JP2015181891A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce an SAR (Specific Absorption Ratio) or reduce nonuniformity of Bdistribution in an MRI apparatus in a high magnetic field.SOLUTION: The magnetic resonance imaging apparatus adjusts a posture of a subject 103a or 103b in imaging to such an extent that it is not the burden on a patient as the subject, and improves at least one of a local specific absorption ratio of a prescribed site and uniformity of a high frequency magnetic field distribution of a region of interest, more than that of before the adjustment. The adjustment is performed by a posture adjusting member 121 provided in the MRI apparatus. The posture adjusting member 121 is arranged on a table 107 on which the subject is placed and can change the arrangement position and/or height.

Description

The present invention relates to magnetic resonance imaging (MRI).
ing) technology, and more particularly, to a technology for controlling the amount of RF absorption (SAR (Specific Absorption Ratio)) in vivo.

  The MRI apparatus is a medical image diagnostic apparatus that causes magnetic resonance to occur in nuclei in an arbitrary cross section that crosses an examination target, and obtains a tomographic image in the cross section from a generated magnetic resonance signal. A radio wave (Radio Frequency wave, hereinafter referred to as RF), which is a type of electromagnetic wave, is transmitted to the inspection object, and the spin of the atomic nucleus in the inspection object is excited, and then a nuclear magnetic resonance signal generated by the nuclear spin is received. The inspection object is imaged. RF transmission to the inspection object is performed by the RF transmission coil, and reception of the nuclear magnetic resonance signal from the inspection object is performed by the RF reception coil.

  In recent years, with the aim of improving the SNR (Signal to Noise Ratio) of images, the static magnetic field strength tends to increase, and the spread of high magnetic field MRI devices (super high magnetic field MRI devices) with a static magnetic field strength of 3 T (Tesla) or higher. Has begun.

  In the MRI apparatus, in consideration of safety to the living body, the amount of RF absorption (SAR (Specific Absorption Ratio); specific absorption rate) in the living body is regulated to fall within a predetermined range. Due to the higher magnetic field of the device, the frequency used is higher and the SAR is also increasing. Therefore, there is a need for a technique for reducing SAR absorbed by the entire living body (hereinafter, whole body SAR) and SAR generated locally in the living body (hereinafter, local SAR).

  As a technique for reducing the SAR, there is a technique for reducing the SAR by placing a material having a high dielectric constant around the head (see, for example, Patent Document 1). In addition, there is a technique for reducing the whole body SAR by utilizing the change of the whole body SAR by changing the posture of the human body (for example, see Non-Patent Document 1).

As the magnetic field is increased, the RF frequency used for inducing the magnetic resonance phenomenon is increased with the increase of the magnetic field, and image unevenness is likely to occur. For example, an MRI apparatus having a static magnetic field strength of 3T (Tesla) (hereinafter referred to as a 3T MRI apparatus) uses an RF having a frequency of 128 MHz. In the living body, the wavelength of this RF is about 30 cm, which is approximately the same scale as the abdominal section, and the phase changes. Due to this phase change, the irradiation RF distribution and the spatial distribution of the rotating magnetic field (hereinafter referred to as high-frequency magnetic field distribution, B ₁ ) generated by the RF and inducing a magnetic resonance phenomenon become non-uniform, resulting in image unevenness. Thus, ultra-high field MRI apparatus, a technique for reducing the unevenness of distribution of the rotating magnetic field B ₁ is being required.

US Patent Application Publication No. 2011/0152670

Wang Z, et al., Possible Male and Frame Numeric Body Models for Field Calculus in MRI, Proc. ISMRM 2008, pp. 75

  In the technique disclosed in Patent Document 1, a high dielectric constant material is disposed so as to cover the entire periphery of the head to reduce SAR. However, since this technique covers the entire periphery of the head, the patient's openness and comfort are impaired. Moreover, head temperature may rise by SAR, and the heat dissipation effect in that case may be impaired.

In Patent Document 1, there is also a disclosure of the heterogeneous reduction of B ₁ distribution. However, with the technique disclosed in Patent Document 1, as described above, the patient's openness and comfort may be impaired, and the heat dissipation effect may be impaired.

  Non-Patent Document 1 discloses a reduction in the value of whole body SAR in a human body model, but does not describe a method for controlling a local SAR. In the literature, it is shown that the SAR decreases when the position of the arm is greatly changed and the arm is raised above the head. However, the burden on the patient due to raising the arm over the head for a long time is large. Moreover, it cannot be applied to patients who have difficulty raising their arms over the head for a long time.

The present invention has been made in view of the above circumstances, in the MRI apparatus, reduction in SAR, or, as intended for the reduction of non-uniformity of B ₁ distribution.

  The present invention adjusts the posture of the subject at the time of imaging within a range that does not impose a heavy burden on the patient who is the subject, and at least one of the local specific absorption rate of the predetermined part and the uniformity of the high-frequency magnetic field distribution of the region of interest, Improve from before adjustment. The adjustment is made by a posture adjusting member provided in the MRI apparatus. The posture adjusting member is arranged on a table on which the subject is placed, and is a member capable of changing the arrangement position and / or height.

According to the present invention can be reduced in the MRI apparatus, SAR, or unevenness in _{B 1} distribution.

It is a block diagram of the MRI apparatus of embodiment of this invention. (A) is explanatory drawing for demonstrating the simulation aspect of embodiment of this invention, (B) is explanatory drawing for demonstrating an example of the human body model used for simulation of embodiment of this invention. . (A) is a simulation result of the B ₁ distribution in the human body model in the lower arm posture of the embodiment of the present invention, and (B) is a simulation result of the B ₁ distribution in the human body model in the initial posture of the embodiment of the present invention. a simulation result, (C) is a B ₁ simulation result of the distribution of the human body model of the arm on the attitude of the embodiments of the present invention. (A) is the simulation result of the SAR distribution in the human body model of the adult male in the lower arm posture of the embodiment of the present invention, and (B) is the human body model of the adult man in the initial posture of the embodiment of the present invention. (C) is a simulation result of the SAR distribution in the human body model of an adult male in the above-arm posture according to the embodiment of the present invention. (A) is a simulation result of the SAR distribution in the human body model of the child in the lower arm posture according to the embodiment of the present invention, and (B) is the SAR in the human body model of the child in the initial posture of the embodiment of the present invention. It is a simulation result of distribution, (C) is a simulation result of the SAR distribution in the human body model of the child in the upper arm posture of the embodiment of the present invention. The B ₁ simulation of uniformity of distribution results in a human model of embodiments of the present invention is a graph showing. (A) is explanatory drawing for demonstrating arrangement | positioning of the attitude | position adjustment part of embodiment of this invention, (B) is the schematic of the attitude | position adjustment part of embodiment of this invention. (A) And (B) is explanatory drawing for demonstrating the heat dissipation promotion part of the attitude | position adjustment part of embodiment of this invention. It is explanatory drawing for demonstrating arrangement | positioning of the attitude | position adjustment part of the modification of embodiment of this invention. (A) And (B) is explanatory drawing for demonstrating the attitude | position adjustment part which has the height adjustment function of embodiment of this invention. (A) is explanatory drawing for demonstrating the marker light irradiation part with which the conventional magnetic resonance imaging device is provided, (B) is explanatory drawing for demonstrating the marker light irradiation part of embodiment of this invention. is there. It is a functional block diagram of the computer 109 of the embodiment of the present invention.

  Hereinafter, a first embodiment to which the present invention is applied will be described with reference to the drawings. Note that, throughout the drawings for explaining the embodiments, unless otherwise specified, the same reference numerals are given to components having the same functions, and repeated descriptions thereof are omitted. Note that the present invention is not limited thereby.

  First, the overall configuration of the MRI apparatus of this embodiment will be described. FIG. 1 is a block diagram of the MRI apparatus 100 of the present embodiment.

  As shown in the figure, the MRI apparatus 100 of the present embodiment includes a magnet 101 that generates a static magnetic field, a gradient coil 102 that generates a gradient magnetic field, a shim coil 112 that adjusts the static magnetic field uniformity, a sequencer 104, An RF transmission coil (transmission coil) 114 that irradiates (transmits) a high-frequency magnetic field (RF); an RF reception coil (reception coil) 115 that detects (receives) a nuclear magnetic resonance signal generated from the subject 103; A table 107 on which the subject 103 is placed, a gradient magnetic field power source 105, a high-frequency magnetic field generator 106, a receiver 108, a shim power source 113, and a computer 109 that controls each part of the MRI apparatus 100 and realizes imaging. A posture adjusting unit 121.

  The gradient coil 102 and shim coil 112 are connected to a gradient magnetic field power source 105 and a shim power source 113, respectively. The transmission coil 114 and the reception coil 115 are connected to the high-frequency magnetic field generator 106 and the receiver 108, respectively.

  The sequencer 104 sends commands to the gradient magnetic field power supply 105, the shim power supply 113, and the high-frequency magnetic field generator 106 according to instructions from the computer 109 to generate a gradient magnetic field and RF, respectively. RF is irradiated (transmitted) to the subject 103 through the transmission coil 114.

  A nuclear magnetic resonance signal generated from the subject 103 by irradiating (transmitting) RF is detected (received) by the receiving coil 115 and detected by the receiver 108. A magnetic resonance frequency used as a reference for detection by the receiver 108 is set by the computer 109 via the sequencer 104.

  The detected signal is sent to the computer 109 through an A / D conversion circuit, where signal processing such as image reconstruction is performed. The result is displayed on the display device 110 connected to the computer 109. The detected signals and measurement conditions are stored in the storage device 111 connected to the computer 109 as necessary.

  The computer 109 of this embodiment is an arithmetic device provided with CPU and memory, for example. Each function realized by the computer 109 is realized by, for example, loading a program stored in the storage device 111 in advance into a memory and executing the program.

  The magnet 101, shim coil 112, and shim power supply 113 constitute a static magnetic field forming unit that forms a static magnetic field space. The gradient magnetic field coil 102 and the gradient magnetic field power source 105 constitute a gradient magnetic field application unit that applies a gradient magnetic field to the static magnetic field space. The transmission coil 114 and the high-frequency magnetic field generator 106 constitute a high-frequency magnetic field transmission unit that irradiates (transmits) a high-frequency magnetic field (RF) to the subject 103 arranged in the static magnetic field. The receiving coil 115 and the receiver 108 constitute a signal receiving unit that detects (receives) a nuclear magnetic resonance signal generated from the subject 103.

  The posture adjustment unit 121 of the present embodiment adjusts the posture of the subject 103 by supporting at least one part of the subject 103. At this time, at least one of the index indicating the local specific absorption rate (SAR) and the index indicating the homogeneity of the high-frequency magnetic field distribution in the region of interest of the supporting part is the posture adjusting unit 121. The part is supported at a position that is higher than the index in the initial state where it is not placed.

  Prior to the detailed description of the posture adjustment unit 121 of the present embodiment, it will be described that the SAR changes by changing the height of a predetermined part of the subject 103. Hereinafter, in the present embodiment, a case where a human body is used as the subject 103 will be described as an example. In the present embodiment, a horizontal magnetic field type apparatus is used as the MRI apparatus 100, and the transmission coil 114 has a cylindrical shape. Further, in the following description, as shown in FIG. 2A, the body axis direction of the human body 103 placed on the table 107 in the supine position is selected from the z direction and the two directions orthogonal to the magnetic field direction. The x direction and the other direction are the y direction. Here, in order to simplify the description, the side surface of the transmission coil 114 is shown as transparent.

  First, a numerical simulation result of a change in the RF irradiation distribution when the posture of the human body 103 is changed will be shown. Here, the posture of the human body 103 is changed by moving the position of the arm up and down.

  FIG. 2 shows an example of the human body model 201 used in this numerical simulation. In the human body model 201 shown in FIG. 2, the position of the arm 201a is on the side of the body (torso). Hereinafter, in the present specification, a natural posture in which the position of the arm 201a is arranged at a position away from the side surface of the trunk by a predetermined distance, regardless of whether the posture is the supine position (the supine position) or the prone position (the prone position). Is called the initial posture.

  This time, the human body model 201 in the initial posture, the human body model 201 in the posture in which the position of the arm 201a is moved to the back side (lower side) from the initial posture (hereinafter referred to as the lower arm posture), and the supine position. A human body model 201 having a posture (hereinafter referred to as an upper arm posture) in which the position of the arm 201a is moved to the ventral side (upper side) from the initial posture is prepared and simulated.

  The simulation method used is the FDTD (Finite-Difference Time-Domain) method.

  The simulation condition was that the resonance frequency of the transmission coil 114 was 128 MHz, and the human body model 201 was placed so that the abdomen (per kidney) 201b of the human body model was at the center position of the transmission coil 114 in the z direction. The human body model 201 had a height of 173 cm and a weight of 65 kg. These adopted the average size of Japanese adult men.

FIGS. 3A to 3C show the simulation results of the B ₁ distribution in each human body model 201. FIG. FIG. 3A shows simulation results 301, 302, and 303 for the lower arm posture, FIG. 3B for the initial posture, and FIG. 3C for the upper arm posture, respectively.

From these figures, the position of the arms 201a, it can be seen _{that B 1} distribution changes. This is because the position of the arm 201a is changed, show that RF propagation behavior is changing, the position of the arms 201a, B ₁ distribution well, suggesting that even distribution of the SAR has changed Yes.

  4A to 4C show the simulation results of the SAR distribution in each human body model 201. FIG. 4A shows simulation results 401, 402, and 403 for the lower arm posture, FIG. 4B for the initial posture, and FIG. 4C for the upper arm posture, respectively. The illustrated cross section includes a section having the largest local SAR in the three-dimensional SAR data in the human body model, and arrows 411, 412, and 413 in the figure are the sections. From these figures, it can be seen that the SAR distribution changes depending on the position of the arm 201a.

  The numerical values shown in the lower part of each figure are values obtained by dividing the value of the local SAR by the value of the whole body SAR (hereinafter referred to as local SAR / whole body SAR). The local SAR / whole body SAR is an index indicating that the smaller the value, the lower the local SAR and the safer from the viewpoint of the local SAR. Hereinafter, in this specification, it is called a SAR index.

  That is, by adjusting the position of the arm 201a, that is, the posture so that the SAR index becomes small, it is possible to ensure higher safety from the viewpoint of the local SAR. For example, in FIGS. 4A to 4C, the SAR index is the smallest in the initial posture of FIG. 4B. Therefore, among these three postures, this initial posture is the safest.

  Next, other conditions are the same, and simulation results in which only the size of the human body model 201 is changed are shown in FIGS. 5 (A) to 5 (C). Here, the SAR distribution when a child-sized human body model 201 is used is shown. The used human body model has a height of 109 cm and a weight of 18 kg.

  FIG. 5A shows simulation results 421, 422, and 423 for a lower arm posture, FIG. 5B shows an initial posture, and FIG. 5C shows an upper arm posture. The illustrated cross section includes a section having the largest local SAR in the three-dimensional SAR data in the human body model, and the arrows 431, 432, and 433 in the figure are the sections. From these figures, it can be seen that the SAR distribution (SAR index) changes depending on the position of the arm 201a even in a child-sized human body model.

  Also in this figure, the numerical value shown below each figure is the SAR index. In the example of FIGS. 5A to 5C, it can be seen that the local SAR / whole body SAR is the smallest in the on-arm posture of FIG. 5C.

  From the above simulation results, it can be seen that the SAR distribution (SAR index) varies depending on the position of the arm 201a, that is, the posture, regardless of whether the human body model 201 is an adult size or a child size. However, the posture with the highest safety differs depending on the size. That is, in the case of the human body model 201 of an adult size, the initial posture is the highest in the case of the human body model 201 in the child size, and the highest posture is the highest.

  Here, focusing on the relationship between the position of the transmission coil 114 and the arm 201a, when the position of the arm 201a is close to the center of the transmission coil 114 in the y direction in both the adult-sized human body model 201 and the child-sized human body model 201. In addition, the SAR index is the smallest.

  Generally, the closer the human body model 201 is to the transmission coil 114, the higher the local SAR tends to be. By making the position of the arm 201a closer to the center of the transmission coil 114 in the y direction, the distance between the transmission coil 114 and the arm 201a is increased, and the SAR index is considered to be the smallest.

6, a simulation result of uniformity of the B ₁ distribution in the human body model of an adult size, indicated by numerical values. (A) is a simulation result of a lower arm posture, (b) is an initial posture, and (c) is a simulation result of an upper arm posture. Note that the uniformity of the _{B 1} distribution, using an index _{U SD} represented by the following formula (1). This, the index _{U SD} is the value dev standard deviation of _{B 1} value in the human body model _{(B 1),} it is divided by the mean value mean of the _{B 1} value _{(B 1).} Hereinafter, it is referred to as a uniformity index.
As the value of uniformity index U _SD is small, indicating a high uniformity of the B ₁ distribution. In the example of FIG. 6, when the initial orientation of (b), uniformity index is the smallest, B ₁ it can be seen that a high uniformity of distribution. From this result, by adjusting the position of the arms 201a, it is understood that the uniformity of the B ₁ distribution changes. In the case of this simulation, the value of the uniformity index in each position, when comparing the SAR index shown in FIG. 4, as the uniformity of the B ₁ distribution is high attitude, SAR index is small.

[Arrangement of posture adjustment unit]
The outline of the present embodiment will be described based on the results of these numerical simulations. Here, the case where the predetermined part of the human body 103 supported by the posture adjustment unit 121 is the arm 103a will be described as an example. A state in which the posture adjustment unit 121 is not arranged is referred to as an initial state.

As described above, the SAR index or the uniformity index changes by changing the position of the arm 103a. Therefore, the position of the arms 103a, by the predetermined position, the local SAR or unevenness of the B ₁ distribution can be reduced from the initial state. The posture adjustment unit 121 of the present embodiment supports the arm 103a so as to realize such a posture. That is, in this embodiment, the posture of the human body 103 is adjusted by adjusting the position of the arm 103a.

  The position of the arm 103a to be adjusted is the height of the arm 103a from the table 107. The posture adjustment unit 121 according to the present embodiment supports the arm 103a so that the arm 103a is arranged at a position in the vertical direction where the SAR index or the uniformity index is the best in a range that is not unreasonable for the subject 103.

  FIG. 7A shows a schematic diagram of the arrangement of the posture adjustment unit 121 of the present embodiment. As shown in the drawing, the posture adjustment unit 121 of the present embodiment is provided independently of the MRI apparatus 100 and is detachably installed on a table on which the subject (human body) 103 is placed. Then, it is arranged between the arm 103 a of the subject 103 and the table 117. The arrangement position on the table 117 can be freely adjusted.

[Shape of posture adjustment unit]
FIG. 7B illustrates an example of the posture adjustment unit 121 according to this embodiment. As shown in the figure, the posture adjustment unit 121 of the present embodiment has, for example, a solid (not hollow) rectangular parallelepiped shape. The width w and the length l in the longitudinal direction of the posture adjustment unit 121 are set to a width and a length in which the arm 103a of the subject 103 can be disposed. Further, for example, as shown in FIGS. 4 and 5, the height h is a height at which the arm 103a can be disposed at a position where the SAR index is minimum within the simulated range, and is preliminarily stored.

  The height h of the posture adjustment unit 121 may be set to a height at which the arm 103a can be disposed at a position where the uniformity index is best within a simulated range, as in the example illustrated in FIG.

Note that the shape of the posture adjustment unit 121 is not limited to this. The arm 103a of the body 103, at least one of the indices of an index indicating the uniformity of the B ₁ distribution of indices and ROI shows local SAR arm 103a is in a position to increase from the index of the initial state, the arms 103a It only needs to be supported.

[Material for posture adjustment unit]
As a general rule, the material (material) of the posture adjusting unit 121 is a material having low conductivity and low relative dielectric constant. That is, it is made of a material whose conductivity is equal to or lower than a predetermined first threshold and whose relative dielectric constant is equal to or lower than a predetermined second threshold. This is because if the electrical conductivity is high or the relative dielectric constant is high, the RF propagation loss increases, the wavelength shortening effect increases, and the RF propagation behavior changes greatly.

  Note that a material having a high relative dielectric constant may be used for the posture adjustment unit 121. That is, the posture adjusting unit 121 may be formed of a material having a conductivity that is equal to or lower than a predetermined first threshold and a relative dielectric constant that is equal to or higher than a predetermined third threshold. In this case, the RF attenuation effect is suppressed by suppressing the conductivity, and the wavelength shortening effect due to the high dielectric constant of the material is used.

That is, by using such a material, using both the shape of the posture adjustment unit 121, the position of the predetermined part (arm 103a) due to the arrangement, and the change in RF propagation behavior due to the wavelength shortening effect of the material, reduced or reduction of _{B 1} non-uniformity of the local SAR. However, when a material having a high relative dielectric constant is used, an optimal arrangement position for reducing a local SAR and an optimal relative dielectric constant of a desired part (arm 103a) in the human body 103 are determined by the relative dielectric constant. A simulation including the posture adjustment unit 121 having a high rate is performed and determined.

  Further, in the human body that is the subject 103, the amount of heat generation is increased and the temperature is increased in a region where the RF irradiation is performed and the local SAR is increased. Therefore, if the heat can be efficiently radiated from the heat-generated area, the temperature rise can be suppressed. In order to realize this, the posture adjustment unit 121 according to the present embodiment may include a heat radiation promoting unit that promotes heat radiation on a surface in contact with the subject 103.

  An example of the heat dissipation promoting unit 500 is an uneven structure. FIG. 8A and FIG. 8B are schematic views in the case where a concavo-convex structure is provided on the surface of the posture adjusting unit 121 in contact with the human body 103. FIG. 8A shows an example in which a groove 501 is dug in one direction to realize a concavo-convex structure (heat dissipation promoting portion 500), and FIG. 8B shows grooves 501 and 502 dug in two directions. It is an example which realized the structure (heat dissipation promotion part 500).

  Compared with the case where the arm 103a is in contact with the surface without the groove 501 (and 502), when the groove 501 (and 502) is dug, the heat dissipation effect from the region 103c where the temperature rise is high in the arm 103a is high. Conceivable. Specifically, the posture adjusting unit 121 having a concavo-convex structure is easy to dissipate heat due to the air flow (indicated by white arrows in the figure) by the grooves 501 (and 502).

  Further, as the heat radiation promoting unit 500, a material that can absorb heat generated in the arm 103a as efficiently as possible may be used. That is, a material that efficiently dissipates the heat in the arm 103 a to the outside of the subject 103 may be used as the heat dissipation promoting unit 500. For example, a liquid gel or granular gel (hereinafter referred to as a cooling sheet) having a high cooling effect is used as the heat dissipation promoting unit 500. Or it is good also as a structure which covers a cooling sheet with a material with high heat conductivity as the heat dissipation promotion part 500. FIG.

  As described above, the MRI apparatus 100 according to the present embodiment receives a high-frequency magnetic field transmission unit that transmits a high-frequency magnetic field to the subject 103 arranged in the static magnetic field, and a nuclear magnetic resonance signal generated from the subject 103. A signal receiving unit, a table 107 on which the subject 103 is placed, and a posture adjusting unit 121 that adjusts the posture of the subject 103 by supporting at least one part of the subject 103. . Then, the posture adjustment unit 121 is in an initial state in which at least one of the index indicating the local specific absorption rate of the part and the index indicating the homogeneity of the high-frequency magnetic field distribution in the region of interest is not arranged in the posture adjustment unit 121. The part is supported at a position lower than the index.

  Normally, when the human body is sleeping on the table 107 during imaging, the position of the arm 103a is often on the back side. However, in this embodiment, by providing the posture adjustment unit 121, the position of the arm 103a can be arranged at a desired height.

Then, the height of the posture adjustment unit 121 is set such that the arm 103a is disposed at a position where the local SAR is reduced from the initial state and the local SAR is minimized within a changeable range. Therefore, higher safety can be ensured from the viewpoint of local SAR. On the other hand, the height of the attitude adjustment unit 121, a position where the uniformity of the B ₁ distribution is improved from the initial state, to configure the arm 103a is disposed at a position where the uniformity of the B ₁ distribution is maximum Thus, a higher quality image can be obtained in a short time. Thereby, an image with the same image quality can be obtained in a shorter time, and the SAR can be reduced accordingly.

Since the subject 103 only has to put his arm on the posture adjustment unit 121, there is almost no burden and difficulty. That is, according to this embodiment, in a range that does not burden the patient, the posture of the subject, can be adjusted in position to reduce the non-uniformity of the SAR or B ₁ distribution. Therefore, a high-quality image can be obtained safely.

<Modification 1 of the posture adjustment unit>
In the above embodiment, the case where the posture adjustment unit 121 is provided independently of the MRI apparatus 100 and can be placed on the table 107 is described as an example, but the present invention is not limited to this. For example, the posture adjustment unit 121 may be configured integrally with the reception coil 115. Further, the posture adjusting unit 121 may be configured to be detachably attached to the receiving coil 115.

  FIG. 9 shows an example in which the posture adjustment unit 121a is integrated with the reception coil 115. Here, an example is shown in which the receiving coil 115 for the trunk is arranged above and below the trunk portion 103b. Here, the case where the attitude | position adjustment part 121a is attached to the receiving coil 115 arrange | positioned under the trunk | drum 103b is illustrated.

  The receiving coil 115 is conventionally set by the engineer on the body portion 103b of the human body 103 during imaging. Therefore, by attaching the posture adjusting unit 121a to the receiving coil 115 in advance, the posture can be adjusted with the same amount of work as before. Moreover, if the arrangement | positioning of the attitude | position adjustment part 121a is not appropriate, there exists a possibility that the effect of attitude | position adjustment will not be acquired or the performance of the receiving coil 115 will fall. Such a situation can be reduced by integrating the receiving coil 115.

<Modification 2 of the posture adjustment unit>
Further, the posture adjustment unit 121 may be configured to be able to adjust the height. That is, the posture adjustment unit 121 of the present embodiment may be configured by a member capable of adjusting the height from the table 107 of at least a part (arm 103a) of the subject 103 to be supported. In such a configuration, after setting the posture adjustment unit 121, the engineer can adjust the height according to the subject 103.

  As a structure which can adjust height, for example, you may be comprised with the member which can adjust height in steps. As an example of such an adjustment member 112, FIG. 10A shows an example constituted by a plurality of plate-like members (spacers). The posture adjusting unit 121b of this example is configured by stacking (stacking) a plurality of thin plate-like spacers 601. The height h of the posture adjustment unit 121b is adjusted by increasing or decreasing the spacer 601.

  Moreover, as a structure which can adjust height, you may be comprised by the member which can adjust height continuously, for example. Such a mechanism is realized by, for example, a jack. As such an example, FIG. 10B shows an example of a posture adjusting unit 121c having a screw-type jack mechanism. The posture adjusting unit 121c of this example adjusts the height by rotating the screw 602. The posture adjustment unit 121c of this example is somewhat complicated in structure as compared with a configuration in which a plurality of spacers are stacked, but can perform stepless height adjustment.

  Thus, when the posture adjustment unit 121 has a height adjustment mechanism, the engineer adjusts the height so that the arm 103a is disposed near the center of the transmission coil 114 in the y direction. Therefore, in order to support this adjustment, the MRI apparatus 100 may be configured to be able to grasp the position of the arm 103a when adjusting the height of the posture adjustment unit 121.

  For example, the MRI apparatus 100 includes a marker light irradiation unit that emits marker light as a configuration that supports the grasping of the position of the arm 103a. For example, a laser marker is used as the marker light. 11A shows a state of laser marker irradiation by the marker light irradiation unit 900 of the conventional MRI apparatus, and FIG. 11B shows a state of laser marker irradiation by the marker light irradiation unit 700 of the MRI apparatus 100 of the present embodiment. It is a figure for demonstrating.

  The laser marker by the conventional marker light irradiation unit 900 is a so-called positioning marker used to place the region of interest of the subject 103 at the center position of the static magnetic field space. Therefore, as shown in FIG. 11A, one marker light irradiation unit 900 is provided in each direction so as to irradiate the laser markers (901, 902) in the vertical direction (y direction) and the horizontal direction (x direction), respectively. Is provided.

  With such laser markers (901, 902), only one arm 103a can be positioned. For this reason, in this embodiment, the marker light irradiation units 900 are provided as many as the number of positions that can be determined by all the parts (here, the arms 103a) supported by the posture adjustment unit 121.

  Specifically, as shown in FIG. 11B, two marker light irradiation units 700 that irradiate the laser marker 702 in the x direction are provided on each side of the subject 103, for a total of two. In addition, three marker light irradiation units 700 that irradiate the laser marker 701 in the y direction are disposed in total, two on the upper part of the arm 103a and one on the upper part of the body 103b.

  By providing two marker light irradiation units 700 that irradiate laser markers in the x direction, the laser marker 702 can be irradiated from both sides of the subject 103, and the heights of both arms (positions in the y direction) can be set to desired positions, respectively. Can lead to.

  Further, by providing three marker light irradiation units 700 that irradiate laser markers in the y direction, the positions of both arms in the x direction can be adjusted. This is used to maintain an appropriate distance between the two because the local SAR may rise when the distance between the transmission coil 114 and the arm 103a is short.

  Note that when the accuracy of adjusting the height and position of the arm 103a by the posture adjusting unit 121 is not required, a conventional laser marker as shown in FIG. 11A may be used as it is.

As described above, when the position of the arm 103a is adjusted, the local SAR can be reduced by adjusting the distance between the arm 103a and the transmission coil 114 as large as possible. At this time, B ₁ and obtain the map, adjust the position of the arm 103a using the map data may be determined.

That is, as shown in FIG. 12, the computer 109 according to the present embodiment acquires a B ₁ map (B ₁ distribution) of the imaging region, and determines whether or not the posture adjustment unit 121 is appropriately arranged according to the B ₁ map. A determination unit 801 may be further provided. In FIG. 12, other functions realized by the computer 109 are omitted.

Specifically, appropriateness determination unit 801 obtains the B ₁ map data each time to change the position of the arm 103a. Then, a uniformity index is calculated from the B ₁ map data and compared with a predetermined threshold value of B ₁ uniformity. As a result, when the calculated uniformity index becomes a value smaller than a predetermined threshold value, it is determined as appropriate and the adjustment of the position of the arm 103a is completed.

  At this time, a position adjustment completion message may be presented to the user. On the other hand, when the calculated uniformity index is equal to or greater than a predetermined threshold value, the user is informed that the adjustment of the position of the arm 103a has not been completed. The presentation is realized by a method of displaying on the display device 110, for example.

Further, propriety determination unit 801 estimates the local SAR distribution from the acquired B ₁ map data, based on the local SAR distribution data estimation may be configured to perform determination of the properness. For example, the information of the electric field is estimated by substituting the information of the magnetic field (B ₁ ) into the equation using the Maxwell equation representing the relationship between the magnetic field and the electric field. The local SAR distribution data is calculated by matching the information of the living body (density / conductivity) with the information of the electric field. In this case, each time the position of the arm 103a is changed, the local SAR distribution data is estimated by the above method. Then, it may be determined whether the position of the arm 103a is appropriate based on the estimated local SAR distribution. The presentation of the result is the same as above.

  Thus, by providing the suitability determination unit 801, the position of the arm 103a can be adjusted with higher accuracy for patients of various body types.

  Note that the method of estimating the local SAR distribution can also be used to determine the height of the posture adjustment unit 121. That is, the best height of the posture adjustment unit 121 is determined based on the estimated local SAR distribution data.

  In each of the above embodiments and modifications, the case where the imaging target region (region of interest) is the abdomen and the position of the arm 103a is adjusted has been described as an example. However, the imaging target part and the position adjustment target part are not limited to these. The same adjustment can be made for various parts of the human body (subject 103). Examples of the various parts include a head, a neck, a knee, a wrist, and an ankle.

  For example, when the adjustment target is a knee, the simulation results show that the local SAR greatly changes depending on the distance between the knees, the height (distance from the bed), and the knee bending angle. In this case, the posture adjusting unit 121 has a shape that can adjust the interval, height, and knee bending angle of both knees. Thus, the posture adjustment unit 121 may have a different shape depending on the part.

  The position for supporting the part to be adjusted may be determined in advance by simulation or the like. For example, the posture adjustment unit 121 is created and prepared for each part and for each attribute of the subject. At this time, the shape is an optimum shape for each part. Further, for example, the support position may be adjusted by the suitability determination unit 801.

  Note that the local SAR reduction method of the present embodiment is applicable to various imaging fields including medical use.

  DESCRIPTION OF SYMBOLS 100: MRI apparatus, 101: Magnet, 102: Gradient magnetic field coil, 103: Subject (human body), 103a: Arm, 103b: Torso part, 103c: Area | region where temperature rise is high, 104: Sequencer, 105: Gradient magnetic field power supply, 106: high frequency magnetic field generator, 107: table, 108: receiver, 109: calculator, 110: display device, 111: storage device, 112: shim coil, 113: shim power source, 114: transmission coil, 115: reception coil, 117 : Table, 121: Posture adjustment unit, 121a: Posture adjustment unit, 121b: Posture adjustment unit, 201: Human body model, 201a: Arm, 201b: Torso, 301: Simulation result, 302: Simulation result, 303: Simulation result, 401 : Simulation result, 402: simulation result, 4 3: simulation result, 411: arrow indicating a location with the largest local SAR, 412: arrow indicating a location with the largest local SAR, 413: arrow showing a location with the largest local SAR, 421: simulation result, 422: simulation result 423: simulation result, 431: arrow indicating a location with the largest local SAR, 432: arrow indicating a location with the largest local SAR, 433: arrow showing a location with the largest local SAR, 500: heat dissipation promoting unit, 501: Groove: 600: heat dissipation promoting unit, 601: spacer, 602: screw, 700: marker light irradiation unit, 701: laser marker, 702: laser marker, 801: suitability determination unit, 900: marker light irradiation unit, 901: laser marker, 902: Laser marker

Claims (20)

A high-frequency magnetic field transmission unit that transmits a high-frequency magnetic field to a subject arranged in a static magnetic field;
A signal receiver for receiving a nuclear magnetic resonance signal generated from the subject;
A table on which the subject is placed;
A magnetic resonance imaging apparatus comprising: a posture adjusting unit that adjusts the posture of the subject by supporting at least one part of the subject. The magnetic resonance imaging apparatus according to claim 1,
In the posture adjustment unit, at least one of an index indicating the local specific absorption rate of the part and an index indicating the uniformity of the high-frequency magnetic field distribution in the region of interest is improved from the index in the initial state where the posture adjustment unit is not disposed. The magnetic resonance imaging apparatus is characterized in that the part is supported at a position where the magnetic resonance imaging is performed. The magnetic resonance imaging apparatus according to claim 2,
The magnetic resonance imaging apparatus, wherein the position for supporting the part is a predetermined position. The magnetic resonance imaging apparatus according to claim 1,
The posture adjusting unit has a different shape depending on the part. The magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus, wherein the posture adjustment unit is detachably installed on a table on which the subject is placed. The magnetic resonance imaging apparatus according to claim 1,
The signal receiving unit includes a receiving coil for receiving the nuclear magnetic resonance signal,
The magnetic resonance imaging apparatus, wherein the posture adjustment unit is integral with the receiving coil. The magnetic resonance imaging apparatus according to claim 1,
The signal receiving unit includes a receiving coil for receiving the nuclear magnetic resonance signal,
The magnetic resonance imaging apparatus, wherein the posture adjustment unit is detachably attached to the receiving coil. The magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus, wherein the posture adjustment unit includes a member capable of adjusting a height of the part supported by the posture adjustment unit from the table. The magnetic resonance imaging apparatus according to claim 8,
The magnetic resonance imaging apparatus, wherein the posture adjustment unit includes a member capable of adjusting the height stepwise. The magnetic resonance imaging apparatus according to claim 8,
The magnetic resonance imaging apparatus, wherein the posture adjustment unit includes an apparatus capable of continuously adjusting the height. The magnetic resonance imaging apparatus according to claim 1,
A marker light irradiating unit for irradiating marker light for positioning the subject;
The magnetic resonance imaging apparatus according to claim 1, wherein the marker light irradiation units are arranged in a number that can determine the positions of all the parts supported by the posture adjustment unit. The magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus, wherein the posture adjustment unit includes a heat dissipation promotion unit that promotes heat dissipation on a surface in contact with the subject. The magnetic resonance imaging apparatus according to claim 12,
The heat radiation acceleration unit includes a concavo-convex structure. The magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus, wherein the posture adjusting unit is formed of a material having a conductivity equal to or lower than a predetermined first threshold and a relative dielectric constant equal to or lower than a predetermined second threshold. The magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus, wherein the posture adjustment unit is formed of a material having a conductivity equal to or lower than a predetermined first threshold and a relative dielectric constant equal to or higher than a predetermined third threshold. The magnetic resonance imaging apparatus according to claim 12,
The heat radiation acceleration unit is formed of a cooling sheet. The magnetic resonance imaging apparatus according to claim 9,
The magnetic resonance imaging apparatus, wherein the member is a plurality of spacers formed to be stackable. The magnetic resonance imaging apparatus according to claim 1,
A magnetic resonance imaging apparatus, further comprising: a suitability determination unit that acquires a high-frequency magnetic field distribution in an imaging region and determines the suitability of the posture adjustment unit according to the high-frequency magnetic field distribution. The magnetic resonance imaging apparatus according to claim 18,
The magnetic resonance imaging apparatus, wherein the suitability determination unit calculates high-frequency magnetic field uniformity from the high-frequency magnetic field distribution, and determines that the high-frequency magnetic field uniformity is appropriate when the high-frequency magnetic field uniformity is equal to or less than a predetermined uniformity. The magnetic resonance imaging apparatus according to claim 18,
The magnetic resonance imaging apparatus, wherein the suitability determining unit estimates the distribution of the local specific absorption rate from the high-frequency magnetic field distribution, and determines the suitability from the estimation result.