Local magnetic field inhomogeneity correction pad

Abstract

To provide a local magnetic field inhomogeneity correction pad which, compared to conventional magnetic field distortion elimination cushions or local magnetic field inhomogeneity correction pads, is capable of correcting local magnetic field inhomogeneity even in MRI examinations at low magnetic fields of less than 1.5 tesla, thereby improving image clarity.SOLUTION: A pad 1 comprises a gas-barrier bag body 11 and calcined alum 12, where the bag body 11 is filled with calcined alum 12 obtained by heating, dehydrating and drying potassium aluminum sulfate. The pad 1 is provided with a recessed portion, and is wound for use such that a subject's neck is placed against the recessed portion. The pad 1 further comprises an opening / closing valve that allows adjustment of the filling amount of the calcined alum 12. The bag body 11 is formed of a synthetic resin sheet having flexibility to deform along the contour of the human body and elasticity allowing the calcined alum 12 to flow and thereby change the shape of the bag body.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a pad for correcting local magnetic field non-uniformity in MRI (Magnetic Resonance Imaging) examinations. [Background technology]

[0002] In recent years, MRI scans have become widely used as a test to image organs and blood vessels. MRI stands for magnetic resonance imaging, and is a test that does not use X-rays like X-rays, but instead uses strong magnets and radio waves to depict the state of the body as cross-sectional images. Specifically, RF (Radio Frequency) waves are irradiated onto the human body in a strong magnetic field, and weak signals generated by hydrogen atoms are collected and imaged. Approximately two-thirds of the human body is made up of water, and this image focuses on hydrogen atoms, which have a natural abundance of 99.985%. This MRI scan can detect various lesions inside the body, such as in the brain, ovaries, prostate, lower abdomen, spine, and limbs. However, depending on the part being imaged, there are problems such as uneven density and blurring of the image, especially in parts with complex shapes such as the neck and fingers.

[0003] There are several causes of image density unevenness and blurring, one of which is due to localized magnetic field inhomogeneity. This is because the human body itself is a magnetic material, so simply entering the MRI machine induces magnetic field disturbances, and when the human body is irradiated with the RF magnetic field, inhomogeneity also occurs. When the RF magnetic field becomes inhomogeneous, image density unevenness and blurring occur, resulting in adverse effects such as image unevenness and blurring. One solution to these localized magnetic field inhomogeneities is a technique called active shimming. Active shimming can adjust the disturbed magnetic field by passing an electric current through the shim coils of an MRI device. However, active shimming alone is unable to ensure sufficient local magnetic field homogeneity in areas with complex shapes, such as the neck or fingers, and is therefore not a complete solution. Therefore, a magnetic field distortion removal cushion for MRI devices is known that can further correct localized magnetic field inhomogeneity (see Patent Document 1).

[0004] The magnetic field distortion removal cushion is configured with a gas-barrier bag containing air, which contains an aqueous solution containing an iron- or cobalt-based material dissolved therein, and granular material for uniformly distributing the aqueous solution within the bag. This cushion can be attached to the lower, upper, or neck area of ​​a subject lying supine or prone on an MRI bed, or other areas that come into contact with the subject's body during imaging. This cushion enables the fat suppression pulse, an important function for obtaining MRI images, to function with a high probability, regardless of individual differences in the subject's body shape, enabling images with suppressed fat areas to be obtained. The magnetic field distortion elimination cushion of Patent Document 1 uses a water-soluble solution containing dissolved iron or cobalt-based materials, which eliminates local magnetic field nonuniformity and improves image clarity to a certain extent. However, the magnetic field distortion removal cushion of Patent Document 1 has a problem in that although it can be applied to so-called high magnetic field MRI devices, it cannot be used in low magnetic fields of less than 1.5 tesla.

[0005] Also known is a pad for correcting local magnetic field nonuniformity, which comprises a bag made of a gas barrier material, an aqueous solution in which a manganese-based material is dissolved, and granules for dispersing the aqueous solution uniformly inside the bag, and which is characterized in that the divalent manganese ion concentration in the manganese-based material is 0.83 to 1.66 g / L (see Patent Document 2).By using a manganese-based material to adjust the manganese ion concentration to a predetermined concentration, local magnetic field nonuniformity can be corrected more effectively than with conventional magnetic field distortion removal cushions, and image clarity can be improved. However, like the magnetic field distortion removal cushion of Patent Document 1, the pad of Patent Document 2 can be applied to so-called high magnetic field MRI devices, but has the problem that it cannot be used in low magnetic fields of less than 1.5 tesla. Previously, images taken with low-field MRI devices (less than 1.5 Tesla) had problems such as low image quality and long imaging times, but advances in computer technology have resolved these issues, and low-field MRI devices are becoming more common. Currently, low-field MRI devices are also being used in relatively small hospitals. The inventors conducted tests in low magnetic fields using pads made of manganese-based materials with several different manganese concentrations, and found that pads made of manganese-based materials had high signal strength and were therefore difficult to use in low magnetic fields. As mentioned above, the human body itself is a magnetic body, but the human body is a diamagnetic body (-11 to -7 × 10 -6 cm 3 / g), and air (3.01 × 10 -7 cm 3 / g), the magnetic susceptibility is smaller than that of the human body. Therefore, the magnetic susceptibility changes suddenly at the boundary between the human body and air, causing the magnetic field strength to fluctuate and resulting in uneven fat suppression effects in the subcutaneous fat tissue. Therefore, it is speculated that the change in magnetic susceptibility can be made gentler by covering the human body surface with a substance that alleviates the sudden change in magnetic susceptibility on the human body surface. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 5212026 [Patent Document 2] Patent No. 6609751 Summary of the Invention [Problem to be solved by the invention]

[0007] In view of the above situation, the present invention aims to provide a local magnetic field inhomogeneity correction pad that corrects local magnetic field inhomogeneity even in low-field MRI examinations of less than 1.5 Tesla, thereby improving image clarity, compared to conventional magnetic field distortion removal cushions and local magnetic field inhomogeneity correction pads. [Means for solving the problem]

[0008] To solve the above problems, the local magnetic field inhomogeneity correction pad of the present invention is characterized in that the inside of a bag made of a gas barrier material is filled with burnt alum (potassium aluminum sulfate) powder. By filling the inside of the bag with burnt alum powder, local magnetic field inhomogeneity can be corrected even in MRI examinations with low magnetic fields of less than 1.5 Tesla, improving the clarity of the captured images. This will be described in detail in the examples below, showing comparative experimental data. Burnt alum is made by heating, dehydrating, and drying raw alum. The static magnetic field strength of the permanent magnets used in MRI devices is preferably 0.5 tesla or less, and more preferably 0.4 tesla or less. Covering the surface of the human body with a bag filled with burnt alum powder can moderate changes in magnetic susceptibility.

[0009] In addition, the local magnetic field inhomogeneity correction pad of the present invention is characterized in that the inside of a bag made of a gas barrier material is filled with powder of burnt alum (potassium aluminum sulfate) and solid or hollow foamed resin granules of 0.3 to 2 mm, and the volumetric content of burnt alum is 70% or more. The expanded resin granules are, for example, expanded polystyrene, but other thermoplastic resins may also be used. By mixing the expanded granules with burnt alum in the bag, when the subject applies a load, the expanded granules flow smoothly along the shape of the load and deform into a concave shape. In other words, the expanded polystyrene can be prevented from becoming uneven. By making the volumetric content of burnt alum 70% or more, the clarity of the image can be improved. The volumetric content of burnt alum is preferably 80% or more, and more preferably 90% or more.

[0010] In the local magnetic field inhomogeneity correcting pad of the present invention, it is preferable that an on-off valve for circulating air between the inside and outside of the bag is attached to the outer cover of the bag. Moreover, it is preferable that the on-off valve is a check air valve, and the bag is deformed by an external load to discharge the air inside the bag to the outside, thereby enabling the bag to maintain its shape. The burnt alum powder and foamed resin granules may be mixed inside the bag, but if an opening / closing valve is attached to the local magnetic field inhomogeneity correction pad, it is preferable to divide the burnt alum powder into small bags of a specified size and store it inside the bag separately from the foamed resin granules in order to prevent the filter from clogging with the burnt alum during degassing. [Effects of the Invention]

[0011] The local magnetic field nonuniformity correction pad of the present invention has the effect of correcting local magnetic field nonuniformity even in MRI examinations with low magnetic fields of less than 1.5 Tesla, thereby improving the clarity of images, compared to conventional magnetic field distortion removal cushions and local magnetic field nonuniformity correction pads. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a plan view of a local magnetic field nonuniformity correcting pad according to a first embodiment of the present invention; [Figure 2] Cross-sectional view of the pad of Example 1 [Figure 3] A perspective view of the phantom used in the verification of Example 1. [Figure 4] Shimming installation diagram [Figure 5] Comparison of static magnetic field disturbances when shimming is installed sagitally [Figure 6] Comparison of static magnetic field disturbances when shimming is installed in the coronal and oil regions [Figure 7] Comparison of static magnetic field disturbances when shimming is installed in the coronal and water regions [Figure 8] Comparison of static magnetic field disturbances when shimming is installed in the coronal, oil, and water regions (1) [Figure 9] Comparison of static magnetic field disturbances when shimming is installed in the coronal, oil, and water regions (2) [Figure 10] Image of the pad in use in Example 1 DETAILED DESCRIPTION OF THE INVENTION

[0013] An example of an embodiment of the present invention will be described in detail below with reference to the drawings. Note that the scope of the present invention is not limited to the following examples and illustrated examples, and many modifications and variations are possible. [Example]

[0014] FIG. 1 shows a plan view of a local magnetic field inhomogeneity correcting pad (hereinafter simply referred to as "pad") of Example 1. As shown in FIG. 1, pad 1 has a recess 1a, and is used by placing the recess 1a against the neck of a subject and wrapping it around the neck. Pad 1 is filled with burnt alum (not shown). Pad 1 is provided with an on-off valve 13, which allows the amount of burnt alum filled to be adjusted. If on-off valve 13 is a check air valve, the bag body will deform under an external load, discharging air from inside the bag to the outside, thereby allowing the shape of the bag to be maintained. Alternatively, the pad 1 may be shaped like a scarf and wrapped around the neck. By covering the neck, it is possible to smooth out changes in magnetic susceptibility. The neck is the most uneven area, making it the most susceptible to magnetic field distortion, and it can more securely fill the entire circumference of the neck than a U-shaped pad.

[0015] Fig. 2 shows a cross-sectional view taken along the line AA of Fig. 1. As shown in Fig. 2, the pad 1 comprises a gas-barrier bag body 11 and burnt alum 12, and the bag body 11 is filled with the burnt alum 12. The gas barrier bag 11 is made of a synthetic resin sheet that is flexible enough to conform to the contours of the human body and stretchable enough to allow the burnt alum 12 to flow and change the shape of the bag. The synthetic resin sheet may be made of, for example, polyurethane, polyester, or polyvinyl chloride. However, it is not limited to these, and other synthetic resin sheets may be used as long as they have gas barrier properties.

[0016] In this example, verification was performed using a phantom (human anatomical model) using a dumbbell-shaped container. Figure 3 shows perspective views of the phantom used in the verification of Example 1, where (1) shows the dumbbell-shaped container before being filled with water and oil, (2) shows the phantom before being filled with water and oil, and (3) shows the phantom wrapped in padding. As shown in Figure 3(1), a dumbbell-shaped container 30 is used as the phantom. The dumbbell-shaped container 30 is a well-known container that can be used for training, etc., and can be filled with any solution by opening and closing the lid 3d. As shown in Figure 3(2), the phantom 3 is such a dumbbell-shaped container 30, half filled with water 4 and half filled with oil 5. Salad oil was used as the oil 5. The reason for using a dumbbell-shaped container 30 as the phantom 3 is that the irregular shape of the container makes it easy to physically create an inhomogeneous static magnetic field. Another advantage of the dumbbell shape is that it can be verified that the thick part 3b corresponds to the head, the narrow part 3a corresponds to the neck, and the thick part 3c corresponds to the shoulders.

[0017] As shown in Figure 3(3), a pad 1 is wrapped around the constricted portion 3a shown in Figure 3(1). When wrapping the pad 1, the pad is fully air-released before wrapping it around the constricted portion 3a. The thickness of the pad 1 when wrapped around the constricted portion 3a is approximately 8 mm. As shown in Figures 3(2) and 3(3), the disturbance of the static magnetic field was verified when nothing was wrapped around the phantom 3 and when the pad 1 was wrapped around the constricted portion 3a. The verification results will be described later with reference to Figures 5 to 9.

[0018] Next, we will explain how to install shimming. Here, shimming refers to adjustments made to an MRI device to improve the uniformity of the static magnetic field. There are passive shimming and active shimming, but in this specification, we refer to active shimming, which uses current to pass through shim coils or the like to create a gradient magnetic field and improve the uniformity of the magnetic field. Shimming is installed in areas where static magnetic field disturbances are likely to occur in order to improve local uniformity. FIG. 4 is an explanatory diagram of how to set up shimming, where (1) shows shimming set up in the sagittal direction, and (2) to (5) show shimming set up in the coronal direction. In Figure 4(1), the shimming 4a is placed in a sagittal orientation and includes the constricted portion 3a shown in Figure 3(1). In contrast, in Figures 4(2) to 4(5), the shimming is placed in a coronal orientation. Specifically, in Figure 4(2), the shimming 4b is placed in the oil 5 portion. In Figure 4(3), the shimming 4c is placed in the water 4 portion. In Figure 4(4), the shimming 4d is placed in a manner that includes the constricted portion 3a, and includes both the water 4 and the oil 5. In Figure 4(5), the shimming 4e is placed in a manner that includes the entire phantom, and includes both the water 4 and the oil 5. In this way, tests were conducted by changing the placement method of the shimming (4a to 4e), and the disturbances in the static magnetic field were compared. The following verification was carried out using a low-magnetic field MRI device.

[0019] The case where shimming 4a is installed as shown in Figure 4(1) will be described. Figure 5 compares the disturbance of the static magnetic field when shimming is installed sagitally, with (1) no pad, (2) a comparative pad, and (3) an example pad. The comparative pad is a commercially available IC pad made of manganese-based material. When shimming 4a is installed, when no pad is wrapped around the phantom 3, signal loss is observed at regions (P1, P2) as shown in Figure 5(1). When the comparative pad is wrapped around the phantom 3, signal loss is observed at regions (P3, P4) as shown in Figure 5(2). However, when the example pad 1 is wrapped around the phantom 3, signal loss over a wide area like regions (P3, P4) is not observed, and signal loss is also improved at regions (P1, P2) as shown in Figure 5(3). Furthermore, when the pad of the comparative example was wrapped around the phantom 3, uneven fat suppression was observed at the region P5 as shown in Figure 5(2), but when the pad 1 of the embodiment was wrapped around the phantom 3, it was found that uneven fat suppression was improved at the region P5 as shown in Figure 5(3).

[0020] The case where shimming 4b is installed as shown in Figure 4(2) will be described. Figure 6 is a comparison diagram of the disturbance of the static magnetic field when shimming is installed in the coronal and oily areas, where (1) shows no pad, (2) shows the pad of the comparative example, and (3) shows the pad of the example. When shimming 4b is installed, signal loss is observed at the regions (P6, P7) as shown in Figure 6(1) or (2) when no pad is wrapped around the phantom 3 or when the pad of the comparative example is wrapped around it. However, when pad 1 is wrapped around the phantom 3, it was found that the signal loss at the regions (P6, P7) is improved as shown in Figure 6(3).

[0021] The case where the shimming 4c shown in Figure 4(3) is installed will be described. Figure 7 is a comparison diagram of the disturbance of the static magnetic field when the shimming is installed in the coronal and water portions, where (1) shows no pad, (2) shows the pad of the comparative example, and (3) shows the pad of the embodiment. When the shimming 4c is installed and the pad is not wrapped around the phantom 3 or the pad of the comparative example is wrapped around it, uneven fat suppression was observed at the region P9 as shown in Figure 7(1) or (2). However, when the pad 1 is wrapped around the phantom 3, it was found that the uneven fat suppression at the region P9 is improved as shown in Figure 7(3).

[0022] The case where the shimming 4d shown in Figure 4(4) is installed will be explained. Figure 8 is a comparison diagram of the disturbance of the static magnetic field when the shimming is installed in the coronal and oil and water parts, (1) shows no pad, and (2) shows the case where the pad of the embodiment is installed. When the shimming 4d is installed and the pad 1 is not wrapped around the phantom 3, as shown in Figure 8(1), the disturbance of the static magnetic field is 10 ,P 11 ) uneven fat suppression was observed, and 12 ~P 15 ) signal loss was observed, but when pad 1 was wrapped around phantom 3, the signal loss was observed at the area (P 10 ,P 11 ) and uneven fat suppression is improved in the area (P 12 ~P 15 ) and found that signal loss improved.

[0023] The case where the shimming 4e shown in Figure 4(5) is installed will be explained. Figure 9 is a comparison diagram of the disturbance of the static magnetic field when the shimming is installed in the coronal and oil and water parts, (1) shows without a pad, and (2) shows with a pad of the embodiment. When the shimming 4e is installed and the pad 1 is not wrapped around the phantom 3, as shown in Figure 9(1), 16 ,P 17 ) uneven fat suppression was observed, and 18 ,P 19) signal loss was observed, but when pad 1 was wrapped around phantom 3, the signal loss was observed at the area (P 16 ,P 17 ) and uneven fat suppression is improved in the area (P 18 ,P 19 ) and found that signal loss improved.

[0024] The above verification showed that when nothing was wrapped around the device, signal loss and uneven fat suppression occurred depending on the location and range of shimming. The occurrence rate also varied depending on the shimming position. When the comparative pad was wrapped around the device, there was often no improvement or a tendency for the results to worsen. However, when Pad 1 filled with alum (potassium aluminum sulfate) was wrapped around the device, there was a clear improvement compared to when no pad was wrapped around the device or when the comparative pad was wrapped around the device. Furthermore, when Pad 1 was wrapped around the device, the occurrence rate depending on the shimming position was also comparable, with consistent results regardless of the shimming position. These results confirm that using alum as a filler in the local magnetic field inhomogeneity correction pad can improve uneven fat suppression and signal loss. It is also inferred that the dependency of shimming on the location can be improved.

[0025] Figure 10 shows an image of the pad of Example 1 in use, where (1) shows the pad not in use and (2) shows the pad in use. As shown in Figure 10(1), when the pad 1 is not in use, the magnetic field 10 is disturbed due to the body shape of the subject 9, but by using the pad 1, the disturbance in the magnetic field 10 can be improved as shown in Figure 10(2).

[0026] (Other Examples) The pad of Example 1 is filled with burnt alum powder, but in addition to the burnt alum powder, solid or hollow foamed resin particles with an average diameter of about 1 mm may be filled, in which the volume fraction of the burnt alum powder is 70% or more. [Industrial Applicability]

[0027] The present invention is useful as a pad for correcting local magnetic field inhomogeneity in examinations using MRI apparatuses with low magnetic fields of less than 1.5 Tesla. [Explanation of symbols]

[0028] 1 pad 1a Recess 3. Phantom 3a Waist 3b,3c Thick part 3d lid part 4a~4e Shimming 9. Subjects 10 magnetic field 11 Bag body 12 Burnt alum 13 On-off valve 30 Dumbbell-shaped container P1~P 19 part

Claims

1. A pad for correcting local magnetic field inhomogeneity, with a bag made of gas barrier material filled with burnt alum (potassium aluminum sulfate) powder.

2. A local magnetic field inhomogeneity correcting pad, the inside of which is filled with powder of burnt alum (potassium aluminum sulfate) and solid or hollow foamed resin granules of 0.3 to 2 mm, the volume ratio of which is 70% or more.

3. 3. The local magnetic field inhomogeneity correcting pad according to claim 1, wherein an opening / closing valve for circulating air between the inside and outside of the bag is attached to the outer cover of the bag.

4. A local magnetic field inhomogeneity correction pad as described in claim 3, wherein the opening / closing valve is a check air valve, and the bag body is deformed by an external load, thereby discharging air inside the bag body to the outside of the bag body, thereby enabling the shape of the bag body to be maintained.

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