Realization of the Low Frequency Transmission Beam Computed Tomography for the Abdominal Tissue Sound Speed Imaging

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Ultrasound travel time tomography for the quantitative sound speed image of the human abdominal tissue medium has been investigated. The realization of the present technique becomes difficult due to the large sound wave attenuation when transmitting the wave within an internal organ. In addition, existence of the spinal cord violates the weak scattering assumption assumed in the reconstruction of the image. To encounter the problems, low frequency transmission tomography was introduced to decrease the attenuation of the sound wave. Here, travel time along the path between the transmitter and receiver was assumed to be path integration of the inverse sound speed distributions. By this means, the image can be reconstructed from the data whose information is confined along the ray paths, as a result, the signal from the spinal cord can be excluded. As a means to reconstruct the image with small number of observation path data as well as excluding the spinal cord region, smoothed path algebraic reconstruction technique (SPART) was proposed. The validity of the method was demonstrated through the numerical simulation examination. It was also demonstrated that travel time of the transmitted waves by the narrow apertured transducers on the human body surface met well with the straight ray propagation model. Finally, it was shown that the rough estimate of the sound speed image could be obtained by using only 16 transducer elements and their combination of 120 small number of path data.