Quantification of skin motion over the liver using optical tracking

The purpose of this study was to quantify skin motion over the liver when patients are repositioned during image-guided interventions. Four human subjects with different body habitus lay supine on the interventional radiology table. The subjects held their arms up over their heads and down at their sides for 13 repositioning trials. Precise 3-D locations of the four skin fiducials permitted deformable skin motion to be quantified. For the first two occasions, the average skin motion was 1.00±0.82 mm in the arms-up position and 0.94±0.56 mm in the arms-down position, a small, but not statistically significant difference. Three out of the four subjects exhibited increased skin motion in the arms-up position, suggesting that patient-positioning technique during CT imaging may have an effect on the skin-motion component of registration error in image-guided interventions. The average skin motion was 0.65±0.39 mm for Subject 1 and 1.32±0.78 mm for Subject 2, a significant difference. Subjects 3 and 4 demonstrated a similar amount of skin motion (0.86±0.55mm and 0.89±0.59mm, respectively). The subject with the largest body habitus demonstrated significantly less skin motion, an observation that is difficult to explain. The skin fiducial on the xiphoid process exhibited significantly less skin motion than the other fiducials, suggesting that certain anatomic locations could influence motion of the fiducial, and subsequently, the introduced error.