Purpose The purpose of this study is to assess the effects upon lethal ice (< −30°C) proportions in different heat load phantoms while varying the size and number of cryoprobes at 2 cm spacing. Materials and Methods Thermocouples at 0.5, 1.0 and 1.5 cm intervals from 1.7 or 2.4 mm diameter cryoprobes were held by jigs accommodating 1–4 cryoprobes. Agar phantoms (N=24) used 3 sets of baseline temperatures at approximately 6°C, 24°C and 39°C. Temperatures during 15 minutes freeze cycles were correlated with actual thermocouple locations seen within the ice by computed tomography (CT). Diameters and surface areas of the −30°C lethal isotherm were assessed over time as percentages of the overall iceball. Results The high heat load, 39°C, phantom experiments showed the greatest impact upon percentage lethal zones for all probe configurations. Single, double, triple and quadruple probe arrangements of 2.4mm cryoprobes at 15 minutes had average lethal ice diameters of 1.2, 3.3, 4.1 and 4.9 cm, comprising 13 %, 46 %, 51 % and 56 % surface areas of lethal ice, respectively. Surface areas and diameters of lethal ice made by 1.7mm cryoprobes were 71%and 84% of the 2.4 mm cryoprobes, respectively. Lethal ice resides <1 cm behind the leading edge for nearly all probe configurations and heat loads. Conclusion Single cryoprobes have very low percentages of lethal ice. Multiple cryoprobes overcome both the high heat load of body temperature phantoms and help compensate for lower freeze capacity of thinner cryoprobes.

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