Figure 17a. US guidance of hepatic cryoablation. (a) Sonogram shows an echogenic 5-mm cryoprobe (arrow) placed centrally within a relatively isoechoic colon metastasis (arrowheads). (b) Sonogram obtained at the partial freeze stage (ie, at 3 minutes) demonstrates that the ice ball (arrow) has extended to the lateral margin of the tumor, but the anterior margin (arrowheads) is still visible. (c) Sonogram obtained at the complete freeze stage (ie, at 8 minutes) shows the ice ball (arrow), which now completely encompasses and extends beyond the anterior margin of the tumor, indicating a successful ablation.