Effects of CT threshold value to make a surface bone model on accuracy of shape-based registration in a CT-based navigation system for hip surgery

Abstract To clarify the effects of different CT threshold values used to make computer models on the accuracy of surface registration in a CT-based navigation system, a simulation study was performed using CT data of 30 patients who underwent total hip arthroplasty with navigation guidance. Surface models of the pelvis for use in clinical applications were made by contouring the periosteal boundary at threshold ranging from 140 to 260 Hounsfield units (HU) (mean, 200 HU). In each case, the threshold was determined to be approximately 200 HU, based on the balance between soft tissue noise and surface defects. Ten pelvic surface models were made from each set of CT data, using 10 different CT threshold values ranging from 50 to 320 HU with an increment of 30 HU. The center of the acetabulum was defined as the target point in each set of CT data, so that each set of 10 surface models should have the same reference point for measuring the positional and rotational differences among the models after registration. The average residue of registration reached its nadir was minimum with the 200 HU models, and there were no significant differences in residue of registration among the models made at thresholds ranging from 110 to 320 HU. The target registration errors for position and rotation both showed strong correlation with the residue of registration (R=0.879 and 0.880, respectively). We thus conclude that accurate surface registration can be obtained with computer models made at thresholds ranging from 110 to 320 HU by assessing the balance between soft tissue noise and bone surface defects.