Computerized methods for X-ray-based small bone densitometry

Animal models have been widely used to correlate in vivo changes in bone mineral density (BMD) with changes in disease state of bone. In small animal models, e.g. the hindlimb suspension model of bone loss, a non-invasive assessment of BMD is required. X-ray radiography has been surpassed in some cases by quantitative computed tomography (QCT) and dual X-ray absorptiometry (DEXA) quantitation. However, there are drawbacks in using the computerized methods, especially for small animals. In this paper, we present image-processing algorithms to quantitatively determine bone area and mineral density in digitized radiographs. Image calibration is based on a calibration step wedge, and the algorithm automatically detects the steps and computes the calibration data. In addition, we demonstrate how the algorithm can accurately determine the cortical outline of the bone and provide reliable data and statistics for small animal studies. A downloadable implementation example for the popular NIH Image package is provided.