Reduction in Beam Positioning Error During HIFU Ablation Studies in Tissue Phantoms

Measurements of high intensity focused ultrasound (HIFU) induced temperature rise using thermocouples in tissue phantoms are subject to several types of error which must be accounted for in order to accurately assess the thermal field and predict the outcome of clinical procedures. Thermocouple artifacts due to viscous heating is one source of error. A second source of error involves displacement of the beam relative to the targeted thermocouple junction, due to the difficulty in precisely positioning the very narrow beam. This paper presents an iterative method for removing inaccuracies due to positioning error from the measured temperature data. The refined data is used to quantify the effect of blood flow through large vessels on the efficacy of HIFU procedures. It was determined that blood flow cooling effect causes an order of magnitude decrease in thermal dose at the target within 2 mm of the blood vessel, potentially resulting in incomplete ablation of the tumor. The technique also reveals that thermocouple artifacts exist in significant proportions from about 0.5 to 2.2 times the computed temperature rise in the initial few seconds. The iterative method can aid in clinical procedure planning, especially in predicting the proper HIFU intensity and duration for complete destruction of tumors.