Artifacts and thresholding in X-ray CT of a cortical bone and titanium composite.

PURPOSE: X-Ray CT has the potential to provide precise and accurate data from which the mechanical properties of bone can be calculated. Such data would be useful in understanding the response of bone tissue to implants. Various artifacts can, however, degrade accuracy of the data. The aim of this study was to measure the artifactual errors produced in CT of a simulated femoral bone-titanium composite and to propose a method to correct for them.
METHOD: A composite phantom that simulates cortical bone and a titanium implant was designed and constructed. The phantom was scanned and the image data were analyzed over a range of thresholds with image analysis software developed for this study. The outer (OD) and inner (ID) diameter and the CT number of the cortical bone, with and without titanium, were measured over a range of cortical thicknesses.
RESULTS: While ID can be accurately measured by choosing the proper threshold (800 HU), OD, even at optimal threshold, will be underestimated by approximately 2%. If a proper threshold is selected, CT number can also be accurately determined. Errors of up to 5%, however, are produced by titanium unless corrected by proper threshold selection.
CONCLUSION: Intramedullary titanium is not a deterrent to obtaining accurate measurements of cortical bone dimensions and properties. Proper choice of thresholds for image analysis of CT scan data can yield accuracy and precision of < or = 2%.