This invention relates to a Magnetic Resonance Imaging scanner and in particular to a housing for such a scanner which minimizes heating of helium held within the housing.
Magnetic Resonance Imaging (MRI) scanners typically utilize 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 minimize 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 form the helium vessel, a number of layers of aluminized Mylar (registered trade mark—®) polyester sheets and insulation mesh, and then the outer vessel. This structure is evacuated during manufacture to minimize transfer from the outer vessel by convection.
To create the vacuum in the housing, it is necessary to provide a port for connection to a vacuum pump. The pumping down to the state required can take a few days due to the need for migration of the molecules through the port once low pressure are achieved. To assist in this in this process, it is desirable to provide a large port to increase the chances of trapped air molecules chancing upon the exit and a passageway through the Mylar® foil insulation and also the radiation shield.
Unfortunately, in prior art arrangements, once the vacuum is created and the port is closed by a cap there is a path for radiation from the cap which is relatively hot through the hole in the Mylar® aluminized polyester sheets and the heat shield to the helium vessel itself. This leads to undesirable heating of the helium vessel which leads to expensive helium loss.
According to the invention there is provided a housing for a superconducting magnet, which housing comprises an outer vacuum vessel housing a coolant vessel for, in use, holding a volume of coolant for cooling a superconducting magnet, a radiation shield for shielding the coolant vessel from radiated heat, a hole through the radiation shield adjacent a pump-out port and a closure member located over the hole and its periphery but adapted to be spaced apart therefrom during manufacture to permit passage of molecules therebetween during pumping out of the outer vacuum vessel, wherein the closure member comprises, at least in part, a ferrous material such that after the pumping out operation, energizing an associated superconducting magnet draws the closure member inwards to abut the periphery of the hole to close the hole to prevent radiation of heat therethrough.
Preferably, the closure member is spaced apart from the hole by a means which permits the relative inwards movement of the closure member but prevents outwards movement. The means may be a web of flexible material provided at locations about the periphery but in the preferred embodiment is a clip.
After energizing the magnet, the closure member abuts the periphery and it may be retained thereto by an adhesive. In the preferred embodiment it is retained by a clip. In this particular case the clip is the same that provided the spaced apart relationship. In its preferred form the clip is resiliently deformable and comprises a first location bounded by walls formed in the clip and a second location also formed by walls formed in the clip. The closure member is retained at the first location until the magnets are energized; this causes an inward attractive force which is sufficient to deflect the walls of the clip to allow passage of the clip into the second location. By virtue of the resilience of the clip, the walls revert back to their original shape to retain the panel at the second location.
The closure member may be a homogenous ferrous material, or may have a discrete ferrous part or parts. This latter option will be advantageous in order that the part which abuts the radiation shield may be formed from a compatible material to avoid material mismatch problems for example differential corrosion (the radiation shield is often made of high grade aluminum).
Preferably, the closure member is formed to have a reflective surface to minimize thermal radiation.
A specific embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
As is shown in
The housing needs to be evacuated to prevent heat transfer and is provided with a cylindrical pump out port 7. The outer end is provided with a flange to permit the attachment of a pump (not shown). To improve the pump down process a circular hole 8 of diameter a is formed in the radiation shield 4.
A closure member 9 is provided which is in the form of a disc having a diameter b which is greater than a such that the closure member 9 overlaps the periphery of the hole 8. Four clips 10 are provided (three of which are shown) engaging the periphery and the closure member 9 to hold the closure member in position in a spaced apart and centered relationship to the hole 8. This provides a generally cylindrical clearance gap 11 which facilitates air removal during pump down as shown by the flow path indicated by arrow 14.
The clips 10 and the closure member 9 are shown in greater detail in
An important feature of the clip 10 is the shallow curving nature of the retaining wall 105 which assists in the smooth inward movement of the closure member 9 and the sharper profile after the peak of the curve to provide a secure retention in the closed position.
It will be appreciated that the precise profile of the clip may be varied and materials other than plastics may be used such as a metal. While the invention has been described with particular reference to MRI scanners, it will be appreciated that the invention may be applied to housings for any superconducting magnets. Similarly, while the description makes particular reference to helium coolant, the invention is applicable to magnets cooled by any suitable cryogen, such as nitrogen, hydrogen, neon and so on.
Number | Date | Country | Kind |
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0801116.5 | Jan 2008 | GB | national |