Abstracts

Rationale: Cryogenics is used as a powerful tool in therapy (cooling) and surgery (freezing) of the brain. However, the application field of cryogenics is mainly restricted by the surface of the brain. Cryogenics still does not possess a specific feature of radiation surgery, because regular cryogenic probes cannot penetrate deep inside the brain without the risk of damage to nearby healthy tissue. We aim to ensure the feasibility of using cryogenic exposure in the depths of the brain to destroy abnormal or diseased tissue with minimal damage to the nearby healthy neurons. In this study, the new micro cryogenic probe has been developed specifically for the brain surgery. The micro probe is capable of a deep freeze at its very tip while the main part of the probe is heat-insulated and does not affect the tissue. Methods: The method of freezing is based on the phase transition - liquid refrigerant is boiling inside of the cryogenic probe. The silver tip at the end of the probe provides a good heat contact between the inner and outer sides. The rest of the probe surface has a thermal isolation. The combination of a silver tip and thermal isolation, provides the cryogenic probe with a high freezing rate at its tip, while the main part of the probe does not affect the healthy tissue during the operation. Commercially available refrigerant with a boiling point of -50^oC is used in the experiment. Since the phase transition is a natural stabilizer, freezing temperature is maintained a constant with high accuracy. The dimensions of the probe were chosen equal to or less than the electrodes used in neurosurgery, and ranged in outer diameter from 0.7 mm to 2.5 mm. Results: The cryogenic probe with a diameter of 0.7 mm was used to monitor the freezing of a mixture of agar and gelatin gels. The temperature of the mixture was maintained at 36.6^oC by means of water bath. Once freezing starts, an ice ball is gradually growing during about 5 minutes and then becomes stable when reaching the size of about 4.5 mm, as is seen in Figure1. As is evident from the theoretical model, the ice ball dimension depends on probe diameter and boiling point of the refrigerant. For the given boiling point of refrigerant and probe diameter, the experimental result is in good agreement with theoretical estimation. A similar experiment was performed with the rat brain in vivo. Two weeks after freezing, the rat brain was extracted and sliced for examination. As seen from Figure 2, size of ablated area of the brain correlated with the frozen gel ball. Conclusions: A new reliable cryogenic probe that satisfied the rigid conditions of neurosurgery has been developed. The probe is able to ablate by means of freezing a specified volume deep inside of the brain without cryogenic damage to healthy tissue. Minimally invasive treatment is caused by the small diameter of the probe and proper thermal isolation of its passive part.