ALGEBRAIC RECONSTRUCTION OF SPATIAL DISTRIBUTIONS OF REFRACTIVE INDEX AND ATTENUATION IN TISSUES FROM TIME-OF-FLIGHT AND AMPLITUDE PROFILES.

Two-dimensional distributions of refractive index and attenuation were measured in transverse sections through intact isolated organs, using reconstruction techniques. Profiles of time-of-flight (TOF) and/or amplitude of 10 MHz pulses through the specimen were obtained by rectilinearly scanning two opposing transducers along either side of the specimen in the plane of interest. The received pulses were digitized at a rate of one 8-bit sample per 10 ns for 512 samples, and were analyzed with a computer algorithm which calculated the TOF of the pulse to within plus or minus 10 ns and/or its amplitude. TOF's through tissue, normalized by TOF through water, were used to calculate velocity and, hence, refractive index within the specimen, using an algebraic reconstruction technique (ART). A similar approach was taken to calculate attenuation distributions. Images obtained represented acoustic velocities and/or attenuation in individual cross sections within the tissue specimen with a resolution of 64 by 64 elements ( less than 2 mm**2). The disadvantage of TOF and attenuation reconstruction is that transmission scanning is rquired. Advantages over B- and C-scan imaging are: (1) dynamic changes in receiver gain are not required; (2) attenuation occurs on only one traversal through tissue; and (3) the absolute values of important acoustic parameters (velocity and attenuation) are determined which may have significant diagnostic value.