Because of its nature as an NSS, destruction of tumor tissue must be complete while minimizing damage to the surrounding healthy parenchyma. Expansion of the iceball into other adjacent organs, such as the colon, is also a concern. To achieve NSS, real-time monitoring of the cryolesion is very important. Open and laparoscopic cryosurgery permit monitoring by direct observation, ultrasonog-raphy, and thermocouples. Direct observation of the iceball during the freeze-thaw cycle is the simplest monitoring. The periphery of the iceball can be estimated by the change in tissue appearance, as well as the formation of ice crystals on the surface of the frozen tissue (Fig. 2). Intraoperative ultrasonography accurately delineates tumor size, cryoprobe placement, and depth of freezing
. An anechoic, avascular sphere with an advancing, hyperechoic rim characterizes the real-time intraoperative ultrasound image of renal cryolesions (Fig. 3) . However, ultrasound monitoring of the complete iceball is difficult because the frozen tissue shows an acoustic shadow, resulting in visibility of only the near edge of the iceball. Only the placement of thermocouples can allow an accurate predictor of tissue destruction as tissue temperature. In magnetic resonance imaging (MRI)- or computed tomographic (CT)-guided cryoablation, each axial imaging becomes the good monitoring modality. With MRI, the short T2 of ice leaves the frozen region black against a background of normal tissue
. CT imaging detects a sharply defined margin around frozen tissue, with a decrease of attenuation of approximately 30 Hounsfield unit (HU) in frozen tissue compared with adjacent unfrozen tissue .
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