JPWO2013002232A1 - Magnetic resonance imaging apparatus and method of measuring gradient magnetic field output waveform thereof - Google Patents

Abstract

A means for accurately measuring the gradient magnetic field waveform itself is provided. Further, a means capable of actually measuring the magnetic field component generated in the direction orthogonal to the magnetic axis of the gradient magnetic field is provided. For this purpose, reference measurement for acquiring a signal without using a test gradient magnetic field and main measurement for acquiring a signal using a test gradient magnetic field are performed. In that case, a signal is acquired from the rod-shaped region where the slice in the magnetic axis direction of the test gradient magnetic field and the slice in the direction orthogonal thereto intersect. The bar-shaped areas are at least three areas having different positions. The measured value of the test gradient magnetic field and the magnetic field component orthogonal to the magnetic axis direction are calculated by calculating the NMR signal acquired in the reference measurement and the NMR signal acquired in the main measurement.

Description

  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 artifacts caused by gradient magnetic field output errors.

  An MRI system that performs tomography using the NMR phenomenon measures the NMR signals generated by the nuclear spins that make up the body of the subject, especially the human body, and the shape and function of the head, abdomen, limbs, etc. It is a device for imaging in three dimensions. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field, frequency-encoded, and measured as time series data. The measured NMR signal has a coordinate position in k-space that is uniquely determined according to the 2-axis or 3-axis encoding amount, so it must be two-dimensionally or three-dimensionally arranged in k-space coordinates and then subjected to Fourier transform processing. The image is made with.

  The arrangement in k-space coordinates is determined in advance based on the gradient magnetic field output set at the time of sequence design, but there are errors in the waveform shape and area between the set gradient magnetic field output and the actual gradient magnetic field output. Then, there is a difference between the predicted k-space coordinates and the actual coordinates, and there is a problem that artifacts such as image distortion occur when imaged in this state.

  Factors of gradient magnetic field output error include inhomogeneous static magnetic field, gradient magnetic field offset, eddy current, and system response. Of these, static magnetic field inhomogeneity and gradient magnetic field offset are often corrected using short pre-measurements such as shimming and offset adjustment before actual imaging, whereas gradient magnetic field output errors due to eddy currents and system response are Since the measurement takes time, it is difficult to perform in advance measurement and is a component that does not depend on the subject. Therefore, it is often performed by hardware adjustment when installing the MRI apparatus.

  The effect of eddy currents and system response on gradient magnetic field output basically works in the direction of hindering the output, so to correct the error, it is strong enough to compensate for the output error (high intensity, high slew rate) Although it is necessary to output the correction magnetic field and superimpose it on the original gradient magnetic field output, it is very expensive to apply the correction magnetic field with high accuracy, so that an error that cannot be corrected remains, resulting in poor image quality. Sometimes.

  As a countermeasure for such an error component, there is a means of measuring a gradient magnetic field output waveform including an error with high accuracy in advance and imaging it using coordinates in k space corresponding to the actual gradient magnetic field output. Become important.

  As a technique for actually measuring the gradient magnetic field output waveform, for example, in Non-Patent Document 1, after applying a test gradient magnetic field, a large number of excitation pulses are applied, and a large number of FIDs with different elapsed times after application of the test gradient magnetic field are measured. Thus, a technique for measuring the distortion of the gradient magnetic field caused by the eddy current is described. In Non-Patent Document 2, after excitation of a predetermined thin slice, a sequence for acquiring a signal by applying a test gradient magnetic field and a reference sequence for acquiring a signal without applying a test gradient magnetic field are implemented. There is described a method of actually measuring a gradient magnetic field output waveform of a test gradient magnetic field by calculation between signals obtained in a sequence.

  Patent Document 1 describes a technique for approximating an error component of a gradient magnetic field output using an equivalent circuit model and correcting k-space coordinates of an NMR signal using a result estimated by an approximate expression. .

International Publication No. 2010/047245

Wysong RE: A simple method of measuring gradient induced eddy currents to set compensation network: Magn Reson Med 29, 119-121 (1993) Peter Latta: Simple phase method for measurement of magnetic field gradient waveforms: MAGNETIC RESONAMCE IMAGING 25, 1272-1276 (2007) J.I Jackson et.Al., Selection of a Convolution Function for Fourier Inversion Using Gridding, IEEE Trans.Med Imaging, vol10, pp.473-478, 1991

  The method of Non-Patent Document 1 measures only the effect of eddy current generated after application of a test gradient magnetic field, and the gradient magnetic field output waveform itself is affected not only by the effect of eddy current but also by the response of the entire gradient magnetic field system. Cannot be measured.

  The method of Non-Patent Document 2 is an effective method that can obtain the gradient magnetic field output waveform itself by a relatively simple procedure, but it is not possible to measure the magnetic field component due to the eddy current generated in the direction orthogonal to the test gradient magnetic field. Can not. In the sequence of filling the k-space while reversing the polarity of the readout gradient magnetic field as in the echo planer method, eddy currents are generated on the orthogonal axes by the strong readout gradient magnetic field, and the influence of the eddy currents depends on the polarity of the readout. Can cause N / 2 artifacts. Such artifacts can be effectively reduced if the eddy current components generated on the orthogonal axes can be known.

  In addition, in the method of Non-Patent Document 2, it is necessary to acquire NMR signals in thin slices in order to eliminate the influence of position fluctuations such as eddy currents, and the measured gradient magnetic field output waveform cannot provide a sufficient SNR. .

  Accordingly, an object of the present invention is to provide an MRI apparatus provided with means for measuring the gradient magnetic field waveform itself with high accuracy, and in particular, to provide an MRI apparatus capable of actually measuring a component orthogonal to the magnetic axis.

  The MRI apparatus of the present invention that solves the above problems acquires a nuclear magnetic resonance signal from a plurality of regions of a subject without using a test gradient magnetic field, and uses a test gradient magnetic field to obtain a nuclear magnetic resonance from a plurality of regions of the subject. Get the signal. Then, using these nuclear magnetic resonance signals, an actual measurement value of the test gradient magnetic field is calculated. Using the calculated actual measurement value, the magnetic field component in the magnetic axis direction of the test gradient magnetic field is at least one orthogonal to the magnetic axis direction. Calculate the magnetic field component in one direction.

  Each calculated magnetic field component can be used, for example, for correction of k-space coordinates where the nuclear magnetic resonance signal obtained in the imaging sequence is arranged.

  According to the present invention, the gradient magnetic field output waveform can be measured easily and accurately. Further, according to the present invention, not only the magnetic field component of the magnetic axis of the gradient magnetic field but also the magnetic field component due to the eddy current in the direction orthogonal to the magnetic axis can be measured simultaneously. By using the measured gradient magnetic field output waveform and orthogonal magnetic field components, coordinate correction with high accuracy can be performed in image reconstruction, and an image in which distortion and artifacts are suppressed can be obtained.

The block diagram which shows the whole outline | summary of the MRI apparatus with which this invention is applied Functional block diagram of the arithmetic processing unit The flowchart which shows an example of the operation | movement procedure by 1st embodiment. Diagram showing the measurement pulse sequence used in the first embodiment The figure explaining the excitation position excited by the measurement pulse sequence of FIG. It is a figure which shows the relationship between a gradient magnetic field waveform and k space coordinate, (a) shows a waveform, (b) shows k space (kx) coordinate. It is a figure which shows the example which correct | amends a gradient magnetic field error on k space coordinate, (a) shows the case where an orthogonal component does not arise, (b) shows the case where correction | amendment of an orthogonal component is required. It is a figure which shows the result which confirmed the effect of correction | amendment on the image, (a) shows before correction | amendment, (b) shows after correction | amendment. The flowchart which shows an example of the operation | movement procedure by 2nd embodiment. The figure which shows the measurement pulse sequence used in 2nd embodiment The figure explaining the excitation position excited by the measurement pulse sequence of FIG. The figure which shows an example of the display screen of the measurement tool of this invention Diagram showing an example of gradient magnetic field waveform measurement results The figure which shows the other example of the measurement result of a gradient magnetic field waveform

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, an outline of an MRI apparatus to which the present invention is applied will be described with reference to FIG. 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 obtains a tomographic image of the subject 101 using the NMR phenomenon, and includes a static magnetic field generating magnet 102, a gradient magnetic field coil 103 and a gradient magnetic field power supply 109, an RF transmission coil 104 and an RF transmission unit 110, The RF receiving coil 105, the signal detection unit 106, the signal processing unit 107, the measurement control unit 111, the overall control unit 108, the display / operation unit 113, and the top plate on which the subject 101 is mounted is a static magnetic field generating magnet. And a bed 112 to be taken in and out of the interior of 102.

  The static magnetic field generating magnet 102 includes a static magnetic field generation source including a permanent magnet, a normal conducting magnet, or a superconducting magnet, and generates a uniform static magnetic field around the subject 101. Depending on the direction of the static magnetic field, there are a vertical magnetic field method and a horizontal magnetic field method. If the vertical magnetic field method is used, the static magnetic field is applied in the direction perpendicular to the body axis of the subject 101. generate.

  The gradient magnetic field coil 103 is composed of coils wound in the three-axis directions of X, Y, and Z, which are the real space coordinate system (stationary coordinate system) of the MRI apparatus, and each gradient magnetic field coil is a gradient magnetic field that drives it. A current is supplied to the power source 109. Specifically, the gradient magnetic field power supply 109 of each gradient coil is driven according to a command from the measurement control unit 111 described later, and supplies a current to each gradient coil. Thereby, gradient magnetic fields Gx, Gy, and Gz are generated in the three-axis directions of X, Y, and Z. By combining gradient magnetic fields in three axial directions, a cross section (slice plane) in an arbitrary direction can be selected and imaged.

  When imaging a two-dimensional slice plane, a slice gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 101, orthogonal to the slice plane and orthogonal to each other. The phase encoding gradient magnetic field pulse (Gp) and the readout (lead-out) gradient magnetic field pulse (Gf) are applied in the remaining two directions, and position information in each direction is encoded into the NMR signal (echo signal). However, the phase encoding gradient magnetic field and the read gradient magnetic field may not be distinguished depending on the imaging method.

  The RF transmission coil 104 is a coil that irradiates the subject 101 with an RF pulse, and is connected to the RF transmission unit 110 and supplied with a high-frequency pulse current. As a result, an NMR phenomenon is induced in the spins of atoms constituting the living tissue of the subject 101. Specifically, the RF transmission unit 110 is driven in accordance with a command from the measurement control unit 111, and the high frequency pulse is amplitude-modulated and amplified, and then supplied to the RF transmission coil 104 disposed close to the subject 101 By doing so, the subject 101 is irradiated with the RF pulse.

  The RF receiving coil 105 is a coil that receives an echo signal emitted by the NMR phenomenon of spin that constitutes the living tissue of the subject 101. The received echo signal is connected to the signal detecting unit 106 and is received by the signal detecting unit 106. Sent.

  The signal detection unit 106 performs processing for detecting an echo signal received by the RF receiving coil 105. Specifically, in accordance with a command from the measurement control unit 111, the signal detection unit 106 amplifies the received echo signal, divides it into two orthogonal signals by quadrature detection, and each of them is a predetermined number (for example, 128). , 256, 512, etc.), and each sampling signal is A / D converted into a digital quantity and sent to the signal processing unit 107. Therefore, the echo signal is obtained as time-series digital data (hereinafter referred to as echo data) composed of a predetermined number of sampling data.

  The signal processing unit 107 performs various processes on the echo data, and sends the processed echo data to the measurement control unit 111.

  The measurement control unit 111 mainly transmits various commands for collecting echo data necessary for reconstruction of the tomographic image of the subject 101 to the gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106. And a control unit for controlling them. Specifically, the measurement control unit 111 operates under the control of the overall control unit 108 described later, and controls the gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106 based on a predetermined pulse sequence, Echo data necessary for reconstructing the image of the imaging region of the subject 101 by repeatedly performing the irradiation of the RF pulse and the application of the gradient magnetic field pulse to the subject 101 and the detection of the echo signal from the subject 101 Control the collection.

  In the repetition, the application amount of the phase encoding gradient magnetic field is changed in the case of two-dimensional imaging, and the application amount of the slice encoding gradient magnetic field is further changed in the case of three-dimensional imaging. Values such as 128, 256, and 512 are normally selected as the number of phase encodings, and values such as 16, 32, and 64 are normally selected as the number of slice encodings. With these controls, echo data from the signal processing unit 107 is output to the overall control unit 108.

  As the pulse sequence for controlling the measurement control unit 111, various pulse sequences are prepared in advance depending on the imaging method, and are stored in the storage unit 115 as a program. In the MRI apparatus of this embodiment, in addition to a general imaging pulse sequence for the subject 101, a pulse sequence for measuring a gradient magnetic field output waveform is provided. The gradient magnetic field waveform measurement pulse sequence is implemented as a pre-measurement or as a measurement when the apparatus is installed. Details of the gradient magnetic field waveform measurement pulse sequence will be described later.

  The overall control unit 108 controls the measurement control unit 111 and controls various data processing and processing result display and storage, and includes an arithmetic processing unit 114 having a CPU and a memory, an optical disc, And a storage unit 115 such as a magnetic disk. Specifically, the measurement control unit 111 is controlled to execute the collection of echo data, and when the echo data is input from the measurement control unit 111, the arithmetic processing unit 114 converts the encoded information applied to the echo data. Based on this, it is stored in an area corresponding to the k space in the memory. Hereinafter, the description that the echo data is arranged in the k space means that the echo data is stored in an area corresponding to the k space in the memory.

  A group of echo data stored in an area corresponding to the k space in the memory is also referred to as k space data. The arithmetic processing unit 114 performs processing such as signal processing and image reconstruction by Fourier transform on the k-space data, and displays the resulting image of the subject 101 on the display / operation unit 113 described later. And is recorded in the storage unit 115.

  The display / operation unit 113 includes a display unit for displaying the reconstructed image of the subject 101, a trackball or a mouse and a keyboard for inputting various control information of the MRI apparatus and control information for processing performed by the overall control unit 108. Etc., and an operation unit. The operation unit is disposed in the vicinity of the display unit, and an operator interactively controls various processes of the MRI apparatus through the operation unit while looking at the display unit.

In addition to the above-described image reconstruction, the arithmetic processing unit 114 has a function of calculating a gradient magnetic field output waveform by calculating a result obtained by executing the gradient magnetic field waveform measurement pulse sequence.
FIG. 2 shows a functional block diagram of the arithmetic processing unit 114.

  As shown in the figure, the arithmetic processing unit 114 includes an image reconstruction unit 1141 that reconstructs an image using the NMR signal obtained by executing the imaging sequence, and a pulse sequence according to the imaging conditions specified by the display / operation unit 113. A sequence calculation unit 1142 for calculation, a gridding unit 1143 for calculating data values on lattice points in k space in order to perform operations such as Fourier transform using NMR signals, and a gradient magnetic field waveform measurement sequence Is provided with a gradient magnetic field calculation unit 1144 that calculates an output waveform of the gradient magnetic field using a signal obtained by executing the above. The measured data of the gradient magnetic field output waveform calculated by the gradient magnetic field calculation unit 1144 is stored in the storage unit 115, and is used to correct k-space coordinates of the NMR signal in the gridding unit. That is, the gridding unit 1143 includes a correction unit that corrects k-space coordinates of the NMR signal.

  Based on the configuration of the MRI apparatus, the operation of the MRI apparatus, particularly, the gradient magnetic field waveform measurement operation will be mainly described below.

<First embodiment>
First, prior to the measurement of the gradient magnetic field waveform, error magnetic field components other than the gradient magnetic field system such as offset magnetic field adjustment and residual magnetic field adjustment (in the case of a permanent magnet device) are adjusted in advance. After these adjustments, gradient magnetic field output waveform measurement is performed. FIG. 3 shows the procedure for measuring the gradient magnetic field output waveform.

  As shown in the figure, the gradient magnetic field waveform measurement is roughly divided into a reference measurement 301-1 for acquiring a signal without using a gradient magnetic field to be measured (hereinafter referred to as a test gradient magnetic field) and a test gradient magnetic field. The measurement step 301 consisting of the main measurement 301-2 for acquiring the signal, and the eddy current component (hereinafter referred to as the gradient magnetic field waveform and the gradient magnetic field) generated by performing the calculation using the NMR signal acquired in the measurement step 301 in the direction orthogonal to the magnetic axis. , Which is also referred to as an orthogonal component). The measurement step 301 is executed for a plurality of excitation positions. The same measurement is repeated as many times as necessary to improve SNR. This is the repeat count loop 302. Integration processing 303 is performed on the result of the same measurement. The measurement step 301 to the calculation step 304 are executed a plurality of times with different magnetic axes of the gradient magnetic field. This is the X, Y, Z axis loop 305. Details of each step will be described below.

<< Measurement Step 301 >>
In this measurement 301-2, a signal is measured using a test gradient magnetic field as a read gradient magnetic field. Since this signal is accompanied by phase rotation (time-series change in phase) according to the applied amount of the test gradient magnetic field, theoretically, a gradient magnetic field waveform can be obtained by calculating the time change of the phase of the signal. become. However, the phase rotation received by the signal includes not only the test gradient magnetic field but also the effect of eddy currents caused by the slice gradient magnetic field. The reference measurement 301-1 is performed to remove an eddy current component due to the slice gradient magnetic field.

  An example of a measurement sequence used in the reference measurement 301-1 and the main measurement 301-2 is shown in FIG. Here, as an example, the case where the magnetic axis is the X axis and the Y axis component is measured as the orthogonal component is shown.

In both the reference measurement and the main measurement, a spherical phantom as shown in FIG. 5 (a) is placed at the center of the magnetic field, and a very thin excitation thickness slice is set at a distance Dx, Dy from the magnetic field center.
In the first excitation, first, an X-axis slice selection gradient magnetic field and a 90-degree RF pulse and a Y-axis slice selection gradient magnetic field and a 180-degree RF pulse are applied, and the slice selected by the 90-degree RF pulse and the 180-degree RF pulse are applied. Excites the area (bar-shaped area) at the intersection of the slices selected by the pulse.

  For example, in the first excitation, the excitation position 1 (−Dx, Dy) is excited as shown in FIG. 5 (a). After the excitation, the reference measurement 301-1 samples the FID signal without applying the test gradient magnetic field. In this measurement 301-2, the FID signal is sampled while applying a readout gradient magnetic field coaxial with the magnetic axis (X axis) of the slice gradient magnetic field as the test gradient magnetic field. This FID signal includes the influence of the test gradient magnetic field component and the eddy current component generated on the orthogonal axis thereby.

  In the second excitation, the frequency of the RF pulse is changed without changing the slice gradient magnetic field, so that the slice position to be selected is changed, and the same excitation and sampling of the FID signal are performed as in the first excitation. For example, in the second excitation, a signal is acquired from excitation position 2 (Dx, Dy). Similarly, signals are acquired at different positions (Dx, -Dy) and (-Dx, -Dy) for the third and fourth times. FIGS. 4 and 5 show the case where signals are acquired from four excitation positions, but it is only necessary to have signals from three excitation positions for the calculation of gradient magnetic field waveforms, which will be described later. The signal acquisition from can be omitted.

  In these four signal acquisition steps, both the reference measurement and the main measurement are arranged in a nested manner within the repetition time TR, thereby acquiring signals from four regions without extending the overall measurement time. be able to.

  In the measurement step of the present embodiment, rod-like excitation is performed, so that the signal intensity is insufficient with one measurement. Therefore, sufficient signal strength can be obtained by increasing the number of repetitions (repetition number loop 302).

  The signal (sampling data) acquired by repeating the measurement step 301 is integrated in the integration processing step 303 and then used in the calculation step 304.

<< Calculation step 304 >>
The gradient magnetic field output waveform and the orthogonal component are calculated by calculation between the signals acquired in the measurement step 301. This processing is performed by the gradient magnetic field calculation unit 1144 of the arithmetic processing unit 114.
First, the signal _{Sgrad1 at the} excitation position 1 obtained by this measurement can be _expressed by the following equation (1).

Here, K is a complex constant, Dx is the distance from the magnetic field center to the slice excitation position, Δr is the slice thickness, and Gx (t) is the time change of the actually measured gradient magnetic field (test magnetic field) intensity, that is, , Meaning gradient magnetic field waveform. Further, I (t) is an eddy current component due to the slice gradient magnetic field, and φ0 is an initial value of the phase. Here, k (t) can be expressed by the following equation (2), and using this, equation (1) becomes equation (3).

式(4)中、B0(t)を含む項は、傾斜磁場印加に伴う静磁場B0の渦電流成分である。The signal S _ref obtained by the reference measurement is given by setting Gx = 0 in Equation (2), and the phase difference between the signal S _{grad1 of} this measurement and the signal S _{ref of the} reference measurement is given by Equation (4). Given.

In Expression (4), the term including B0 (t) is an eddy current component of the static magnetic field B0 accompanying the gradient magnetic field application.

Similarly, the phase differences between the main measurement signal and the reference measurement signal at the remaining three excitation positions 2 to 4 are expressed by equations (5) to (7).

By solving the simultaneous equations using any one of the equations (4) to (7) described above, the relationship between the phase value at each position and k _x , k _y can be calculated. The following formulas (8) and (9) are examples calculated using formulas (4) to (6), but the term including B0 is deleted and either Dx · kx or Dy · ky is obtained. As long as it is a combination, the combinations of formulas (4) to (7) are arbitrary.

The change in the unit time Δt of the phase difference between the main measurement and the reference measurement is equivalent to the application amount of the test gradient magnetic field per unit time as shown in the following equation (10). Similarly, the eddy current component orthogonal to the test gradient magnetic field is expressed by the following equation (11).

Therefore, the application amount Gx (t) and the coaxial component Gy (t) per unit time are derived from Equation (12) derived from Equation (8) and Equation (10), and from Equation (9) and Equation (11). Each is obtained by the equation (13). Thus, Gx (t) and Gy (t) can be understood from the phase change of the NMR signal.

こうして、テスト傾斜磁場Gxを印加した場合の出力波形Gx(t)、直交成分Gy(t)、B0成分が算出される。Also, for the term including B0 (B0 component of eddy current) erased in Equations (8) and (9), the combination of Equation (4) and Equation (6) or the combination of Equation (5) and Equation (7) By solving the simultaneous equations, the following equations (14) and (15) can be obtained.

Thus, the output waveform Gx (t), orthogonal component Gy (t), and B0 component when the test gradient magnetic field Gx is applied are calculated.

<< Repetition Step 305 >>
The calculation step 304 described above is repeated with different combinations of the magnetic and orthogonal axes of the test magnetic field. Specifically, the following six combinations are executed.

Magnetic axis: X axis, orthogonal axis: Y axis Magnetic axis: X axis, orthogonal axis: Z axis Magnetic axis: Y axis, orthogonal axis: X axis Magnetic axis: Y axis, orthogonal axis: Z axis Magnetic axis: Z axis, Orthogonal axis: X axis Magnetic axis: Z axis, orthogonal axis: Y axis Fig. 5 (b) shows the case where the magnetic axis is the X axis (or Z axis) and the orthogonal axis is the Z axis (or X axis). (c) shows a case where the magnetic axis is the Y axis (or Z axis) and the orthogonal axis is the Z axis (or Y axis). As shown in the figure, in these six combinations, it is a measurement step to excite a position where two orthogonal slices that are separated from the gradient magnetic field center by a predetermined interval intersect and to acquire signals for three or more excitation positions. The procedure is the same as that described in 301 and calculation step 304.

  As described above, the output waveform Gx (t) and orthogonal component Gy (t) obtained for each axis are stored in the storage unit 115 and used for correction calculation of k-space coordinates where the NMR signal is arranged when the imaging pulse sequence is executed. Is done. The B0 eddy current component can be used to correct the phase of NMR signal sampling (A / D).

  The measured waveform data can be stored in the storage unit 115 as discrete measured waveform data, but an approximate waveform may be created and stored as approximate waveform data. The calculation of the approximate waveform can be performed in the gradient magnetic field calculation unit 1144 by simple function fitting, linear interpolation processing, or the like. Further, the approximate waveform can be calculated by modeling the response of the gradient magnetic field system. The response function can be obtained by creating a model formula such as an RLC circuit and optimizing it so as to match the actual measurement value.

  As described above, the gradient magnetic field waveform measurement according to the present embodiment can be performed in a short time by a relatively simple procedure. In addition, when performing gradient magnetic field waveform measurement as a procedure when installing the device separately from imaging, multiple types of gradient magnetic fields with different sizes are measured and the results are stored in the storage unit 15, which is necessary for imaging and other calculations. It is also possible to read and use various data.

<< k-space coordinate correction >>
Next, correction of k-space coordinates using the gradient magnetic field output waveform Gx (t) and the orthogonal component Gy (t) obtained by the above procedure will be described.

First, the k-space coordinate shift caused by the distortion of the gradient magnetic field waveform will be described with reference to FIG. FIG. 6A shows an ideal trapezoidal readout gradient magnetic field waveform 601 created at the time of software design and a waveform 602 in which the gradient magnetic field waveform rises and falls slowly due to various influences. During application of the read gradient magnetic field, data sampling is performed with a sampling time determined by the frequency band BW of the imaging condition. Each acquired sampling data is arranged on the coordinates of the readout axis on the k space calculated from the intensity / time product of the readout gradient magnetic field using the following equation (16).

  FIG. 6B shows data arranged in k-space coordinates, the upper side is when the read gradient magnetic field is the ideal waveform 601 and the lower side is when the actual waveform 602 is present. As can be seen from the figure, there is an error in the position between the coordinates at the rise and fall due to the dullness in the actual waveform.

グリッディング処理は、k空間の格子上に存在しないデータを、格子点から距離に応じて重み付けした周囲のデータから補間して求める処理であり、公知の手法(例えば、非特許文献3記載の技術)を用いることができる。In the case of an overall shift in which the positional deviation of k-space coordinates is simple, only a first-order phase gradient is generated on the image after the Fourier transform, which is not a big problem. However, when there is a difference in density between coordinates, image distortion becomes a big problem. Therefore, using the Gx component (Equation (12)) and Gy component (Equation (13)) calculated from the measured values, the Kx and Ky coordinates of the k space are obtained from the following equations (17) and (18), and the gridding process is performed. I do.

The gridding process is a process for obtaining data that does not exist on the k-space grid by interpolating from surrounding data weighted according to the distance from the grid point, and is a known method (for example, the technique described in Non-Patent Document 3). ) Can be used.

  FIG. 7 shows an example in which the gradient magnetic field error is corrected on the k-space coordinates in the EPI sequence. FIG. 7 (a) shows a case where an eddy current component (orthogonal component) in the Y direction does not occur, and only correction in the kx direction has to be performed using Equation (17). FIG. 7 (b) shows a case where an eddy current component in the Y direction is generated at the rise and fall of the readout gradient magnetic field (X axis), and in that case, the kx direction is obtained by Equation (17) and Equation (18). And correct the coordinates for the ky direction.

  FIG. 8 shows the result of confirming the effect of the above-described correction on the result of imaging using a spherical phantom. Fig. 8 (a) is the image before correction, (b) is the image after correction, the upper is the standard window width / window level (WW / WL) image, and the lower is the WW / WL different from the standard. An image is shown. In the image before correction, distortion occurs at the edge of the image due to the gradient magnetic field output error, or ghost artifacts appear on the top and bottom of the image, whereas in the image after correcting the k-space coordinates, It can be seen that distortion and ghost artifacts have almost disappeared.

式中、G_x1は、磁軸をX軸として、式(12)より求めた傾斜磁場成分、G_x2は、磁軸をY軸として、式(13)より求めた直交成分である。またG_y1は、磁軸をY軸として、式(12)より求めた傾斜磁場成分、G_y2は、磁軸をX軸として、式(13)より求めた直交成分である。In the above description, while the gradient magnetic field (Gx) of one axis is being applied, the description is based on the assumption that nothing is applied to the orthogonal axis (Gy). For radial measurement and spiral measurement in which gradient magnetic fields are simultaneously applied to two axes, the two-axis gradient magnetic field waveforms are measured and corrected by the following equations (19) and (20).

In the equation, G _x1 is a gradient magnetic field component obtained from Equation (12) with the magnetic axis as the X axis, and G _x2 is an orthogonal component obtained from Equation (13) with the magnetic axis as the Y axis. G _y1 is a gradient magnetic field component obtained from equation (12) with the magnetic axis as the Y axis, and G _y2 is an orthogonal component obtained from equation (13) with the magnetic axis as the X axis.

ここで、γは磁気回転比、ΔB0(t)は時間的に変動するB0渦電流成分である。<< A / D phase correction >>
The B0 eddy current component obtained by Equation (15) can be used for phase correction when sampling the NMR signal. The AD phase is affected by the phase rotation caused by the static magnetic field in the time from the center of the RF pulse to the center of the sampling window. The phase rotation caused by the static magnetic field is caused by the B0 eddy current obtained by the equation (15), and becomes a phase offset Δθ (t) [rad] expressed by the following equation (21).

Here, γ is a magnetic rotation ratio, and ΔB0 (t) is a B0 eddy current component that varies with time.

  By correcting the AD phase with the offset Δθ at the time of image reconstruction, the influence of the B0 eddy current component on the image quality can be eliminated.

  In the above-described correction of k-space coordinates and the like, it is assumed that the measured gradient magnetic field output waveform and the readout gradient magnetic field actually used in the imaging pulse sequence have the same axis and amplitude. When actual measurement of the gradient magnetic field output waveform is performed as pre-measurement of the main imaging, the same gradient magnetic field as the readout gradient magnetic field of the main imaging is used as the test gradient magnetic field of the pre-measurement, and the result is k at the time of image reconstruction of the main imaging What is necessary is just to apply to spatial coordinate correction.

  Alternatively, a gradient magnetic field output waveform and orthogonal components are obtained for a plurality of types of gradient magnetic fields assuming various imaging pulse sequences and stored as a table, and a gradient magnetic field that matches the readout gradient magnetic field of the imaging pulse sequence in actual imaging. The k-space coordinate correction can be performed by selecting the output waveform and the orthogonal component from the table. If there is no data that coincides with the readout gradient magnetic field, it is possible to interpolate from a plurality of gradient magnetic field output waveforms having the same axis and close amplitude, and to perform k-space coordinate correction using the interpolated data. When the gradient magnetic field waveform data is stored as approximate waveform data, approximate waveform data can be used.

  As described above, the MRI apparatus of this embodiment and the method of measuring the output waveform of the gradient magnetic field include an imaging unit including a static magnetic field generation unit, a gradient magnetic field generation unit, a high frequency magnetic field generation unit, and a high frequency magnetic field detection unit, A control unit that controls the operation of the imaging unit based on an imaging sequence, and a calculation unit that performs calculations including image reconstruction using the nuclear magnetic resonance signal detected by the high-frequency magnetic field detection unit, The gradient magnetic field measurement unit that measures the gradient magnetic field generated by the gradient magnetic field generation unit, the gradient magnetic field measurement unit, the first measurement to obtain a nuclear magnetic resonance signal from the subject without using the test gradient magnetic field, The second measurement for acquiring a nuclear magnetic resonance signal from the subject using the test gradient magnetic field is controlled, and the calculation unit is acquired by the nuclear magnetic resonance signal acquired by the first measurement and the second measurement. Test tilt using nuclear magnetic resonance signal A gradient magnetic field calculation unit for calculating an actual measurement value of the magnetic field, the gradient magnetic field calculation unit, the first calculation for calculating the magnetic field component of the test gradient magnetic field in the magnetic axis direction, and at least one orthogonal to the magnetic axis direction And a second calculation for calculating the magnetic field component in the direction.

  According to the present embodiment, the k-space coordinates for arranging the NMR signals at the time of imaging are corrected using the actually measured gradient magnetic field output waveform and the eddy current component in the orthogonal axis direction, so that distortion and artifacts due to the gradient magnetic field error are corrected. Can be obtained.

<Second Embodiment>
In the first embodiment, a bar-shaped region that is an intersection of two orthogonal slices is used as an excitation position, NMR signals are acquired from a plurality of excitation positions, a gradient magnetic field output waveform of a test gradient magnetic field, and orthogonal eddy currents Although the case where components are measured has been described, the present embodiment is characterized in that a plurality of parallel slices are set as excitation positions. Hereinafter, the gradient magnetic field output waveform measurement operation by the MRI apparatus of the present embodiment will be described focusing on differences from the first embodiment.

  The procedure for measuring the gradient magnetic field output waveform is shown in FIG. The measurement of this embodiment includes a measurement step 901, an integration step 903, a calculation step 904, and a waveform addition step 906. It is the same as the procedure shown in FIG. 3 that the measurement step 901 is included in the repetition loop 902 and that the entire measurement step 901 is included in the X, Y, Z axis loop 905.

An example of the measurement pulse sequence executed in the measurement step 901 is shown in FIG. Here, as an example, the case where the X-axis test gradient magnetic field is measured as the test gradient magnetic field is shown.
The measurement pulse sequence of FIG. 10 also includes a reference measurement 901-1 for acquiring a signal without using a test gradient magnetic field and a main measurement 901-2 for acquiring a signal by using a test gradient magnetic field. It is the same as the sequence.

  However, the 180-degree RF pulse is not used in the measurement pulse sequence of FIG. Accordingly, the region selected by the 90-degree RF pulse and the slice selective gradient magnetic field on the X axis is excited. A plurality of slices having different slice positions are excited by changing the frequency of the RF pulse without changing the slice selection gradient magnetic field. Excitation positions excite a plurality of sets of very thin excitation thickness slices at positions symmetrically away from the gradient magnetic field center. FIG. 11 shows the excitation position in the sphere phantom. In the example shown, three sets of left and right slices are excited, but the number of slices is not limited to three sets (6 slices) and is arbitrary.

  In this measurement 901-2, an NMR signal is measured while applying a readout gradient magnetic field coaxial with the slice gradient magnetic field. Here again, the reference measurement and the main measurement are repeated as necessary, and the results are integrated (repetition loop 902, integration 903). At this time, by arranging each signal measurement step in a nested manner in one TR, it is possible to suppress time extension due to the number of repetitions.

ここで、Kは複素定数、Dxは磁場中心からスライス励起位置までの距離、Δrはスライス厚、Gx(t)は実測する傾斜磁場強度の時間変化(つまり、傾斜磁場波形)を意味する。リファレンス計測で得られたNMR信号は、式(22)においてGx＝0と置くことにより求められ、本計測のNMR信号との位相差は、次式(24)で表わされる。

B0を含む項は、静磁場B0の渦電流成分である。Next, calculation of the gradient magnetic field output waveform using the measured NMR signal (step 904) will be described. An NMR signal from a slice at a distance Dx from the center of the gradient magnetic field can be expressed by equations (22) and (23).

Here, K is a complex constant, Dx is the distance from the center of the magnetic field to the slice excitation position, Δr is the slice thickness, and Gx (t) is the time change of the actually measured gradient magnetic field strength (that is, the gradient magnetic field waveform). The NMR signal obtained by the reference measurement is obtained by setting Gx = 0 in the equation (22), and the phase difference from the NMR signal of the main measurement is represented by the following equation (24).

The term including B0 is an eddy current component of the static magnetic field B0.

On the other hand, for a signal from a slice at a symmetrical position with respect to the center of the gradient magnetic field, the phase difference between the main measurement and the NMR signal is calculated by the same calculation as the above expressions (22) and (23). It is represented by

複数組、ここでは3組のスライスについて、それぞれ上述の計算を行い、波形加算ステップ906で得られた傾斜磁場出力波形を加算する。Since the eddy current component of the static magnetic field B0 in the equations (24) and (25) can be eliminated by the following equation (26), k (t) is obtained. Using this, the gradient magnetic field output waveform is calculated by Equation (27). That is, the application amount Gx (t) per unit time of the test gradient magnetic field is obtained from the phase change of the NMR signal by the equation (27).

The above-described calculation is performed for each of a plurality of sets, that is, three sets of slices, and the gradient magnetic field output waveforms obtained in the waveform addition step 906 are added.

  The above steps 901 to 906 are repeated while changing the axis of the test gradient magnetic field (step 905), and finally gradient magnetic field waveforms are obtained for all the X, Y, and Z axes. FIGS. 11B and 11C show the Y-axis excitation position and the Z-axis excitation position, respectively. Correcting k-space coordinates at the time of imaging using the obtained gradient magnetic field waveform is the same as in the first embodiment. However, in this embodiment, since the orthogonal component is not measured, correction including the orthogonal component is not performed.

  As described above, in the MRI apparatus of this embodiment and the method of measuring the output waveform of the gradient magnetic field, the magnetic field measurement unit generates a nuclear magnetic resonance signal for each of a plurality of parallel slices of the subject. In addition, the gradient magnetic field calculation unit calculates an actual measurement value of the test gradient magnetic field using the acquired nuclear magnetic resonance signal for each of the plurality of slices, and then adds the actual measurement values.

  According to the present embodiment, by performing multi-slice measurement, signal measurement of a plurality of slices can be performed in TR, so that it is possible to suppress an increase in measurement time due to addition. Moreover, since the gradient magnetic field output waveform which is a result obtained by a plurality of slices can be added, the measurement accuracy can be improved.

  In this embodiment, the eddy current component in the direction orthogonal to the test gradient magnetic field is not measured, but by combining with the first embodiment, that is, by combining the selection by the 180 degree RF pulse after the 90 degree RF pulse excitation. Measurement of orthogonal components is also possible.

  Finally, an embodiment of a measurement tool for performing gradient magnetic field output waveform measurement will be described.

  The measurement tool can be mounted as a UI on the display / operation unit 113 (FIG. 1) of the MRI apparatus. An example of the display screen is shown in FIG. The display screen includes buttons 1201 and 1202 for instructing start / stop of measurement, a check box 1203 for instructing measurement conditions, and a waveform display unit 1204 for displaying an input waveform and a waveform as a result.

  In the measurement tool of this embodiment, when a start button 1201 is pressed, an operation as shown in FIG. 3 or FIG. 9 is started, a measurement sequence is executed, and an arithmetic process using the result is performed. The operation stops when the stop button 1202 is pressed. The calculation may be performed in parallel with the execution of the measurement sequence when data that can be calculated is collected, or may be performed after all the measurements are completed.

  The test gradient magnetic field used in the measurement sequence has a predetermined input waveform (trapezoidal waveform) as a default, and it can be used as it is in the measurement sequence or can be changed arbitrarily by the operator. . For example, the amplitude and rising / falling slope of the input waveform displayed on the waveform display unit 105 can be changed using a pointing device or the like. When the input waveform is determined, the measurement sequence is executed using the set input waveform as the input waveform of the test gradient magnetic field. Although not shown in FIG. 12, the operator may arbitrarily set the number of repetitions of the repetition loops 302 and 902 in advance.

  As a calculation result, as shown in FIG. 13, gradient magnetic field output waveforms are obtained for the X, Y, and Z axes. When the orthogonal axis component and the B0 component of the eddy current are measured together with the gradient magnetic field output waveform, the orthogonal axis component of the eddy current and the B0 component not shown are obtained as shown in FIG. The operator can specify one of the magnetic field components by checking the X, Y, and Z check boxes 1203, and by checking the B0 and Cross check boxes, the orthogonal axis component of the eddy current and B0 Components can be displayed on the waveform display screen. In the example shown in FIG. 12, the waveform display screen 1204 includes a trapezoidal waveform 1211 that is an input waveform, an actually measured value (dotted line) 1212 of the gradient magnetic field waveform, an actually measured value 1213 (dotted line) of the orthogonal axis component of the eddy current, An approximate waveform (solid line) fitting the measured values is displayed.

  According to the present invention, there is provided means for simply and accurately measuring the gradient magnetic field output waveform of the MRI apparatus. By using the gradient magnetic field output waveform actually measured by this means, various problems in the MRI apparatus caused by the gradient magnetic field error with respect to the input waveform, in particular, image distortion and artifacts can be solved.

  102 Static magnetic field generation system, 103 High frequency magnetic field generation system, 104 Gradient magnetic field generation system, 105 Reception system, 108 Overall control unit, 111 Measurement control unit, 113 Display / operation unit, 114 Arithmetic processing unit, 115 Storage unit, 1141 Component, 1144 Gradient field calculator

Claims (18)

An imaging unit including a static magnetic field generation unit, a gradient magnetic field generation unit, a high-frequency magnetic field generation unit, and a high-frequency magnetic field detection unit;
A control unit for controlling the operation of the imaging unit based on an imaging sequence;
Using the nuclear magnetic resonance signal detected by the high-frequency magnetic field detection unit, a calculation unit that performs calculations including image reconstruction,
In a magnetic resonance imaging apparatus comprising:
The control unit includes a gradient magnetic field measurement unit that measures a gradient magnetic field generated by the gradient magnetic field generation unit, and the gradient magnetic field measurement unit acquires a nuclear magnetic resonance signal from a subject without using a test gradient magnetic field. Controlling the first measurement and the second measurement for acquiring a nuclear magnetic resonance signal from the subject using the test gradient magnetic field,
The calculation unit includes a gradient magnetic field calculation unit that calculates an actual measurement value of the test gradient magnetic field using the nuclear magnetic resonance signal acquired in the first measurement and the nuclear magnetic resonance signal acquired in the second measurement. The gradient magnetic field calculation unit calculates the first magnetic field component in the magnetic axis direction of the test gradient magnetic field, and the second calculation calculates the magnetic field component in at least one direction orthogonal to the magnetic axis direction. And a magnetic resonance imaging apparatus. In the magnetic resonance imaging apparatus according to claim 1,
The region for acquiring the nuclear magnetic resonance signal is a region where a slice in the magnetic axis direction of the test gradient magnetic field and a slice in a direction perpendicular thereto intersect. In the magnetic resonance imaging apparatus according to claim 1,
A magnetic resonance imaging apparatus comprising: a correction unit that corrects k-space coordinates where a nuclear magnetic resonance signal obtained in the imaging sequence is arranged using an actual measurement value calculated by the gradient magnetic field calculation unit. In the magnetic resonance imaging apparatus according to claim 3,
The correction unit uses the measured value in the magnetic axis direction of the test gradient magnetic field calculated by the gradient magnetic field calculation unit and the measured value in the direction orthogonal thereto, and coordinates of the image in the image reconstruction for at least two directions, A magnetic resonance imaging apparatus comprising a coordinate correction unit for correcting each of the magnetic resonance imaging apparatuses. In the magnetic resonance imaging apparatus according to claim 1,
The magnetic field measurement unit generates a nuclear magnetic resonance signal for each of a plurality of parallel slices of the subject,
The gradient magnetic field calculation unit calculates an actual measurement value of the test gradient magnetic field using the acquired nuclear magnetic resonance signal for each of a plurality of slices, and then adds the actual measurement values. . In the magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus characterized in that the test gradient magnetic field used by the gradient magnetic field measurement unit is the same gradient magnetic field as the gradient magnetic field set in the imaging sequence. In the magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus, wherein the gradient magnetic field calculation unit performs a third calculation for calculating an eddy current component superimposed as a static magnetic field component by the test gradient magnetic field. In the magnetic resonance imaging apparatus according to claim 7,
The magnetic resonance imaging apparatus, wherein the arithmetic unit performs the image reconstruction by correcting a phase offset based on the eddy current component. In the magnetic resonance imaging apparatus according to claim 3,
The magnetic resonance imaging apparatus, wherein the gradient magnetic field calculation unit performs a fourth calculation for calculating an approximate waveform of an actual measurement value of the test gradient magnetic field obtained by calculation. The magnetic resonance imaging apparatus according to claim 9,
The fourth calculation includes fitting of the actual measurement value using a fitting function,
The magnetic resonance imaging apparatus, wherein the correction unit corrects k-space coordinates where a nuclear magnetic resonance signal obtained in the imaging sequence is arranged using the fitting function. The magnetic resonance imaging apparatus according to claim 9,
The fourth calculation includes a process of linearly interpolating the actual measurement value,
The magnetic resonance imaging apparatus, wherein the correction unit corrects k-space coordinates where a nuclear magnetic resonance signal obtained in the imaging sequence is arranged, using an actually measured value obtained by linear interpolation. In the magnetic resonance imaging apparatus according to claim 1,
The gradient magnetic field measurement unit uses a test gradient magnetic field of a plurality of magnetic axes as a test gradient magnetic field, and performs gradient magnetic field measurement for each axis,
The magnetic resonance imaging apparatus, wherein the gradient magnetic field calculation unit calculates an actual measurement value for each of test gradient magnetic fields having a plurality of magnetic axes. In the magnetic resonance imaging apparatus according to claim 3,
The gradient magnetic field measurement unit uses a plurality of types of test gradient magnetic fields as test gradient magnetic fields, and performs gradient magnetic field measurement for each test gradient magnetic field,
The gradient magnetic field calculation unit calculates an actual measurement value for each of a plurality of types of test gradient magnetic fields,
The correction unit performs the correction using an actual measured value of the same gradient magnetic field as the gradient magnetic field set in the imaging pulse sequence among the actual measured values of the plurality of types of test gradient magnetic fields. Resonance imaging device. In the magnetic resonance imaging apparatus according to claim 1,
A magnetic resonance imaging apparatus comprising a display unit for displaying a result calculated by the gradient magnetic field calculation unit. A method for measuring an output waveform of a gradient magnetic field in a magnetic resonance imaging apparatus,
Using a phantom, repeat the signal acquisition step to excite a bar-shaped area that is the intersection of two orthogonal slices at a certain distance from the center of the gradient magnetic field and acquire a nuclear magnetic resonance signal without applying a read gradient magnetic field A first measurement step of acquiring a nuclear magnetic resonance signal from at least three rod-like regions having different positions;
Using the phantom, a second measurement step of exciting at least three rod-like regions having different positions and applying a readout gradient magnetic field as a test gradient magnetic field to obtain a nuclear magnetic resonance signal;
Using the nuclear magnetic resonance signals acquired in the first and second measurement steps, a first calculation step for calculating a gradient magnetic field output waveform of the test gradient magnetic field,
A second calculation step of calculating a magnetic field component generated in a direction orthogonal to the magnetic axis of the test gradient magnetic field using the nuclear magnetic resonance signals acquired in the first and second measurement steps. Measuring method of gradient magnetic field waveform. The gradient magnetic field waveform measuring method according to claim 15,
A first repetition step for repeating the first measurement step and the second measurement step;
And a step of integrating the nuclear magnetic resonance signals obtained in the first and second measurement steps after the repetition, respectively, and a gradient magnetic field waveform measuring method. The gradient magnetic field waveform measuring method according to claim 15,
The first measurement step, the second measurement step, the first calculation step, and the second calculation step include a second repetition step that repeats by changing an axis of the test gradient magnetic field. Gradient magnetic field waveform measurement method. The gradient magnetic field waveform measuring method according to claim 15,
A gradient magnetic field waveform measurement method comprising a third calculation step of calculating an approximate waveform of the gradient magnetic field output waveform calculated in the first calculation step.

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