Development and characterization of an innovative synthetic tissue-mimicking material for high intensity focused ultrasound (HIFU) exposures

While many tissue-mimicking phantoms have been developed for ultrasound imaging applications, none is suitable for exploration of the high temperature and pressure regimes involved in High Intensity Focused Ultrasound (HIFU). HIFU dosimetry studies are usually performed on biological tissues, but this approach has two drawbacks: 1) tissues are opaque and development of coagulative lesions cannot be visually observed in real-time, and 2) the natural heterogeneous structure of tissue may complicate direct comparison with numerical models. To address these issues, a new optically transparent tissue phantom was developed. It is a polyacrylamide hydrogel with a thermally sensitive indicator protein (Bovine Serum Albumin, 3 - 9%) that becomes optically diffusive when denatured. We describe various measurements undertaken to characterize this material and to demonstrate how well it matches tissue in terms of bulk acoustic and thermal properties. In summary, this new phantom material simulates many of the lesion-forming characteristics of soft tissue under HIFU exposures.