Accuracy of a pulse-echo boundary tomogram

An ultrasonic tomogram was developed with a view to providing the outline of a cross section of the ulna in vivo. The construction and design of the instrument was undertaken to obtain a resolution as close as possible to theoretical limits. The apparatus was constructed to be used in conjunction with a system for assessing bone mineral content using a conventional clinical x-ray set. It had been expected that the information from the two instruments would make it possible to obtain a precise spatial plot of bone density. It was thought that the tomograph would respond only to reflections at the bone--soft tissue interface, where there is a large change in acoustic impedance, but in vivo trials have shown that other soft tissue interfaces give rise to reflection, which makes the performance unsatisfactory. However, some interesting results have been obtained from solid objects in water, which have shown that there is a limitation on the accuracy of the plot depending on the angle of incidence of the ultrasonic beam and not dependent on errors of mechanical or electronic origin. Some theory is put forward to account for irregularities in plots for these objects.