Interstitial instrumentation for therapeutic ultrasonic heating: modeling the discrete blood vessels

Instrumentation for interstitial ultrasound (US) heating is an important emerging technology in thermotherapy of deep seated tumors or those hard to reach by external devices. The instrumentation has special significance in case of radio-and/or chemotherapy resistant lesions. Its efficacy strongly depends on local tissue properties, especially local blood vessels. We evaluate effects of the vessels on temperature distribution elevated from basal by deposition of ultrasound energy. In the proposed model, we take into account several micron diameter vessels in proximity to the US four-applicator array. At large distances from the array, the volume is assigned a modified effective thermal conductivity. Our Finite Element Analysis of the so-defined problem shows that modelling under the assumption of constant, basal temperature across the vessels' lumen leads to erroneous results. The simulations agree best with experiments if fixed nodal temperature is applied at 60% of the lumen. We specify requirements on the array to avoid local underheating that could lead to performance failure of the instrumentation.