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Image guided percutaneous thermal ablation is widely used for patients with primary or secondary liver tumors who do not qualify for surgical resection. The COVER-ALL study is a randomized Phase II clinical trial that evaluates the impact of using software aid in confirming probe position and ablation coverage. Current practice in the trial involves acquisition of a pre-procedure contrast enhanced computed tomography (CECT) scan for gross tumor volume (GTV) definition and non-contrast CT after probe placement, followed by a biomechanical model-based deformable image registration (Morfeus) between the two scans to map the GTV onto the non-contrast CT for position confirmation. CT scan length that covers the entire liver is needed for Morfeus. In this work, we investigated an alternative workflow with a reduced length non-contrast CT using image padding on the first 50 COVER-ALL trial patients. The full-length non-contrast CT was first cropped to a fixed thickness, ranging from 2.5-7.5 cm, along the GTV. The remaining volume was padded with the CECT based on intensity-based deformable image registration (DIR). Morfeus DIR was performed between the CECT and resultant padded non-contrast CT to map the GTV segmentation from CECT to padded non-contrast CT. The GTV mapping results were compared to the original GTV mapping results performed on the full-length CT. The median mapping differences using cropping thickness of 7.5 cm was 1.2 (0.5-2.3) mm, with only 3 cases having larger than 5 mm discrepancies. The comparable DIR performance suggests the feasibility of acquiring a reduced-length non-contrast CT to maintain image registration accuracy.