Magnetic Resonance Imaging System
By attaching the RF shield to the inner surface of the gradient magnetic field coil near its turn center, the temperature rise due to eddy currents is suppressed, addressing the challenges of RF shield heating and enabling larger MRI apparatus diameters and effective RF shielding.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- FUJIFILM CORP
- Filing Date
- 2022-09-02
- Publication Date
- 2026-06-19
AI Technical Summary
Existing MRI systems face challenges in suppressing temperature rise due to eddy currents in RF shields caused by gradient magnetic fields, particularly in high-speed imaging techniques, while maintaining effective RF shielding and allowing for larger patient openings.
The RF shield is attached to the inner surface of the gradient magnetic field coil, specifically near the turn center where the gradient magnetic field is strongest, reducing thermal resistance and eddy current heating without additional cooling structures.
This configuration effectively suppresses temperature rise in the RF shield, enabling thinner RF shields and larger MRI apparatus diameters without the need for dedicated cooling, while maintaining RF shielding efficacy.
Smart Images
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Abstract
Description
Technical Field
[0001] The present invention relates to a configuration of an RF shield attached to the inner surface of a gradient magnetic field coil in a magnetic resonance imaging apparatus to block magnetic coupling between a high-frequency magnetic field by a high-frequency irradiation (hereinafter referred to as RF) coil and the gradient magnetic field coil.
Background Art
[0002] A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) is an apparatus that obtains a cross-sectional image showing the physical and chemical properties of a subject by using the nuclear magnetic resonance phenomenon that occurs when the subject placed in a uniform static magnetic field is irradiated with high-frequency pulses, and is particularly used for medical purposes.
[0003] An MRI apparatus mainly includes a magnet device that generates a uniform static magnetic field in an imaging space into which a subject is inserted, a gradient magnetic field coil that generates a spatially gradient magnetic field in a pulsed manner to impart position information to the imaging space, an RF coil that irradiates the subject with high-frequency pulses, a reception coil that receives a nuclear magnetic resonance signal from the subject, and a computer system that processes the received signal and displays an image.
[0004] As a main means for improving the performance of an MRI apparatus, there is an improvement in the intensity of the static magnetic field generated by the magnet device. The stronger the static magnetic field, the clearer the image obtained. Therefore, the development of MRI apparatuses has been continuously directed towards improving the magnetic field intensity. Also, for the gradient magnetic field coil, an improvement in the intensity of the generated gradient magnetic field and a shortening of the pulse interval are aimed at for improving the image quality and shortening the imaging time. The RF coil causes a nuclear magnetic resonance phenomenon in the nuclear spin and acquires a magnetic resonance signal as an electromagnetic wave. For this reason, for obtaining a clear image, an improvement in the ratio of the magnetic resonance signal to noise (SN ratio) by irradiating the subject with a high-frequency electromagnetic field having a frequency determined by the static magnetic field intensity in a uniform distribution is required.
[0005] In a typical horizontal magnetic field MRI system, the gradient coils are installed in a roughly concentric manner on the inner circumference of the static magnetic field magnet, which is a roughly cylindrical magnetic device. Furthermore, the RF coil is also roughly concentrically cylindrical and is installed on the inner circumference of the gradient coil, roughly concentric with the static magnetic field magnet and the gradient coil.
[0006] The electromagnetic field generated by passing a current with a frequency determined by the static magnetic field strength through the RF coil not only irradiates the subject but also creates a magnetic coupling between the RF coil and the gradient magnetic field coils located on its outer circumference. For this reason, an RF shield is installed between the RF coil and the gradient magnetic field coils to prevent the electromagnetic field generated by the RF coil from leaking to the gradient magnetic field coils. The RF shield is also roughly concentric with the RF coil and is typically made of a thin, non-magnetic conductive material such as copper, aluminum, or stainless steel in MRI devices.
[0007] The gradient coil generates a pulsed gradient magnetic field in the imaging space where the subject is placed. However, the RF shield between the gradient coil and the subject is made of a thin conductive plate material that receives the gradient magnetic field pulses, generating eddy currents. This Joule heating caused by eddy currents is the reason for the temperature rise of the RF shield.
[0008] In recent years, MRI systems have increasingly utilized high-speed imaging techniques such as echo-planar imaging (EPI). These high-speed imaging techniques are achieved by rapidly raising and lowering high-intensity gradient magnetic field pulses multiple times. Consequently, eddy currents generated in the RF shield are also increasing, and suppressing the increased temperature rise associated with eddy current heating is a challenge.
[0009] Conventionally, a method for reducing eddy current heating in RF shields due to gradient magnetic fields is known, as shown in Patent Document 1, which involves making slits in the thin plate of the RF shield or creating a tile-like pattern to reduce the amount of eddy current generated. This is because the frequency of the gradient magnetic field pulse (~several kHz) is lower than the frequency of the high-frequency electromagnetic field (several tens to several hundred MHz) generated by the RF irradiation coil, so the shield is molded to have low impedance to the high-frequency electromagnetic field and high impedance to the gradient magnetic field. However, increasing the number of slits or tile divisions increases the resistance to the high-frequency electromagnetic field as well, thus reducing the effectiveness of the RF shield. Therefore, in this method, the generation of eddy currents must be tolerated to the extent that the effectiveness of the RF shield is not reduced.
[0010] Furthermore, methods to suppress the temperature rise due to heat generation in the RF shield may include providing a cooling structure using water or air cooling, as shown in Patent Document 2. On the other hand, in recent MRI devices, there is a tendency to make the openings larger so that patients do not feel a sense of occlusion during imaging. For this reason, gradient coils, RF coils, and RF shields are becoming thinner and more concentric cylindrical with respect to the static magnetic field magnet, and it is desirable that the cooling structure be simplified. [Prior art documents] [Patent Documents]
[0011] [Patent Document 1] Special Publication No. 03-050543 [Patent Document 2] Japanese Patent Publication No. 2006-116300 [Overview of the Initiative] [Problems that the invention aims to solve]
[0012] As shown in Patent Document 1, in order for the RF shield to reduce the magnetic coupling between the RF coil and the gradient magnetic field coil while suppressing the generation of eddy currents due to the gradient magnetic field, the RF shield needs to be made of a conductive material in the form of thin plates in the shape of tiles or strips with slits. On the other hand, as shown in Patent Document 2, cooling is necessary to suppress the rise in temperature due to heat generation caused by eddy currents. Furthermore, in order to enlarge the opening into which the patient is inserted, the gradient magnetic field coil, RF coil, and RF shield, which are located within the static magnetic field magnet and are generally concentric cylindrical in shape, need to be made thinner.
[0013] The present invention has been made in view of these circumstances, and aims to provide an MRI device that can suppress the temperature rise of the RF shield without a dedicated water-cooling or air-cooling structure by attaching the RF shield to the inner surface of the gradient magnetic field coil near the center of the gradient magnetic field coil turn where the gradient magnetic field is strong, thereby reducing the thermal resistance with the gradient magnetic field coil. [Means for solving the problem]
[0014] To solve the above problems, the present invention provides an MRI apparatus comprising: a magnetic pole that generates a static magnetic field in the imaging space; a gradient coil that generates a dynamic magnetic field with linear magnetic field strength with respect to position in the imaging space; a high-frequency coil that generates a high-frequency magnetic field in the imaging space; and an RF shield located between the gradient coil and the high-frequency coil to reduce electromagnetic coupling between the high-frequency electromagnetic field and the gradient coil, wherein the RF shield is installed by adhering or attaching it to the surface of the gradient coil, and the adhesive or attachment position is in the region near the turn center around which the X or Y gradient coil conductor constituting the gradient coil is wound. [Effects of the Invention]
[0015] According to the MRI apparatus using the RF shield installation method of the present invention, the temperature rise caused by eddy current heating in the RF shield due to energizing the gradient magnetic field coil is suppressed, and since the temperature rise of the RF shield is suppressed without additional cooling structures, it is possible to make the shield thinner and realize a large-diameter MRI apparatus.
Brief Description of the Drawings
[0016] [Figure 1] It is a diagram showing a configuration example of an RF shield according to Example 1. [Figure 2] It is a schematic external perspective view of a horizontal magnetic field type MRI apparatus according to Example 1. [Figure 3] It is a schematic external perspective view showing another mode of the MRI apparatus of Example 1. [Figure 4] It is a developed view showing the conductor shape of the X gradient magnetic field coil of the gradient magnetic field coil used in the horizontal magnetic field type MRI apparatus according to Example 1. [Figure 5] It is a plan view showing the conductor shape of the X gradient magnetic field coil of the gradient magnetic field coil used in the open type MRI apparatus according to Example 1. [Figure 6] It is a partial developed view showing the adhesion or attachment position to the RF shield and the inner cylindrical surface of the gradient magnetic field coil used in the MRI apparatus according to Example 2. [Figure 7] It is a partial developed view showing the adhesion or attachment position to the RF shield and the inner surface of the gradient magnetic field coil used in the open type MRI apparatus according to Example 3.
Modes for Carrying Out the Invention
[0017] Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Examples
[0018] As shown in FIG. 2, a general horizontal magnetic field type MRI apparatus has a cylindrical magnetic pole 1 composed of a superconducting coil, and generates a static magnetic field in the imaging space 2 in the direction indicated by the arrow 3. The subject 4 is carried into the imaging space 2 by a movable bed 5 to obtain an image. Such an MRI apparatus mainly has a concentric cylindrical gradient magnetic field coil 6 and an RF coil 7 inside the magnetic pole 1 that generates a static magnetic field using a superconducting coil. These are normal conducting coils that respectively obtain position information for image acquisition and generate magnetic resonance to obtain signals.
[0019] The RF shield is located between the gradient magnetic field coil 6 and the RF coil 7 and is installed on the inner surface of the gradient magnetic field coil 6 by adhesion or attachment using double-sided tape. These are covered with a cover (not shown) that is integral with the magnetic poles. In addition, as other major components of the MRI apparatus, there are a power supply device that supplies current to superconducting coils such as the gradient magnetic field coil and the RF coil, a water-cooling or air-cooling device for cooling these superconducting coils and the power supply device, and a computer system for operation and image display. However, these are omitted in the figure.
[0020] Fig. 3 shows a configuration example of an MRI apparatus called an open type, which is another general type of MRI apparatus. Disk-shaped upper and lower magnetic poles 1 that generate a static magnetic field are arranged above and below the imaging space 2, and a static magnetic field is generated in the direction of arrow 3 in the imaging space 2. The subject 4 is carried into the imaging space 2 by a movable bed 5 to acquire an image, which is the same as in the case of Fig. 2.
[0021] In the open-type MRI apparatus of this configuration, between the upper and lower magnetic poles, in addition to being supported by structures such as columns, it may be connected by a magnetic return yoke 8 having a schematic C-shaped form as shown in Fig. 3, and is particularly found in apparatuses with a static magnetic field strength of 1 tesla or less. In such an open-type MRI apparatus, the gradient magnetic field coil 6 and the RF coil 7 are also disk-shaped and arranged above and below the imaging space 2, like the magnetic poles, and are covered with a cover (not shown) that is integral with them.
[0022] The RF shield is also generally disk-shaped and is located between the gradient magnetic field coil of the upper and lower magnetic poles and the RF shield, and is installed on the imaging space side surface of the gradient magnetic field coil. In addition, as other major components, there are a power supply device for energizing superconducting coils such as the gradient magnetic field coil and the RF coil, a cooling device for cooling these superconducting coils and the power supply device, and a computer system for operation and image display. However, these are omitted in Fig. 3 in the same way as the apparatus in Fig. 2.
[0023] As shown in Figure 3, the gradient magnetic field is a pulsed, time-varying magnetic field in the imaging space 2, where the component of the static magnetic field in direction 3 has a gradient such that its strength is proportional to the distance from the center. The gradient magnetic field coil 6 is composed of three sets of coils, for example, with the direction of the static magnetic field 3 being the Z direction, so that gradient magnetic fields can be arbitrarily generated in the three orthogonal XYZ directions of the imaging space 2, and pulsed current can be independently passed through each coil according to the direction of the gradient magnetic field.
[0024] The gradient magnetic field coil 6 consists of three sets of gradient magnetic field coils for X, Y, and Z directions, but each gradient magnetic field coil in each direction may consist of a main coil and a shield coil. In the horizontal magnetic field type MRI device shown in Figure 2, the unfolded view of the main coil of the X gradient magnetic field coil that generates the gradient magnetic field in the X direction is composed of four spiral-shaped gradient magnetic field coil conductors 10, as shown in Figure 4.
[0025] The gradient magnetic field coil conductor 10 is formed by winding a plate or rod of good conductor material such as copper or aluminum, or by grooving a plate material. Each of the four spiral shapes has a region 11 near the center of the turn, and although these positions are far from the imaging space 2 that generates the gradient magnetic field 9, the intensity of the pulsed magnetic field generated by the X gradient magnetic field coil is strongest on the inner cylindrical surface of the gradient magnetic field coil. Similarly, the Y gradient magnetic field coil has a shape that is roughly the same as the X gradient magnetic field coil rotated 90 degrees around the Z axis at the center of the coaxial cylinder.
[0026] Figure 5 shows an example of the conductor shape configuration of the X gradient coil in the open-type MRI apparatus shown in Figure 3. Figure 5 shows the conductor 10 of the X gradient coil installed on the lower magnetic pole of the static magnetic field magnet. The direction of the static magnetic field is perpendicular to the plane of the paper, and the X gradient coil generates a pulsed magnetic field in the imaging space 2 located midway between the upper and lower magnetic poles in the Z direction, where the magnetic field strength of the Z component is approximately proportional to the position in X from the center.
[0027] The gradient magnetic field coil conductor 10 is formed by winding copper plates or copper wires, or by groove processing on a flat plate, similar to horizontal magnetic field type MRI devices. Similar to horizontal magnetic field type MRI devices, the intensity of the pulsed magnetic field generated by the gradient magnetic field coil is strongest in the region 11 near the center of the turn, which is outside the imaging space 2, on the surface of the gradient magnetic field coil. In an open-type MRI device, there are two turn center regions 11 on the surface of each gradient magnetic field coil installed on the upper and lower magnetic pole surfaces for each gradient magnetic field axis. Note that the Y gradient magnetic field coil has a shape obtained by rotating the X gradient magnetic field coil approximately 90 degrees around the Z axis.
[0028] Figure 1(A) shows a partially unfolded view of the RF shield 12 according to Embodiment 1 in a horizontal magnetic field type MRI apparatus. The RF shield 12 is a thin plate or mesh structure made of a conductive material such as copper, stainless steel, or aluminum, and the shape and thickness of the RF shield pattern 13 are determined so that the high-frequency magnetic field (hereinafter referred to as RF magnetic field) from the RF coil is uniformly irradiated into the imaging space 2 and the magnetic coupling with the gradient magnetic field coil is reduced. The RF shield 12 is installed on the inner surface of the gradient magnetic field coil, but in this embodiment it is bonded or attached to the inner cylindrical surface of the gradient magnetic field coil.
[0029] For bonding, for example, an epoxy resin adhesive is used, and for attaching, for example, an acrylic adhesive tape is used. The RF shield may be made by directly bonding or attaching the conductive material to the inner surface of the gradient coil, but if a pattern shape is used to reduce the generation of eddy currents due to the gradient coil, for example, by attaching a pattern of conductive material to a sheet of fiber-reinforced plastic (hereinafter referred to as FRP) and integrating it as an RF shield 12, which is then bonded or attached to the inner cylindrical surface of the gradient coil, an improvement in positional accuracy with respect to the gradient coil can be expected.
[0030] At this time, the region 11 near the center of the turn of the X gradient magnetic field coil conductor is the region where the intensity of the pulsed magnetic field generated by the X gradient magnetic field coil is closest, and large eddy currents are generated in the RF pattern 13 of the conductive material constituting the RF shield. The eddy currents generate Joule heat due to the electrical resistance of the RF pattern, causing the temperature of the RF shield to rise. For this reason, measures are needed to prevent the temperature rise from becoming excessive, and generally, cooling means such as air cooling or cooling with a coolant are used. Alternatively, methods such as limiting the energizing pulses of the gradient magnetic field coil to prevent an excessive temperature rise may also be taken.
[0031] Therefore, in the first embodiment of the present invention, the RF shield 12 is bonded or attached 14 to the inner cylindrical surface of the gradient coil so as to include at least the central turn region 11 of the gradient coil conductor. As a result, the heat generated by eddy currents in the RF shield is cooled by the gradient coil, thus preventing excessive temperature rise. Generally, gradient coils have cooling means such as water cooling or air cooling, so excessive temperature rise can be suppressed in both cases. In general, the eddy current heat generated in the RF shield is less than 1 / 10 of the heat generated when the gradient coil is energized, so the effect of this embodiment on the temperature rise of the gradient coil itself is small.
[0032] Furthermore, in this embodiment, the pattern 13 of the conductive material constituting the RF shield is in the shape of a strip 13(a) as shown in Figure 1(B), or a tile shape 13(b) as shown in Figure 1(C), and there is a slit structure 15 without a pattern between these strips or tiles. It has been found that the eddy currents generated in these strip-shaped or tile-shaped RF shield patterns 13 have the highest current density at the edges of the strips or tiles, and it is effective in suppressing temperature rise when the strips or tiles are bonded or attached 14 to the inner cylindrical surface of the gradient coil in line with the edges of the strips or tiles.
[0033] When the entire surface of the RF shield is bonded or attached to the inner cylindrical surface of the gradient magnetic field coil, the temperature rise suppression effect of the RF shield is maximized. However, when the entire surface is attached, the position of the RF shield relative to the RF coil may shift during installation, or wrinkles may form if the RF shield is a thin film. Also, if the RF shield peels off during use, peeling over a wide area can cause a temperature rise.
[0034] Experimental and analytical studies have shown that if approximately 30% or more of the RF shield's surface area is bonded or attached to the inner surface of the gradient coil, a temperature rise suppression effect comparable to that of full-surface attachment can be obtained, even when imaging using echo-planar imaging (EPI), which is known to generate a large amount of heat from the RF shield due to the gradient magnetic field.
[0035] In an actual horizontal magnetic field MRI system with a static magnetic field strength of 1.5 Tesla, an RF shield formed by attaching a copper strip-shaped RF shield pattern, approximately 20 micrometers thick and 40 mm wide, to both sides of a resin sheet approximately 25 micrometers thick, was attached to the inner cylindrical surface of a gradient coil via double-sided tape. When imaging was performed using echo-planar imaging (EPI), it was found that the temperature rise of the RF shield remained below 40 degrees Celsius when the area of the double-sided tape attachment was 15% or more of the surface area of the RF shield.
[0036] On the other hand, temperature rise analysis of the same system revealed that the heat transfer coefficient from the RF shield to the gradient coil in the area where double-sided tape was not applied was about 0.2 times that of the area where double-sided tape was applied.
[0037] Therefore, in the case of insulation where there is no heat transfer in the parts not attached with double-sided tape, considering the 15% of the surface area of the RF shield which is the area where double-sided tape was attached in the above experiment, and the 0.2 times heat transfer coefficient of the 85% area not attached with double-sided tape based on the experimental and analytical results, 15 + 85 × 0.2 = 15 + 17 = 32%, that is, approximately 1 / 3 of the area of the RF shield should be attached to the surface of the gradient magnetic field coil by adhesive or double-sided tape.
[0038] Thus, by adhering or attaching the RF shield not to the entire surface but only to the inner cylindrical surface of the gradient coil 14, wrinkles, lifting during installation, and positional displacement due to differences in thermal expansion with the gradient coil can be prevented, thereby improving the installation accuracy relative to the gradient coil conductor position. This can suppress a decrease in the irradiation efficiency of the high-frequency electromagnetic field and localized heat generation of the RF shield. In addition, compared to the case where the RF shield is molded integrally with the gradient coil in resin, the positional accuracy relative to the gradient coil conductor is improved, and the RF shield becomes easier to replace during long-term use. [Examples]
[0039] Figure 6 shows a method for adhering or attaching the RF shield 12 to the inner cylindrical surface of the gradient coil according to Embodiment 2. In this embodiment, the RF shield 12 is adhering or attached 14 to the inner cylindrical surface of the gradient coil so as to cover the area near the turn center of the gradient coil conductor. This embodiment is suitable when the conductive member of the RF shield is in a mesh shape. When the conductive member of the RF shield is in a sheet shape, it is sufficient to adhering or attach 14 only to the edges of the RF shield 12 and the area near the turn center of the gradient coil conductor, and according to this embodiment, improved positional accuracy of the RF shield and a reduction in the number of adhesive points can be achieved. [Examples]
[0040] Figure 7 shows the method of bonding or attaching the RF shield 12 to the surface of the gradient magnetic field coil according to Example 3. This example shows the installation configuration of the RF shield 12 for an open-type MRI device. In this example, the RF shield 12 is disc-shaped, but there are a total of four regions 11 near the turn center of the X-axis gradient magnetic field coil conductor and the Y-axis gradient magnetic field coil conductor on one side of the upper and lower magnetic poles, so the corresponding parts and the ends of the RF shield pattern 13 are bonded or attached 14 to the surface of the gradient magnetic field coil on the imaging k-space side.
[0041] According to this embodiment, the temperature rise associated with eddy current heating generated in the RF shield when the gradient coil is energized is suppressed not only for horizontal magnetic field type MRI devices but also for open type MRI devices.
[0042] In Examples 1 to 3, the RF shield 12 can be either a single-piece or segmented structure, and the shape can be selected for manufacturing purposes. Similar effects can be expected if the bonding or attachment position to the gradient magnetic field coil is the same. [Explanation of symbols]
[0043] 1 magnetic pole 2. Imaging space 3. Show the static magnetic field and its direction → 4. Subjects 5. Movable bed 6. Gradient field coils 7 RF coils 8 Return York 9 Gradient magnetic field 10. Gradient field coil conductor 11 Turn Center Area 12 RF Shield 13 RF Shielding Patterns 13(a) Strip-shaped RF shielding pattern 13(b) Tile-shaped RF shield pattern 14. Position of adhesion or attachment of RF shield to gradient magnetic field coil 15 slits
Claims
1. A magnetic resonance imaging device, The imaging space includes a magnetic pole that generates a static magnetic field, a gradient coil that generates a dynamic magnetic field with linear magnetic field strength with respect to position within the imaging space, a high-frequency coil that generates a high-frequency magnetic field within the imaging space, and an RF shield located between the gradient coil and the high-frequency coil to reduce electromagnetic coupling between the high-frequency magnetic field and the gradient coil. The RF shield is installed by partially adhering or attaching it to the surface of the gradient magnetic field coil, and the adhesive or attachment position is in the region near the turn center where the X or Y gradient magnetic field coil conductor constituting the gradient magnetic field coil is wound. A magnetic resonance imaging apparatus characterized by the following features.
2. A magnetic resonance imaging apparatus, The imaging space includes a magnetic pole that generates a static magnetic field, a gradient coil that generates a dynamic magnetic field with linear magnetic field strength with respect to position within the imaging space, a high-frequency coil that generates a high-frequency magnetic field within the imaging space, and an RF shield located between the gradient coil and the high-frequency coil to reduce electromagnetic coupling between the high-frequency magnetic field and the gradient coil. The RF shield is installed by adhering or attaching it to the surface of the gradient magnetic field coil, and the adhesive or attachment position is in the region near the turn center where the X or Y gradient magnetic field coil conductor constituting the gradient magnetic field coil is wound. The RF shield is constructed by attaching a pattern of strip-shaped, thin-film conductive material to a sheet-like member made of insulating material, and the RF shield is bonded or attached to the gradient magnetic field coil at the ends of the pattern of the conductive material or along the grooves between the patterns. A magnetic resonance imaging apparatus characterized by the following features.
3. A magnetic resonance imaging apparatus according to claim 2, The RF shield, formed in sheet form, has at least one-third of its area bonded or attached to the gradient magnetic field coil. A magnetic resonance imaging apparatus characterized by the following features.
4. A magnetic resonance imaging apparatus according to any one of claims 1 to 3, The magnetic pole, the gradient magnetic field coil, the high-frequency coil, and the RF shield are arranged in a generally concentric cylindrical shape and are generally coaxial with the imaging space, and the RF shield is bonded or attached to the inner cylindrical surface of the gradient magnetic field coil. A magnetic resonance imaging apparatus characterized by the following features.
5. A magnetic resonance imaging apparatus according to any one of claims 1 to 3, The magnetic pole, the gradient magnetic field coil, the high-frequency coil, and the RF shield are a pair of roughly circular discs facing each other across the imaging space, and the RF shield is bonded or attached to the surface of the gradient magnetic field coil on the imaging space side. A magnetic resonance imaging apparatus characterized by the following features.