Abstract:
A cryosurgical system providing for temperature control of individual cryoprobes so as to simplify and increase treatment flexibility in cryoablation procedures. The cryosurgical system provides individual control of multiple cryoprobes in a closed-loop refrigeration circuit. The individual control allows the simultaneous use of multiple cryoprobes in a procedure. Typically six to eight probes are used but additional probes and control thereof is contemplated by this invention. The primary refrigeration circuit&#39;s compressor can also be utilized to generate pressurized hot vapor for heating the probe ends. In order to direct the pressurized hot vapor to the probe ends, an internal valving and control system reverses the direction of pressurized gas flow through the cryoprobes, delivering the hot gas immediately to the ends by bypassing the heat exchangers. Thus each cryoprobe can be independently controllable to provide full, partial or no freezing or heating at any time.

Description:
PRIORITY CLAIM 
       [0001]    The present application claims priority to U.S. Provisional Application Ser. No. 60/866,288, filed Nov. 17, 2006 and entitled “CRYOPROBE THERMAL CONTROL FOR A CLOSED-LOOP CRYOSURGICAL SYSTEM”, which is herein incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to cryoprobes for use in cryosurgical systems and more specifically to the individual thermal control of multiple cryoprobes for a closed-loop cryosurgical system including the ability to reverse flow for probe heating. 
       BACKGROUND OF THE INVENTION 
       [0003]    Cryosurgical probes are used to treat a variety of diseases. Cryosurgical probes quickly freeze diseased body tissue, causing the tissue to die after which it will be absorbed by the body, expelled by the body, sloughed off or replaced by scar tissue. Cryothermal treatment can be used to treat prostate cancer and benign prostate disease. Cryosurgery also has gynecological applications. In addition, cryosurgery may be used for the treatment of a number of other diseases and conditions including breast cancer, liver cancer, glaucoma and other eye diseases. 
         [0004]    A variety of cryosurgical instruments variously referred to as cryoprobes, cryosurgical probes, cryosurgical ablation devices, cryostats and cryocoolers have been used for cryosurgery. These devices typically use the principle of Joule-Thomson expansion to generate cooling. They take advantage of the fact that most fluids, when rapidly expanded, become extremely cold. In these devices, a high pressure gas mixture is expanded through a nozzle inside a small cylindrical shaft or sheath typically made of steel. The Joule-Thomson expansion cools the steel sheath to a cold temperature very rapidly. The cryosurgical probes then form ice balls which freeze diseased tissue without undue destruction of surrounding healthy tissue. 
         [0005]    The use of cryosurgical probes for cryoablation of prostate is described in Onik, Ultrasound-Guided Cryosurgery, Scientific American at 62 (January 1996) and Onik, Cohen, et al., Transrectal Ultrasound-Guided Percutaneous Radial Cryosurgical Ablation Of The Prostate, 72 Cancer 1291 (1993). In this procedure, generally referred to as cryoablation of the prostate, several cryosurgical probes are inserted through the skin in the perineal area (between the scrotum and the anus) which provides the easiest access to the prostate. The probes are pushed into the prostate gland through previously placed cannulas. Placement of the probes within the prostate gland is visualized with an ultrasound imaging probe placed in the rectum. The probes are quickly cooled to temperatures typically below −100° C. The prostate tissue is killed by the freezing, and any tumor or cancer within the prostate is also killed. The body will absorb some of the dead tissue over a period of several weeks. Other necrosed tissue may slough off through the urethra. The urethra, bladder neck sphincter and external sphincter are protected from freezing by a warming catheter placed in the urethra and continuously flushed with warm saline to keep the urethra from freezing. 
         [0006]    Rapid re-warming of cryosurgical probes is desired. Cryosurgical probes are warmed to promote rapid thawing of the prostate, and upon thawing the prostate is frozen once again in a second cooling cycle. Moreover, the probes cannot be removed from frozen tissue because the frozen tissue adheres to the probe. Forcible removal of a probe which is frozen to surrounding body tissue leads to extensive trauma. Thus many cryosurgical probes provide mechanisms for warming the cryosurgical probe with gas flow, condensation, electrical heating, etc. 
         [0007]    Some devices utilize separate gas types for reheating. Ben-Zion, Fast Changing Heating and Cooling Device and Method, U.S. Pat. No. 5,522,870 (Jun. 4, 1996) applies the general concepts of Joule-Thomson devices to a device which is used first to freeze tissue and then to thaw the tissue with a heating cycle. Nitrogen is supplied to a Joule-Thomson nozzle for the cooling cycle, and helium is supplied to the same Joule-Thomson nozzle for the warming cycle. Preheating of the helium is presented as an essential part of the invention, necessary to provide warming to a sufficiently high temperature. 
         [0008]    Various cryocoolers use mass flow warming, flushed backwards through the probe, to warm the probe after a cooling cycle. Lamb, Refrigerated Surgical Probe, U.S. Pat. No. 3,913,581 (Aug. 27, 1968) is one such probe, and includes a supply line for high pressure gas to a Joule-Thomson expansion nozzle and a second supply line for the same gas to be supplied without passing through a Joule-Thomson nozzle, thus warming the catheter with mass flow. Longsworth, Cryoprobe, U.S. Pat. No. 5,452,582 (Sep. 26, 1995) discloses a cryoprobe which uses the typical fin-tube helical coil heat exchanger in the high pressure gas supply line to the Joule-Thomson nozzle. The Longsworth cryoprobe has a second inlet in the probe for a warming fluid, and accomplishes warming with mass flow of gas supplied at about 100 psi. The heat exchanger, capillary tube and second inlet tube appear to be identical to the cryostats previously sold by Carleton Technologies, Inc. of Orchard Park, N.Y. 
         [0009]    Still other Joule-Thomson cryocoolers use the mechanism of flow blocking to warm the cryocooler. In these systems, the high pressure flow of gas is stopped by blocking the cryoprobe outlet, leading to the equalization of pressure within the probe and eventual stoppage of the Joule-Thomson effect. Examples of these systems include Wallach, Cryosurgical Apparatus, U.S. Pat. No. 3,696,813 (Oct. 10, 1973). These systems reportedly provide for very slow warming, taking 10-30 seconds to warm sufficiently to release frozen tissue attached to the cold probe. Thomas, et al., Cryosurgical Instrument, U.S. Pat. No. 4,063,560 (Dec. 20, 1977) provides an enhancement to flow blocking, in which the exhaust flow is not only blocked, but is reversed by pressurizing the exhaust line with high pressure cooling gas, leading to mass buildup and condensation within the probe. 
         [0010]    Each of the above mentioned cryosurgical probes builds upon prior art which clearly establishes the use of Joule-Thomson cryocoolers, heat exchangers, thermocouples, and other elements of cryocoolers. However, the prior art fails to provide a system in which each probe is independently controlled during a heating and freezing cycle. Furthermore, there remains a need for a cryoprobe that does not require a separate energy source and circuit or separate gas supply and lines for heating so as to minimize and reduce the cost of each probe. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention is directed to a system that simplifies and adds more flexibility to cryoablation procedures. As the individual cryoprobes are directed to various treatment areas it is known that a selectable freeze performance would increase system efficiencies as well as provide greater safety to the patient. The present invention provides individual control of multiple cryoprobes in a closed-loop refrigeration circuit. The individual control allows the simultaneous use of multiple cryoprobes in a procedure. Typically six to eight probes are used but additional probes and control thereof is contemplated by this invention. Thus each cryoprobe will be independently controllable to provide full, partial or no freezing at any time. 
         [0012]    The present invention allows for individual control of the cryoprobes through switchable valving on the high pressure delivery tubes of the primary refrigerant circuit for each probe. The refrigerant is channeled either through the heat exchangers and to the probe ends or back to the compressor via bypass tubing. Restrictor elements in the bypass tubing are utilized to balance the mass flow in the circuit when rerouting refrigerant out of the probes. A heat exchanger is added to the bypass line for rejecting excess heat in the return refrigerant flow line. 
         [0013]    The present invention further provides an energy means for heating the tips of the cryoprobes in a closed-loop cryosurgical system in order to thaw the cryoprobe produced iceballs created during the freezing treatment and/or release the probes from the frozen tissue. In a first embodiment, the present invention provides an alternative to the separate electrical heater element commonly used on cryoprobes in closed-loop cryosurgical procedures. The primary refrigeration circuit&#39;s compressor is utilized to generate pressurized hot vapor for heating the probe ends. In order to direct the pressurized hot vapor to the probe ends, an internal valving and control system reverses the direction of pressurized gas flow through the cryoprobes, delivering the hot gas immediately to the ends by bypassing the heat exchangers. Heat control at the tips is controlled by the temperature sensor feedback. Thus the present invention eliminates the need for a separate energy source and circuit system for heating the cryoprobes. The elimination of the heater system further results in smaller diameter and less expensive probes. 
         [0014]    The above summary of the various representative embodiments of the invention is not intended to describe each illustrated embodiment or every implementation of the invention. Rather the embodiments are chosen and described so that other skilled in the art may appreciate and understand the principles and practices of the invention. The figures in the detailed description that follows more particularly exemplify these embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]    These as well as other objects and advantages of this invention, will be more completely understood and appreciated by referring to the following more detailed description of the presently preferred exemplary embodiments of the invention in conjunction with the accompanying drawings of which: 
           [0016]      FIG. 1  is a side view of an embodiment of a cryosurgical system according to the prior art. 
           [0017]      FIG. 2  is a schematic view of a heat exchanger system for use in a cryosurgical system of the prior art. 
           [0018]      FIG. 3  is a schematic view of a cryosurgical system according to an embodiment of the present invention. 
           [0019]      FIG. 4  is a schematic view of a cryosurgical system according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The present invention builds off prior art cryosurgical systems in which a manifold is used to distribute refrigerant to multiple probes. The present invention includes the means to separately heat and cool the individual probes. A prior art closed loop cryosurgical system  100  is illustrated in  FIG. 1 . Cryosurgical system  100  can include a refrigeration and control console  102  with an attached display  104 . Control console  102  can contain a primary compressor to provide a primary pressurized, mixed gas refrigerant to the system and a secondary compressor to provide a secondary pressurized, mixed gas refrigerant to the system. Control console  102  can also include controls that allow for the activation, deactivation, and modification of various system parameters, such as, for example, the flow rates, pressures, and temperatures of the refrigerants. Display  104  can provide the operator the ability to monitor, and in some embodiments adjust, the system to ensure it is performing properly and can provide real-time display as well as recording and historical displays of system parameters. One exemplary console that can be used with an embodiment of the present invention is used as part of the Her Option® Office Cryoablation Therapy available from American Medical Systems of Minnetonka, Minn. 
         [0021]    The high pressure primary refrigerant is transferred from control console  102  to a cryostat heat exchanger module  110  through a flexible line  108 . The cryostat heat exchanger module  110  transfers the refrigerant into and receives refrigerant out of one or more cryoprobes  114 . The particular cryoprobe configuration will depend on the application for which the system is used. For example, a uterine application will typically use a single, rigid cryoprobe, while a prostate application will use a plurality of flexible cryoprobes (which is shown in the embodiment of  FIG. 1 ). If a single, rigid cryoprobe is used, the elements of the cryostat heat exchanger module  110  may be incorporated into a handle of the cryoprobe. 
         [0022]    In the prior art, as depicted in  FIG. 1 , when a plurality of flexible cryoprobes are used, a manifold  112  is connected to cryostat heat exchanger module  110  to distribute the refrigerant among the several cryoprobes. The cryostat heat exchanger module  110  and cryoprobes  114  can also be connected to the control console  102  by way of an articulating arm  106 , which may be manually or automatically used to position the cryostat heat exchanger module  110  and cryoprobes  114 . Although depicted as having the flexible line  108  as a separate component from the articulating arm  106 , cryosurgical system  100  can incorporate the flexible line  108  within the articulating arm  106 . 
         [0023]    Referring now to  FIG. 2 , there can be seen a prior art embodiment of a cryostat heat exchanger module  110 . The cryostat  110  may contain both a pre-cool heat exchanger, or pre-cooler  118 , and a recuperative heat exchanger, or recuperator  120 . A vacuum insulated jacket  122  surrounds the cryostat  110  to prevent the ambient air from warming the refrigerant within the cryostat  110  and to prevent the outer surface of the cryostat  110  from becoming excessively cold. High pressure primary refrigerant  124  enters the cryostat  110  and is cooled by high pressure secondary refrigerant  128  that is expanded to a lower temperature in the pre-cool heat exchanger  118 . The resulting low pressure secondary refrigerant  130  then returns to the secondary compressor to be repressurized. Since the secondary refrigerant does not flow into the probes  114  (which are brought into direct contact with the patient), a higher pressure can be safely used for the secondary refrigerant  128 ,  130  than the primary refrigerant  124 ,  126 . 
         [0024]    The high pressure primary refrigerant  124  then flows into the recuperator  120  where it is further cooled by the low pressure primary refrigerant  126  returning from the manifold  112 . The low pressure primary refrigerant  126  is colder than the high pressure primary refrigerant because it has undergone Joule-Thompson expansion in the plurality of probes  114 . Recuperator  120  is preferably incorporated into the cryostat  110 . Alternatively, tubing coils inside each probe  114  may act as recuperative heat exchangers in order to reduce insulation requirements and return low pressure refrigerant to the console. 
         [0025]    After leaving the recuperator, high pressure primary refrigerant  124  flows into the manifold  112 , where it is distributed into multiple flexible probes  114 . In one presently contemplated embodiment, six probes are connected to the manifold, but one of skill in the art will recognize that greater or fewer probes may be used depending on the needs of a particular procedure. In each probe  114 , the refrigerant  124  flows into a Joule-Thompson expansion element, such as a valve, orifice, or other type of flow constriction, located near the tip of each probe  114 , where the refrigerant  124  is expanded isenthalpically to a lower temperature. In one presently preferred embodiment, the Joule-Thompson expansion elements are capillary tubes. The refrigerant then cools a heat transfer element mounted in the wall of the probe, allowing the probe to form ice balls that freeze diseased tissue. The refrigerant then follows low pressure primary refrigerant path  126 , exits the manifold  112 , travels through the recuperator  120  (where it serves to further cool the high pressure primary refrigerant  124 ), flows past the precooler  118  and back to the primary compressor in the console, where it is compressed back into high pressure refrigerant  124  so that the above process can be repeated. 
         [0026]    The present invention replaces the manifold system and the electric heater with a valve control system for independent thermal control of each probe. Referring now to  FIG. 3 , a cryosurgical system  200  for eight Joule-Thomson cryoprobes incorporating an individual control system is illustrated schematically. In general, high pressure primary refrigerant  124  is divided into a separate fluid path for each respective probe after passing through oil separator filter  201 . In the embodiment illustrated in  FIG. 3 , eight separate refrigerant lines  224   a - h  are included. After primary refrigerant  124  is divided into refrigerant lines  224   a - h , a probe control valve  202  is inserted into each line. The probe control valve  202  is a three way valve, preferably a three way solenoid valve, for selectively directing gas away from cryostat  210 . Gas directed away from cryostat  210  is directed ultimately back to gas mix compressor  203 . Valves  202  can each selectively allow all gas to pass through into the probes, reroute all gas back to the compressor  203 , or allow a predetermined amount of gas to both the probes and the compressor  203 . Return flow to compressor  203  of refrigerant lines  224   a - h  first passes through restrictor  204  in each respective line for mass flow balancing of the entire system  200 . Restrictor  204  can be, for example, capillary tubing, orifices, or needle valves. Refrigerant lines  224   a - h  are then combined to a single refrigerant line  205 . The combined refrigerant line  205  is in communication with oil separator filter  201  by way of adjustable solenoid valve  206  for pressure balancing. Refrigerant line  205  is directed through gas mix  207  before entering gas mix compressor  203 . Refrigerant line  205  can also include a bypass flow heat rejecter for rejecting excess heat in the refrigerant returning to the compressor. 
         [0027]    When flow bypass valves  202  are closed, refrigerant lines  224   a - h  enter the cryostat  210  and each line is cooled by high pressure secondary refrigerant  128 . A secondary refrigerant line  128  flows through oil separator  229 , then into condenser  230 . Secondary refrigerant line  128  is expanded to a lower temperature through capillary  231  and then directed to the pre-cool heat exchanger  218 . The resulting low pressure secondary refrigerant  236  then returns to the secondary compressor  232  to be repressurized. Since the secondary refrigerant  128  does not flow into the probes  214  (which are brought into direct contact with the patient), a higher pressure can be safely used for the secondary refrigerant  128 ,  230  than the primary refrigerant lines  124 . 
         [0028]    Cryostat heat exchanger module  210  may contain both a pre-cool heat exchanger, or pre-cooler  218 , and a recuperative heat exchanger, or recuperator  220  for each refrigerant line  224   a - h  respectively. A vacuum insulated jacket  222  surrounds the cryostat  210  to prevent ambient air from warming the refrigerant within the cryostat  210  and to prevent the outer surface of the cryostat  210  from becoming excessively cold. 
         [0029]    The high pressure primary refrigerant lines  224   a - h  direct primary refrigerant  124  into the recuperator  220  where it is further cooled by the low pressure primary refrigerant lines  226   a - h  returning from the probes  214 . The low pressure primary refrigerant lines  226   a - h  are colder than the high pressure primary refrigerant lines  224   a - h  because a low pressure primary refrigerant has undergone Joules-Thompson expansion in the probes  214 . Recuperator  220  is preferably incorporated into the cryostat  210 . Alternatively, tubing coils inside each probe  214  may act as recuperative heat exchangers in order to reduce insulation requirements and return low pressure refrigerant to the console. 
         [0030]    After leaving the recuperator  220 , high pressure primary refrigerant  124  flows into the vacuum insulated bellows section  223 . Instead of the typical manifold where refrigerant is distributed into multiple flexible probes, the present invention utilizes couplers  225  to provide for the connection of disposable probe ends for contamination protection and durability. In one presently contemplated embodiment, eight probes  214  are individually connected to the gas mix compressor  203 , but one of skill in the art will recognize that greater or fewer probes may be used depending on the needs of a particular procedure. In each probe  214 , high pressure primary refrigerant  124  flows into a Joule-Thompson expansion element  227 , such as a valve, orifice, or other type of flow constriction, located near the tip of each probe  214 , where the high pressure primary refrigerant  124  is expanded isenthalpically to a lower temperature. In one presently preferred embodiment, the Joule-Thompson expansion elements  227  are capillary tubes. A low pressure primary refrigerant  228  then cools a heat transfer element mounted in the wall of the probe  214 , allowing the probe to form ice balls that freeze diseased tissue. The low pressure primary refrigerant  228  then follows low pressure primary refrigerant lines  226   a - h  and travels through the recuperator  220  (where it serves to further cool the high pressure primary refrigerant  124 ), flows past the precooler  218  and back to the primary compressor  203  in the console, where it is compressed back into high pressure primary refrigerant  124  so that the above process can be repeated. The present invention requires active control of the valves  204  to maintain mass flow through the system when one or more individual probes are turned off. 
         [0031]    In an alternate embodiment, as illustrated in  FIG. 4 , the present invention includes a method to reverse the flow of the pressurized gas to avoid the heat exchangers so that hot gas can enter the probe for thawing the iceballs. The hot refrigerant gas flowing from the gas mix compressor is warm enough to heat the probes but it must be directed to the probes without flowing through the heat exchanger system. 
         [0032]    As the heat cycle occurs after cooling, the system first must have the capability to individually cool each probe. Referring now to  FIG. 4 , a schematic view of a cryosurgical system  300  for eight Joule-Thomson cryoprobes  314  is illustrated incorporating an individual heating and cooling control system. In general, high pressure primary refrigerant  124  is divided into a separate fluid path for each respective probe after passing through oil separator filter  301 . In the embodiment illustrated in  FIG. 4 , eight separate refrigerant fluid lines  324   a - h  are included. After primary refrigerant  124  is divided into refrigerant lines  324   a - h , a probe control valve  302  is inserted into each line. The probe control valve  302  is a three way valve, preferably a three way solenoid valve, for selectively directing gas away from cryostat  310 . Gas directed away from cryostat  310  is directed ultimately back to gas mix compressor  303 . Return flow of high pressure primary refrigerant  124  first passes through restrictor  304  in each respective line for mass flow balance of the entire system  300 . Refrigerant lines  324   a - h  are then combined to a single refrigerant line  305 . The combined refrigerant line  305  is in communication with oil separator filter  301  by way of adjustable solenoid valve  306  for pressure balancing. Combined refrigerant line  305  is directed through gas mix dryer  307  before entering gas mix compressor  303 . 
         [0033]    When flow bypass valves  302  are closed, high pressure primary refrigerant  124  enters the cryostat  310  and each refrigerant line is cooled by high pressure secondary refrigerant  328 . High pressure secondary refrigerant  328  flows through oil separator  329 , and then through condenser  330  before it is expanded to a lower temperature through capillary  331 . Secondary low pressure refrigerant  336  is then directed to pre-cool heat exchanger  318 . The resulting low pressure secondary refrigerant  336  then returns to the secondary compressor  332  to be repressurized. Since the secondary refrigerant  328  does not flow into the probes  314  (which are brought into direct contact with the patient), a higher pressure can be safely used for the secondary refrigerant line  128  than the primary refrigerant lines  324   a - h.    
         [0034]    Cryostat heat exchanger module  310  may contain both a pre-cool heat exchanger, or pre-cooler  318 , and a recuperative heat exchanger, or recuperator  320  for each refrigerant line  324   a - h  respectively. A vacuum insulated jacket  322  surrounds the cryostat  310  to prevent the ambient air from warming the refrigerant within the cryostat  310  and to prevent the outer surface of the cryostat  310  from becoming excessively cold. 
         [0035]    The high pressure primary refrigerant  124  then continues into the recuperator  320  where it is further cooled by the low pressure primary refrigerant  338  returning from the probes  314 . The low pressure primary refrigerant  338  is colder than the high pressure primary refrigerant  124  because it has undergone Joule-Thompson expansion in the plurality of probes  314 . Recuperator  320  is preferably incorporated into the cryostat  310 . Alternatively, tubing coils inside each probe  314  may act as recuperative heat exchangers in order to reduce insulation requirements and return low pressure refrigerant to the console. 
         [0036]    After leaving the recuperator  320 , high pressure primary refrigerant  124  flows into the vacuum insulated bellows section  323 . Instead of the typical manifold where refrigerant is distributed into multiple flexible probes, the present invention utilizes couplers  325  to provide for the connection of disposable probe ends for contamination protection and durability. In one presently contemplated embodiment, eight probes  314  are individually connected to the gas mix compressor  303 , but one of skill in the art will recognize that greater or fewer probes may be used depending on the needs of a particular procedure. In each probe  314 , the high pressure primary refrigerant  124  flows into a Joule-Thompson expansion element  327 , such as a valve, orifice, or other type of flow constriction located near the tip of each probe  314 , where the high pressure primary refrigerant  124  is expanded isenthalpically to a lower temperature. In one presently preferred embodiment, the Joule-Thompson expansion elements are capillary tubes. The low pressure primary refrigerant  338  then cools a heat transfer element mounted in the wall of the probe  314 , allowing the probe to form ice balls that freeze diseased tissue. The low pressure primary refrigerant  338  then follows low pressure primary refrigerant line  326   a - h , travels through the recuperator  320  (where it serves to further cool the high pressure primary refrigerant  124 ), flows past the precooler  318  and back to the primary compressor  303  in the console, where it is compressed back into high pressure refrigerant  124  so that the above process can be repeated. The present invention requires active control of the valves  304  to maintain mass flow through the system when one or more individual probes  314  are turned off. 
         [0037]    After the cooling cycle has begun, the high pressure primary refrigerant  124  can be used to rethaw the probes  314 . High pressure primary refrigerant  124  passes through oil separator filter  301  before high pressure primary refrigerant  124  is divided into a separate fluid path for each respective probe  314 . However, to warm the probes  314 , high pressure primary refrigerant  124  flows into a three way control valve  340  that selectively directs high pressure primary refrigerant  124  to bypass the precooler  318  and recuperator stage  320  of cryostat  310 . High pressure primary refrigerant  124  flows through two way valve  342  that is selectively in communication with pressure relief needle valve  343  that allows excess high pressure primary refrigerant  124  to flow back to gas mix compressor  303  under certain pressure conditions. 
         [0038]    High pressure primary refrigerant  124  then continues into the heat exchanger  320  through three way diverter valve  344  from where high pressure primary refrigerant  124  is divided into flow refrigerant lines  326   a - h  and then directed to probes  314 , respectively. The reverse flow scheme avoids the capillary tubes  327  before the probe tips. On the return flow, the refrigerant lines  326   a - h  can be directed back to the original return path at valve  302 . It is envisioned that the reverse flow line could include a heater element for increasing the temperature of high pressure primary refrigerant  124 . It is further envisioned that the lines could be insulated to decrease heat loss of high pressure primary refrigerant  124 . 
         [0039]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it will be apparent to those of ordinary skill in the art that the invention is not to be limited to the disclosed embodiments. It will be readily apparent to those of ordinary skill in the art that many modifications and equivalent arrangements can be made thereof without departing from the spirit and scope of the present disclosure, such scope to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent structures and products.