In vivo change in ultrasonic backscattered energy with temperature in motion-compensated images

Ultrasound is an attractive modality for non-invasive imaging to monitor temperature of tumorous regions undergoing hyperthermia therapy. Previously, we predicted monotonic changes in backscattered energy (CBE) of ultrasound with temperature for certain sub-wavelength scatterers. We also measured CBE values similar to our predictions in bovine liver, turkey breast muscle, and pork rib muscle in both 1D and 2D in in vitro studies. To corroborate those results in perfused, living tissue, we measured CBE in both normal tissue and in implanted human tumors (HT29 colon cancer line) in 7 nude mice. Images were formed by a phased-array imager with a 7.5 MHz linear probe during homogeneous heating from 37 degrees to 45 degrees C in 0.5 degrees C steps and from body temperature to 43 degrees C during heterogeneous heating. We used cross-correlation as a similarity measure in RF signals to automatically track feature displacement as a function of temperature. Feature displacement was non-uniform with a maximum value of 1 mm across all specimens during homogeneous heating, and 0.2 mm during heterogeneous heating. Envelopes of image regions, compensated for non-rigid motion, were found with the Hilbert transform then smoothed with a 3 x 3 running average filter before forming the backscattered energy at each pixel. Means of both the positive and negative changes in the BE images were evaluated. CBE was monotonic and accumulated to 4-5 dB during homogeneous heating to 45 degrees C and 3-4 dB during heterogenous heating to 43 degrees C. These results are consistent with our previous in vitro measurements and support the use of CBE for temperature estimation in vivo during hyperthermia.