Experimental studies in transmission ultrasound computed tomography.
The reconstruction of the speed-of-sound distribution within a target can be achieved by CT techniques from measurements on transmitted ultrasonic pulses. The mathematical relationship between speed-of-sound imaging and the conventional CT situation is explained. An experimental system, which has been developed to investigate speed-of-imaging and other forms of in-vivo ultrasound CT, is described, along with the techniques used for data acquisition and image reconstruction. These include measurement of pulse time-of-flight by the threshold or cross-correlation methods. Techniques for reducing artifacts in speed-of-sound images are also described, such as median filtering and modified Shepp-Logan filtering. These techniques have been used to obtain high quality speed-of-sound images of various phantoms. Images of tissue in-vitro have been less satisfactory, because of refraction and attenuation effects. Ways of overcoming these difficulties in an improved system are proposed.