Abstract

Noninvasive temperature imaging would enhance the ability to uniformly heat tumors at therapeutic levels. Ultrasound is an attractive modality for this purpose. 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, and pork muscle in 1D. Those measurements were corrected manually for changes in the axial position of echo signals with temperature. To investigate the effect of temperature on CBE in 2D, we imaged 1-cm thick samples of bovine liver, turkey breast, and pork muscle during heating in a water bath. Images were formed by a Terason 2000 imager with a 7 MHz linear probe. Employing RF signals permitted the use of cross-correlation as a similarity measure for automatic tracking of feature displacement as a function of temperature. Feature displacement across the specimen was non-uniform with typical total displacements of 0.5 mm in both axial and lateral directions. Apparent movement in eight image regions in each specimen was tracked from 37 to 50^oC in 0.5^oC steps. Envelopes of motion-compensated image regions were found with the Hilbert transform then smoothed with a 3x3 running average filter before forming the backscattered energy at each pixel. Our measure of CBE compared means of both the positive and negative changes in the BE images. CBE was monotonic and differed by about 4 dB at 50^oC from its value at 37^oC. Relatively noise-free CBE curves from tissue volumes of less than 1 cm³ supports the use of CBE for temperature estimation.

Keywords: diagnostic ultrasound, hyperthermia, noninvasive thermometry