11B-3 A Unified Transmission/Reflection Acoustic Tomography Scheme for Small Animal Tissue Characterization

One of the central desiderata of the preclinical cancer research is a reliable imaging system making the noninvasive characterization of different tissue types possible. While spatially resolved reconstruction of speed of sound and acoustic attenuation is known to be significantly useful in differentiating between different tissue changes in human female breast, the two parameters may also contribute to the characterization of different tissue types and tissue changes in mouse. In addition, some limitations in the conventional pulse echo imaging may partly be overcome by computing a multidirectional compound image. Also for this purpose a spatially resolved reconstruction of the speed of sound is vital for a precise registration of the individual echo images, since different tissue types of mouse possess different speeds of sound. A standard ultrasound scanner was utilized in conjunction with a custom designed add-on system to make the simultaneous acquisition of echo and transmission data possible. The add-on system is capable of positioning ultrasound transducers of the scanner in such a way that the data required for full angle compounding as well as transmission tomography may be acquired. This made a unified system for reflection as well as transmission mode computed tomography possible. The reconstruction of the acoustic speed and acoustic attenuation was carried out under the assumption of a straight line propagation model in the first step, which is corrected iteratively with the help of an accurate ray tracing algorithm. The full angle echo data was utilized to compute the compound images. The reconstructed acoustic speed was employed to correct the individual compound images for registration errors. The reconstruction results of naked mouse cadaver show that the straight ray approximation tends to limit the spatial resolution seriously due to highly inhomogeneous acoustic speed of the mouse tissue. The refraction correction improves the spatial resolution - while it has no significant effect on the contrast resolution. The reconstructed acoustic speed for different organs was found to be in excellent conformity with the standard literature values for respective organs. The refraction correction carried out in compound images with the help of ray tracing corrected most of the arc and circle artifacts so that different organs became visible separately. The reconstructed attenuation coefficient conformed reasonably to the standard literature. The results show that the method may differentiate between tissue types and enhance image definition with the help of spatial compounding.