JP6546837B2 - Magnetic resonance imaging apparatus and method - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a correction technique of static magnetic field inhomogeneity.

  A magnetic resonance imaging (hereinafter referred to as “MRI”) apparatus measures nuclear magnetic resonance (hereinafter referred to as “NMR”) signals generated by nuclear spins constituting an object, in particular, a tissue of a human body. , An apparatus for imaging two or three dimensional forms and functions of limbs etc. In imaging, different phase encoding is added to the NMR signal by the gradient magnetic field and frequency encoding is performed to measure it as time-series data. The measured NMR signal is reconstructed into an image by being subjected to two-dimensional or three-dimensional Fourier transform.

  Related prior art documents include, for example, Patent Document 1 and Non-Patent Document 1.

Japanese Patent Application Laid-Open No. 60-209154

W. Zhang et al., “An improved Maxwell term compensation method for FSE images”, ISMRM Proceedings 556 (2013)

  As described in the above-mentioned prior art documents, when a gradient magnetic field is applied by the MRI apparatus, the static magnetic field by the high-order Maxwell term of the gradient magnetic field as shown in Equation 1 at position (x, y, z) Unevenness occurs and affects the image quality.

Therefore, various studies have been made so far to correct such high-order Maxwell terms. However, in order to correct such high-order Maxwell terms, complicated processes need to be added, and a large capacity shim power supply is required.

  In order to solve the above problems, an object of the present invention is to provide an MRI apparatus capable of correcting static magnetic field inhomogeneity generated when a gradient magnetic field is applied using a simple and inexpensive configuration, and an operation method thereof. It is to do.

  In order to achieve the above object, in the present invention, a magnetic field generating unit that generates a static magnetic field that is uniform in the space for containing the subject, a gradient magnetic field superimposed on the static magnetic field, and a high frequency magnetic field that irradiates the subject A detection unit that detects an NMR signal generated from a subject as an echo signal, and an image processing unit that images the detected echo signal, the magnetic field generation unit calculates an average value of gradient magnetic fields, and the average The values are used to provide an MRI apparatus configured to correct static magnetic field inhomogeneity based on gradient magnetic fields.

  Further, in order to achieve the above object, in the present invention, a static magnetic field that is uniform in a space for containing an object, a gradient magnetic field superimposed on the static magnetic field, and a magnetic field generation unit that generates a high frequency magnetic field to irradiate the object A method of operating an MRI apparatus comprising: a detection unit for detecting an NMR signal generated from a subject as an echo signal; and an image processing unit for imaging the detected echo signal, wherein the magnetic field generation unit is a gradient magnetic field The present invention provides an operation method of an MRI apparatus which calculates an average value of gradient magnetic field control signals for applying V.sub.0, and uses the average value to correct static magnetic field inhomogeneity based on the gradient magnetic field.

  According to the present invention, it is possible to provide an MRI apparatus capable of correcting static magnetic field inhomogeneity due to application of a gradient magnetic field with a low capacity power supply.

The figure for demonstrating the whole structure of a MRI apparatus based on an Example. BRIEF DESCRIPTION OF THE DRAWINGS The block diagram which shows an example of a gradient magnetic field generation system based on an Example. FIG. 2 is a block diagram of an example of an arithmetic circuit that performs time averaging according to the first embodiment. FIG. 5 is a waveform diagram of each part of the arithmetic circuit performing time average calculation according to the first embodiment. FIG. 7 is a block diagram of an example of an arithmetic circuit that performs moving average calculation according to a second embodiment. FIG. 7 is a waveform diagram of each part of the arithmetic circuit that performs moving average calculation according to the second embodiment.

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail according to the attached 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 will be omitted.

  First, the general outline of an example of an MRI apparatus according to each embodiment will be described based on FIG. FIG. 1 is a block diagram showing an entire configuration of an embodiment of an MRI apparatus. This MRI apparatus obtains a tomographic image of a subject using NMR phenomenon, and as shown in FIG. 1, the MRI apparatus comprises a static magnetic field generation system 2 and a gradient magnetic field generation system 3 which constitute a magnetic field generation unit. A transmission system 5, a reception system 6 as a detection unit, a signal processing system 7 as an image processing unit, a sequencer 4 constituting a control unit, and a central processing unit (CPU) 8 are provided.

  If the static magnetic field generation system 2 of the magnetic field generation unit is a vertical magnetic field type, the space around the subject 1 in the direction orthogonal to the body axis is a static magnetic field uniform in the body axis direction. A static magnetic field generation source of a permanent magnet system, a normal conduction system or a superconducting system is arranged around the subject 1.

  The gradient magnetic field generation system 3 of the magnetic field generation unit includes a gradient magnetic field coil 9 for applying a gradient magnetic field in three axial directions of X, Y and Z which are coordinate systems (stationary coordinate system) of the MRI apparatus, and respective gradient magnetic field coils. The gradient magnetic field power supply 10 is driven, and by driving the gradient magnetic field power supply 10 of each coil in accordance with an instruction from the sequence controller 4 described later, gradient magnetic fields GX, GY, Apply GZ. At the time of imaging, slice direction gradient magnetic field pulses (Gs) are applied in the direction orthogonal to the slice plane (imaging cross section) to set the slice plane with respect to the subject 1, and the remaining 2 orthogonal to the slice plane The phase encoding direction gradient magnetic field pulse (Gp) and the frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction to encode position information of each direction in the echo signal.

  The sequencer 4 is control means for repeatedly applying a high frequency magnetic field pulse (hereinafter referred to as "RF pulse") and a gradient magnetic field pulse in a predetermined pulse sequence, and operates under control of the CPU 8 to acquire data of tomographic images of the subject 1 To the transmitting system 5, the gradient magnetic field generating system 3 and the receiving system 6 as required.

  The transmission system 5 irradiates an RF pulse to the subject 1 in order to cause nuclear magnetic resonance to nuclear spins of atoms constituting the living tissue of the subject 1. The high frequency oscillator 11, the modulator 12, and the high frequency amplifier 13, and a high frequency coil (transmission coil) 14a on the transmission side. The RF 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 RF pulse is amplified by the high frequency amplifier 13 and then disposed close to the subject 1 The RF pulse is irradiated to the subject 1 by supplying to the high frequency coil 14a.

The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the living tissue of the subject 1, and comprises 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. After the NMR signal of the response of the subject 1 induced by the electromagnetic wave emitted from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged in proximity to the subject 1 and amplified by the signal amplifier 15, The quadrature phase detector 16 divides the signal into two orthogonal signals at a timing according to a command from the sequencer 4, converts each into a digital quantity by the A / D converter 17, and sends it to the signal processing system 7.
The signal processing system 7 performs various data processing and displays and stores processing results, and includes an external storage device such as an optical disk 19 and a magnetic disk 18 and a display 20 formed of a liquid crystal panel or the like. When data from the receiving system 6 is input to the CPU 8, the CPU 8 executes 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 an external storage device. Recording on the magnetic disk 18 of FIG.

  The operation unit 25 is for inputting various control information of the MRI apparatus and control information of processing performed by the signal processing system 7 and comprises a track ball or mouse 23 and a keyboard 24. The operation unit 25 is disposed close to the display 20, and the operator interactively controls various processes of the MRI apparatus through the operation unit 25 while looking at the display 20.

  In FIG. 1, the high frequency coil 14a on the transmission side and the gradient magnetic field coil 9 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 horizontal magnetic field type, it is installed so as to surround the subject 1. Further, the high frequency coil 14b on the receiving side is installed to face or surround the subject 1.

  At present, the imaging target nuclide of the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent substance of a subject, as widely used in clinical practice. By imaging information on the spatial distribution of the proton density and the spatial distribution of the relaxation time of the excited state, the form or function of the human head, abdomen, limbs, etc. can be imaged two-dimensionally or three-dimensionally.

  In the first embodiment, a static magnetic field that is uniform in the space for containing the subject, a gradient magnetic field superimposed on the static magnetic field, a magnetic field generating unit that generates a high frequency magnetic field for irradiating the subject, and an NMR signal generated from the subject And an image processing unit which is a signal processing system for imaging the detected echo signal, and the magnetic field generation unit calculates an average value of the gradient magnetic field control signals and 6 is an embodiment of an MRI apparatus and method configured to correct static magnetic field inhomogeneity based on gradient magnetic fields using the average value.

  The configuration and operation of the MRI apparatus of the first embodiment will be described using FIGS. 2, 3A, and 3B. As shown in FIG. 2, the gradient magnetic field generation system 3 of the magnetic field generation unit of this embodiment includes a gradient magnetic field control circuit 22, a gradient magnetic field power supply 10, an average operation value circuit 23, a high order shim control circuit 24, and a shim power supply 18. , And gradient magnetic field coils 9. As shown in the figure, a control signal for controlling the high-order shim control circuit 24 is input from the sequencer 4.

Next, the operation of the MRI apparatus to which the average value calculation circuit 23 of this embodiment is added will be described. The gradient magnetic field control signals (G _X , G _Y, G _Z ) generated by the sequencer 4 are gradient magnetic field control The magnetic field is supplied to the gradient coil 9 through the circuit 22 and the gradient power supply 10 to generate gradient magnetic fields GX, GY and GZ. At this time, G _X and G _Y among the gradient magnetic field control signals are input to the average value calculation circuit 23. The average value calculation circuit 23 calculates an average value for correcting (Gx ² + Gy ² ) Z ² of the static magnetic field inhomogeneity due to the high-order Maxwell term of Equation 1 described above.

FIG. 3A shows an example of the circuit configuration of the average value calculation circuit 23, and FIG. 3B shows an example of the waveform of each part of the average value calculation circuit 23. In FIG. 3B, the gradient magnetic field control signals G _X and G _Y , the (G _X ² + G _Y ² ) calculated based on them, and the time average value Σ (G _X ) which is the output of the average value calculation circuit 23 in order from the top. ² + G _Y ² ) / t is shown. Here, t is an average value calculation interval. As described above, in the averaging circuit 23 of this embodiment, the time average value calculating circuit 25 calculates the time average value per unit time of (G _X ² + G _Y ² ).

  Here, the unit time of the average value calculation section, that is, the width of the window for calculating the average value is stored in advance in the storage device such as the magnetic disk etc., the type of pulse sequence and the unit time for each imaging condition. The value may be read and used, or may be variable by the user using the input control device 21 or the like. The preferred range of unit time is longer than the time width of one gradient pulse / 2 and shorter than the repetition time (TR). For example, several milliseconds to several tens of milliseconds.

  With the configuration of the gradient magnetic field generation system of this embodiment shown in FIGS. 3A and 3B, the high-order shim control circuit 24 inputs to the shim power supply 18 based on the time average value obtained by the calculation based on the gradient magnetic field control signal. The correction value is determined, and the correction current is supplied to the shim coil from the shim power supply 18 to which this correction value is input, thereby correcting the static magnetic field inhomogeneity generated by the gradient magnetic fields GX and GY.

According to the configuration of the present embodiment, as shown in FIG. 3B, the correction value maximum value of (G _X ² + G _Y ² ) is Σ (G _X ² + G _{Y Y} ) by averaging in each average value calculation section t. ² ) / t can be a value equal to or less than half of the correction amount maximum value, so correction of static magnetic field inhomogeneity generated by high-order gradient magnetic fields is performed using a low capacity and inexpensive shim power supply 18 It becomes possible.

  The MRI apparatus of this embodiment is different from that of the first embodiment in that moving average value calculation is performed instead of time average value calculation in the average value calculation circuit 23 of FIG. 2, and the other configuration is the same. This difference will be described.

FIGS. 4A and 4B show an example of the configuration of the average value calculation circuit 23 for calculating the average value by moving average and the waveforms of the respective parts in the MRI apparatus of the present embodiment. The time average value calculation circuit 25 that calculates an average value for each unit time based on the gradient magnetic field control signal of the first embodiment is a moving average value calculation circuit 26 that calculates a moving average value for a fixed time. As shown in FIG. 4B, the output of the moving average value calculation circuit 26 in the present embodiment moves sequentially as the average value calculation section t moves, but as in the first embodiment, averaging in the average value calculation section t is performed. Thus, Σ (G _X ² + G _Y ² ) / t can be reduced to half or less. Therefore, as in the first embodiment, it is possible to correct the static magnetic field inhomogeneity generated by the high-order gradient magnetic field using the low-capacity and inexpensive shim power supply. Furthermore, in the present embodiment, since the moving average value is used, it is possible to correct static magnetic field inhomogeneity with better follow-up.

  The average value calculation interval t in the present embodiment, that is, a preferable range of unit time, is longer than time width / 2 of one gradient magnetic field pulse and shorter than repetition time (TR), preferably one of the gradient magnetic field pulses. ~ A few minutes. For example, several milliseconds to several tens of milliseconds.

  The present invention is not limited to the embodiments described above, but includes various modifications. For example, the embodiments described above have been described in detail for better understanding of the present invention, and are not necessarily limited to those having all the configurations of the description. In addition, part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. In addition, with respect to a part of the configuration of each embodiment, it is possible to add, delete, and replace other configurations.

  Furthermore, although each configuration, gradient magnetic field generation system, etc. mentioned above explained the example of the hardware constitution which realizes the part or all of them with a circuit, software is designed by designing a part or all of them with a program etc. It goes without saying that it may be realized by

Reference Signs List 1 object 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 central 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 (transmission coil)
14b High frequency coil (reception coil)
Reference Signs List 15 signal amplifier 16 quadrature phase detector 17 A / D converter 18 shim power supply 19 display 20 storage device 21 input control device 22 gradient magnetic field control circuit 23 average value calculation circuit 24 high-order shim control circuit 25 time average value calculation circuit 26 movement Average value calculation circuit

Claims (8)

A magnetic field generating unit that generates a static magnetic field that is uniform in a space for containing the subject, a gradient magnetic field superimposed on the static magnetic field, and a high frequency magnetic field that is applied to the subject;
A detection unit that detects a nuclear magnetic resonance (NMR) signal generated from the subject as an echo signal;
An image processing unit for imaging the detected echo signal;
The magnetic field generation unit calculates an average value of the gradient magnetic fields, and uses the average value to correct static magnetic field inhomogeneity based on the gradient magnetic fields.
Magnetic resonance imaging (MRI) apparatus characterized by The MRI apparatus according to claim 1,
The magnetic field generation unit includes an arithmetic circuit that calculates the average value, and a shim control circuit that controls a shim power supply using the average value.
MRI apparatus characterized by The MRI apparatus according to claim 2, wherein
The arithmetic circuit calculates the average value based on gradient magnetic field control signals Gx and Gy for applying gradient magnetic fields in two directions of the coordinate system (X, Y, Z) of the device.
MRI apparatus characterized by The MRI apparatus according to claim 3, wherein
The arithmetic circuit calculates a time average value of (Gx ² + Gy ² ) as the average value,
MRI apparatus characterized by The MRI apparatus according to claim 3, wherein
The arithmetic circuit calculates a moving average value of (Gx ² + Gy ² ) as the average value,
MRI apparatus characterized by The MRI apparatus according to claim 3, wherein
The shim control circuit outputs a correction value for correcting (Gx ² + Gy ² ) Z, which is static magnetic field inhomogeneity based on the gradient magnetic field, to the shim power supply based on the average value.
MRI apparatus characterized by A static magnetic field that is uniform in the space for containing the subject, a gradient magnetic field superimposed on the static magnetic field, a magnetic field generation unit that generates a high frequency magnetic field that irradiates the subject, and an NMR signal generated from the subject A method of operating an MRI apparatus, comprising: a detection unit for detecting as; and an image processing unit for imaging the detected echo signal,
The magnetic field generation unit calculates an average value of gradient magnetic field control signals for applying the gradient magnetic field, and corrects static magnetic field inhomogeneity based on the gradient magnetic field using the average value.
Method of operating an MRI apparatus characterized by A method of operating an MRI apparatus according to claim 7, wherein
The magnetic field generation unit uses the gradient magnetic field control signals Gx and Gy for applying the gradient magnetic field applied in the X and Y 2 directions of the coordinate system of the device as the average value (Gx ² + Gy ² ) Calculate the time average value or moving average value of
Method of operating an MRI apparatus characterized by

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