Noncontact Measurement of Bone Density Distribution Using Ultrasonic Computed Tomography : The 1st Report: Numerical Simulation and Experimental Measurement

Numerical simulations and experiments are carried out to apply ultrasonic computed tomography (CT) for a non-invasive measurement of bone density distribution. When an ultrasonic ray passes through such an object as bone, it propagates along refracted and curved paths. In such a case, the reconstruction methods commonly used will lead to inexact or even false results. To overcome this difficulty encountered in ultrasonic CT, the following procedure was developed. First, an approximate distribution of sound velocity is determined with usual reconstruction method on the assumption of straight-line propagation. Subsequently, after the corrected curved ray paths have been computed based on the result, a more exact sound velocity distribution can be reconstructed as the solutions of linear algebraic equations. This procedure is confirmed to be very effective from the present numerical simulation for a cylindrical object with high sound velocity. Through the measurements of the forefingers, the sound velocity distributions corresponding to the above first approximation could be obtained. The results indicate that the shape of the finger is enlarged due to the sound ray refraction. Because the present subjects are all young and healthy persons, there are no remarkable differences with respect to the maximum sound velocity of the finger