GB2344873A - Cryogen supply apparatus - Google Patents

Abstract

A cryogen supply unit is disclosed for controlling the flow of a Joule-Thompson fluid through a cryosurgical instrument. The supply unit includes a fluid flow path manifold block (16) formed as an assembly of three sub-units (16a, 16b, 16c). The manifold block includes internal fluid passageways and carries functional devices including pressure regulators (36, 82), open/closed valves (54, 56, 90, 92), connector sockets (30, 22, 86) and fluid pressure sensors (40, 46). The manifold avoids the need for conventional floating or suspended tubes to link the functional devices. A defrosting method is also described for defrosting the Joule-Thompson instrument by applying generally equal pressures of cryogen fluid to the inlet and exhaust channels of the instrument.

Description

1 2344873 CRYOGEN SUPPLY APPARATUS The present invention relates to

apparatus for use in controlling the flow of cryogenic fluid through a cryosurgical instrument such as a cryosurgical probe, to produce, for example, cooling or defrosting elfects. The invention is especially suitable for supplying cryogen gas which produces cooling within the instrument by a Joule 711ompson Q-T) throttling efFect, but the invention may be used with other fluid supplies.

Typically, the flow of cryogenic fluid is controlled by means of a control unit or console to which the cryosurgical instnunent is coupled by inlet and exhaust lines. The console may include an internal supply of cryogen fluid, or it may be coupled to an external supply. For a J-T system a replaceable bottle supply of high pressure gas is commonly used.

The control console includes regulators, valves and one or more pressure sensors, to control the pressure of the gas supplied to the instnunent, and to allow different modes of operation, such as pre-freeze purging, freeze, and defrost. The regulators, valves and pressure sensors are coupled together within the console by floating or suspended tubing, to form a fluid flow circuit. Where junctions are necessary, a junction piece,is interposed between two pieces of tubing.

In devising one aspect of the present invention, it has been appreciated that the above prior art construction is relatively complicated, requires a large number of fluid tight tube joints to be made, and is vulnerable to leaking at any of the fluid joints. This is especially true for high pressure portions of the fluid circuit in a J- T systeni, in which the fluid pressure will typically be about 750 pst This results in the console being relatively expensive to construct, and difficult to maintain. The console may also require particularly careful handling and transportation, to avoid creating leaks or weakening the joints between the suspended tubing.

The present invention has been devised bearing the above in mind.

In contrast to the above prior art, one broad aspect of the present invention is to employ a fluid circuit manifold having a plurality of fluid flow paths, and carrying at least one functional device communicating with an internal fluid flow path.

2 As used herein, the term "functional device" covers broadly any device for controlling, directing, handling, or sensing, fluid flow in a flow path. For example, the term includes devices such as valves (e.g. open/closed valves, or variable aperture valves), regulators (e.g. fixed pressure regulators or variable pressure regulators), sensors (e.g. pressure sensors or temperature sensors), and inlet and outlet connectors.

As used herein, the term "manifold" covers broadly any integral structure defining a plurality of flow paths. The structure may itself be an assembly of sub-units coupled together to form an integral structure, provided that the sub- units do form an integral unit.

The invention can provide significant advantages over the prior art, by reducing the number of couplings required in the fluid circuit, and therefore reduce the number of potential leak points in the systern. The manifold may also provide a convenient mounting for the one or more fimction devices, and can avoid the need for suspended tubing between the devices. This can reduce manufacturing costs, and provide a more durable product with improved reliability.

Although manifolds are known in certain engineering applications, such a system has not been proposed hitherto for use in the present highly specialised field of cryogen fluid control, in particular for providing a cryogenic fluid circuit, and for carrying one or more fimCtiOnal devices. Moreover, such a system has not been proposed for handling high pressure J-T cryogen fluids.

In one preferred form, the manifold is in the form of a block, which is drilled and/or otherwise machined to define internal passageways, and mounting ports and/or apertures for the fimctional devices.

It may be convenient to form the manifold as a plurality of sub-units or modules, which are then secured together to form. the overall manifold unit. Once assembled, the sub-units are rigidly fixed to each other, and it is very rare to have to disassemble the manifold. This means that the seals between the sub-units will not be prone to leakage, and will not deteriorate with as a result of movement or repeated disassembly.

Preferably, the manifold (or manifolds if more then one manifold is used) defines at least the majority of a fluid circuit within the console, and supports at least 3 the majority of the fimctional devices for the fluid circuit. In an especially preferred form, the manifold carries all of the fluid circuit fimctional devices.

Another aspect of the invention addresses the problem of generating rapid reheating (also referred to as "defrosting") of a cryosurgical instrument after the instrument has been operated during a freeze cycle. Various techniques are known in the art, including back flushing gas through the cryosurgical instrument, or allowing natural re-heating, or using an electric heater to heat the instrument.

It would be desirable to provide a re-heating system which is versatile, yet simple to implement, and which does not require a large number of additional system components.

In accordance with this aspect, the invention provides a unit in which is operable to re-heat a J-T cryosurgical instrument by application of generally equal pressures of fluid to the inlet and exhaust channels of the instrument.

With such a system, defrosting or reheating is achieved by the gas in the exhaust channel being re-pressurised, and generating heat. The defrost action is achievable extremely quickly by application of relatively high pressure gas, for example, more than 300 ps to both the inlet and exhaust channels of the instrument.

It will be appreciated that the defrost feature can be implemented very conveniently in a conventional J-T system. In a preferred form, the system includes a valve means operable to couple together the inlet and exhaust channels of or to, the cryosurgical instrument, and to cut-ofF the normal exhaust vent (for example, an atmospheric vent).

In a closely related aspect, the invention provides a control unit for controlling the flow of cryogen fluid through a cryosurgical instrument cooled by the Joule Ihompson effect, the control unit comprising first and second fluid path channels for coupling to the cryosurgical instrument, and being operable in a first operating mode to supply pressurised cryogen fluid to the first channel and to exhaust expended fluid from the second channel, and being operable in a second operating mode to supply pressurised cryogen fluid to both the first and second channels at substantially equal pressures.

4 In a closely related aspect, the invention provides a control unit for controlling the flow of cryogen fluid through a cryosurgical instrument cooled by the Joule Thompson effiect, the control unit comprising a fluid flow path circuit comprising:

an inlet for receiving pressurised cryogen fluid; a coupler for coupling first and second fluid path channels to a cryosurgical instrument; an exhaust outlet for exhausting expended fluid; and valve means operable: in a first operating mode to couple the inlet to the first channel of the coupler, and to couple the second channel to the exhaust outlet; and in a second operating mode to decouple the second channel from the exhaust outlet, and to couple the inlet to the first and second channels of the coupler.

Preferably, the control unit includes valve means for selectively coupling the inlet to the first channel of the coupler.

Preferably, the control unit includes a pressure regulator for regulating the fluid is pressure received through the inlet.

Preferably, the control unit comprises an electronic control circuit for controlling the operation of the valve means.

In a yet further related aspect, the invention provides a method of operating a cryosurgical instrument, the instrument including first and second channels and a throttling constriction for generating cooling by Joule 11ompson expansion of gas passing through the constriction from the first channel to the second channel, the method comprising:

during a cooling cycle, applying pressurised cryogen to the first channel, and exhausting expanded cryogen from the second channel; and during a re-heat cycle following the cooling cycle, applying generally equal pressures of cryogen fluid to the first and second channels.

Preferably, the defrost cycle follows the cooling cycle immediately.

Preferably, the method finther comprises a venting cycle following the re heating cycle, in which the first and second channels are vented at generally the same rate as each other. Preferably, this is achieved by allowing the first and second channels to be coupled together while they are vented.

An embodiment of the invention is now described by way of example only, with reference to the accompanying drawings, in which:

Fig. I is a schematic view of a J-T control console; Fig. 2 is a schematic front view of the fluid manifold of the console; Fig. 3 is a schematic exploded rear view of the fluid manifold showing the fimctional devices (but not showing internal fluid flow paths); Fig. 4 is a schematic rear view of a first part of the manifold; Fig. 5 is a schematic side view of the first part of the manifold; Fig. 6 is a schematic rear view of a second part of the manifold; Fig. 7 is a schematic side view of a third part of the manifold; Fig. 8 is a schematic rear view of the third part of the manifold; Fig. 9 is a schematic fluid flow diagram showing the fluid circuit in each part of the manifold; Fig. 10 is a schematic diagram illustrating the sealing arrangement between the manifold parts; and Fig. I I is a schematic section through the tip of a typical J-T cryosurgical probe.

Referring to Fig. I of the drawings, a control console 10 for a J-T cryosurgical system includes a case 12 containing an electronics module 14 and a fluid flow circuit defined by a manifold block 16. An inlet conduit 18 supplies high pressure fluid, for example, nitrous oxide from an external source (not shown) to the rear of the manifold block 16. A variable pressure regulator (descriW below) is mounted within the block 16, and is controlled by a manually operable rotary control 20 projecting from the front of the block 16 through the front panel of the case 12. The block 16 also carries a socket 22 for coupling to a cryosurgical instrument (not shown), to supply fluid to the instrument, and to receive exhaust gas from the instrument. The socket projects from the front of the block, through the front panel of the case 12. T'he expended exhaust gas exits the block 16 through an atmospheric exhaust line 24 coupled to the rear of the block 16.

As described in detail below, the manifold block 16 contains internal fluid passages, and carries various finictional devices (including the variable pressure 6 regulator) to form a multi-fimetional fluid circuit, to enable the console 10 to operate in various fimetional modes.

Referring to Figs. 2 and 3, in this embodiment, the manifold block 16 is formed by three sub-units 16a, l6b and 16c. Splitting the block into sub-units can enable more intricate configurations of fluid passages to be machined in each sub- unit, to form an overall path configuration which might be virtually impossible to produce from a single unit inauffold block. It can also facilitate modular assembly and testing of the different sub-units 16a-16c if desired.

In this embodiment, the sub-units are made from solid aluminium, which is relatively easy to machine. Figs. 3-8 show the internal flow paths within each sub-unit.

Ilese flow paths can be drilled or otherwise machined in the aluminium blocks after the blocks have been shaped. Some of the flow paths are depicted in the drawings as not extending to an exterior surface of the block. The skilled man will appreciate that such flow paths in a block are formed by extending the path to the surface of the block, and subsequently fitting a plug to close the unwanted opening in the exterior surface.

However, for the sake of clarity, such extensions and plugs are not shown explicitly in the drawings; only the "fimetional" flow paths interconnecting various fiinctional devices are illustrated.

The first sub-unit. 16a (Figs. 2 to 5 and 9) is generally rectangular, and comprises a rear inlet aperture 30 for receiving an input particulate filter/connector 32 for communicating with the high pressure inlet conduit 18. Tlie first sub- unit 16a also includes a second through aperture 34 for receiving the variable pressure regulator 36.

The apertures 30 and 34 communicate via a first internal passage 38 to supply input fluid from the input connector 32 to the regulator 36.

In order to monitor the pressure of fluid entering the block 16, an inlet pressure sensor switch 40 is mounted externally on the side of the first sub-unit 16a. The inlet pressure sensor 40 is threadedly engaged in a threaded bore 42 which communicates with the inlet aperture 30 by a second internal passage 44. Ile inlet pressure sensor is provided to monitor whether the inlet pressure exceeds a certain minimum threshold, for example, 600 psi. It will be appreciated that if the inlet pressure falls below a certain threshold, then it may be impossible to generate a sufficient cooling effect in the cryosurgical instrument. The electronics control module 14 monitors the 7 output from the inlet sensor switch 40, and generates a warning signal should the pressure be too low for normal operation.

In order to monitor the output pressure from the regulator 36, a second pressure sensor 46 is mounted externally on the side of thefirst sub-unit 16a, above the inlet pressure sensor 40. The second pressure 46 is threadedly engaged in a bore 48 which communicates directly with the aperture 34 housing the regulator 36. The output from the second sensor 46 is fed to the electronics module to provide a visual indication of the output pressure on the console front panel The fluid output from the first sub-unit 16a of the block 16 exits through a bore 50 which is aligned with a cornplementary input bore 52 in the opposing face of the second sub unit l6b.

The second sub unit 16b (Figs. 2, 3, 6 and 9) is generally L-shaped, and carries two externally mounted, electronically controlled open/closed valves, referred to as a freeze valve 54 and a purge valve 56; the t i ology refers merely to the operating modes of the console, the valves operating here merely to control the routing of fluid through the second sub-unit. The input bore 52 leads to the freeze valve inlet port 58 on the surface of the second sub-unit 16b, and in parallel via a third internal passage 60 to the purge valve inlet port 62 on the surface of the sub-unit l6b.

Adjacent to the freeze valve inlet port 58 is a freeze valve outlet port 64 for receiving fluid passed by the freeze valve 54 when the valve is open. The freeze valve outlet port 64 is coupled by a fourth internal passage 66 to a first outlet 68 to the third sub-unit 16c.

Adjacent to the purge valve inlet port 62 is a purge valve outlet port 70 for receiving fluid passed by the purge valve 56 when the valve is open. The purge valve outlet port 70 is coupled by a fifth internal passage 72 extending into the base of the "L" shape, to a second outlet 74 to the third sub-unit 16c.

The valves 54 and 56 are mounted on the rear surface of the second subunit 16b in alignment with the pairs of ports 58 & 64, and 62 & 70, respectively. The valves are retained in position by bolts extending into threaded holes 76 in the rear surface of the second sub-unit l6b.

The third sub-unit 16c (Figs. 2, 3, 7, 8 and 9), has a rectangular shape similar to the first sub-unit 16a. Internally, the third sub-unit 16c carries the instrument 8 connector socket 22 in a connector through aperture 80. It also carries an internal fixed regulator 82 mounted within a through aperture 84, and an internal exhaust socket 86 mounted within a rear aperture 88.

Externally mounted on the side of the third sub-unit 16c are two fin-ther electronically controlled open/closed valves, referred to herein as a defrost valve 90 and an exhaust valve 92. As before, the terminology refers to the operating modes of the console. Each valve is aligned with an inlet port 94, 96 respectively, and an output port 98, 100, respectively, on the side surface of the third sub-unit 16c. Tlie valves are secured in position by bolts received in threaded holes 76 in the side surface of the third sub-unit 16c.

On the side face of the third sub-unit 16c facing the second sub-unit l6b are a first inlet bore 102 and a second inlet bore 104 aligned respectively with the first outlet 68 and the second outlet 74 of the second sub-unit 16b. The first inlet bore 102 leads directly into the connector aperture 80 to communicate with the supply port of the connector socket 22 (Le. to communicate with the supply line to the cryosurgical instrument).

The second inlet bore 104 is coupled by a sixth internal passage 106 to the inlet to the fixed pressure regulator 82. The output from the fixed pressure regulator 82 is coupled by a seventh internal passage 108 also to the supply port of the connector socket 22. There are therefore two parallel fluid paths through the second and third sub-units l6b and 16c, to the supply port of the instrument socket. The first is a high pressure, or "freeze" path through the freeze valve 54. The second is a low pressure, or "purge" path through the purge valve 56, and through the fixed (low) pressure regulator 82.

The exhaust port of the instrument socket 22 is coupled by an eighth internal passage 110 to the input port 96 for the exhaust valve 92. 'Me output port 100 for the exhaust valve communicates directly with the exhaust connector 86 in the rear exhaust aperture 88, to allow the flow of exhaust gas from the manifold block 16 when the exhaust valve 92 is open.

A ninth internal passage 112 couples the input port 96 for the exhaust port 92 to the input port 94 for the defrost valve 90. The output port 98 for the defrost valve communicates directly with the supply port of the instrument connector (in paraHel 9 with the first inlet bore 102 and the seventh internal passage 108). It can be seen that the defrost valve 90 is therefore operative, when open, to couple together the supply and exhaust ports of the instrument socket 22, and thereby to equalise the pressure on the delivery and exhaust sides of the J-T cooling nozzle within the cryosurgical instrument.

Ile manifold block sub-units 16a-16c are secured together by bolts. In the present embodiment, the first and third sub-units are attached independently to the second sub-unit. Ihe first sub-unit 16a includes counter-sunk through bores 120, and the second sub-unit 16b includes threaded holes 122 for receiving bolts inserted through the bores 120. The third sub-unit 16c similarly includes counter- sunk through bores 124, and the second sub-unit 16b includes threaded holes 126 for receiving bolts inserted through the bores 124.

The fluid connections between the first and second sub-units 16a and 16b, and between the second and third sub-units 16b and 16c, are rendered fluid- tight by means of a sealing arrangement illustrated in Fig. 10. One of the faces is machined to have a small annular pocket 130 encircling the bore, and a resilient 0-ring 132 is placed w the pocket. A bridging tube or spigot 134 is inserted within the 0-ring 132. When the two faces are tightened against each other, the 0-fing 132 is compressed to form a highly reliable seal. The pocket 130 provides a seat for the 0-ring, and the tube 134 serves to prevent the 0-ring from deforming inwardly, which might block or constrict the fluid path. Such a sealing arrangement is dwIe and inexpensive, yet can provide a perfect seal even for high fluid pressure. The seal is located within the manifold block 16, and it is very rare that the sub-units of the manifold block will be separated or moved relative to each other after the initial construction. Ilerefore, the seal life will not be reduced by external forces or by repeated disassembly or adjustment of the seal.

A circular spot face 136 is also used to provide a good sealing surface around the pocket 130. Spot faces 138 are also used around each of the ports 58, 62, 64, 70, 94, 96, 98, 100 to provide sealing surfaces to co-operate with the seals built into the valves 54, 56, 90 and 92.

In use, the valves 54, 56, 90 and 92 are controlled by the electronics module 14 to provide difFerent console operating modes. Mie electronics module 14 typically includes a microprocessor or microcontroller (not shown) executing a software control routine, to control the electronic signals sent to the valves.

The diffierent operation modes of the console 10 can be best appreciated from Fig. 9. In this figure the broken fines represent the boundaries between the different sub-units 16a- 16c of the manifold block 16.

In a passive or rest mode, the freeze valve 54, the purge valve 56 and the defrost valve 90 are closed, while the exhaust valve 92 is open. In this mode no fluid is passed to the cryosurgical instrument. ne open exhaust valve 92 means that the system is always vented to atmosphere at rest. This is the default fail- safe mode of the console, and so normally-closed valves are used for the freeze valve 54, the purge valve 56 and the defrost valve 90, while a normallyc-open valve is used for the exhaust valve 92.

In a purging mode, the freeze valve 54 and the defrost valve 90 remain closed, but the purge valve 56 is opened to match the state of the exhaust valve 92. During this phase low pressure fluid is supplied to the cryosurgical instnunent, and is exhausted through the atmospheric exhaust output. The low pressure gas serves to remove any residual moisture in the cryosurgical instrument, prior to freezing. The low pressure is created by the fixed pressure regulator 82 through which the fluid is routed by the purge valve 56. (It will be appreciated that the fluid pressure is normally about 750 ps but this is reduced to a much lower pressure of about 300 psi by the fixed regulator 82).

To enter the freeze mode, the purge valve 56 is closed, and the freeze valve 56 is opened to allow flill-pressure fluid to pass to the cryosurgical instrument. The defrost valve 90 remains closed, such that the expanded gas exits the manifold through the exhaust channel.

After freezing has been completed, the defrost mode is entered by closing the exhaust valve 92, and opening the defrost valve 90. During this cycle, the freeze valve 54 remains open to supply high pressure gas to the instrument socket. The effect of the open defrost valve 90 is to apply the high pressure fluid simultaneously to both the supply and exhaust sides of the cryosurgical instrument. This equalises the pressure causing a rapid warming effect by re-pressurisation of the expanded gas on the exhaust side of the J-T nozzle within the cryosurgical instrument.

11 After defrosting, the freeze valve 54 is closed, and the exhaust valve 92 opened to vent the fluid within the system. During this venting, the defrost valve 90 is kept open to ensure that no significant pressure is developed between the supply and exhaust sides of the J-T nozzle; Various defrost techniques are known in the prior art, including back flushing warm gas through the cryosurgical instrument. However, back flushing can cause complications. For example, Fig. 11 illustrates a typical J-T probe tip. The probe includes concentric inner and outer walls 140 and 142, defining and inlet channel 144 and an exhaust channel 146 in the probe. The inner wall 140 is crimped near its tip to form a constriction 148. During freezing, the J-T throttling efflect takes place at the constriction 148 as high pressure gas is forced through the constriction and expands in the probe tip. The expanded gas then exits through the exhaust channel 146.

It will be appreciated that if gas is backflushed through the probe at any significant rate (required for rapid defrosting), the backflushed gas will itself undergo J-T throttling as it passes through the constriction 148. Therefore, the act of backflushing will itself create cooling, which is contrary to the defrost flinction, and reduces the defrost efficiency.

Compared to prior art techniques, the defrost feature used in the preferred embodiment described above can provide rapid and efficient defrosting without the problem of cooling being generated by gas flow through the constriction 148. Ile equal pressures of cryogen fluid in the inlet and exhaust channels 144 and 146 ensures that there is negligible flow through the constriction.

It will be also be appreciated that during the venting cycle (following the defrost cycle) the defrost valve 90 is maintained open to maintain equal pressures in the inlet and exhaust channels of the instrument, as any pressure differential might cause undesirable rapid re-cooling.

Although a multi-part manifold 16 has been described, it will be appreciated that, if desired, the manifold could be formed as an integral unit. Also, although the illustrated manifold has a block form, it will be appreciated that any suitable shape and configuration of manifold may used as desired.

It is emphasised that the foregoing description is merely illustrative of a preferred form of the invention. The skilled man will appreciate that many 12 modifications may be made within the scope and/or principles of the invention. Features believed to of particular importance are defined in the appended claims. However, the Applicant claims protection for any novel feature or idea described herein and/or illustrated in the drawings whether or not emphasis has been placed thereon.

13

Claims (1)

1. A cryosurgical control unit for controlling the flow of cryogen fluid through a cryosurgical instrument, wherein the control unit comprises a manifold defining a plurality of internal fluid flow paths and carrying at least one fimctional device communi eating with a said internal flow path.

2. A unit according to claitn 1, wherein the manifold is in the form of a block.

3. A unit according to claim 1 or 2, wherein the manifold is an assembly of subunits.

4. A unit according to claim 3, wherein at least one of the sub-units is in the form of a block.

5. A unit according to claim 3 or 4, wherein the sub-units are secured to each other in face-to-face relation.

6. A unit according to claim 3, 4 or 5, comprising scaling means for forming a fluid-tight seal between two co-operating apertures in the subunits, the sealing means comprising a bridging tube, and an 0-ring around the bridging tube.

7. A unit according to claim 6, wherein the 0-ring seats in a recess in the face of at least one of the manifold sub-units.

8. A unit according to any preceding claim, wherein the control unit contains a fluid flow path circuit including a plurality of functional devices, at least a majority of 30 the fluid circuit being implemented by means of the manifold.

14 9. A unit according to claim 8, wherein the manifold carries at least a majority of the flinctional devices. 10. A unit according to any preceding claim, wherein the manifold carries at least 5 one pressure regulator.

11. A unit according to claim 10, wherein the manifold carries at least one fixed pressure regulator.

12. A unit according to claim 10 or 11, wherein the manifold carries at least one variable pressure regulator. 13. A unit according to any preceding claim wherein the manifold carries at least one flow control valve. 15 14. A unit according to claim 13, wherein the manifold carries at least one open/closed valve. 15. A unit according to any preceding claim, wherein the manifold defines a first 20 flow path from an inlet connector through a variable pressure regulator to an instrument connector for communicating with the cryosurgical instrument. 16. A unit according to claim 15, wherein the manifold carries a first valve for controlling the flow through the first flow path. 25 17. A unit according to claim 15 or 16, wherein the manifold defines a second flow path coupled to the first flow path, the second flow path passing through a fixed pressure regulator to said instrument connector. 30 18. A unit according to claim 17, wherein the manifold carries a second valve for controlling the flow through the second flow path.

19. A unit according to any preceding claim, wherein the manifold defines an exhaust flow path from the or an instrument connector through an exhaust control valve to an exhaust connector.

20. A unit according to any preceding claim, wherein the manifold defines a pressure equalisation path from the delivery part of the or an instrument connector, through an equalisation control valve, to the exhaust part of said instrument connector.

21. A cryosurgical control unit for controlling the flow of cryogen fluid through a cryosurgical instrument, the control unit comprising a fluid flow path circuit including a plurality of fimetional devices, wherein at least a majority of the fluid circuit routing is implemented by means of a manifold defining a plurality of internal flow paths and carrying the respective fimctional devices for the circuit implemented by the manifold.

22. A cryosurgical control unit which is operable to re-heat a J-T cryosurgical instnunent by application of generally equal pressures of fluid to the inlet and exhaust channels of the instrument.

23. A cryosurgical control unit according to claim 22, comprising valve means for coupling together the inlet and exhaust channels of the instrument.

24. A control unit for controlling the flow of cryogen fluid through a cryosurgical instrument cooled by the Joule 11ompson effect, the control unit comprising first and second fluid path channels for coupling to the cryosurgical instrument, and being operable in. a first operating mode to supply pressurised cryogen fluid to the first channel and to exhaust expended fluid from the second channet and being operable in a second operating mode to supply pressurised cryogen fluid to both the first and second channels at substantially equal pressures.

25. A control unit for controlling the flow of cryogen fluid through a cryosurgical instrument cooled by the Joule Thompson effect, the control unit comprising a fluid flow path circuit comprising:

16 an inlet for receiving pressurised cryogen fluid; a coupler for coupling first and second fluid path channels to a cryosurgical instrument; an exhaust outlet for exhausting expended fluid; and valve means operable: in a first operating mode to couple the inlet to the first channel of the coupler, and to couple the second channel to the exhaust outlet; and in a second operating mode to decouple the second channel from the exhaust outlet, and to couple the inlet to the first and second channels of the coupler.

26. A unit according to claim 25, wherein the valve means comprises a pressure equalisation valve coupled between the first and second channels for coupling the channels together.

27. A unit according to claim 26, wherein the valve means comprises an exhaust valve coupled between the second channel and the exhaust outlet for selectively coupling the second channel to the outlet.

28. A unit according to claim 25, 26 or 27, wherein the valve means comprises an inlet valve coupled between the inlet and the first channel of the coupler.

29. A unit according to claim 28, fluther comprising a pressure regulator coupled between the inlet and the inlet valve.

30. A unit according to claim 28 or 29, further a fluid path for passing low pressure purge fluid to the first channel of the coupler.

31. A unit according to claim 30, wherein the low pressure fluid path comprises a fixed pressure regulator and a purge valve for selectively controlling the flow of fluid through the low pressure fluid path.

32. A unit according to claim 26 and claim 27 and claim 28 and claim 30 and claim 3 1, wherein the unit is operable in:

17 a purge mode in which the inlet valve and the pressure equalisation valve are closed, and the low pressure valve and the exhaust valve are open.

33. A unit according to claim 26 and claim 27 and clam 28 and claim 30 and claim 3 1, wherein the unit is operable in: a freeze mode in which inlet valve and the exhaust vale are open, and the low pressure valve and the pressure equalisation valve are closed.

34. A unit according to claim 26 and claim 27 and claim 28 and claim 30 and claim 3 1, wherein the unit is operable in: a defrost mode in which inlet valve and the pressure equalisation valve are open, and the low pressure valve and the exhaust valve are closed.

35. A unit according to claim 26 and claim 27 and claim 28 and claim 30 and claim 3 1, wherein the unit is operable in: a vent mode in which the inlet valve and the low pressure valve are closed, and the pressure equalisation valve and the exhaust valve are open.

36. A unit according to two or more of claims 32, 33, 34 and 35.

37. A method of operating a cryosurgical instrument, the instrument including first and second channels and a throttling constriction for generating cooling by Joule Thompson expansion of gas passing through the constriction from the first channel to the second channel, the method comprising:

during a cooling cycle, appbying pressurised cryogen to the first channel, and exhausting expanded cryogen from the second channel; and during a re-heat cycle following the cooling cycle, applying generally equal pressures of cryogen fluid to the first and second channels.

38. A method according to claim 37, flu-ther comprising a venting cycle following the re-heating cycle, in which the first and second channels are vented at generally the same rate as each other.

18 39. A cryosurgical system comprising a control unit as defined in any of claims 1 to 36.

40. A system according to claim 39, wherein the system is a JouleThompson system.

41. A system according to claim 39 or 40, fluther comprising a cryosurgical instrument coupled or coupleable to the control unit.

42. A system according to claim 41, wherein the instrument is a cryosurgical probe.

43. A manifold, or a fluid circuit, or a control unit, or a method of operation, being substantially as hereinbefore described with reference to any of the accompanying 15 drawings.