Shannon entropy can detect myocardial anisotropy without specular echo gating in vitro.

Abstract

Ultrasonic images are often corrupted by specular echoes that obscure features near interfacial boundaries. This problem can be especially severe for in vitro examination of excised tissues (e.g., myocardium) which must often be sliced into thin sections to permit examination by other techniques in addition to ultrasound (e.g., optical, MRI, and histology), in order to characterize infarct region size and myofiber orientation. In many of these studies, myocardial anisotropy is obscured without removal of specular echoes by gating. Here, it is demonstrated that entropy imaging enables visualization of myocardial anisotropy without gating. Short axis slice of mouse heart (n=4) was C-scanned on a 55×55 grid, with 0.1 mm stepsize. At each point one 8-bit, 2048 point, time-averaged A-line was acquired. These were analyzed using three signal receivers, log variance, log energy (sum of squared), and Shannon entropy, to produce one pixel value at each location in the C-scan. Diffusion-tensor-weighted magnetic resonance at 12 T was used to validate the ultrasonic result. Among the three signal receivers, only entropy clearly resolves myocardial anisotropy without specular echo removal. [This work supported by NIH CA119342, and NSC-095-SAF-I-564-051-TMS.]