GB2454475A

GB2454475A - A Heat Shield for an MRI scanner

Info

Publication number: GB2454475A
Application number: GB0721799A
Authority: GB
Inventors: Trevor Bryan Husband; Stephen Paul Trowell; Philip Alan Charles Walton
Original assignee: Siemens Magnet Technology Ltd
Current assignee: Siemens Magnet Technology Ltd
Priority date: 2007-11-07
Filing date: 2007-11-07
Publication date: 2009-05-13
Also published as: GB0721799D0

Abstract

A radiation shield for a MRI scanner housing is formed of a plurality of thermally conductive tracks interspaced by electrically insulating material (e.g a plastics material) which are formed to provide heat conduction paths to a cooling area of the shield. The amount of conductive material used, for example copper, is reduced and the formation of eddy currents prevented or at least reduced.

Description

A Radiation Shield And A MaQnetic Resonance lmaciing Scanner This invention relates to a Magnetic Resonance Imaging scanner and in particular to a shield for such a scanner which minimises heating of helium held within a housing of the scanner.

Magnetic Resonance Imaging (MRI) scanners utilise large superconducting magnets which require cooling to liquid helium temperatures for successful operation. A containment structure is provided to enclose the magnets and to hold a large volume of the liquid helium to provide the cooling. Liquid helium is very expensive and thus the structure is designed to minimise its loss through heating from the environment where the scanner is located. A multilayer structure is provided which is designed to prevent heat passing into the helium by conduction, convection and radiation.

The structure comprises a helium vessel which is innermost, a radiation shield spaced apart from the helium vessel, a number of layers of aluminised Mylar foil and insulation mesh, and then the outer vessel. This structure is evacuated during manufacture to minimise heat transfer from the outer vessel by convection.

The radiation shield is formed of a high grade aluminium to provide a highly reflective surface to minimise radiation of heat into the inner helium vessel. A problem with such shields is that they permit the generation of eddy currents which oppose the field produced by the scanner magnets leading to in-efficiencies and, in particular, may make the interpretation of the scan more difficult particularly if the eddy currents are not evenly distributed.

The present invention arose from the realisation that the material used in the heat shield could be tailored to provide the required heat conduction properties whilst minimising the generation of eddy currents.

According to the invention there is provided a heat shield for a MRI scanner comprising a plurality of conductive elements interspaced by an electrically insulating material which conductive elements aligned to provide heat conduction paths to at least one cooling area, in use, coupled to a cooling source.

By providing electrically insulating material interspacing the conductive elements, eddy currents are prevented from flowing across the conductive elements. The conductive elements still assist in the cooling of the radiation shield by being aligned with the desired heat conduction paths to the cooling area. Preferably, the conductive elements are metal. In the described embodiment copper is used embedded in a plastics material. There is also less conductive material used in the shield which further reduces the formation of eddy currents, weight and cost.

In the described embodiment, there is one cooling area but there could be two or more cooling areas.

The material may be made by an injection moulding process and then the moulded material taken and formed into the shield. Alternatively, it will be advantageous to form the shield from the material by direct injection moulding.

A specific embodiment of the invention will now be described, by way of example only, with reference to the drawing in which: Figure 1 shows a radiation shield for a scanner in accordance with the invention; Figure 2 shows an enlarged view of part of the surface of the radiation shield shown in figure 1.

As is shown in figure 1, a radiation shield 1 of a Magnetic Resonance Imaging scanner is formed as a generally cylindrical annular structure with two annular end faces 2, 3 one of which is visible, an inner cylinder 4 and an outer cylinder 5.

Within the shield 1 is located a helium vessel containing helium to cool a set of superconducting magnets (not shown). About the shield 1 is located a number of insulation layers of reflective Mylar material and insulating mesh 6 and an outer containment vessel 7 also of generally cylindrical configuration. The radiation shield I is contained within a vacuum in the outer containment vessel 7.

A refrigeration unit (not shown) supplies coolant to a cooling head 8 in good thermal contact to a cooling area on the shield 1. This maintains the shield I at a temperature, when in use, of about 52 Kelvin.

There will exist a temperature gradient from the cooling area to the lower part of the shield 1 and hence the heat will be conducted in the direction of the arrows flowing circumferentially from the lower part of the shield to the cooling area at the top of the shield. The invention arose from a realisation by the inventors that it is desirable to provide material aligned with the flow paths to provide the conduction but that it is not necessary to promote flow paths in direction at an angle to them. Thus, the shield 1 is formed from a material of copper tracks embedded in electrically insulating material which maximise conduction material to provide the circumferential flow paths. The electrically insulating material reduces the amount of conductive material at an angle to the flow paths and prevents the creation of eddy currents.

The material is shown in greater detail in figure 2 which is an enlargement of area labelled II. Area II shows that the shield 1 is formed by an injection moulding technique in which a large number of copper tracks 20 embedded and separated by an insulating plastics material 21. The copper tracks 20 extend in the circumferential direction to encourage heat flow in the desired circumferential paths to the cooling area and cooling head 8. The insulating plastics material 21 prevents eddy currents flowing between the copper tracks 20 and also reduces the amount of copper matenal used which in turn reduces the eddy currents further. A low emissivity coating 22 is applied which is shown partially removed for the purposes of illustration. This provides a reflective surface to the heat shield to prevent heat absorption from the external environment. This may be sprayed on or applied as an adhesive tape or applied in other ways.

In this embodiment the end faces 2, 3 are formed from high grade aluminium but in alternative embodiments may be itself formed of conductive tracks interspaced by insulating material. These may be formed to provide conduction paths which flow generally upwards about the annulus as shown by the flow arrows depicted on the end face 2.

To assist efficient cooling at the cooling area, the copper tracks 20 may be linked in that region and braiding provided for coupling the tracks to the cooling head 8.

In the described embodiment, the conductive tracks are provided as one layer but it will be appreciated that they may be over-wound to provide multiple layers extending outwardly in a radial direction from an axis of the scanner.

Claims

Claims 1. A heat shield for a MRI scanner comprising a plurality of conductive elements interspaced by an electrically insulating material which conductive elements aligned to provide heat conduction paths to at least one cooling area, in use, coupled to a cooling source.
2. A heat shield as claimed in claim 1 wherein the conductive elements are formed of a metal.
3. A heat shield as claimed in claim 2 wherein the metal is copper.
4. A heat shield as claimed in any preceding claim wherein a low emissivity layer is applied over the conductive elements and electrically insulating material to prevent heat transfer to the shield.
5. A heat shield as claimed in any preceding claim wherein the conductive elements extend circumferentially about the heat shield.
6. A heat shield as claimed in any preceding claim wherein the heat shield has end faces formed with a plurality of conductive elements interspaced by an electrically insulating material which provide heat conduction paths at least in part to the cooling area.
7. A heat shield for an MRI scanner substantially as hereinbefore described with reference to the drawing.

7. A heat shield for an MRI scanner substantially as hereinbefore described with reference to the drawing.

Amendments to the Claims have been filed as follows Claims 1. A heat shield for a MRI scanner comprising a plurality of conductive elements embedded within, and interspaced by, an electrically insulating material which conductive elements are aligned to provide heat conduction paths to at least one cooling area, in use, coupled to a cooling source.

2. A heat shield as claimed in claim 1 wherein the conductive elements are formed of a metal.

3. A heat shield as claimed in claim 2 wherein the metal is copper.

4. A heat shield as claimed in any preceding claim wherein a low emissivity layer is applied over the conductive elements and electrically insulating material to Q 15 prevent heat transfer to the shield.

O 5. A heat shield as claimed in any preceding claim wherein the conductive C\J elements extend circumferentially about the heat shield.

6. A heat shield as claimed in any preceding claim wherein the heat shield has end faces formed with a plurality of conductive elements embedded within, and interspaced by, an electrically insulating material which conductive elements provide heat conduction paths at least in part to the cooling area.