[Sequential MRI and CT monitoring in cryosurgery--an experimental study in rats]

Cryosurgery is one of treatments of cancer, such as carcinomas of the face, oral cavity, prostate, breast, rectum and liver. But the method of cryosurgery seemed to be not yet completely established. The most important problem in this procedure is to define the extent of the cryolesion. But the trial with MRI and CT has not been reported. The purpose of this study is to investigate whether the image diagnosis such as Magnetic Resonance (MR) and CT is useful for the determination of the region during and after cryosurgery. The animal experiments were performed using 13 Wistar rats with inoculated Walker 256 cancer on both sides of femoral regions subcutaneously in the concentration of 1 million cells. After 7 days, cryosurgery was done by contacting for 15 sec with the absorbent cotton ball dipped in liquid nitrogen to the surface of right femoral tumor 3 to 6 times. The left side of tumor was intact. MRI was performed with a 0.1 Tesla ASAHI MR Mark-J and CT images were obtained using GE 9800. From the freezing to thawing, LF image (Low Flip Angle gradient echo image of Tr = 100 msec, Te = 18 msec, 60 degrees of flip angle) of MR and plain CT were taken every one minute. After thawing is over, SE image of Tr = 1500 msec, Te = 90 msec, IR images of Tr = 1200 msec, Td = 400 msec, and plain and contrast enhanced CT were carried out. Two and 7 days after cryosurgery, LF, SE and IR images of MR and CT with and without contrast medium were obtained with corresponding pathological examinations. The frozen cryolesion was of no signal intensity on the LF MR image and hypodensity on plain CT. Identification of cryolesion became possible during the cryosurgery. The cryolesion immediately after thawing showed higher intensity on the SE and LF images and hypodensity on enhanced CT. Therefore, the extent of cryosurgery can be diagnosed easily by these methods during and immediately after cryosurgery. In the follow-up studies after cryosurgery, the histological changes such as necrosis or tissue reaction were well represented by MRI and enhanced CT, but insufficiently by plain CT. From these experimental results, it may be concluded that MRI and CT are useful for monitoring the process of cryosurgery during and after the procedure.