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Multipath Artifact Corrections in Ultrasonic Transmission Tomography1

This paper deals with computed imaging techniques for ultrasonic transmission tomography. Cross-sectional images of the refractive index in soft biological tissues can be obtained by measuring the arrival-time of signals that propagate between two transducers. An alternative method to obtain images is to measure the attenuation in the path between the pair of transducers. In practice, images of the refractive index exhibit a better quality than images of the attenuation coefficient. In this report we have shown through computer simulation that some of the degradations in images of the attenuation coefficient are due to the existence of multiple rays that link the transmitting and the receiving transducers. This condition is known as multipath. We have demonstrated two methods to reduce the influence of multipath on reconstructions of the attenuation coefficient. The first method employs homomorphic signal processing to estimate the attenuation coefficient for one of the linking rays. The second method applies median filtering in the projection space to remove the artifacts caused by multipath. The correction techniques were tested on data obtained by scanning tissue equivalent phantoms. Results obtained indicate that the correction methods significantly improve the quality of images of the attenuation coefficient.