CN106782998A - Open self-shileding magnetic resonance image-forming superconducting magnet - Google Patents

Abstract

An open self-shielding magnetic resonance imaging superconducting magnet, consisting of a pair of planar fingerprint shim coils (1), a pair of first main magnetic field coils (2), a pair of second main magnetic field coils (3) and a pair of The shielding coil (4) is composed of four pairs of superconducting coils arranged up and down along the z coordinate, symmetrical about the center. The planar fingerprint shimming coil group (1) includes 15 groups of active shimming coils, which are layered and symmetrically arranged at the closest point to the center point, and the first main magnetic field coil (2) and the second main magnetic field coil (3 ), the shielding coil (4) is arranged in the outermost layer. The first main magnetic field coil (2) and the second main magnetic field coil (3) pass forward current to provide main magnetic field strength. The shielding coil (4) passes a reverse current to generate a magnetic field opposite to the main magnetic field, so as to eliminate the stray magnetic field of the magnet to the external space. The planar fingerprint shimming coil group (1) is used for compensating the magnetic field in the central area, so as to improve the uniformity of the magnetic field of the magnet in the spherical area.

Description

Open self-shielding MRI superconducting magnet

technical field

The invention relates to a superconducting magnet used in the field of magnetic resonance whole-body imaging, in particular to an open self-shielding superconducting magnet.

Background technique

Magnetic field uniformity is one of the most important indicators in a magnetic resonance imaging (MRI) system, and its size reflects the performance of the magnet in the MRI system. It is generally required that the peak-to-peak non-uniformity of the magnetic field in a 40cm spherical imaging space that can accommodate the width of a human body should be less than 10ppm (one millionth). By designing magnets with high uniformity and using shimming technology to achieve high uniformity magnetic field, such as iron sheet shimming, room temperature coil shimming, low temperature coil shimming, etc.

General MRI magnets are pipe-shaped, which can provide high magnetic field uniformity. But the tube-shaped magnets create a sense of confinement for patients, and some patients can become claustrophobic as a result. Although some short-cavity magnet structures have been produced by shortening the length of the pipeline in recent years, they still cannot meet the requirements of openness. In addition, the tube-forming imaging magnet can only be used for medical imaging, and cannot perform real-time medical diagnosis and treatment, that is, interventional treatment. From the perspective of the development of interventional therapy and medical diagnostic technology, a more open and compact magnet is needed to meet the needs of medical interventional therapy. Interventional treatment generally needs to meet the minimum vertical gap greater than 0.5 meters, so that patients can enter the magnetic field, and the maximum vertical gap is greater than 0.75 meters, so that some medical detection devices can be placed. The largest diameter generally needs to be less than 1.5 meters.

Therefore, it is necessary to develop a compact superconducting magnet for whole-body imaging with a large open space, which can provide patients with a wider field of view on the one hand, and provide doctors with real-time treatment operating space on the other hand.

The current fully open MRI products are mainly permanent magnets that provide a central magnetic field below 1T, or superconducting ferromagnetic magnets that provide the main magnetic field with superconducting magnets and ferromagnetic magnets that provide magnetic shielding and shimming. The open whole-body imaging magnet usually adopts a "C"-shaped cantilever or "Π"-shaped vertical structure, and its outer diameter is generally greater than 2 meters. WO/2007/094844 provides an open magnetic resonance imaging permanent magnet structure with a central magnetic field up to 1T. WO/1998/007362 provides a magnetic resonance imaging permanent magnet with double-sided structure. Chinese patent CN302850947S provides a two-column open Π-type superconducting ferromagnetic magnetic resonance magnet. The main problem of open permanent magnet imaging magnets and open superconducting ferromagnetic imaging magnets is that there is a large space for stray fields generated by magnetic flux leakage. In order to avoid damage to the surrounding electrical equipment, iron choke mechanisms with large weight and volume are required. Shrinking the stray field space leads to difficulties in hospital installation and operation by doctors; and for permanent magnets, the magnetic field strength generated by them is also low, generally lower than 0.7T, and the imaging resolution is low.

A few companies and research institutes have also developed superconducting magnets for open MRI. For the open structure, the spatial distribution of the superconducting coils of the full superconducting magnet is limited and can only be separated, that is, it is composed of multiple pairs of superconducting coils distributed symmetrically about the uniform area. This structure is often difficult after the magnet is processed and installed. To obtain a good magnetic field uniformity, in order to obtain a high uniformity magnetic field for imaging, more shimming devices are required, occupying more space, and more time is required for shimming operations. Such as Hitachi and Philips related products. The Chinese patent CN02824552 of Philips only adopts an open magnet structure of a pair of superconducting coils, which requires subsequent use of many ferromagnetic shims to correct the inhomogeneity of the magnetic field. Chinese patent CN102360690A adopts 5 pairs of superconducting coils, and the uniformity of the magnetic field is high, but the internal structure is complicated, which is not conducive to manufacturing, and the outer diameter is greater than 2 meters, which is not conducive to the doctor's interventional surgery.

Another problem with open magnetic resonance imaging magnets using fully superconducting coils is that magnetic shielding increases the cost and technical difficulty of magnet manufacturing. For the open superconducting magnet structure with ferromagnetic shielding, the magnet system is too bulky after adding ferromagnetic shielding. For an open superconducting magnet that uses active shielding, the ratio of the highest field to the central field is generally larger. In order to avoid high magnetic fields in the superconducting coil, the superconducting magnet is generally designed with a larger outer diameter and a smaller gap between the upper and lower poles. This in turn increases the volume of the magnet and deteriorates its openness, which is not conducive to interventional treatments such as surgical operations by doctors. Therefore, it is necessary to invent a new magnet structure to overcome this problem.

Contents of the invention

The purpose of the present invention is to overcome the problems of large volume, insufficient openness and low uniformity of the magnetic field of the existing magnetic resonance superconducting magnet system, and propose an open self-shielding superconducting magnet. The open superconducting magnet structure of the present invention can obtain larger open space, smaller magnet size, higher magnetic field uniformity and weaker stray field, and is suitable for medical diagnosis, especially for interventional treatment.

The superconducting magnet of the present invention is composed of four pairs of superconducting coils, and the four pairs of superconducting coils are arranged up and down along the z coordinate and are symmetrical about the center. The four pairs of superconducting coils include a pair of planar fingerprint shim coils, a pair of first main magnetic field coils, a pair of second main magnetic field coils and a pair of shielding coils. The planar fingerprint shim coil group is arranged closest to the center point, the first main magnetic field coil and the second main magnetic field coil are arranged outward in sequence, and the shielding coil is arranged on the outermost layer. Four pairs of superconducting coils are placed in the same cryogenic Dewar.

The first main magnetic field coil and the second main magnetic field coil pass forward current to provide main magnetic field strength. The shielding coil passes a reverse current to generate a magnetic field opposite to the main magnetic field to offset the stray magnetic field of the magnet to the external space, making the 5G envelope of the magnet smaller. The planar fingerprint shim coil group includes 15 sets of active shim coils including axial first-order to third-order coils, radial first-order coils, radial second-order coils and radial third-order coils. The 15 sets of active shim coils adopt Layers of stacked forms are placed in symmetrical spatial positions. According to the spatial geometric position, size and magnetic field strength of the central area of the first main magnetic field coil, the second main magnetic field coil and the shielding coil, determine the shimming strength and superconducting wire specification of each order shim coil of the planar fingerprint shimming coil group And the size of the current, the magnetic field in the central area is compensated, and the uniformity of the magnetic field of the magnet in the spherical area is improved.

The magnet of the present invention can be realized by using a low-temperature or high-temperature superconducting wire, and has the following performance characteristics:

(1) In a 360mm imaging space, the unevenness can be less than 1ppm, which can meet the whole body imaging requirements.

(3) When the central magnetic field is 1.5T, the stray field of 5 Gauss is distributed in the ellipsoidal range of less than 5 meters in the radial direction and about 4.8 meters in the axial direction, so it has good electromagnetic compatibility.

(4) The entire magnet has a compact structure, the maximum coil diameter is less than 1.4m, and the minimum distance between the two poles of the coil is greater than 0.5m, which is very beneficial for doctors to perform surgical interventions, and the magnet does not require high site space. The maximum distance between the two poles of the coil is greater than 0.75 meters, and the diameter of the area greater than 0.75 meters is greater than 1.1 meters, which is not limited by the shape of obese patients and can also be used to place other small medical devices.

(5) Due to the use of a planar fingerprint shimming coil group, and the coil group is wound with a superconducting wire, it can carry a large current greater than 100A, so it has a strong shimming ability and can completely replace the subsequent iron shim The field operation reduces the space used by the iron sheet shimming, and simplifies the cumbersome operation of the iron sheet shimming.

Description of drawings

Fig. 1 Electromagnetic structure diagram of open compact high-uniformity superconducting coil, 1 planar fingerprint shim coil group, 2 first main magnetic field coil, 3 second main magnetic field coil, 4 shielding coil;

Fig. 2 is a schematic diagram of an active shim coil, wherein Fig. 2a is an axial coil, and Fig. 2b is a radial coil;

Fig. 3 is the engineering sectional view of magnet of the present invention;

Figure 4 shows the equipotential line distribution of the magnetic field uniformity in the imaging space with a diameter of 360mm;

Fig. 5 is a distribution diagram of 5 Gauss lines of the magnet system when the central magnetic field is 1.2T.

detailed description

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

Figure 1 shows the structure of the open magnetic resonance magnet of the present invention. As shown in FIG. 1 , the present invention consists of four pairs of superconducting coils symmetrical about the center, including a planar fingerprint shim coil group 1 , a first main magnetic field coil 2 , a second main magnetic field coil 3 and a shielding coil 4 . The planar fingerprint shim coil group 1 is arranged closest to the center point, and the first main magnetic field coil 2 and the second main magnetic field coil 3 are arranged outward in sequence, and the shielding coil 4 is located in the outermost layer. The first main magnetic field coil 2 and the second main magnetic field coil 3 jointly provide the main magnetic field, and the shielding coil 4 generates a magnetic field opposite to the main magnetic field to offset the stray magnetic field generated by the main magnet in the outer space, thereby obtaining the 5G envelope of the magnet The range is small. Planar fingerprint shim coil group 1 includes 15 sets of active shim coils, namely: axial first-order coil Z1, axial second-order coil Z2, axial third-order coil Z3, two sets of radial first-order coils X, Y , four sets of radial second-order coils ZX, ZY, X2-Y2, XY, six sets of radial third-order coils Z2X, Z2Y, Z(X2-Y2), ZXY, X3, Y3, and the coils of each order are stacked in layers Forms are placed in symmetrical spatial positions. Fig. 2 is a schematic diagram of an active shim coil, wherein Fig. 2a is an axial coil, and Fig. 2b is a radial coil. According to the measured magnetic field distribution, currents of different magnitudes and directions are passed to each group of fingerprint shim coils to compensate the magnetic field in the central area to improve the uniformity of the magnetic field of the magnet in the spherical area.

Fig. 3 is the engineering sectional view of magnet of the present invention, and the cylindrical space between the first main magnetic field coil 2 is larger, and the vertical length of its cylindrical space is 860 millimeters, and diameter is 1120 millimeters, and the cylindrical space between the second main magnetic field coil 3 The space is the smallest, the vertical length of the cylindrical space is 560 mm, and the diameter is 1280 mm; among all the coils, the diameter of the second main magnetic field coil 3 is the largest, only 1280 mm, so the superconducting magnet has a large space openness and compactness The coil structure fully meets the requirements of interventional therapy.

Fig. 4 is the calculation result of the uniformity of the magnetic field with an imaging space diameter of 360mm. The non-uniformity at the edge of the region is about 1.5ppm, indicating that the magnet of the present invention can provide a reasonable uniform magnetic field for use in medical nuclear magnetic resonance imaging.

Fig. 5 is the distribution characteristic of the 5 Gauss line of the magnet of the present invention. When the central magnetic field is 1.2T, the 5 Gauss stray field is distributed within the scope of the ellipsoid domain less than 3.1 meters in the radial direction and less than 5 meters in the axial direction.

When the magnet is used at less than 1.2T, the magnet of the present invention can be wound with NbTi superconducting wire, realized by using NbTi phase-embedded conductor (WIC), and the total consumption of superconducting wire is less than 300kg.

Claims (4)

1. An open type self-shielding magnetic resonance imaging superconducting magnet, is characterized in that, described superconducting magnet is made up of four pairs of superconducting coils, and four pairs of superconducting coils are arranged up and down along the z coordinate, symmetrical about the center; The four pairs of coils include a pair of planar fingerprint shimming coils (1), a pair of first main magnetic field coils (2), a pair of second main magnetic field coils (3) and a pair of shielding coils (4); The field coil group (1) is arranged closest to the central point, and the first main magnetic field coil (2) and the second main magnetic field coil (3) are arranged outward in sequence, and the shielding coil (4) is arranged on the outermost layer; Magnetic field coil one (2), the second main magnetic field coil (3) pass forward current, provide main magnetic field intensity; Shielding coil (4) pass reverse current, produce the magnetic field opposite with main magnetic field, to eliminate the magnet to external space Stray magnetic field; planar fingerprint shimming coil group (1) compensates the magnetic field in the central area to improve the magnetic field uniformity of the magnet in the spherical area; four pairs of superconducting coils are placed in the same low-temperature Dewar. 2. according to the described open type self-shielding magnetic resonance imaging superconducting magnet of claim 1, it is characterized in that, described planar fingerprint shim coil group (1) comprises 15 groups of active shim coils, respectively: axial First-order coil Z1, axial second-order coil Z2, axial third-order coil Z3, two sets of radial first-order coils X, Y, four sets of radial second-order coils ZX, ZY, X2-Y2, XY, six sets of diameter To the third-order coils Z2X, Z2Y, Z(X2-Y2), ZXY, X3, Y3; all the coils of each order of the planar fingerprint shim coil group (1) are placed in symmetrical spatial positions in the form of layer upon layer . 3. according to the described open type self-shielding magnetic resonance imaging superconducting magnet of claim 1, it is characterized in that, according to described first main magnetic field coil (2), the second main magnetic field coil (3) and shielding coil (4 ), determine the shimming strength, superconducting wire specification and current size of the shim coils of each order in the planar fingerprint shim coil group (1). 4. The open self-shielding magnetic resonance imaging superconducting magnet according to claim 1, wherein the outermost diameter of the open self-shielding magnetic resonance imaging superconducting magnet is less than 1.4 meters, and the minimum distance between the upper and lower poles is greater than 0.5 meters, the maximum spacing is greater than 0.75 meters, and the area with the maximum spacing is greater than 1.1 meters in diameter.