MRI-MONITORED CRYOSURGERY IN THE RABBIT BRAIN

The inability to observe the transient, irregular shape of the frozen region that develops during cryosurgery has inhibited the application of this surgical technique to the treatment of tumors in the brain and deep in visceral organs. We used proton NMR spin-echo and spoiled gra dient-echo imaging to monitor the development of frozen lesions during cryosurgery in the rabbit brain and the resulting postcryosurgical ch anges up to 4 hr after freezing. Spoiled gradient-echo images (TE = 14 ms; TR = 50 ms) were acquired during freezing and thawing at a rate o f 15 s/slice. Although the frozen region itself is invisible in MR ima ges, its presence is distinguished easily from the surrounding unfroze n soft tissue because of the large contrast difference between frozen and unfrozen regions. T-2-weighted spin-echo images (TE = 100 ms, TR = 2s) obtained after thawing suggest that edema forms first at the marg in of the region that was frozen (cryolesion) and then moves into the region's core. Histological examination showed complete necrosis in th e cryolesion and a sharp transition to undamaged tissue at the margin of the lesion and its image. Blood-brain barrier (BBB) damage was inve stigated using gadolinium-DTPA. The region of edema in the T-2-weighte d spin-echo images was coincident with the area of BBB damage in the G d-DTPA-enhanced T-1-weighted spin-echo images (TE = 33 ms, TR = 400 ms ) and both were distinguishable as areas of high signal relative to th e surrounding normal tissue. The results of these experiments indicate that MR can both effectively monitor the cryosurgical freezing and th awing cycle and characterize the postcryosurgical changes in tissue du ring follow-up.