the following model equation.
- K P proportional gain K I is an integral gain
- K D is the derivative gain
- Diff (t) in equation (9) is defined by the following equation.
- delay is a time delay when calculating the input of PID control.
- the relationship between gamp (t) and the output gradient magnetic field waveform is as follows.
- gout (t) is the output gradient magnetic field waveform stored in the RAM 22 in step 305 of FIG.
- h (t) is the response function of the eddy current shown in Equation (4), and is calculated by the above-described processing and is known. Therefore, when h (t) calculated in step 303 is convolved with gamp (t), which is the sum of the input gradient magnetic field waveform and the control component u (t) of the gradient magnetic field power supply, the most output gradient magnetic field waveform gout Find u (t) giving a gradient magnetic field waveform close to (t).
- gamp (t) is the sum of the input gradient magnetic field waveform and the control component u (t) of the gradient magnetic field power supply
- gout Find u (t) giving a gradient magnetic field waveform close to (t).
- step 801 delay, K P , K I , and K D are all initialized to zero.
- step 802 only the delay is changed to search for the optimum delay.
- the optimum value is a value that minimizes the evaluation amount e2 of the following equation.
- the search for delay at which the evaluation amount e2 is minimized is performed in the same manner as the search processing for the coefficients ⁇ 1 to ⁇ n and ⁇ 1 to ⁇ n of the response function described above. That is, an all-point search may be performed within a specific range, or a search algorithm typified by the golden section method may be used.
- Diff (t) is obtained.
- step 803 the optimum K P is searched.
- the condition under which K P is optimal is a value that minimizes the evaluation quantity e2 in equation (12).
- the K I in step 804 searches the K D in step 805.
- the response function of the control circuit for the gradient magnetic field power source finally obtained is expressed by the following equation.
- the response function h (t) of the eddy current and the response function of the control circuit (control component u (t)) are calculated.
- the magnetic field waveform can be calculated.
- the parameters of the response function h (t) (formula (4)) and u (t) for solving Equation (11) are obtained for individual data measured for each space as shown in FIG. It is stored in the RAM 22 of the signal processing system 7 and used for pulse sequence calculation and image reconstruction in actual imaging.
- FIG. 10 shows an example of the estimation result of the output gradient magnetic field waveform when the control component u (t) of the gradient magnetic field power supply is not added.
- Output gradient magnetic field waveform calculation process 102 >> Next, returning to the block diagram of FIG. 2, the processing 102 performed by the output gradient magnetic field waveform calculation unit 82 will be described.
- an imaging pulse sequence is calculated by the sequence calculation unit 83 according to the imaging purpose, and a high-frequency pulse (amplitude, envelope), slice selection gradient magnetic field pulse (input gradient magnetic field waveform) included in the calculated imaging pulse sequence is calculated.
- a read gradient magnetic field pulse input gradient magnetic field waveform 1114) and the like are stored in the RAM 22.
- the output gradient magnetic field waveform of the imaging pulse sequence is calculated using the response function 1112 calculated by the response function calculation means 81 and the set input gradient magnetic field waveforms 1113 and 1114.
- FIG. 11 shows a flowchart of the calculation process of the output gradient magnetic field waveform.
- step 1101 the visual field center position in the three-axis directions of X, Y, and Z is acquired from the coordinate position of the imaging visual field.
- the visual field center position is calculated from the following equation.
- Axis represents each axis of X, Y, and Z (hereinafter the same)
- FOV (Axis, min) and FOV (Axis, max) are the minimum of the imaging field of view of each axis specified according to the imaging conditions Coordinate position and maximum coordinate position.
- step 1102 from the response function h (t) for each space stored in the RAM 22 and the control component u (t) of the gradient magnetic field power supply, the response function h (t ) And control component u (t) are selected and acquired.
- the acquisition of the response function h (t) and the control component u (t) of the gradient magnetic field power supply may be obtained from the result calculated at the position closest to the visual field center position, or the result calculated for each space May be interpolated to obtain the response function h (t) and the control component u (t) of the gradient magnetic field power supply.
- interpolating follow the formula below.
- h (Axis, t) and u (Axis, t) are the response function at the visual field center position of each axis and the control component of the gradient magnetic field power supply circuit.
- P1 and P2 represent the calculation positions of the response function and the control component of the gradient magnetic field power supply circuit that are closest in the front-rear direction of the visual field center position.
- step 1103 an input gradient magnetic field waveform with respect to the X, Y, and Z directions is acquired.
- the input gradient magnetic field waveform is set according to the imaging pulse sequence and is stored in the RAM 22, and is the slice gradient magnetic field pulse 1113 or the read gradient magnetic field pulse 1114 shown in FIG. In either case, the processing contents are the same.
- step 1104 a waveform obtained by synthesizing the three-axis input gradient magnetic field waveform represented by the following equation is calculated.
- gin (Axis, t) is the acquired input gradient magnetic field waveform of each axis.
- step 1105 the output gradient magnetic field waveform is estimated using the response function h (t) and the control component u (t) of the gradient magnetic field power supply for the input gradient magnetic field waveform for each axis.
- the estimation of the output gradient magnetic field waveform is expressed by the following equation as a form in which equation (11) is expanded to three axes.
- gest (Axis, t) is an estimation result of the output gradient magnetic field waveform of each axis obtained by calculation.
- the waveform of each axis is synthesized according to the following equation.
- the output gradient magnetic field waveforms of the slice gradient magnetic field and readout gradient magnetic field estimated as described above are recalculated for the high-frequency magnetic field pulse (Fig. 2: 103) and the correction of the k-space coordinate position of the NMR signal (Fig. 2: Used for processing 104).
- High-frequency magnetic field pulse calculation process 103 the high-frequency magnetic field pulse is recalculated using the output gradient magnetic field waveform gest (t) and the input gradient magnetic field waveform gin (t) of the slice gradient magnetic field obtained by the output gradient magnetic field waveform calculation unit 82.
- Recalculation of the high frequency magnetic field pulse calculates the envelope and frequency of the high frequency pulse according to the following formula.
- RF env '(t) is the envelope of the high frequency magnetic field pulse after recalculation
- RF env (t) is the envelope of the high frequency magnetic field pulse before recalculation
- RF freq ' (t) is the high frequency magnetic field pulse after recalculation
- RF freq (t) is the frequency of the high-frequency magnetic field pulse before recalculation.
- the amplitude, frequency, and irradiation time of the envelope of the high-frequency magnetic field pulse are changed in proportion to the intensity of the output gradient magnetic field waveform of the slice gradient magnetic field. Based on these equations, it is possible to make changes in consideration of device restrictions and imaging conditions.
- phase of the high-frequency magnetic field pulse may be changed with time, and as a result, frequency modulation may be realized.
- the calculation of the phase for realizing the frequency modulation follows the following equation.
- g_standard is the gradient magnetic field strength [T / m] that is the reference for frequency calculation of the high-frequency magnetic field pulse
- distance is the distance from the magnetic field center to the excitation position [m]
- ⁇ is the gyromagnetic ratio [Hz / T]
- ⁇ t Is the phase change interval time [s] of the high-frequency magnetic field pulse.
- the range in which the phase can be changed is 0 to 360 [deg]
- the range of distance is also limited. Therefore, in order to widen the range in which the distance can be set, it is preferable that the phase change is small. Therefore, in order to minimize the phase change, the g_standard and the frequency of the high frequency magnetic field pulse are calculated according to the following formula.
- Average [] is the mean value calculation process within the high frequency magnetic field pulse application section
- RF freq ' is the frequency of the high frequency magnetic field pulse after recalculation without time change
- RF freq is before recalculation without time change The frequency of the high frequency magnetic field pulse.
- Another example that needs to be changed is the suppression of the irradiation time of the high-frequency magnetic field pulse.
- the irradiation time of the high-frequency magnetic field pulse changes, but depending on the pulse sequence, the irradiation time may change, especially the irradiation time may be extended. It may not be preferable.
- the recalculation procedure of the high frequency magnetic field pulse that does not change the irradiation time will be described with reference to the flowchart shown in FIG.
- step 1021 and 1022 the output gradient magnetic field waveform for each axis is calculated, and the output gradient magnetic field waveform for each axis is synthesized. These processes are performed by the output gradient magnetic field waveform calculation unit 82, and are the same as the processes performed in step 1103 and step 1104 in FIG.
- step 1031 a high frequency magnetic field pulse is calculated according to equations (19) to (21).
- step 1032 the irradiation time increase rate ⁇ RF is calculated.
- the increase rate ⁇ RF is calculated according to the following equation.
- gin (t) is the input gradient magnetic field waveform
- gest (t) is the estimation result of the output gradient magnetic field waveform obtained from the equation (18)
- StartTime is the high frequency magnetic field pulse irradiation start time
- EndTime is the high frequency magnetic field pulse irradiation end time Represents.
- the increase rate beta RF determines whether exceeds 1.0. If an increase rate beta RF exceeds 1.0, so that the irradiation time of the high-frequency magnetic field pulse after the calculation is extended. In that case, the process moves to step 1034 to recreate the input gradient magnetic field waveform of the slice gradient magnetic field.
- the reconstruction of the input gradient magnetic field waveform follows the following formula. By multiplying the input gradient magnetic field waveform by the irradiation time increase rate ⁇ RF , the intensity of the input gradient magnetic field waveform is increased, thereby shortening the application time of the high-frequency magnetic field pulse. At this time, since only the entire input gradient magnetic field waveform is increased uniformly, no special shim coil or gradient magnetic field coil is required.
- step 1021 of processing 102 After re-creating the input gradient magnetic field waveform, return the result to step 1021 of processing 102, calculate the output gradient magnetic field waveform (steps 1021 and 1022), recalculate the high-frequency magnetic field pulse (step 1031), and increase the irradiation time
- the rate ⁇ RF is calculated (step 1032). If the calculated increase rate ⁇ RF is 1.0 or less, the process is terminated, and if it is not 1.0 or less, the input gradient magnetic field waveform is created again. At this time, gin (t) in Equation (26) is the input gradient magnetic field waveform recreated immediately before.
- the irradiation time of the high-frequency magnetic field pulse can be finally prevented from extending compared to the irradiation time of the high-frequency magnetic field pulse before recalculation. it can. This eliminates the need to change the pulse sequence other than the slice gradient magnetic field pulse when using the recalculated high-frequency magnetic field pulse.
- FIG. 13 shows an image 1301 obtained using the high-frequency magnetic field pulse before recalculation and an image 1302 obtained using the high-frequency magnetic field pulse after recalculation. Both images are the imaging results of ultra-short TE measurement using the VERSE method. Compared to the image 1301 obtained using the high-frequency magnetic field pulse before the recalculation, the image 1302 obtained using the high-frequency magnetic field pulse after the recalculation is a background signal (a signal from other than the imaging section) indicated by the arrow. It can be seen that a good excitation profile is obtained because of less contamination.
- ⁇ k-space coordinate recalculation process 104 the k-space coordinate recalculation process 104 performed by the k-space coordinate calculation unit 84 in FIG. 2 will be described.
- the NMR signal is arranged using the output gradient magnetic field waveform gest (t) and the input gradient magnetic field waveform gin (t) of the readout gradient magnetic field obtained by the output gradient magnetic field waveform calculation unit 82.
- FIG. 14 shows an example of a flowchart showing a recalculation procedure of k-space coordinates. This processing procedure is also the same processing flow as the high-frequency magnetic field pulse recalculation flowchart shown in FIG. 12, and the input gradient magnetic field waveform of the readout gradient magnetic field is recreated based on the recalculation result of the k-space coordinates.
- the k-space coordinates are calculated using the input gradient magnetic field waveform gin (t) and the output gradient magnetic field waveform gest (t) of the read gradient magnetic field obtained in the process 102.
- the recalculation of k-space coordinates using the output gradient magnetic field waveform gest (t) follows the following formula.
- ⁇ is the magnetic rotation ratio [Hz / T].
- This equation is obtained by replacing gin (j) with gest (j) in the equation for calculating k-space coordinates using the general input gradient magnetic field waveform gin (t) (the following equation (28)).
- step 1042 the reduction rate ⁇ k of the k space is calculated.
- the reduction rate ⁇ k is calculated according to the following equation.
- EndTime represents the read gradient magnetic field pulse application end time (or NMR signal measurement end time).
- step 1043 it is determined whether or not the reduction rate ⁇ k is less than 1.0.
- the increase rate ⁇ k is less than 1.0
- the k-space coordinate k ′ (t) after the recalculation is reduced with respect to the k-space coordinate k (t) before the recalculation.
- the process proceeds to step 1044, where the input gradient magnetic field waveform of the read gradient magnetic field is recreated.
- the reconstruction of the input gradient magnetic field waveform follows the following formula.
- the intensity of the input gradient magnetic field waveform is increased, thereby expanding the range of the k-space coordinates.
- the input gradient magnetic field waveform can only be increased uniformly, so that no special shim coil or gradient magnetic field coil is required.
- FIG. 15 shows an image 1501 obtained using k-space coordinates before recalculation and an image 1502 obtained using k-space coordinates after recalculation. Both images are the imaging results of EPI measurement, and the readout gradient magnetic field is applied in the horizontal direction in the image. Compared to the image 1501 obtained using the k-space coordinates before recalculation, the image 1502 obtained using the k-space coordinates after recalculation has reduced distortion of the image indicated by the arrow. It can be seen that the compensation for the readout gradient magnetic field is properly compensated.
- Imaging is performed using the high-frequency magnetic field pulse corrected in the process 103, and the acquired NMR signal is reconstructed using the k-space coordinates corrected in the process 104.
- an external signal is not mixed into the selected slice, and an image in which image distortion and artifacts are suppressed can be obtained.
- Non-Patent Document 1 In the method using a fixed input gradient magnetic field waveform (Non-Patent Document 1), it can be applied only to a specific pulse sequence, whereas in the present embodiment, the output gradient magnetic field waveform for any input gradient magnetic field waveform can be estimated. There is no restriction that a fixed input gradient magnetic field waveform must be used.
- each coefficient of the model formula is obtained as a criterion for judging the appearance of the image, but in this embodiment, based on the output gradient magnetic field waveform. Since the coefficient of the model formula is calculated, it is not necessary to rely on human judgment, and a highly accurate model formula can be calculated.
- the configuration of the digital signal processing unit 8 shown in FIG. 2 and the four characteristic processes 101 to 104 are performed in the digital signal processing unit 8 are the same as in the first embodiment.
- the present embodiment is characterized in that the process 101 for calculating the response function of the control circuit for the eddy current and gradient magnetic field power supply is obtained by a simpler procedure than the first embodiment. That is, in the first embodiment, the measurement of the output gradient magnetic field waveform was performed twice each in the state in which the control of the gradient magnetic field power supply circuit was invalidated and in the valid state. Only measure the magnetic field waveform.
- the processing flow of this embodiment will be described with reference to the flowchart of FIG.
- step 1601 the output gradient magnetic field waveform is measured for each space.
- the control parameters of the gradient magnetic field power supply are not changed, and the apparatus is in a normal state. That is, the control parameter of the gradient magnetic field power source is in a valid state.
- the measurement result of the output gradient magnetic field waveform is stored in the RAM 22. This process is the same as the process performed in steps 302 and 305 of FIG. 3 showing the flow of the first embodiment.
- step 1602 the input gradient magnetic field waveform and the measured output gradient magnetic field waveform stored in the RAM 22 are read, and the response function of the control circuit of the eddy current and gradient magnetic field power supply is calculated.
- step 1701 an eddy current response function is calculated.
- This process is the same as the process of step 303 in the first embodiment described above. That is, a response function h (t) that minimizes the evaluation amount e1 of the following equation (31) is searched.
- gout (t) is the output gradient magnetic field waveform stored in the RAM 22 in step 1601
- gin (t) is the input gradient magnetic field waveform.
- h (t) is an eddy current response function defined by equation (4), and the coefficients ⁇ 1 to ⁇ n and ⁇ 1 to ⁇ n of equation (4) are searched in the same manner as in step 303 in the first embodiment.
- Equation (31) has the same shape as Equation (5) of the first embodiment, but the output gradient magnetic field waveform gout (t) of Equation (5) is measured with the control circuit of the gradient magnetic field power supply disabled.
- the output gradient magnetic field waveform gout (t) of Expression (31) includes distortion due to the control circuit of the gradient magnetic field power supply. Therefore, the calculated response function h (t) of the eddy current includes not only the eddy current but also the characteristics of the control circuit of the gradient magnetic field power supply.
- step 1702 a response function of the control circuit of the gradient magnetic field power supply is calculated.
- This process is the same as the process in step 306 of the first embodiment. That is, a search is made for gamp (t) that minimizes the evaluation amount e2 of the equation (32).
- Gamp (t) in Expression (32) is defined by Expression (33).
- u (t) is a control component of the gradient magnetic field power source defined by equation (9), but the eddy current response function h (t) obtained in step 1701 includes the control of the gradient magnetic field power source. Since the response component of the circuit is also included, the component that could not be expressed by the model of the eddy current response function (equation (4)) is applied to equation (9).
- the response function h (t) of the eddy current is recalculated in step 1703. This is by defining the response function u (t) of the control circuit of the gradient magnetic field power supply, so that each coefficient ( ⁇ 1 to ⁇ n and ⁇ 1 to ⁇ n) of the current response function h (t) (equation (4)) is optimal. This is because the value changes. Recalculate the response function h (t) by replacing gin (t) in equation (31) with gamp (t) obtained in step 1702 and each coefficient of h (t) ⁇ ⁇ ⁇ that minimizes the evaluation quantity e1 By searching.
- a response function taking into account the eddy current response function and the control component of the gradient magnetic field power source is obtained. The response function is obtained for each piece of data measured for each space and stored in the RAM 22 of the signal processing system 7.
- the procedure of the process 102 for estimating the output gradient magnetic field waveform and the processes 103 and 104 for recalculating the high-frequency magnetic field pulse and k-space coordinates using the response function stored in the RAM 22 is the same as that of the first embodiment. The description is omitted.
- the number of times of calculating the response function is increased, the number of times of measurement of the output gradient magnetic field waveform can be reduced, and the work of changing the control parameter of the gradient magnetic field power source can be omitted, with a simpler procedure.
- the response function required to recalculate the excitation profile can be calculated.
- Other effects are the same as in the first embodiment.
- the configuration of the digital signal processing unit 8 shown in FIG. 2 and the four processes 101 to 104 performed by the digital signal processing unit 8 are the same as in the first embodiment.
- the present embodiment is characterized in that the response function of the control circuit of the eddy current and the gradient magnetic field power supply is obtained without being applied to a specific model equation.
- the processing flow of this embodiment is shown in FIG. Steps 1801 and 1802 in FIG. 18 are the same as steps 1601 and 1602 in FIG. 16 described in the second embodiment.
- step 1801 a plurality of types of input gradient magnetic field waveforms are processed in step 1801. Processing to measure the output gradient magnetic field waveform, processing to directly derive the response function of the eddy current and the gradient power supply control circuit without using a model formula in step 1802, processing to correct the derived response function in step 1803 It is.
- the eddy current transfer characteristic H ( s) can be calculated from equation (3.1). Reprint equation (3.1). However, when the value of Gin (s) becomes zero, the value of H (s) diverges and a correct value cannot be obtained. In order to prevent this, in the first and second embodiments, the response function that gives the optimum evaluation value is derived by applying the response function of the eddy current to the model formula.
- the response function h (t) calculated from the equation (3.1) or the equation (3.2) obtained by inverse Laplace transform is used without using the model equation.
- a plurality of types of input gradient magnetic field waveforms used for calculating the response function of the eddy current and the gradient magnetic field power supply control circuit are prepared.
- the output gradient magnetic field waveform is measured using a plurality of types of input gradient magnetic field waveforms, and stored in the RAM 22.
- An input gradient magnetic field waveform having a steep shape as much as possible is preferable. This is because a gently changing waveform does not include a high frequency range, and a high frequency response cannot be calculated.
- it is desirable to use an impulse wave but because it is difficult to output an impulse wave due to restrictions such as the output performance of the gradient magnetic field power supply, in this embodiment, a plurality of trapezoidal waves as shown in FIG. 19 are used. Use.
- step 1802 the transfer characteristic H (s) is calculated by equation (3.1).
- the zero point appearing in the spectrum (Laplace transform type) of one input gradient magnetic field waveform (trapezoidal wave) is replaced with the spectrum data of another input gradient magnetic field waveform (trapezoidal wave) to avoid the zero point.
- Gin1 (s) is a Laplace transform type of trapezoidal wave 1901
- Gout1 (s) is a Laplace transform type of output gradient magnetic field waveform when trapezoidal wave 1901 is used as an input gradient magnetic field waveform
- Gin2 (s) is Laplace transform type of trapezoidal wave 1902
- Gout2 (s) is a Laplace transform type of output gradient magnetic field waveform when trapezoidal wave 1902 is used as the input gradient magnetic field waveform.
- FIG. 20 shows an outline of the amplitude spectrum obtained by Laplace transforming the trapezoidal wave group shown in FIG.
- the positions where the zeros appear in the spectrums 2001 and 2002 of the trapezoidal waves 1901 and 1902 are different between the spectrum 2001 and the spectrum 2002.
- the zero point can be avoided.
- two trapezoidal waves are shown in FIG. 19, three or more trapezoidal waves may be prepared.
- Equation (34) among the plurality of input gradient magnetic field waveforms, the combination of the input gradient magnetic field waveform having the largest spectrum value (not zero) and the output gradient magnetic field waveform resulting therefrom is selected. What is necessary is just to use for calculation of a transfer characteristic.
- step 1803 a process for eliminating the influence of noise from the response function h (t) calculated in step 1802 is performed. This is because Gout1 (s) and Gout2 (s) are actually measured data, and noise is superimposed, and there are cases where correct transfer characteristics cannot be obtained. In the present embodiment, such noise is removed by performing two-stage filter processing in the Laplace space.
- the transfer characteristic obtained by the equation (34) is multiplied by the optimum filter of the following equation to suppress the influence of noise.
- H ′ (s) is the transfer characteristic after applying the optimum filter
- ⁇ (s) is the optimum filter.
- ⁇ (s) a Wiener filter that is a known technique is used as the optimum filter ⁇ (s).
- the definition formula of the Wiener filter is as follows.
- N (s) is a noise component of Gout1 (s)
- S (s) is a signal component excluding the noise of Gout1 (s).
- N (s) and S (s) are calculated according to a known technique, where N (s) is the highest frequency signal value of the power spectrum of Gout1 (s), and N (s ) Is subtracted S (s).
- a noise removal filter is executed on the transfer characteristic after the optimum filter is applied.
- a median filter is used as a noise removal filter.
- H ′′ (s) is the transfer characteristic after the median filter is applied.
- the response function that combines the eddy current and the gradient power supply control circuit can be obtained by inverse Laplace transform of Equation (37).
- ILT [] represents the inverse Laplace transform.
- the procedure for estimating the output gradient magnetic field waveform and recalculating the high-frequency magnetic field pulse / k-space coordinates using the response function obtained as described above is the same as in the first embodiment.
- the response function can be obtained without applying a specific model formula, and various hardware configurations ( It can be flexibly applied to a gradient magnetic field coil or the like) or a software configuration (control method of gradient magnetic field power supply).
- the present invention is not limited to the above-described embodiment, and various modifications can be made. It is.
- the model formula that expresses the distortion of the output gradient magnetic field waveform due to the eddy current and the distortion of the output gradient magnetic field waveform by the control circuit of the gradient magnetic field power supply is not limited to that exemplified in the embodiment, and other model formulas may be used. Is possible.
- the control circuit of the eddy current and the gradient magnetic field power source is taken up, and the case where the response function by them is used for the calculation of the output gradient magnetic field waveform has been described.
- Other components such as residual magnetic fields with hysteresis found in MRI apparatus using permanent magnets, for example, can be added to the calculation of the output gradient magnetic field waveform.
- the processing procedure of the flowchart shown for description of each embodiment is an example, there is a process which can be omitted, and it is also possible to add another process as needed.
- the present invention is used when a high-frequency magnetic field pulse is calculated from an output gradient magnetic field waveform calculated for a slice gradient magnetic field, or when k-space coordinates where an NMR signal is arranged are corrected by an output gradient magnetic field waveform calculated for a read gradient magnetic field.
- the readout gradient magnetic field also includes a two-direction gradient magnetic field in a pulse sequence such as a radial scan or a spiral scan that does not distinguish between the readout gradient magnetic field and the phase encoding gradient magnetic field.
- the outline of the present invention clarified by the description of each of the above embodiments is as follows. That is,
- the MRI apparatus of the present invention is An imaging means comprising a static magnetic field generating means, a gradient magnetic field generating means, a high frequency magnetic field generating means and a high frequency magnetic field detecting means, a control means for controlling the operation of the imaging means based on an imaging sequence, and the high frequency magnetic field detecting means detected
- the computing means uses an input waveform input to the gradient magnetic field generating means and an output waveform generated by the gradient magnetic field generating means according to the input waveform, and a response function of a plurality of elements that affect the output waveform.
- Gradient magnetic field calculation for calculating an output waveform from an input waveform of a gradient magnetic field pulse set in the imaging sequence, using a response function calculating unit that calculates a response function that is a sum, and a response function calculated by the response function calculating unit And k-space coordinates of the nuclear magnetic resonance signal obtained by the correction of the high-frequency magnetic field pulse set in the imaging sequence and / or the nuclear magnetic resonance signal obtained by the imaging sequence using the calculated value of the output waveform calculated by the gradient magnetic field calculating unit Compensating means for correcting the above is provided.
- the response function calculating means calculates a response function for each of a plurality of elements that change the output waveform, and the gradient magnetic field calculating means uses a response function that is a sum of the response functions of the plurality of elements to output the output waveform. Perform the calculation.
- the response function calculating means includes a model setting means for setting a response function model for each element, a difference between a calculated value of the output waveform calculated by the response function model set by the model setting means, and an actually measured value of the output waveform.
- Model optimization means for optimizing the response function model so as to minimize the response function model, and the response function model optimized by the model optimization means is used as the response function of each element.
- the response function calculating means uses a plurality of input values as the input values, and uses a combination of a plurality of input values and corresponding measured values, directly from the relationship between the input values and the measured values of the output waveform, Calculate the response function.
- the response function calculation unit includes a filter unit that filters the response function after calculation.
- the filter means includes a Wiener filter and a median filter.
- the gradient magnetic field generating means includes a control circuit that controls an input waveform based on an output waveform of the generated gradient magnetic field,
- the plurality of elements that change the output waveform include distortion of the output waveform accompanying control by the control circuit.
- the response function calculating unit calculates a response function for each of a plurality of gradient magnetic fields having different positions with respect to the center of the static magnetic field generated by the static magnetic field generating unit.
- the gradient magnetic field pulse calculated by the gradient magnetic field calculation means is a slice selection gradient magnetic field in the imaging sequence, and the compensation means uses a high-frequency pulse set in the imaging sequence based on a calculated value of the slice selection gradient magnetic field. High frequency pulse correction means for correcting is provided.
- the high-frequency pulse correction means includes irradiation time calculation means for calculating a change in irradiation time of the high-frequency pulse before and after correction, and when the change in irradiation time calculated by the irradiation time calculation means exceeds a predetermined increase rate, The input value of the gradient magnetic field calculating means is changed, and the high-frequency pulse is corrected using the estimated value of the output waveform estimated by the gradient magnetic field calculating means from the changed input value.
- the gradient magnetic field pulse calculated by the gradient magnetic field calculation means is a readout gradient magnetic field in the imaging sequence, and the compensation means determines that the nuclear magnetic resonance signal obtained in the imaging sequence is based on the calculated value of the readout selective gradient magnetic field.
- K-space coordinate correcting means for correcting the arranged k-space coordinates is provided.
- the correction means includes k-space size calculation means for calculating a change in the size of k-space coordinates before and after correction, and the change in the size of the k-space calculated by the k-space size calculation means exceeds a predetermined reduction rate.
- the input value of the gradient magnetic field calculating means is changed, and the k-space coordinates are corrected using the recalculated value of the output waveform calculated from the changed input value by the gradient magnetic field calculating means.
- the method of estimating the output gradient magnetic field waveform generated by the gradient magnetic field generating means of the magnetic resonance imaging apparatus of the present invention is as follows: Inputting an input value of the gradient magnetic field generation means and an actual measurement value of an output gradient magnetic field waveform generated by the gradient magnetic field generation means; Setting a response function model for each of a plurality of factors that affect the output gradient magnetic field waveform; Using each input value and actual measurement value of the input gradient magnetic field waveform to optimize each response function model; Calculating an estimated value of an output gradient magnetic field waveform using the optimized response function model for an input value of a desired gradient magnetic field; It is characterized by including.
- the plurality of factors include an eddy current generated by a gradient magnetic field and an output of a control circuit provided in the gradient magnetic field generating means.
- the actual measurement value includes a first actual measurement value when the control circuit of the gradient magnetic field generation unit is invalidated, and a second actual measurement value when the control circuit is enabled,
- the response function of the output gradient magnetic field waveform by the control circuit is obtained using the first actual measurement value and the second actual measurement value.
- 1 subject 2 static magnetic field generation system, 3 gradient magnetic field generation system, 4 sequencer, 5 transmission system, 6 reception system, 7 signal processing system, 8 digital signal processing unit (CPU), 9 gradient magnetic field coil, 10 gradient magnetic field power supply , 11 High-frequency transmitter, 12 modulator, 13 high-frequency amplifier, 14a high-frequency coil (transmitting coil), 14b high-frequency coil (receiving coil), 15 signal amplifier, 16 quadrature detector, 17 A / D converter, 18 magnetic disk , 19 optical disk, 20 display, 21 ROM, 22 RAM, 23 trackball or mouse, 24 keyboard, 25 operation unit, 81 response function calculation unit, 82 gradient magnetic field waveform calculation unit, 83 sequence calculation unit, 84 k-space coordinate calculation unit 85 Image reconstruction unit.
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Abstract
Description
まず、渦電流を補償する傾斜磁場パルスを出力する方法(特許文献1、特許文献2)では、時間的・空間的に高次の複雑な変化を示す渦電流について、渦電流により生じる磁場を完全に補正する渦電流補正コイルを実現することは困難かつ高コストであるという問題がある。また、傾斜磁場電源の制御回路の振る舞いについては考慮されておらず、出力傾斜磁場波形の歪みを完全に除去できない。
<第一の実施形態>
本実施形態のMRI装置のディジタル信号処理装置8で行われる処理は、大きく分けて、4つの処理からなる。4つの処理の関係を図2のブロック図を用いて説明する。1つめの処理101は、事前測定として応答関数算出部81で行われる処理であり、渦電流及び傾斜磁場電源の制御回路の応答関数1112を算出する処理である。処理102~104は、実際の撮像において行われる処理である。処理102は、処理101で求めた応答関数1112と、撮像シーケンス毎に設定される入力傾斜磁場波形1113、1114を用いて、それぞれの出力傾斜磁場波形を推定する処理であり、傾斜磁場波形算出部82が行う。処理103は、処理102の結果を用いて、撮像シーケンス高周波磁場パルスを再計算する処理であり、シーケンス計算部83の高周波磁場パルス算出部831が行う。処理104は、処理102の結果を用いて、計測されるNMR信号1115の各サンプリング点に対応するk空間座標を再計算する処理であり、k空間座標算出部84が行う。以下、それぞれの処理について詳説する。
処理101の渦電流及び傾斜磁場電源の制御回路の応答関数算出処理について図3に示すフローチャートを用いて全体の流れを説明する。
渦電流の応答関数は、式(3.1)を逆ラプラス変換することで得られる。
ここで、αi_prevとτi_prevは前回検索時に検出されたαiとτiの最適値である。すなわち、最低2回の繰り替えし検索が必要となる。
ここで、gamp(t)は傾斜磁場電源が出力する傾斜磁場パルス、gin(t)は入力傾斜磁場波形、u(t)は傾斜磁場電源の制御成分である。
実測して得られた出力傾斜磁場波形902と、u(t)をゼロとした式(11)から推定した出力傾斜磁場波形904との間に矢印で示した部分に偏差が生じている。このことから、傾斜磁場電源の制御成分を加味しなければ、高精度に出力傾斜磁場波形を推定できないことが分かる。
次に、図2のブロック図に戻り、出力傾斜磁場波形算出部82で行われる処理102について説明する。実際の撮像にあたっては、撮像目的に応じてシーケンス計算部83により撮像パルスシーケンスが計算され、計算された撮像パルスシーケンスに含まれる高周波パルス(振幅、エンベロープ)、スライス選択傾斜磁場パルス(入力傾斜磁場波形1113)、読出し傾斜磁場パルス(入力傾斜磁場波形1114)などがRAM22に保存される。
ここで、AxisはX、Y、Zの各軸を表し(以下、同じ)、FOV(Axis,min)とFOV(Axis,max)は撮像条件に応じて指定される各軸の撮像視野の最小座標位置と最大座標位置である。
ここで、h(Axis,t)とu(Axis,t)は各軸の視野中心位置における応答関数と傾斜磁場電源回路の制御成分である。また、P1およびP2は、視野中心位置の前後方向でそれぞれ最も近い応答関数および傾斜磁場電源回路の制御成分の算出位置を表す。
各軸の出力傾斜磁場波形を算出した後に、ステップ1106において、下記式に従い各軸の波形を合成する。
以上のように推定されたスライス傾斜磁場及び読出し傾斜磁場の各出力傾斜磁場波形は、それぞれ、高周波磁場パルスの再計算(図2:103)およびNMR信号のk空間座標位置の補正(図2:処理104)に用いられる。
処理103では、出力傾斜磁場波形算出部82で求めたスライス傾斜磁場の出力傾斜磁場波形gest(t)と入力傾斜磁場波形gin(t)を用いて、高周波磁場パルスの再計算を行う。高周波磁場パルスの再計算は下記式に従い、高周波パルスのエンベロープおよび周波数を計算する。
ここで、RFenv'(t)は再計算後の高周波磁場パルスのエンベロープ、RFenv(t)は再計算前の高周波磁場パルスのエンベロープ、RFfreq'(t)は再計算後の高周波磁場パルスの周波数、RFfreq(t)は再計算前の高周波磁場パルスの周波数である。
ここで、g_standardは高周波磁場パルスの周波数計算の基準となる傾斜磁場強度[T/m]、distanceは磁場中心から励起位置までの距離[m]、γは磁気回転比[Hz/T]、Δtは高周波磁場パルスの位相変更間隔時間[s]である。
ここで、Average[ ]は高周波磁場パルス印加区間内での平均値計算処理、RFfreq'は時間的変化のない再計算後の高周波磁場パルスの周波数、RFfreqは時間的変化のない再計算前の高周波磁場パルスの周波数である。式(23)及び式(24)によりg_standardとRFfreq'を算出することで、位相の変化が最小に抑えられ、distanceの上限値を大きく取ることが可能となる。
ここで、gin(t)は入力傾斜磁場波形、gest(t)は式(18)より求めた出力傾斜磁場波形の推定結果、StartTimeは高周波磁場パルス照射開始時刻、EndTimeは高周波磁場パルス照射終了時刻を表す。
その場合にはステップ1034に移り、スライス傾斜磁場の入力傾斜磁場波形の再作成を行う。入力傾斜磁場波形の再作成は次式に従う。
照射時間の増加率βRFを入力傾斜磁場波形に掛けることで、入力傾斜磁場波形の強度を増加させ、それにより高周波磁場パルスの印加時間を短縮させる。この時、入力傾斜磁場波形の全体を一様に増加させるだけであるため、特別なシムコイルや傾斜磁場コイルなどは必要としない。
続いて、図2のk空間座標算出部84が行うk空間座標再計算処理104について説明する。k空間座標再計算処理104では、出力傾斜磁場波形算出部82で求めた読出し傾斜磁場の出力傾斜磁場波形gest(t)と入力傾斜磁場波形gin(t)を用いてNMR信号が配置されたk空間座標の再計算を行う。図14に、k空間座標の再計算手順を示すフローチャートの一例を示す。この処理手順も、図12に示す高周波磁場パルスの再計算のフローチャートと同様の処理の流れで、k空間座標の再計算結果によって読出し傾斜磁場の入力傾斜磁場波形を再作成する。
ここで、γは磁気回転比[Hz/T]である。この式は、一般的な入力傾斜磁場波形gin(t)を用いたk空間座標の算出の式(次式(28))におけるgin(j)をgest(j)に置き換えたものである。
縮小率βkの算出は次式に従う。
ここで、EndTimeは読み出し傾斜磁場パルスの印加終了時刻(もしくはNMR信号の計測終了時刻)を表す。
まず、渦電流を補償する傾斜磁場パルスを出力する方法(特許文献1、特許文献2)では、時間的・空間的に高次の複雑な変化示す渦電流による磁場を完全に補正することは困難かつ高コストであるのに対し、本実施形態では特別な磁場補正用コイルを必要とせず、渦電流による磁場を含めた高周波磁場パルスとk空間座標の再計算を行うことで、励起プロファイルの劣化と画像の歪みやゴースト等のアーチファクトを抑制できる。
本実施形態においても、図2に示すディジタル信号処理部8の構成と、ディジタル信号処理部8で4つの特徴的な処理101~104が行われることは、第一実施形態と同様である。
本実施形態は、渦電流及び傾斜磁場電源の制御回路の応答関数を算出する処理101を、第一実施形態より簡易な手続きで求めることを特徴としている。すなわち、第一実施形態では、傾斜磁場電源回路の制御を無効にした状態と有効にした状態でそれぞれ2回の出力傾斜磁場波形の測定を行ったが、本実施形態では、1回の出力傾斜磁場波形の測定のみ行う。本実施形態の処理の流れを図16のフローチャートを用いて説明する。
ここで、gout(t)はステップ1601でRAM22に格納された出力傾斜磁場波形であり、gin(t)は入力傾斜磁場波形である。h(t)は式(4)で定義される渦電流の応答関数であり、第一実施形態でのステップ303と同様に式(4)の係数α1~αn、τ1~τnを検索する。
ここでu(t)は、式(9)で定義される傾斜磁場電源の制御成分であるが、ステップ1701で求めた渦電流の応答関数h(t)の中には、傾斜磁場電源の制御回路の応答成分も含まれているため、渦電流の応答関数のモデル(式(4))では表現しきれなかった成分を式(9)に当てはめて算出することになる。
その他の効果は、第一実施形態と同様である。
本実施形態においても、図2に示すディジタル信号処理部8の構成と、ディジタル信号処理部8で4つの処理101~104が行われることは、第一実施形態と同様である。本実施形態は、渦電流及び傾斜磁場電源の制御回路の応答関数を特定のモデル式に当てはめずに求めることを特徴としている。本実施形態の処理の流れを図18に示す。図18のステップ1801、1802は、第二実施形態で説明した図16のステップ1601、1602と同様であるが、本実施形態の特有の処理は、ステップ1801において、複数種類の入力傾斜磁場波形を用いて出力傾斜磁場波形を測定する処理、ステップ1802において、モデル式を用いずに渦電流と傾斜磁場電源制御回路の応答関数を直接導出する処理、ステップ1803において、導出した応答関数を補正する処理である。
しかし、Gin(s)の値が零となる場合にはH(s)の値が発散し、正しい値が求められない。
これを防ぐため、第一および第二実施形態では、渦電流の応答関数をモデル式に当てはめて、最適な評価値を与える応答関数を導出した。
ここで、Gin1(s)は、台形波1901のラプラス変換形、Gout1(s)は、入力傾斜磁場波形として台形波1901を用いた場合の出力傾斜磁場波形のラプラス変換形、Gin2(s)は、台形波1902のラプラス変換形、Gout2(s)は、入力傾斜磁場波形として台形波1902を用いた場合の出力傾斜磁場波形のラプラス変換形である。
ここで、H'(s)は最適フィルタ適用後の伝達特性、Φ(s)は最適フィルタである。最適フィルタΦ(s)には、公知の技術であるウィナーフィルタを用いる。ウィナーフィルタの定義式は下記である。
ここで、N(s)はGout1(s)のノイズ成分、S(s)はGout1(s)のノイズを除いた信号成分である。N(s)及びS(s)の求め方は、既知の技術に従い、Gout1(s)のパワースペクトルの最も高周波の信号値をN(s)とし、Gout1(s)のパワースペクトルからN(s)を減算したものをS(s)とする。
ここで、H”(s)はメディアンフィルタ適用後の伝達特性である。
本発明のMRI装置は、
静磁場発生手段、傾斜磁場発生手段、高周波磁場発生手段および高周波磁場検出手段を備えた撮像手段と、撮像シーケンスに基き前記撮像手段の動作を制御する制御手段と、前記高周波磁場検出手段が検出した核磁気共鳴信号を用いて、画像再構成を含む演算を行う演算手段と、を備え、
前記演算手段は、前記傾斜磁場発生手段に入力される入力波形と当該入力波形によって前記傾斜磁場発生手段が発生した出力波形とを用いて、前記出力波形に影響を与える複数の要素の応答関数の合算である応答関数を算出する応答関数算出手段と、前記応答関数計算手段が計算した応答関数を用いて、前記撮像シーケンスに設定された傾斜磁場パルスの入力波形から出力波形を算出する傾斜磁場算出手段と、前記傾斜磁場算出手段が算出した出力波形の計算値を用いて、前記撮像シーケンスに設定された高周波磁場パルスの修正および/または前記撮像シーケンスで得られた核磁気共鳴信号のk空間座標の修正を行う補償手段とを備えたことを特徴とする。
前記応答関数算出手段は、前記出力波形を変化させる複数の要素毎に応答関数を算出し、前記傾斜磁場算出手段は、前記複数の要素の応答関数の合算である応答関数を用いて出力波形の算出を行う。
前記応答関数算出手段は、前記要素毎に応答関数モデルを設定するモデル設定手段と、前記モデル設定手段が設定した応答関数モデルによって算出した出力波形の計算値と、出力波形の実測値との差が最小となるように応答関数モデルを適正化するモデル適正化手段とを備え、前記モデル適正化手段が適正化した応答関数モデルを各要素の応答関数とする。
前記応答関数算出手段は、前記入力値として複数の入力値を用い、複数の入力値とそれに対応する実測値との組合せを用いて、入力値と出力波形の実測値との関係から、直接、応答関数を算出する。
前記応答関数算出手段は、算出後の応答関数にフィルタをかけるフィルタ手段を備える。
前記フィルタ手段は、ウィナーフィルタ、メディアンフィルタを含む。
前記傾斜磁場発生手段は、発生した傾斜磁場の出力波形に基き入力波形を制御する制御回路を備え、
前記出力波形を変化させる複数の要素は、前記制御回路による制御に伴う前記出力波形の歪みを含む。
前記応答関数算出手段は、前記静磁場発生手段が発生する静磁場の中心に対する位置が異なる複数の傾斜磁場について、それぞれ、応答関数を算出する。
前記傾斜磁場算出手段が算出する傾斜磁場パルスは、前記撮像シーケンスにおけるスライス選択傾斜磁場であり、前記補償手段は、前記スライス選択傾斜磁場の計算値に基き、前記撮像シーケンスに設定された高周波パルスを補正する高周波パルス補正手段を備える。
前記高周波パルス補正手段は、補正前後の高周波パルスの照射時間の変化を算出する照射時間計算手段を備え、前記照射時間計算手段が算出した照射時間の変化が所定の増加率を超えるときに、前記傾斜磁場算出手段の入力値を変更し、前記傾斜磁場算出手段が前記変更された入力値から推定した出力波形の推定値を用いて、前記高周波パルスの補正を行う。
前記傾斜磁場算出手段が算出する傾斜磁場パルスは、前記撮像シーケンスにおける読出し傾斜磁場であり、前記補償手段は、前記読出し選択傾斜磁場の計算値に基き、前記撮像シーケンスで得た核磁気共鳴信号が配置されるk空間座標を補正するk空間座標補正手段を備る。
前記補正手段は、補正前後のk空間座標の大きさの変化を算出するk空間サイズ計算手段を備え、前記k空間サイズ計算手段が計算したk空間の大きさの変化が所定の縮小率を超えるときに、前記傾斜磁場算出手段の入力値を変更し、前記傾斜磁場算出手段が変更後の入力値から算出した出力波形の再計算値を用いて、前記k空間座標の補正を行う。
前記傾斜磁場発生手段の入力値およびそれによって前記傾斜磁場発生手段が発生した出力傾斜磁場波形の実測値を入力するステップと、
前記出力傾斜磁場波形に影響を与える複数の因子について、それぞれ、応答関数モデルを設定するステップと、
前記入力した傾斜磁場波形の入力値および実測値を用いて、各応答関数モデルを適正化するステップと、
所望の傾斜磁場の入力値に対し、前記適正化した応答関数モデルを用いて、出力傾斜磁場波形の推定値を算出するステップと、
を含むことを特徴とする。
前記複数の因子は、傾斜磁場により発生する渦電流と、前記傾斜磁場発生手段に備えられた制御回路の出力とを含む。
前記実測値は、前記傾斜磁場発生手段の制御回路を無効にした時の第1の実測値と、前記制御回路を有効にした第2の実測値を含み、
前記応答関数モデルを適正化するステップは、前記第1の実測値および第2の実測値を用いて、制御回路による出力傾斜磁場波形の応答関数を求める。
前記傾斜磁場発生手段の少なくとも2種の入力値およびそれらによって前記傾斜磁場発生手段が発生した出力傾斜磁場波形の実測値を入力するステップと、
前記少なくとも2種の入力値およびそれに対応する実測値を用いて、前記出力傾斜磁場波形に影響を与える複数の因子の影響を含む前記傾斜磁場発生手段の応答関数を算出するステップと、
所望の傾斜磁場の入力値に対し、前記応答関数を用いて、出力傾斜磁場波形の推定値を算出するステップと、を含む。
Claims (16)
- 静磁場発生手段、傾斜磁場発生手段、高周波磁場発生手段および高周波磁場検出手段を備えた撮像手段と、撮像シーケンスに基き前記撮像手段の動作を制御する制御手段と、前記高周波磁場検出手段が検出した核磁気共鳴信号を用いて、画像再構成を含む演算を行う演算手段と、を備えた磁気共鳴イメージング装置であって、
前記演算手段は、前記傾斜磁場発生手段に入力される入力波形と当該入力波形によって前記傾斜磁場発生手段が発生した出力波形とを用いて、前記出力波形に影響を与える複数の要素の応答関数の合算である応答関数を算出する応答関数算出手段と、前記応答関数計算手段が計算した応答関数を用いて、前記撮像シーケンスに設定された傾斜磁場パルスの入力波形から出力波形を算出する傾斜磁場算出手段と、前記傾斜磁場算出手段が算出した出力波形の計算値を用いて、前記撮像シーケンスに設定された高周波磁場パルスの修正および/または前記撮像シーケンスで得られた核磁気共鳴信号のk空間座標の修正を行う補償手段とを備えた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であって、
前記応答関数算出手段は、前記出力波形を変化させる複数の要素毎に応答関数を算出し、前記傾斜磁場算出手段は、前記複数の要素の応答関数の合算である応答関数を用いて出力波形の算出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2に記載の磁気共鳴イメージング装置であって、
前記応答関数算出手段は、前記要素毎に応答関数モデルを設定するモデル設定手段と、前記モデル設定手段が設定した応答関数モデルによって算出した出力波形の計算値と、出力波形の実測値との差が最小となるように応答関数モデルを適正化するモデル適正化手段とを備え、前記モデル適正化手段が適正化した応答関数モデルを各要素の応答関数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であって、
前記応答関数算出手段は、前記入力波形として複数の入力値を用い、複数の入力値とそれに対応する実測値との組合せを用いて、入力値と出力波形の実測値との関係から、直接、応答関数を算出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4に記載の磁気共鳴イメージング装置であって、
前記応答関数算出手段は、算出後の応答関数にフィルタをかけるフィルタ手段を備え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であって、
前記フィルタ手段は、ウィナーフィルタ、メディアンフィルタを含む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であって、
前記傾斜磁場発生手段は、発生した傾斜磁場の出力波形に基き入力波形を制御する制御回路を備え、
前記出力波形を変化させる複数の要素は、前記制御回路による制御に伴う前記出力波形の歪みを含む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であって、
前記応答関数算出手段は、前記静磁場発生手段が発生する静磁場の中心に対する位置が異なる複数の傾斜磁場について、それぞれ、応答関数を算出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であって、
前記傾斜磁場算出手段が算出する傾斜磁場パルスは、前記撮像シーケンスにおけるスライス選択傾斜磁場であり、前記補償手段は、前記スライス選択傾斜磁場の計算値に基き、前記撮像シーケンスに設定された高周波パルスを補正する高周波パルス補正手段を備えたことを特徴とする磁気共鳴イメージング装置。 - 請求項9に記載の磁気共鳴イメージング装置であって、
前記高周波パルス補正手段は、補正前後の高周波パルスの照射時間の変化を算出する照射時間計算手段を備え、前記照射時間計算手段が算出した照射時間の変化が所定の増加率を超えるときに、前記傾斜磁場算出手段の入力値を変更し、前記傾斜磁場算出手段が前記変更された入力値から推定した出力波形の推定値を用いて、前記高周波パルスの補正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であって、
前記傾斜磁場算出手段が算出する傾斜磁場パルスは、前記撮像シーケンスにおける読出し傾斜磁場であり、前記補償手段は、前記読出し選択傾斜磁場の計算値に基き、前記撮像シーケンスで得た核磁気共鳴信号が配置されるk空間座標を補正するk空間座標補正手段を備えたことを特徴とする磁気共鳴イメージング装置。 - 請求項11に記載の磁気共鳴イメージング装置であって、
前記補正手段は、補正前後のk空間座標の大きさの変化を算出するk空間サイズ計算手段を備え、前記k空間サイズ計算手段が計算したk空間の大きさの変化が所定の縮小率を超えるときに、前記傾斜磁場算出手段の入力値を変更し、前記傾斜磁場算出手段が変更後の入力値から算出した出力波形の再計算値を用いて、前記k空間座標の補正を行うことを特徴とする磁気共鳴イメージング装置。 - 磁気共鳴イメージング装置の傾斜磁場発生手段が発生する出力傾斜磁場波形を推定する方法であって、
前記傾斜磁場発生手段の入力値およびそれによって前記傾斜磁場発生手段が発生した出力傾斜磁場波形の実測値を入力するステップと、
前記出力傾斜磁場波形に影響を与える複数の因子について、それぞれ、応答関数モデルを設定するステップと、
前記入力した傾斜磁場波形の入力値および実測値を用いて、各応答関数モデルを適正化するステップと、
所望の傾斜磁場の入力値に対し、前記適正化した応答関数モデルを用いて、出力傾斜磁場波形の推定値を算出するステップと、
を含む傾斜磁場波形推定方法。 - 請求項13に記載の傾斜磁場波形推定方法であって、
前記複数の因子は、傾斜磁場により発生する渦電流と、前記傾斜磁場発生手段に備えられた制御回路の出力とを含むことを特徴とする傾斜磁場波形推定方法。 - 請求項14に記載の傾斜磁場波形推定方法であって、
前記実測値は、前記傾斜磁場発生手段の制御回路を無効にした時の第1の実測値と、前記制御回路を有効にした第2の実測値を含み、
前記応答関数モデルを適正化するステップは、前記第1の実測値および第2の実測値を用いて、制御回路による出力傾斜磁場波形の応答関数を求めることを特徴とする傾斜磁場波形推定方法。 - 磁気共鳴イメージング装置における出力傾斜磁場波形を推定する方法であって、
前記傾斜磁場発生手段の少なくとも2種の入力値およびそれらによって前記傾斜磁場発生手段が発生した出力傾斜磁場波形の実測値を入力するステップと、
前記少なくとも2種の入力値およびそれに対応する実測値を用いて、前記出力傾斜磁場波形に影響を与える複数の因子の影響を含む前記傾斜磁場発生手段の応答関数を算出するステップと、
所望の傾斜磁場の入力値に対し、前記応答関数を用いて、出力傾斜磁場波形の推定値を算出するステップと、を含む傾斜磁場波形推定方法。
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