GB2465556A

GB2465556A - Cryostat suspension arrangement including turret mount

Info

Publication number: GB2465556A
Application number: GB0821105A
Authority: GB
Inventors: Neil Charles Tigwell; Stephen Paul Trowell
Original assignee: Siemens Magnet Technology Ltd; Siemens PLC
Current assignee: Siemens PLC
Priority date: 2008-11-19
Filing date: 2008-11-19
Publication date: 2010-05-26
Anticipated expiration: 2028-11-19
Also published as: GB0821105D0; GB2465556B

Abstract

A cryostat 30 has a vacuum container 39, a thermal shield 38, a turret 35, superconducting magnet coils and a suspension component 31. The magnet coils are positioned within the vacuum container and the turret provides access to the magnet coils. The turret is mounted within the suspension component and the thermal shield and vacuum container are suspended from the suspension component. The suspension component may include an external support frame 32 which may be gantry mounted or ceiling mounted (Fig 4). The suspension component may also include a tubular composite structure of carbon reinforced plastics material or glass reinforced plastics material. Cold head components 41 and magnet ancillaries, such as siphon 55, current lead 49 and pressure relief valve 53 may also be combined within the suspension component. The cryostat may form part of a magnetic resonance imaging system (MRI), and the system may include an integrated patient table, a mounting pedestal and integrated covers. The cold head may be a two-stage refrigerator, and the cryostat may include a cryogen vessel 37 at least partially filled with a cryogen such as helium.

Description

CRYOSTAT

This invention relates to a cryostat, in particular for use with imaging systems.

Conventional magnetic resonance imaging (MRI) scanners or nuclear magnetic resonance systems (NMR) are mounted on support blocks on the floor and the patient is loaded in from one end on a table on a pedestal.

Some examples are described in GB2440350. As illustrated in Fig. 1, a number of coils of superconducting wire are wound onto a former 1. This assembly is housed inside a cryogen vessel 2, which is at least partially filled with a liquid cryogen 2a, for example helium, at its boiling point to keep the coils below their critical point. An outer vacuum container 4 and thermal shield 3 serve to thermally isolate the cryogen vessel 2 from the surrounding atmosphere. Layered insulation 5 may be placed inside the space between the outer vacuum container 4 and the thermal shield 3. A central bore 4a is provided, of a certain dimension to allow access for a patient or other subject to be imaged.

Conventionally, a number of supporting elements 7 are connected between the cryogen vessel 2 and the outer vacuum container (OVC) 4 to bear the weight of the cryogen vessel. These may be tensile bands, tensile rods, straps, compression struts or any known element suitable for the purpose. The elements should have a very low thermal conductivity to minimise heat influx from the outer vacuum container 4, which is typically at ambient temperature, to the cryogen vessel 2. Similar, or alternative, suspension arrangements may be provided to retain the thermal shield 3. Typically, a floor mounted system will require supporting feet 6 or similar, attached to the outer vacuum container to support the weight of the system as a whole. The suspension elements typically pass from the outer vacuum container, through holes in the thermal shield 3 to the cryogen vessel, in order to provide structural integrity against the compression loads from the weight of the cryogen vessel within the cryostat when mounted on the blocks. As a result, the container itself must be made of thick material to support the weight.

Fig. 2 schematically illustrates an alternative arrangement in which a housing is provided in the form of a foot 20, providing floor mounting for the system, and supporting at least some of the weight of the article and the outer vessel. A single housing (foot) may be provided along each side of the outer vacuum container 4, or a plurality of housings (feet) may be placed at selected locations along each side of the outer vacuum container 4. The locations are selected so as to provide an acceptable combination of floor support for the system as a whole, and support for the cryogen vessel 2 within the outer vacuum container 4.

As shown in Fig. 2, the support arrangement typically involves a plurality of upper support elements 22 and a plurality of lower support elements 23. The upper support elements 22 connect between a relatively low point 24 on the cryogen vessel and a relatively high point 26 in the housing 20. The lower support elements 23 connect between a relatively high point 28 on the cryogen vessel and a relatively low point 30 in the housing 20.

Any of the upper support elements 22 and lower support elements 23 may be arranged to be held under tension, or in compression, so as to prevent relative movement of the cryogen vessel and the outer vacuum container and to support the weight of the cryogen vessel.

The various support elements are arranged in calculated angular configurations, determined to provide an acceptable combination of support and resistance to translation and rotation in all possible directions. The number of support elements used is kept to a minimum, since each support element 22, 23 represents a thermal conduction path from the relatively high-temperature outer vacuum container 4 to the much colder cryogen vessel 2.

The suspension elements 22, 23 are each arranged to be as long as is practical, to increase their thermal resistance and to thereby reduce the thermal influx carried by each suspension element. In practice, this means that lower suspension elements 23 are mounted 28 high on the cryogen vessel and are mounted 30 low on the housing 20, while upper suspension elements 23 are mounted 24 low on the cryogen vessel and are mounted 26 high on the housing 20.

While the arrangement shown in Fig. 2 illustrates only the suspension elements required for suspending the cryogen vessel 2 within the outer vacuum chamber 4, similar arrangements are also provided for supporting thermal shield(s) 3. The shield(s) 3 weigh much less than the cryogen vessel 2 and its contents, so the advantages gained from the arrangement by avoiding load bearing on the outer vacuum container 4 are not so significant when applied to the shield(s). However, the arrangement may allow longer, more thermally resistant suspension elements 22, 23 to be used, limiting the thermal influx to the shield(s). Tn addition, it may be simpler to arrange support of the shield(s) 3 in this way in cases where the cryogen vessel 2 is supported in this way, rather than providing two different types of suspension arrangement within a single system. Optionally, the shield(s) may be supported by the same suspension elements 22, 23 which support the cryogen vessel 2.

Mounting the scanner on the floor can also cause problems with vibration being transmitted into the system and hospital workers must keep the area around the supporting feet clean. Furthermore, with either arrangement, the support elements, or at least fittings for the support elements, must be welded in place and so access is required when fitting and to carry out repairs. Supporting elements typically protrude from the surface of the OVC, so integral covers are not practical. The support feet can also make the whole equipment seem rather imposing to a patient.

W02007/147233 proposes simply hanging the MRI magnet from the ceiling, instead of the magnet being floor mounted, leading to reduced vibrations and improved signal to noise performance.

However, this does not address the problems with conventional MRI arrangements of the size of the OVC container, thermal leakage, difficulties of construction and repair, or the user perception.

The present invention provides apparatus as defined in the appended claims.

The present invention avoids the need for structural supports between the outer vacuum container and the cryogen vessel by arranging that the vacuum container and cryogen vessel hang down from a suspension component surrounding the turret of the cryostat, so that the OVC only has to be strong enough to contain the vacuum, rather than also supporting the weight of the magnet in the cryogen vessel. The arrangement gives improved cryogenic performance, visual impact and ease of installation.

An example of a ciyostat in accordance with the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates a cross-sectional view of a conventional solenoidal magnet arrangement for a nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) system; Figure 2 illustrates a second conventional MRT scanner arrangement; Figure 3 illustrates a first example of a cryostat according to the present invention with a gantry supported system, for installations with weak, or vibrating, ceilings.

Figure 4 illustrates a second example of a cryostat according to the present invention using a ceiling mounted system; Figure 5 illustrates a third example of a cryostat according to the present invention based on a floor mounted system, using a tensile suspension configuration; and, Figure 6 illustrates for the cryostats of Figs. 3 to 5, the integration of the turret, suspension component and cryostat casing in more detail.

In the present invention, the arrangement of cold head or refrigerator, turret for magnet ancillaries, and magnet suspension in an MRT cryostat has been optimised for cryogenic performance, visual impact, and ease of installation. The invention enables convection-cooled MRT systems to be used, and hence reduce the helium inventory.

There is a significant advantage in mounting the cold head as close as possible to top dead-centre. This is best achieved by integration of the cold head into the cryostat so that there is no system height penalty.

Figs. 3 and 4 illustrate two embodiments of the system. A cryostat 30 is suspended by a suspension component, or suspension element 31, incoiTporated with a turret of the cryostat, from a support structure 32, such as a gantry (Fig. 3) or strong point (Fig. 4) in a ceiling. The thermal shield and vacuum container depend from the tubular tensile suspension element and the turret, cold head components and magnet ancillaries, such as siphon, current leads and pressure relief valve 53 (Fig. 6) are combined within the suspension element. Suspension of the system from the ceiling or gantry enables the use of a vacuum-only vacuum casing to form the OVC in the cryostat, i.e. the OVC does not need to have any structural functionality, so can be made of thinner material, saving costs, or made of composite. Vibration damping devices may be incorporated into the ceiling, or gantry support, in order to address building-borne vibration, which is otherwise transmitted through the floor supports.

From the turret of the cryostat supply cabling and supply tubes 33 are fed along the gantry, or ceiling and so kept out of the way.

The arrangement of the examples of Figs. 3 and 4 is such that clearance is generated under the magnet, offering the advantage that the system may appear less imposing to the patient, whilst also enabling easier access for interventional procedures since personnel are then able to stand closer to the bore. Furthermore, access under the magnet enables simple patient table docking trolley designs.

In some buildings where MRI scanners are installed, which may not have been purpose built, the floor may be weak, although the ceiling is strong, so by suspending the system mass from the ceiling, there is a clear advantage.

Fig. 5 illustrates the cryostat and suspension element of the present invention configured for a more traditional floor-mounted approach, with the cryostat resting on supporting feet 34. Although, this requires some structural support through the OVC, the support for the magnet is passed via the feet 34 and vacuum casing of the OVC through the suspension element, so avoiding the need for structural supports which pass through the thermal shield and the associated fittings and connections. One benefit is that integrated covers and patient tables are possible with this design. The example of Fig. 5 still requires some structural functionality in the OVC outer shell, but embodies the other benefits of the invention. A common design of magnet may be provided that is suitable either to be suspended from the ceiling, or conventionally floor mounted, giving the customer greater installation options.

Fig. 6 illustrates the internal configuration in more detail showing a cold head 41, turret 35 including current leads, and siphon 55 mounted inside the tubular suspension component 31. A pressure relief valve or burst disc 58 is provided at the top of the turret. The turret 35 and refrigerator 41 are positioned within the suspension component, roughly at the top centre of the cryostat. The example shows the suspension element 31 interfaced with a conventional cryogen vessel 37, but the invention is equally applicable to convection cooled systems without a cryogen vessel where the suspension element interfaces directly with a magnet former.

The turret 35 is integrated into the cryostat. An access tube 48 and refrigerator 41 pass through a turret cover 56. A re-entrant feature 36 is formed in the cryogen vessel to provide the base of the turret. From the surface of the cryogen vessel to the surface of the cryostat, the suspension element 31 provides structural support for the layers of the cryostat and hence the magnet and cryogen vessel. The suspension element is typically made of a composite material, such as carbon or glass reinforced plastic in order to give strength and flexibility. Each of the layers of the cryostat, i.e. the cryogen vessel 37, thermal shield 38 and outer vacuum casing 39, are suspended from the element 31. These layers may be joined to the suspension element by bonding, rather than welding.

A connection 40 connects the thermal shield to a first stage 45 of a double recondensing cold head, or other dual stage refrigerator 41 at 50K to lOOK. A second stage 46 of the refrigerator at 4K provides cooling of the cryogen, typically helium, to recondense boiled off cryogen. However, this type of refrigerator is not essential, provided that sufficient cooling is provided for the magnet. A vacuum port 42 is provided in the suspension element, giving access between the vacuum 43 formed between the thermal shield 38 and vacuum casing (OVC) 39 and a sealed area 47 surrounding the refrigerator and the access tube 48 in the turret. This prevents a pressure differential occurring across the wall of the turret and support element into the OVC. A big pump out port 44 in the vacuum casing 39 allows for pumping and can vent directly into the ceiling void.

A positive current lead 49 is connected from the magnet (not shown) to a stainless steel tube 50 in the access tube 48 and current is coupled out 51 of the access tube via a copper ring and flexible copper cable 52 connected to the stainless tube 50.

The negative current lead (not shown) is earthed through the magnet structure. The access tube has a main vent (not shown) to allow excess gas to escape without damage to the system and a fibreglass tube 53 leads to an auxiliary vent path 54. A permanently installed siphon 55 is also provided through a cover 57 of the access tube.

The siphon is used for filling the magnet with liquid helium. The access tube is open to the cryogen vessel and full of gaseous cryogen providing cooling for the components.

A carbon fibre reinforced plastic (CFRP) or glass reinforced plastic (GRP) suspension tube 31 in tension is stronger than in compression, particularly when lateral displacement forces are considered. Hence, a reduced cross section can be used in comparison with a compressive pillar structure, thereby reducing conduction heat loads.

A minimum amount of helium in the magnet enables use of a small diameter relief pipe in event of quench. A permanently installed siphon and a pulse tube refrigerator cold head allow for acceptable service height. Coupling the cold head directly, and in close proximity, to the shield, turret, and suspension creates high performance thermal intercepts and thereby reduces cryostat heat loads.

The overall effect is a compact system, with ancillary services hidden from view, offering clean lines and improved access for interventional procedures and patient table access.

Claims

CLATMS1. A cryostat comprising a vacuum container, a thermal shield, a turret and superconducting magnet coils; wherein the magnet coils are positioned within the vacuum container; wherein the turret provides access to the magnet coils; wherein the cryostat further comprises a suspension component; wherein the turret is mounted within the suspension component and the thermal shield and vacuum container are suspended from the suspension component.
2. A cryostat according to claim 1, wherein the suspension component comprise a tubular composite structure.
3. A cryostat according to claim 1 or claim 2, wherein the composite is a carbon reinforced plastic, or glass reinforced plastic composite.
4. A cryostat according to any preceding claim, wherein the suspension component is suspended from an external structural support frame.
5. A cryostat according to claim 4, wherein the external structural support frame is ceiling mounted.
6. A cryostat according to any preceding claim, wherein the cryostat further comprises a siphon, a pressure release valve and current leads mounted in the turret.
7. A cryostat according to any preceding claim, wherein the refrigerator is removably mounted in the cryostat.
8. A cryostat according to claim 7, wherein a mechanical refrigerator is mounted within the turret.
9. A cryostat according to any preceding claim, wherein the cryostat further comprises a cryogen vessel.
10. A cryostat according to claim 9, wherein the cryogen vessel is suspended from the suspension component.
11. A magnetic resonance imaging system comprising a cryostat according to any preceding claim.
12. A system according to claim 11, further comprising a mounting pedestal.
13. A system according to claim 11 or claim 12, further comprising integrated covers.
14. A system according to any of claims 11 to 13, further comprising an integratedpatient table.Amendment to the Claims have been filed as followsCLATMS1. A cryostat comprising a vacuum container, a thermal shield, a turret and superconducting magnet coils; wherein the magnet coils are positioned within the vacuum container; wherein the turret provides access to the magnet coils; wherein the cryostat further comprises a suspension component; wherein the turret is mounted within the suspension component and the thermal shield and vacuum container are suspended from the suspension component.2. A cryostat according to claim 1, wherein the suspension component comprise a tubular composite structure.3. A cryostat according to claim 1 or claim 2, wherein the composite is a carbon 0) reinforced plastic, or glass reinforced plastic composite.C) 4. A cryostat according to any preceding claim, wherein the suspension component is suspended from an external structural support frame.5. A cryostat according to claim 4, wherein the external structural support frame is ceiling mounted.6. A cryostat according to any preceding claim, wherein the cryostat further comprises a siphon, a pressure release valve and current leads mounted in the turret.7. A cryostat according to any preceding claim, wherein a refrigerator is removably mounted in the cryostat.8. A cryostat according to claim 7, wherein a mechanical refrigerator is mounted within the turret.9. A cryostat according to any preceding claim, wherein the cryostat further comprises a cryogen vessel.10. A cryostat according to claim 9, wherein the cryogen vessel is suspended from the suspension component.11. A magnetic resonance imaging system comprising a cryostat according to any preceding claim.12. A system according to claim 11, further comprising a mounting pedestal.13. A system according to claim 11 or claim 12, further comprising integrated covers.14. A system according to any of claims 11 to 13, further comprising an integratedpatient table.