Gel phantom study of a cryosurgical probe with a thermosiphon effect and liquid nitrogen-cooled aluminum thermal storage blocks

Abstract

Cryosurgery is a minimally invasive treatment for certain types of cancers. Argon-based cryosurgical devices are available at present, however a large compressed gas cylinder with the pressure of 300 atmospheres is needed. To overcome these drawbacks, we developed a new cryosurgical probe measuring about 50 cm in length with separate lumens inside for liquid and gaseous ethylene to be used as a thermosiphon and liquid nitrogen-cooled aluminum thermal storage blocks. The probe needle was 8 cm in length and 3 mm in outer diameter. To investigate the freezing capabilities of our new cryosurgical system we inserted the needle 5cm into a poly-acrylamide gel phantom warmed to 36.5 ℃. Thermal storage blocks made of aluminum, cooled at -196 ℃ in liquid nitrogen, were attached to the condenser of the probe and replaced with thermal storage blocks every 4 to 5 minutes to compensate for warming. We took digital camera images of the ice ball at the needle and measured the temperature in certain locations of the cryoprobe. Ice ball formation started at one minute after cooling. The sizes (longest diameter × minimum diameter) at 10, 20 and 30 minutes after the start of the procedure were 4.5×2.1, 4.5×3.1 and 4.6×3.7 cm, respectively. During the procedure the minimum temperature of the condenser was -85 ℃ and the needle was -65 ℃. This newly developed compact cryosurgical probe with thermosiphon effect and cooled thermal storage blocks created an ice ball that can be used for cryosurgery within 20 minutes.

Keywords: cryoablation; cryosurgery; cryotherapy; phantom study; thermosiphon.

Fig. 1

Thermosiphon cryoprobe. Thermosiphon effect utilizes convection and gravity so the condenser must be placed at a higher position than the needle during cryosurgical procedure. The upper figure shows external appearance of a thermosiphon cryoprobe and the lower figure indicates the schema of its internal structure. The cryoprobe consists of the needle, the body and the condenser. There is an inner pipe from the body to the needle and gas conduits are located from the upper portion of the outer part of the inner pipe to the condenser. The inner pipe and the gas conduits allow liquid and gaseous ethylene to flow separately. Thick and thin arrows indicate the flow directions of liquid and gaseous ethylene, respectively.

Fig. 2

Thermal storage block. The upper figure shows one set of unfolded thermal storage blocks consisting of two aluminum semicircular columns (a) combined by hinge braces (arrowhead). Lower figure indicates another set of unfolded thermal storage blocks touching the medial side of the condenser (left) and another set of attached thermal storage block fixed by a set of clips (arrow) at the lateral side of the condenser (right). a, aluminum block; b, the body of the cryoprobe; c, the condenser of the cryoprobe; arrowhead, hinge brace; arrow, clip

Fig. 3

Experimental devices used in this study. a, cryoprobe; b, thermal storage block; c, probe holder; d, poly-acrylamide gel phantom; e, ice ball; f, warm water circulation system; g, container filled with warmed water; h, stopwatch; i, digital camera; j, ruler

Fig. 4

Ice ball formation in the warmed gel phantom at 36.5 °C with the aid of a cryosurgical probe with a thermosiphon effect and liquid nitrogen-cooled aluminum thermal storage blocks from the beginning to 30 minutes after the start of cryosurgical procedure. Ice ball formation started around the middle portion (around the orifice of the inner pipe) (arrowhead) of the needle of the cryoprobe (arrow) one minute after the start of cryosurgical procedure (b). Ice ball increases in size over time. Images of a, b, c, d, e, f, g and h indicate the images obtained before procedure and 1, 5, 10, 15, 20, 25 and 30 minutes after the start of cryosurgical procedure.

Fig. 5

Temperature monitoring of several parts of the needle of the cryoprobe during cryosurgical procedure. The temperature of the condenser reached –85 °C, the needle portion outside of the warmed gel reached –78 °C and middle portion (around the orifice of the inner pipe) of the needle reached –65 °C 20 minutes after the start of freezing. During replacement of the liquid nitrogen-cooled aluminum thermal storage blocks every 4 or 5 minutes the needle and condenser become warmer. In the graph, the letters "a" to "e" correspond to the needle portions in legend symbol at the top of the figure.