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U.S. Department of Health and Human Services

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Research Project: Development and Use of Tissue Mimicking Materials (TMMs) for Assessment of HIFU Devices

For device development, quality control, and regulatory assessment, a reusable, hydrogel-based tissue mimicking material (TMM) has been developed that has thermal and acoustic properties close to that of human soft tissue (i.e., acoustic attenuation, speed of sound, nonlinear parameter B/A, thermal diffusivity, and thermal conductivity). The TMM also allows for the embedding of thermocouples and the formation of wall-less vessels that do not deteriorate as a result of continuous flow of blood mimicking fluids through the material.

Cavitation in clinical HIFU procedures generally is to be avoided because of its unpredictable effect on the tissue heating pattern. However, in some cases controlled cavitation is being employed to enhance heating of the target tissue and thus shorten the treatment time. In any event, the occurrence and effect of cavitation in TMMs needs to be well understood in order to evaluate the safety and effectiveness data generated in pre-clinical studies. Therefore, an investigation was undertaken to monitor cavitation behavior using several different techniques, including B-mode imaging and hydrophones and determine its effect on temperature rise in a HIFU TMM containing an embedded thermocouple. Temperature traces obtained at various pressure levels demonstrated a wide range of heating profiles in the TMM due to the occurrence of cavitation. There was good correlation between the various methods to detect the occurrence of cavitation.

A blood mimicking fluid (BMF) has been developed for the acoustic and thermal characterizations of HIFU ablation devices. The BMF is based on a degassed and de-ionized water solution dispersed with low density polyethylene microspheres, nylon particles, gellan gum, and glycerol. A broad range of physical parameters, including attenuation coefficient, speed of sound, viscosity, thermal conductivity, and diffusivity, were characterized as a function of temperature (20–70 °C). The nonlinear parameter B/A and backscatter coefficient were also measured at room temperature. Values of these parameters were similar to those of human blood, making the BMF useful for ultrasound flow imaging and ultrasound-guided HIFU applications. These properties make it a unique HIFU research tool for developing standardized exposimetry techniques, validating numerical models, and determining the safety and efficacy of HIFU ablation devices.

Further with regard to blood mimicking fluids, two BMFs have been developed that can serve as a blood coagulation surrogate in phantoms for bench studies of devices such as those designed to control blood loss in injured vessels. One is based upon the existing BMF by adding a thickening agent and the other is based upon an egg white solution. Evaluation of the BMFs included characterization of the coagulation temperature, viscosity, temperature-dependent attenuation, sound speed, thermal properties, and backscatter coefficient.

A study was performed to determine the thermal effects arising from absorption of ultrasound beams incident upon bone. The safety of simulated HIFU procedures was assessed as a function of transducer/bone separation. Also, an analysis was completed of data acquired in a TMM containing a simulated vessel. Experimental results confirmed computational predictions of a cooling effect due to blood flow through the large vessel, when a HIFU procedure is targeted within about one beam width of the vessel.