GB2100987A - Cryosurgical probe - Google Patents

Abstract

Existing cryosurgical probes are normally cooled by a low boiling point liquid such as liquid nitrogen or by utilizing the Joule-Thomson expansion principle which requires a high pressure gas supply. The present invention adopts the single or multi-stage pulse tube principle to achieve probe tip temperatures down to about 85K using significantly lower gas supply pressures than Joule-Thomson devices. The lower gas supply pressure is well within the capabilities of small industrial compressor units thus making pulse tube cryoprobes available to surgeons in remote areas of the world where cryogenic liquids or high pressure gases are unattainable. <IMAGE>

Description

SPECIFICATION Cryosurgical probe operating on the pulse tube principle Cryogenic surgery has been used on the treatment of certain types of diseases for a number of years.

Cryosurgical instruments used in these treatments usually take the form of cold vapour sprays or metallic probes where the tip of the probe is cooled to sub-zero temperatures. A summary of the various medical treatments and types of instruments which have been used are contained in ref. 1.

Existing cryosurgical probes which are brought into contact with the part of the body being treated are normally cooled by a low boiling point liquid such as liquid nitrogen or by utilizing the Joule Thomson expansion principle by expanding a high pressure gas through a restriction orifice near the probe tip. However, either of these methods of producing low temperatures at the probe tip requires the availability and storage of a cryogenic liquid (e.g.

LN2) or the supply of a high pressure gas ( > 800 psig) in suitable gas cylinders.

In certain regions of the world, particularly in under-developed countries, cryogenic liquids or high pressure gas cylinders are not readily available or are prohibitively expensive.

The principal objective of the present invention is to provide a cryosurgical probe which will operate at relatively low gas pressures ( < 100 psig) which are readily available from small industrial compressor units. The invention achieves this objective by adopt- ing the Pulse Tube refrigeration phenomena which are fully described in ref. 2.

Although the invention described in this document is primarily aimed at using compressed air as the working fluid to produce probe tip temperatures down to about 85K it would equally be used for other available gases.

The method of achieving these objectives will be apparent from reading this specification together with the attached drawing.

Fig. 1 shows the envisaged assembly of a single stage Pulse Tube Cryoprobe which relates to this invention.

The pressurized dry gas supply through a special valve 2 which controls the compression and expansion of the gas in the pulse tube Sat some prescribed frequency depending on the length and diameter of the pulse tube and the desired probe tip temperature.

During each cycle the gas flows along the supply pipe 3 through the regenerator 1 and cold end heat exchanger 4 before entering the pulse tube 5 where it is compressed. A further heat exchanger 6 at the warm end of the pulse tube absorbs some of the heat of compression prior to the expansion phase of the cycle. During the expansion phase the returning gas cools the thermal load heat exchanger 4 and regenerator 1 prior to the next cycle. Thus a temperature gradient is established along the length of the pulse tube 5 which decreases from about room temperature at heat exchanger 6 down to sub-zero temperatures at the thermal load heat exchanger 4.

Various designs of cryoprobe tip 8 can be attached to the thermal load heat exchanger 4 depending an the type of cryosurgical operation being carried out.

An insulating sleeve 9 surrounds the probe tip and thermal load heat exchanger 4 during the cooldown period to improve efficiency of the cycle and prevent ice formation on the probe tip 8 and heat exchanger 4. A suitable insulated handle 7 is attached to the assembly for the convenience of the surgeon.

Although Fig. 1 shows a water cooled heat exchanger 6 at the warm end of the pulse tube this heat exchanger could alternatively be air cooled by introducing a suitable fin design to achieve the necessary heat extraction rate at this end of the pulse tube.

The valve 2 which controfsthe gas compression and expansion cycles can be operated by either some suitable external cycling mechanism or by feed-back pressure sensor from the pulse tube itself.

The physical dimensions of the pulse tube 5 will depend on the gas being used and the ultimate probe tip temperature required. The material of the pulse tube 5 would normally be stainless steel although any material which can withstand the pressures and provide the necessary thermal gradient is acceptable.

To ensure that the gas used as the working fluid has a negligible moisture content a suitable drier must be included in the gas supply system.

If lower probe tip temperatures are required for specific cryosurgical operations than can be achieved by the single stage unit illustrated in fig. 1 then multi-staging of the regenerator-heat exchanger-pulse tube arrangement as described in ref. 2 can be incorporated in the design.

Control and monitoring ofthetemperature of the thermal load heat exchange 4 and cryoprobe tip 8 can be achieved by embedding a suitable thermocouple at the cold end with the output fed to a digital thermometer or similar recording device.

(1) von Leden, H. and Cahan, W.G., "Cryogenics in Surgery", H. K. Lewis and Co. Ltd., London (1971).

(2) Gifford, W. E. and Longsworth, R.E., "Pulse tube refrigeration progress Adv. Cryo. Eng., 10, 69-79 (1964).

Claims (2)

1. I claim the application of the pulse tube refrigeration principle to the cooling of cryosurgical probe tips using any conveniently available compressed gas at relatively low pressures compared to those required forjou!eThomson expansion devices.

2. 1 further claim the use of multi-stage pulse tube arrangements for producing lowercryoprobe tip temperatures than could be achieved by a single stage unit