JP4763267B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI) apparatus that obtains a tomographic image of an arbitrary part of a subject using a magnetic resonance phenomenon, and in particular, various magnetic field fluctuations caused by a residual magnetic field generated when a gradient magnetic field is applied. The present invention relates to a technique for preventing image quality deterioration (image distortion, signal strength reduction, ghost, etc.).

  In an MRI system that obtains tomographic images by controlling gradient magnetic field pulses, the desired amount of application (the area enclosed by the gradient magnetic field pulse waveform and the time axis) is applied by arbitrarily and accurately controlling the waveform and timing of the gradient magnetic field pulse. There is a need.

However, in an MRI apparatus that forms a magnetic circuit using an iron yoke, which is a ferromagnetic material, for example, an MRI apparatus using a permanent magnet (hereinafter referred to as a permanent magnet apparatus), a residual magnetic field with a hysteresis phenomenon is generated by applying a gradient magnetic field. appear. This remanent magnetic field has a magnetic field of remanent magnetization (Residual Magnetization) that is a physical characteristic of the ferromagnetic material because the magnetic pole (pole piece) and iron yoke constituting the magnetic circuit are ferromagnetic. This is caused by remaining on the magnetic pole and iron yoke by application. Specifically, even if the application of the gradient magnetic field is stopped, the magnetic field corresponding to the strength and direction of the applied gradient magnetic field and the application history remains. In particular, the residual magnetic field in the permanent magnet device complicates the magnetic field in the imaging space together with the eddy current, and is a major factor in image quality degradation.
As techniques for correcting this residual magnetic field, techniques disclosed in (Patent Document 1), (Patent Document 2), and (Patent Document 3) are known.

Patent Document 1 discloses a technique in which the maximum gradient magnetic field of the MRI apparatus is added as a preparation pulse before an imaging sequence to make the residual magnetic field a constant value.
Patent Document 2 measures a residual magnetic field for each gradient magnetic field waveform in advance and creates a correction table (or functions), and applies a correction gradient magnetic field according to the gradient magnetic field waveform to be applied when performing imaging. The method is disclosed.

Patent Document 3 measures the residual magnetic field generated by a permanent magnet device using a minimum gradient magnetic field waveform, models the hysteresis curve of the residual magnetic field, and sequentially generates the residual magnetic field based on the application history of the gradient magnetic field. An MRI apparatus that can calculate and correct a residual magnetic field in real time for all gradient magnetic fields used for imaging is disclosed.
U.S. Pat.No. 6043656 JP 2000-157509 A Japanese Patent Application No. 2003-380242

As described above, in the permanent magnet device, a residual magnetic field is generated by applying a gradient magnetic field, which is a major cause of image quality degradation.
According to the inventors' investigation, in addition to the hysteresis of the residual magnetic field based on the application of one gradient magnetic field, the number of application of the gradient magnetic field in the same direction (that is, how many gradient magnetic fields are applied in the same direction) It was found that the residual magnetic field generated greatly fluctuates depending on the dependence, and this residual magnetic field fluctuation has a non-linear dependence on the number of applications in the same direction.

  Such a residual magnetic field fluctuation becomes remarkable particularly when a large gradient magnetic field is applied in the same direction, and an example thereof is an MPG (Motion Probing Gradient) pulse in diffusion weighted imaging (DWI). In this diffusion weighted image, a large image distortion occurs according to the number of times of applying the MPG pulse.

  However, all of the techniques disclosed in the above-mentioned patent documents are based on the premise that the residual magnetic field is corrected based on one application of the gradient magnetic field, and what is related to the residual magnetic field correction according to the number of times of application in the same direction. Is also not disclosed. In addition, since the correction gradient magnetic field to be applied is also primary (that is, the correction magnetic field is applied using a gradient magnetic field coil), it corresponds to a residual magnetic field that remains with a non-linear strength with respect to the number of applications in the same direction. Correcting this is beyond the scope of the disclosed technical idea, and this problem remains undecided.

  Therefore, the MRI apparatus of the present invention has been made to solve such problems, and in the MRI apparatus having a magnetic circuit, the residual magnetic field can be generated in real time according to the number of times the gradient magnetic field is applied in the same direction. The purpose is to correct. In particular, it is intended to perform such correction in a permanent magnet device.

In order to achieve the above object, the MRI apparatus of the present invention is configured as follows. That is,
A static magnetic field generating source for generating a static magnetic field, a magnetic circuit unit serving as a path of a magnetic flux generated by the static magnetic field generating source, a gradient magnetic field generating means for generating a gradient magnetic field, and at least one of the magnetic circuit units by the gradient magnetic field In the magnetic resonance imaging apparatus, the correction unit corrects the residual magnetic field corresponding to the number of times the gradient magnetic field is applied in the same direction. .

  Thereby, the residual magnetic field that exhibits complicated dependence on the number of times of application of the gradient magnetic field can be corrected in real time. As a result, it is possible to dramatically increase the maximum value of the gradient magnetic field strength that can be used as an apparatus, and it is possible to perform imaging with higher resolution and higher functionality. In addition, the SNR and image quality of the image can be improved.

In another embodiment of the MRI apparatus of the present invention, in the above embodiment, the correction means further corrects the residual magnetic field corresponding to the intensity and polarity of the gradient magnetic field generated immediately before. Alternatively, the residual magnetic field is corrected based on measurement data of the residual magnetic field with respect to the number of times of application of the gradient magnetic field acquired in advance.
Thereby, the residual magnetic field which shows a complicated dependence with respect to the frequency | count of application of a gradient magnetic field can be performed in detail and flexibly.

The MRI apparatus of the present invention described above has the following direct effects. That is,
By simply measuring a simple residual magnetic field characteristic in advance, it is possible to correct in real time a residual magnetic field that exhibits a complicated dependence on the number of gradient magnetic field applications.
As a result, it is possible to dramatically increase the maximum value of the gradient magnetic field intensity that can be used as an apparatus, and it is possible to perform photographing with higher resolution and higher functionality. In addition, the SNR and image quality of the image can be improved.

  In addition, the correction gradient magnetic field for correcting the influence of the residual magnetic field can be applied in a superimposed manner on the gradient magnetic field for photographing, and therefore, unlike the method of separately providing a means for controlling the correction gradient magnetic field, the photographing sequence is extended or limited. No harmful effects occur.

The direct effects of the MRI apparatus of the present invention have the following indirect effects in radiography. That is,
The diffusion weighted sequence (DWI), which detects the diffusion of water using a very large gradient magnetic field (MPG), can correct the residual magnetic field according to the number of times the MPG is applied. Can be improved.
Moreover, since the degradation of the static magnetic field uniformity due to the residual magnetic field can be reduced, the image quality of the RF fat suppression image using the chemical shift can be improved.

  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 overall outline of an MRI apparatus according to the present invention will be described with reference to FIGS.
FIG. 1 is a block diagram showing the overall configuration of an MRI apparatus to which a processing method according to the present invention is applied. The MRI apparatus shown in FIG. 1 uses a magnetic resonance phenomenon to obtain a tomographic image of a subject, and as shown in the figure, a static magnetic field generating magnetic circuit 1, a gradient magnetic field generating system 2, a transmitting system 3 and The receiving system 4, the signal processing system 5, the sequencer 6, the central processing unit (CPU) 7, and the operation unit 8 are provided.

  The static magnetic field generating magnetic circuit 1 generates a uniform static magnetic field around the subject 9 in the direction of the body axis or in a direction perpendicular to the body axis. In the space having a certain extent around the subject 9 Permanent magnet type, normal conducting type or superconducting type magnetic field generating means is arranged.

  The gradient magnetic field generation system 2 includes a gradient magnetic field coil 10 wound in three axes of X, Y, and Z, and a gradient magnetic field power source 11 that drives each coil. By driving the gradient magnetic field power supply 11, gradient magnetic fields Gs, Gp, and Gf in the three-axis directions of X, Y, and Z are applied to the subject 9. By applying this gradient magnetic field, a slice plane for the subject 9 can be set, and an echo signal is encoded to provide position information. In addition, based on spatial and temporal information on eddy currents and residual magnetic fields caused by the application of gradient magnetic fields, the shim coils and localized coils that form part of the static magnetic field generation magnetic circuit 1 or the gradient magnetic field generation system 2 The correction current can be applied sequentially.

  The transmission system 3 irradiates a high-frequency signal in order to cause nuclear magnetic resonance to occur in the atomic nucleus constituting the biological tissue of the subject 9 by the high-frequency magnetic field pulse transmitted from the sequencer 6. A high-frequency amplifier 14 and a high-frequency coil 15 on the transmission side, and the high-frequency pulse output from the high-frequency oscillator 12 is amplified by the high-frequency amplifier 14 and then placed in the vicinity of the subject 9. By supplying the coil 15, electromagnetic waves (high frequency signals) are irradiated on the subject 9.

  The receiving system 4 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nuclei constituting the living tissue of the subject 9, and receives a high-frequency coil 16 on the receiving side, an amplifier 17, and quadrature detection The electromagnetic wave (NMR signal) of the response of the subject 9 due to the electromagnetic wave irradiated from the high-frequency coil 15 on the transmission side is arranged on the reception side, which is arranged in the vicinity of the subject 9, and the A / D converter 19 Is detected by a high frequency coil 16 and input to an A / D converter 19 via an amplifier 17 and a quadrature phase detector 18 to be converted into a digital quantity, and further by a quadrature phase detector 18 at a timing according to a command from the sequencer 6. The two sets of sampled collected data are sent to the signal processing system 5.

  The signal processing system 5 uses the echo signal detected by the receiving system 4 to perform image reconstruction calculation and display an image. The echo signal is subjected to processing such as Fourier transform, correction coefficient calculation, image reconstruction, and the sequencer 6. CPU 7 for controlling the image, ROM (read-only memory) 20 for storing time-lapse image analysis processing and measurement programs and invariant parameters used in the execution, measurement parameters and reception system 4 obtained in the previous measurement Temporarily store the echo signal detected in step 1 and the image used for region-of-interest setting and store parameters (such as parameters for setting the region of interest) 21 and the image data reconstructed by the CPU 7. Magneto-optical disk 22 and magnetic disk 23 serving as a data storage unit to be recorded, and image data read from these magneto-optical disk 22 or magnetic disk 23 And a display 24 serving as a display unit for visualizing and displaying as a tomographic image.

  The sequencer 6 serves as a control means for repeatedly applying a high-frequency magnetic field pulse for causing nuclear magnetic resonance to the nucleus constituting the living tissue of the subject 9 in a predetermined pulse sequence, and operates under the control of the CPU 7, Various commands necessary for collecting the tomographic image data of the specimen 9 are transmitted to the transmission system 3, shim coils and localized coils forming part of the static magnetic field generation magnetic circuit 1, the gradient magnetic field generation system 2, and the reception system 4. To send.

  The operation unit 8 inputs control information for processing performed in the signal processing system 5, and includes a trackball, a mouse 25, and a keyboard.

  FIG. 2 is a schematic cross-sectional view of an example of the static magnetic field generating magnetic circuit 1 using a permanent magnet. In the apparatus shown in FIG. 1, a pair of permanent magnets 42a and 42b having different polarities are provided with a uniform magnetic field space 44 into which a subject is inserted. These permanent magnets 42a and 42b are held in contact with a pair of upper and lower rectangular yokes 41a and 41b, and the yokes 41a and 41b are formed by one or more (two in FIG. 2) support columns 47a and 47b. A uniform magnetic field space 44 is secured between the permanent magnets 42a and 42b by being supported at a predetermined distance. Further, the magnetic pole pieces 43a and 43b are held in contact with each other on the surface of the permanent magnets 42a and 42b on the side of the uniform magnetic field space 44. An annular projection 46 having the same shape is provided on the upper and lower sides. Therefore, the effective gap through which the subject can enter is the distance between the annular protrusions of the magnetic pole pieces 43a and 43b. However, in addition to the subject 44, a gradient magnetic field coil 10, an RF irradiation coil (not shown), and an RF receiving coil (not shown) necessary for imaging are arranged in the space between the gaps.

  The pillars 47a and 47b, the yokes 41a and 41b, and the magnetic pole pieces 43a and 43b are made of a ferromagnetic material. Generally, the magnetic pole pieces 43a and 43b are made of a silicon steel plate, The yokes 41a and 41b are iron yokes. Magnetic flux is generated from the permanent magnets 42a and 42b, and a static magnetic field is generated in the uniform magnetic field space via the pole pieces 43a and 43b. The pillars 47a and 47b, the yokes 41a and 41b, and the magnetic pole pieces 43a and 43b are magnetically coupled to the permanent magnets 42a and 42b to form a magnetic circuit serving as a magnetic flux path. The annular protrusion 46 suppresses the magnetic flux from leaking to the periphery of the uniform magnetic field space, and improves the uniformity of the magnetic field between the magnetic pole pieces 43a and 43b, that is, inside the uniform magnetic field space 44.

Next, the correction method corresponding to the number of times of application of the gradient magnetic field in the permanent magnet device having the above-described function will be specifically described.
First, the dependence of the residual magnetic field strength on the number of applied gradient magnetic fields will be described. FIG. 3 shows the results of experiments conducted by the inventors. This Fig. 3 is generated in the direction of the application axis by applying a primary gradient magnetic field with the intensity and application time fixed to 15 [mT / m] and 4000 [us], respectively, in the X direction of the MRI apparatus multiple times. The following residual magnetic field was measured and plotted. The horizontal axis represents the number of applied gradient magnetic fields, and the vertical axis represents the measured residual magnetic field strength.

The residual magnetic field can be calculated from the resonance frequency using a teslameter. Alternatively, an appropriate MRI phantom may be RF-excited to form an image using a gradient magnetic field having such an intensity that the influence of the residual magnetic field can be ignored, and the phase change may be calculated.
From FIG. 3, it is understood that the generated residual magnetic field intensity changes approximately twice as much as the number of times of application of the gradient magnetic field, and saturates when a certain number of times of application is exceeded. That is, it is understood that the residual magnetic field changes nonlinearly with respect to the number of times the gradient magnetic field is applied.

Next, modeling of the dependence of the residual magnetic field strength on the number of applied gradient magnetic fields as shown in FIG. 3 will be described. For example, there are the following two methods for modeling.
(a) A prediction function is applied to data measured on the assumption that the residual magnetic field is exponentially saturated.
(b) The residual magnetic field strength at the unmeasured number of times of application (integer) is complemented with the measured data to create numerical data of the residual magnetic field strength for each number of times of application.

そして、残留磁場補正の際には、予測関数に基づいて傾斜磁場の印加回数に対応して残留磁場を算出して装置毎に残留磁場の補正を行う。 In the case of (a), the coefficient of the prediction function is held for each device as a value unique to the device. If RM is the residual magnetic field strength and n is the number of times the gradient magnetic field is applied, the result of FIG.

When the residual magnetic field is corrected, the residual magnetic field is calculated corresponding to the number of times the gradient magnetic field is applied based on the prediction function, and the residual magnetic field is corrected for each device.

In the case of (b), all the numerical data of the residual magnetic field strength for each number of applied gradient magnetic fields are held for each device as values unique to the device. When the residual magnetic field is corrected, the residual magnetic field is corrected for each apparatus according to the number of times of application of the gradient magnetic field based on the numerical data.
The above modeling is performed independently for each of the X, Y, and Z axes. Therefore, in the case of (a), a prediction function is created in each of the three axis directions, and the residual magnetic field is corrected for each axis based on the prediction function for each axis.

In the case of (b), numerical data of the residual magnetic field strength for each number of applied gradient magnetic fields is created in the three axial directions, and the residual magnetic field is corrected for each axis based on the numerical data for each axis.
In both methods (a) and (b), if there is no difference in the composition and characteristics of the magnets used in the MRI apparatus, only common basic data is retained in each apparatus. In addition, the difference between apparatuses may be absorbed by a scale factor.

  Finally, a method for correcting the residual magnetic field will be described. FIG. 4 shows a flowchart of an example of the correction method. Each processing step is as follows.

  In step 401, the intensity of the gradient magnetic field to be applied n is G (n).

In step 402, it is determined whether G (n) has a polarity opposite to the previous gradient magnetic field strength G (n-1) and whether the strength is less than Rtn XG (n-1) . Here, Rtn is a predetermined coefficient. Instead of using the value multiplied by the ratio to the previous gradient magnetic field G (n−1), a fixed value may be used as the threshold value.

  In step 403, when the determination process in step 402 is true (yes), the number of times of application of the gradient magnetic field counted so far is reset. This reproduces the phenomenon in which the residual magnetic field accumulated by the gradient magnetic field having the same polarity is reset by the reverse polarity gradient magnetic field having an intensity equal to or greater than a certain threshold value and transitions to a single-shot system. If false (no), the process proceeds to the next step 404.

In step 404, it is determined whether G (n) has the same polarity as the previous gradient magnetic field strength G (n-1) and the strength is less than Rtp XG (n-1) . Here, Rtp is a predetermined coefficient. Instead of using a value multiplied by a ratio to the previous gradient magnetic field G (n−1), a fixed value may be used as a threshold value.

  In step 405, when the determination process in step 404 is true (yes), the current number of times of application is held. That is, it is determined that there is no effect of enhancing the residual magnetic field due to the number of times of application when the gradient magnetic field is less. If false (no), the process proceeds to the next step 406.

  In step 406, the number of times of application of the gradient magnetic field is counted, that is, (+1) is added to the number of times of application, and the value is held.

  In step 407, a correction value (correction term Cnum) corresponding to the number of times of application of the gradient magnetic field is calculated with reference to a value modeled based on previously measured data.

  In step 408, the residual magnetic field is corrected based on the correction value (Cnum) calculated in step 407. For this purpose, for example, a gradient magnetic field may be applied by superimposing a correction magnetic field corresponding to this correction value on a single-shot hysteresis correction process (for example, Patent Document 3) which is a conventional correction technique, or a correction value. A gradient magnetic field corresponding to may be applied alone.

  As described above, according to the MRI apparatus of the present invention, it is possible to correct in real time a residual magnetic field that exhibits complicated application frequency dependence by simply measuring a simple residual magnetic field characteristic in advance.

  In general, in a sequence used in normal clinical measurement such as SE and FSE, the ratio of applying a gradient magnetic field in the readout direction and the slice direction excluding the phase encoding direction is very high and the intensity thereof is also large. Therefore, it can be determined that a nonlinear phenomenon that cannot be reproduced by single-shot hysteresis always occurs. If the correction of the present invention is performed, it is possible to dramatically increase the maximum value of the gradient magnetic field intensity that can be used as an apparatus, and it is possible to perform photographing with higher resolution and higher functionality. In addition, the SNR and image quality of the image can be improved. Further, since the fluctuation of the residual magnetic field is modeled and corrected, the correction can be performed in detail and flexibly.

  Furthermore, since the correction gradient magnetic field can be applied in a superimposed manner on the gradient magnetic field for imaging, unlike the method of separately providing a means for controlling the correction gradient magnetic field, there is no adverse effect such as extension or limitation of the imaging sequence.

The above effects of the MRI apparatus of the present invention are particularly effective in the following cases. That is,
In a diffusion weighting sequence (DWI) that detects the diffusion of water using a very large gradient magnetic field (MPG), it is necessary to apply the MPG many times in the same direction. At this time, the fluctuation of the residual magnetic field due to the number of times of MPG application causes image quality degradation. However, according to the present invention, the residual magnetic field generated without special adjustment or correction can be corrected.

  In addition, the degradation of the static magnetic field uniformity due to the residual magnetic field is a great hindrance to RF fat suppression using a chemical shift in the permanent magnet device. However, according to the present invention, the disturbance of the static magnetic field caused by the residual magnetic field can be minimized, and the image quality by RF fat suppression can be improved.

  The above is description of embodiment of the MRI apparatus of this invention. However, the present invention is not limited to the contents disclosed in the above description of the embodiments, and can take other forms based on the gist of the present invention. For example, in the description of the above embodiment, an example in which a permanent magnet is used as a magnetic field generation source has been described. However, the MRI apparatus of the present invention uses not only a permanent magnet but also a superconducting magnet or a normal conduction magnet as a magnetic field generation source. The present invention can also be applied to an MRI apparatus that uses a member as at least a part of a magnetic circuit.

The block diagram which shows the whole structure of the MRI apparatus by this invention. Sectional drawing which shows an example of a static magnetic field generation magnetic circuit. The figure which showed the example of the gradient magnetic field application frequency dependence of the residual magnetic field in this invention. The figure which showed the procedure of the application frequency correction | amendment in this invention.

Explanation of symbols

  1 static magnetic field generation circuit, 2 gradient magnetic field generation system, 3 transmission system, 4 reception system, 5 signal processing system, 6 sequencer, 7 central processing unit (CPU), 8 operation unit, 9 subject, 10 gradient magnetic field coil, 11 Gradient magnetic field power source, 12 High frequency oscillator, 13 Modulator, 14 High frequency amplifier, 15 High frequency coil, 16 High frequency receiver coil, 17 Amplifier, 18 Quadrature phase detector, 19 A / D converter

Claims (5)

A static magnetic field generating source for generating a static magnetic field, a magnetic circuit unit serving as a path of a magnetic flux generated by the static magnetic field generating source, a gradient magnetic field generating means for generating a gradient magnetic field, and at least one of the magnetic circuit units by the gradient magnetic field In a magnetic resonance imaging apparatus having correction means for correcting a residual magnetic field based on residual magnetization generated in part,
The correction means includes counting means for counting the number of times of application of the gradient magnetic field in the same direction according to the intensity and polarity of the gradient magnetic field, and the residual means corresponds to the number of times of application of the gradient magnetic field in the same direction. A magnetic resonance imaging apparatus for correcting a magnetic field. The magnetic resonance imaging apparatus according to claim 1.
The magnetic resonance imaging apparatus characterized in that the correction means corrects the residual magnetic field corresponding to the intensity and polarity of the gradient magnetic field generated immediately before. The magnetic resonance imaging apparatus according to claim 2.
The counting means resets the count value when a gradient magnetic field smaller than a gradient magnetic field having a reverse polarity with a first coefficient multiple is applied to the gradient magnetic field applied immediately before. Resonance imaging device. The magnetic resonance imaging apparatus according to claim 2.
The counting means holds the count value when a gradient magnetic field smaller than the gradient magnetic field having the same polarity as the second coefficient multiple is applied to the gradient magnetic field applied immediately before. Resonance imaging device. The magnetic resonance imaging apparatus according to any one of claims 1 to 4,
The magnetic resonance imaging apparatus, wherein the correction unit corrects the residual magnetic field based on measurement data of the residual magnetic field with respect to the number of times of application of the gradient magnetic field acquired in advance.

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