Digital dynamic range expansion applied to X-ray densitometric analysis of total hip replacement

Due to the presence of the stiff prosthetic stem fitted in the medullary canal during total hip replacement, the surrounding cortex of the femur changes its density over time. This bone remodelling takes place with every type of total hip prosthesis; however, its intensity may vary between prostheses and patients. In the worst cases this process can lead to the late failure of the implant. To monitor such bone density evolution, we are developing a tailored Computer-aided Densitometric Image Analysis system (the major part of this our system uses an 8-bit commercial hardware with 256 levels of grey). The equivalent dynamic range of an X-ray picture is about 10 bits. In this paper we present a method to overcome these hardware limitations by improving the software. Using a double-exposure acquisition it is possible to build a 9-bit image that is good enough for most applications involving bone density measurement.