Characterization of a fat tissue mimicking material for high intensity focused ultrasound applications.

Purpose: Tissue-mimicking materials (TMMs) have a prominent role in validating new high intensity focused ultrasound (HIFU) therapies. Agar-based TMMs are often developed mimicking the thermal properties of muscle tissue, while TMMs simulating fat tissue properties are rarely developed. Herein, twelve agar-based TMMs were iteratively developed with varied concentrations of agar, water, glycerol and propan-2-ol, and characterized for their suitability in emulating the thermal conductivity of human fat tissue.

Methods: Varied agar concentrations (2%, 4%, 6%, 8%, 12%, 16% and 20% w/v) were utilized for developing seven water-based TMMs, while a 20% w/v agar concentration was utilized for developing two water/alcohol-based TMMs (50% v/v water and 50% v/v either glycerol or propan-2-ol) and three alcohol-based TMMs (varied glycerol and propan-2-ol concentrations). Thermal conductivity was measured for all TMMs, and the tissue mimicking material (TMM) exhibiting thermal conductivity closest to human fat was considered the optimum fat TMM and was further characterized using ultrasound (US) and Magnetic Resonance Imaging (MRI).

Results: For the seven water-based TMMs an inverse linear trend was observed between thermal conductivity and increased agar concentration, being between 0.524 and 0.445 W/m K. Alcohol addition decreased thermal conductivity of the two water/alcohol-based TMMs to about 0.33 W/m K, while in the alcohol-based TMMs, increased concentrations of propan-2-ol emerged as a modifier of thermal conductivity. The optimum fat TMM (33.3% v/v glycerol and 66.7% v/v propan-2-ol) exhibited a 0.231 W/m K thermal conductivity, and appeared hypoechoic on US images and with increased brightness on T1-Weighted MRI images.

Conclusion: The optimum fat TMM emulates the thermal conductivity of human fat tissue and exhibits a fat-like appearance on US and MRI images. The TMM is cost-effective and has a long lifespan and possesses great potential for use in HIFU applications as a fat TMM.