Abstracts

Rationale: Regular cryogenic probes cannot be used deep inside of the brain without the risk of damage to nearby healthy tissue. Thermoablation as well as the Gamma Knife cannot guarantee the size of ablated tissue with reasonable accuracy. We have developed, manufactured and tested a set of new cryogenic probes specifically for the surgery deep inside of the brain. The probe represents a powerful freezer located in the very tip of the probe needle while the main part of the needle does not affect the surrounding tissue due to internal thermo-insulation. In this study, we aim to ensure the high precision of cryogenic surgery and a feasibility of monitoring of the frozen tissue deep inside of the brain via MRI. Methods: The freezing occurs as a result of boiling refrigerant inside the micro-evaporator, which is located inside the cryogenic needle. The boiling process is a phase transition which characterized by a stable boiling point. In addition, the body temperature is also stable. Therefore, the frozen volume of tissue is accurately given by the cryogenic needle diameter and boiling temperature of the refrigerant. To avoid potential risks during the MRI procedure, the cryogenic probe should be made of titanium and other metals with low paramagnetic properties or diamagnetic. Titanium, copper, and brass were selected as result of testing the different metals inside of a MRI scanner to construct the new cryogenic probe. Results: Three cryogenic probes with multiple tip diameters were used in the freezing experiment with a gelatin-agar mixture. The experiment shows that the size of the ice ball is rigidly specified by the cryogenic probe diameter and corresponds to the calculated value. The results were confirmed by similar experiments with the rat brain in vivo. The cryogenic probe is compatible with MRI measurements consisting of two parts: refrigerant tank and the cryogenic probe itself. Both parts are connected by flexible copper micro tube. This design is selected to reduce the amount of metal inside the MRI scanner. A special adapter was made to attach the probe to the standard stereotactic frame. Conclusions: The size of the ablated area is uniquely determined by the cryogenic needle diameter and boiling point of the refrigerant. The titanium-based cryogenic probe allows the use of MRI measurements during the course of cryogenic ablation.