Noninvasive two-dimensional temperature imaging for guidance of thermal therapy

Two-dimensional temperature estimation using pulse-echo diagnostic ultrasound has been previously described. Measurement models in both time and frequency domains have been proposed. In a paper by Seip, R. et. al (1995), spectral shifts in the echo data have been shown to be proportional to changes in the local tissue temperature. In works by Simon, C. et. al (1998) and Maass-Moreno, R. et. al (1996), echo shifts were shown to be proportional to local change in tissue temperature. For both the spectral and echo shift methods, the proportionality was shown to be proportional to the local changes in the speed of sound and local tissue expansion. However, temperature images suffer from some artifacts due to the distortion of the imaging beam as it traverses the heated region. In particular, temperature imaging artifacts due to the thermoacoustic lens effects have been reported. In addition, tissue inhomogeneity leads to nonuniform speckle patterns which also lead to errors in the estimated temperature profiles. A reconstructive imaging method employing a physics-based 2D filter and a projection method is presented. The method of projection onto convex sets (POCS) is used. Experimental data was obtained during controlled temperature heating of in vitro bovine muscle using a helical RF ablation probe. Reconstructions using the POCS-based iterative algorithm are shown to produce artifact-free temperature fields. Both spatial and temporal characteristics of the reconstructed temperature conform well with the extent of the heating source and the temporal dynamics of the controlled temperature. These results demonstrate that noninvasive temperature imaging of a relatively large heating region (nearly 3 cm in diameter) can be reliably monitored using pulse-echo ultrasound with appropriate signal and image processing