Estimation of time of flight for ultrasonic reflex-transmission tomography with active contour models

The histological significance of acoustic speed, together with acoustic attenuation, in differentiating breast carcinomas from healthy tissue is well founded. A reflex-transmission system comprising a metallic reflector and a linear array of a standard ultrasound system was developed, the two fixtures being mounted in such a way that they can describe a whole revolution about the object to be imaged thus enabling a tomographic reconstruction, whose accuracy depends largely on that of the time of flight data estimated on the basis of the first arrival signals reflected from the metallic plate. The estimation is an extremely intricate task, its multiple reflections, attenuation, refraction and speckle restrain the utility of conventional techniques like threshold detection and cross correlation. The problem was eluded by using active contour models, as they allow prior information to be formulated in the form of a cost functional, whose minimum will correspond to the optimum estimate for the time of flight.