Magnetic field generator and its manufacturing method

Abstract

There is provided a magnetic field generator (10) and a method of manufacturing it, capable of reducing decline in the magnetic field intensity and deterioration of magnetic field uniformity during transportation. The magnetic field generator (10) includes a pair of plate yokes (12a), (12b). The plate yokes (12a), (12b) are opposed to each other, with their opposing faces provided with permanent magnet assemblies (14a), (14b) respectively. The permanent magnet assemblies (14a), (14b) have faces opposing to each other, to which pole pieces (16a), (16b) are fixed respectively. The permanent magnet assemblies (14a), (14b) include permanent magnets (18). The pole pieces (16a), (16b) include annular projection (24) and permanent magnets (28) for prevention of magnetic flux leakage provided on an outer side surface of the annular projection (24). The permanent magnets (18), (28) include R—Fe—B magnets magnetized to a magnetization rate not smaller than 80% and not greater than 99.9%. When manufactured, the assembled magnetic field generator (10) is heated in its entirety at a temperature not lower than 40° C. and not higher than 70° C.

Description

TECHNICAL FIELD [0001] The present invention relates to a magnetic field generator and a manufacturing method thereof. More specifically, the present invention relates to a magnetic field generator for MRI and a manufacturing method thereof. BACKGROUND ART [0002] Conventionally, a magnetic field generator for MRI receives complete adjustment of magnetic field intensity and magnetic field uniformity before shipment, and then transported to destination in a container for example. [0003] However, despite such deliberate procedures, it sometimes happens that upon arrival, the magnetic field intensity has decreased, or the magnetic field uniformity has deteriorated down below an acceptable limit, and therefore readjustment must be made to the magnetic field generator. [0004] This problem is particularly conspicuous in a magnetic field generator which makes use of magnetic flux leakage prevention magnets in order to reduce magnetic flux leakage and to concentrate magnetic flux from the ma...

AI Technical Summary

Benefits of technology

[0008] According to the present invention, the R—Fe—B magnet (R represents a rare-earth element including Yttrium (Y), Fe represents iron and B represents boron) is controlled to have a final magnetization rate of not smaller than 80% and not greater than 99.9%, whereby it becomes possible to reduce demagnetization caused by time or environmental factors such as temperature increase, of a magnetic field generator which has been assembled without altering its intrinsic coercive force. Therefore, it becomes possible to reduce decline in the magnetic field intensity and deterioration of magnetic field uniformity during transportation of the magnetic field generator, making sure that the magnetic field generator has a high magnetic field uniformity upon arrival at a place of installation.

[0024] In this invention, a magnet is magnetized to a magnetization rate exceeding 99.9% and then demagnetized to a lower magnetization rate. This assures that a magnetic field generated by a magnetic field generator using this magnet is stable over a long time, making possible to reduce decline in the magnetic field intensity and deterioration of magnetic field uniformity during transportation of the magnetic field generator. Further, operating efficiency is high because demagnetizing process is made when the magnet is still in individual state of magnet.

Problems solved by technology

It should be noted here that if the final magnetization rate of the R—Fe—B magnet is less than 80%, it becomes impossible to take full advantage of magnetic characteristics of the magnet, which leads to use of an increased amount of magnet and decreased efficiency.

On the other hand, if the magnet is used at a final magnetization rate exceeding 99.9%, temperature changes and so on during transportation cause a large demagnetization.

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