Ultrasonic Bone Tomography in Live Patients

Abstract

The work described in this thesis involves some of the instrumentation development undertaken for the study of bone mineral loss in the ulna, with a view to being able to quote the actual bone mineral density, with a high resolution, An apparatus for the bone mineral estimation, using x-ray scanning technique has been developed in this Department by J.P. Edwards, The major part of the present project is to develop a means for providing an outline of the cross-section of the ulna, at the level at which the x-ray scanning is performed, by using ultrasonic tomography. The system involves two matching ceramic bowl transducers (PZT-5 A) attached vertically to the bottom of a rotating drum in which all the electronic circuit elements are mounted. One transducer is used as a reference, with a fixed reflecting metallic rod present at its focus, and the other is used as a scanning transducer, Both transducers are immersed in a water bath, which is designed to accommodate an arm vertically, without discomfort to the patient. The limb is held normal to the axis of symmetry of the scanning transducer, in such a way to allow for a smooth rotation of the latter around the limb. The time for a whole complete rotation of 360° is about 25 seconds. An electronic switching technique is used to turn on both transducers simultaneously as transmitters in order to transmit a long duration pulses (120µs) of 6.85 MHz ultrasonic waves, and then as receivers for their corresponding returning echoes. Provision has been made for electrical chopping of the start of the received wave-trains, which removes about 10-15µs worth, The difference in the delay times of the two echoes, which is proportional to the difference in their path lengths, is obtained by comparing the time abrupt cessation of the tail ends of the received signals. Considerable. work has been carried out with regard to the construction of backing and matching layers, but the cessation of the wave-trains could not be made abrupt enough to provide sufficiently distinct events, The abrupt event, however, has been achieved by using a shock excitation with sharp onset, at the end of the transmitted pulses. Signals derived from the reflections of these shocks are amplified and fed into a discriminator which determines which of the two pulses returns earlier, The information from the discriminator provides the input for a servomechanism designed to maintain the scanning bowl focus on the bone surface, by appropriate radial movement. The display system involves a direct mechanical coupling of the servomechanism to a pen, which gives a plot of the outline of the ulna cross-section on a drawing paper attached to the frame of the apparatus with a magnification ratio of 10:1. In this technique, the outline of the ulna cross-section will be carried out in two separate scans, each of 270°; one with the arm pronated, and the other in the supinated position. The two scans are combined to give the complete outline of the cross-section of the ulna, The time for each scan is about 18 seconds, This short scan time means that distortions of the observed profile due to movement by the patient are minimized. It was recognised that the ultimate lateral resolution of the system, depends on the diffraction pattern of the concave bowl transducer, This in turn is dependent on the geometry of the bowl, but as has been shown in Chanter II, is a function of the type of excitation; i.e. continuous wave or impulse, and in the case of the latter the bandwidth of the receiving system also. Consideration has also been given to the limit of axial resolution, which depends on the time discrimination of the two echoes, in Chapter II. In connection with the trials for construction of matching layers, a new method has been developed (Chapter IV) for determining the velocity of sound in solid materials, by using a thin tapered wedge and a converging beam of high frequency ultrasonic waves. In addition to this main project, considerable work has been carried out in the development of an artificial bone-like material (Appendix A), in order to be used as a reference for the bone densitometer.