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(Continued from page 82)
MONITORING ICE BALL FORMATION
When cryosurgery is utilized for destruction of tissue in silent brain areas, the extent of the lesion can only be hypothesized to a distance based upon the characteristics of the probe, tissue and procedure. It is desirable to be able to observe the cryosurgical process so that the surrounding tissue is not destroyed with the lesion. This monitoring can be accomplished with ultrasound, MRI and CT scanning. Ultrasound is extremely versatile and mobile. Its increased quality enables the operator to see the edge of the ice ball clearly. The freezing boundary becomes hyperechoic and the artifact from the probe become decreased over 1 to 3 minutes. Then from 2 to 3 minutes the ice ball decreases in size. The size of the lesion on ultrasound is less than the size of the histological verified lesion by 1-2 mm. This difference may be due to tissue that is initially not destroyed by the cold but later necroses (23). The area being transduced is limited and a bone window with a liquid interface is necessary.
MRI can image an entire region, is also non invasive and can be performed quickly. It is limited by procedures that can be done in the scanner. Open MRIs have only recently become available thus allowing more access to the patient during the procedure. However, the procedure itself must still be performed in the MRI unit. Instruments must be non magnetic as should any support equipment. Although MRI can be performed in real time there would be a delay of no less than two and one half minutes, during which time the ice ball can change in size. Rubinsky (26) tested the feasibility of MRI in cryosurgery by utilizing a 2.35 Tesla magnet on rabbit brains. This specimen had a freeze thaw cycle of 8 and 5 minutes respectively, with a freezing interface velocity of 0.5 mm/min. The images from this small study subject were acquired every 2.5 minutes or 1.5 mm. The freezing interface was easily distinguished by a high contrast change. When thawed, longer acquisition scans were obtained which revealed edema at the cryolesion boundary. When the sections from histology were compared to imaging, the histological lesion extended 1-2 mm beyond the image of the frozen region and therefore MRI underestimated the size of the lesion just as did ultrasound. Therefore, when the freezing front moves slower than 1 mm/min, when the lesion size can be +/- 2 mm and when the operation can be performed inside an MRI scanner it may be feasible to use this modality to monitor the generation of a stereotactic lesion.
CT scanning demonstrates a well demarcated radiolucent sphere that disappears upon thawing. When compared to real time cine CT, the post thaw CT enhanced scan reveals an zone of enhancement no more than 1 mm beyond the real time frozen image, once again confirming the slightly under estimated size of the real time imaging as compared to the final lesion (19). Ultrasound, MRI and CT scanning all underestimate the size of the final lesion but probably accurately image the precise size of the ice (Continued on page 84)
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