GB2438714A

GB2438714A - Shim Tray

Info

Publication number: GB2438714A
Application number: GB0709630A
Authority: GB
Inventors: Richard Gowland; Edgar Charles Malcolm Rayner
Original assignee: Siemens Magnet Technology Ltd
Current assignee: Siemens Magnet Technology Ltd
Priority date: 2006-05-22
Filing date: 2007-05-21
Publication date: 2007-12-05
Anticipated expiration: 2027-05-21
Also published as: CN101078754A; GB2438714B; GB2438429A; GB0709630D0; GB0610024D0; US20070279169A1

Abstract

A shim tray for superconducting magnets is described, being of a one-piece injection moulded construction which facilitates the shimming operation without removal of the lid and which avoids the use of metal components which might interfere with the magnetic field generated. The tray has a lid, and a hinge made by folding or creasing a polymeric material. There is also a resilient member comprised of a spring tongue to retain the shims in each pocket of the tray.

Description

SHIM TRAY

The invention is concerned with shim trays of the type included in superconducting magnet assemblies, which facilitate the shimming operation carried out to correct for inhomogeneities Medical imaging systems which require a strong magnetic field, for example Magnetic Resonance Imaging (MRI) systems, typically employ a superconducting magnet for this purpose. The magnetic field so generated typically has non-uniformities arising from both manufacturing tolerances and the environment in which the magnet is sited.

It is known to correct for these non-uniformities using shims. Although active shims (in the form of resistive coils) are sometimes used, passive shims comprising ferromagnetic sheet material up to about 1mm thick are more common. The passive shims are placed in shim trays or pockets that are incorporated in the system design. Existing designs of shim tray include a separate lid and require packers to retain the shims without movement within the container Hence, each time a tray is filled or has some of the contents removed, a packer must be selected before the lid is replaced.

The present invention offers an improvement to this design such that, in the most preferred embodiment, neither a lid nor packer needs to be selected. A retaining means that is integral with the lid is dispenses with the need to select any components Thus, the number of components needed during a magnet shimming operation is reduced and the process is made easier.

According to the invention, a shim tray comprises the features set out in claim 1 attached hereto.

Preferrably the hinge comprises a fold or crease in polymeric material joining the body and lid of the container. The lid and body may be integrated in a single article which conveniently lends itself to manufacture by injection moulding techniques.

The resilient member may comprise a spring tongue that can be cast as an integral part of the container, fer example on the inside of the lid.

A further preferred embodiment includes an opening in the container, being arranged to facilitate insertion of a shim into the container for retention by the tongue, without removal of the lid The invention will now be described by non-limiting example, with reference to the following figures in which figure 1 shows a shim tray according to the prior art and figure 2 shows a preferred embodiment of the invention.

Referring to figure 1, a shim tray 1 of the prior art includes a container body 2, a lid 3 and a suitably dimensioned packer 4. In use, one or more shims 5 are placed in the body 2 before the lid 3 is replaced with the packer 4 located between the lid 3 and the shims 5 thus serving to retain the latter without allowing movement within the container 1 During the shimming operation, the lid must be removed, the shims added, a suitable spacer selected and the lid replaced.

Referring to figure 2 in a shim tray according to the present invention, the lid 3 is connected to the body 2 by a hinge 8 and is held in the closed position by a snap fit catch 9. Many hinge designs form part of the common general knowledge but, in a preferred embodiment a live' or living' hinge is employed which consists of a fold or crease in polymeric material. This arrange readily lends itself to manufacture of the lid and body as a single integrated unit by techniques such as injection moulding. The use of metal components, which could affect the magnetic field, is avoided.

A number of types of resilient member 7 is possible including a coil spring or an elastomeric block fastened to the lid (or body) by suitable means. In this preferred embodiment, a spring tongue serves as the resilient member 7 The spring tongue has the advantage that it readily lends itself to manufacture with the lid 3 (or the body) as a single integrated unit by, for example, injection moulding technology Morever, inclusion of an opening (slot 10) in the wall of the container means that a shim can be inserted through the slot 10 to be engaged by the tongue and retained, without removing the lid.

A typical shim tray according to the present invention might comprise a plurality of pockets in the body, wherein each pocket is arranged to receive one or more shims. Accordingly, a single hinged lid may be included which covers all pockets or a plurality of lids may be included where each lid serves one or more pockets Thus the most preferred embodiment affords the advantages of: a one-piece item where the lid remains connected with the body when opened and the need for selection of components is dispensed with, ease of manufacture by injection moulding techniques; formation completely in inert material that has minimal effect on the magnetic field and shims may be inserted without removing the lid.

Claims

Claims 1. A shim tray comprising a body and a lid, characterised in

that the body and lid are connected by a hinge.

2. The container of claim 1, where the hinge comprises a fold or crease in polymeric material 3. The container of claim 2, where the body, lid and hinge are cast as a single item in polymeric material 4. The container of claim 3, where further including a resilient member arranged to engage one or more shims placed in the tray, when the lid is closed The container of claim 4, where the resilient member comprises a spring tongue, incorporated in the lid.

6 The container of claim 5, where the lid is retained in the closed position by means of a snap-fit catch.

7 The container of claim 6, and further including an opening, arranged to facilitate the passing or shims therethrough to engage with the spring tongue and by retained thereby.

8. A shim tray according to claim 1, comprising a plurality of pockets and a plurality of lids, each lid being arranged to serve one or more pockets.

9. A container as described in the accompanying text with reference to figure
2.