A. On hepatobiliary phase, 12-mm HCC was seen in S6 (arrows). B. On portal venous phase of post-RFA CT, 52-mm ablative zone was observed in corresponding area (arrowheads). Ablative margin was considered to be sufficient on visual inspection. C. After non-rigid, deformable registration of pre-RFA MRI (left) and post-RFA CT (right) using software, ROI was drawn around index tumor manually on pre-RFA MRI (left, orange circle), and ROI was simultaneously copied on post-RFA CT (right, inner circle) with 5-mm ablative margin (right, outer circle). In software-assisted inspection, ablative margin was determined to be insufficient. However, second-look RFA did not proceed due to unstable vital signs under conscious sedation. D. On ten-month follow-up MRI, LTP (arrows) was observed along mediosuperior margin of ablative zone (arrowheads), and was considered insufficient on software-assisted inspection. CT = computed tomography, HCC = hepatocellular carcinoma, LTP = local tumor progression, MRI = magnetic resonance imaging, RFA = radiofrequency ablation, ROI = region of interest