Technical Papers and Presentations

The five year survival rate for malignant brain and central nervous system cancer was estimated at 35% between 2000 and 2015. Deep brain tumors can be unsuited for conventional surgical intervention. One minimally invasive treatment option is interstitial needle-based therapeutic ultrasound (NBTU). The NBTU probe can be inserted into the tumor through a small burr hole. The probe consists of a piezoelectric transducer which, when excited, produces high frequency acoustic waves that are absorbed by the tumor leading to heating and localized death of cancer cells. In order to effectively ablate the entire area within the tumor margin and minimize collateral damage, model-based control of thermal dose is necessary. We present a thermo-acoustic COMSOL Multiphysics^® simulation of a NBTU probe and validate the model using magnetic resonance thermal imaging (MRTI).

We first model a two-dimensional cross-section of a 90 degree sectored 1.5 mm diameter piezoelectric transducer in a square phantom material and use it to determine the resonant mode by measuring the deformation of the element in frequency domain from 1 to 10 MHz. An eigenmode analysis was also conducted to evaluate the mode shapes at the resonant frequencies. An acoustic pressure field (ACPR) was then calculated from a frequency domain study at the chosen frequency. The ACPR was used as input to a bio heat transfer time domain study in which the probe’s simulated thermal heating was calculated. Cooling after the probe is powered down was also investigated. The results from the time domain study were compared against the measured temperature profile of a realized device measured via MRTI. The model agreed with experimental results. Three dimensional simulations of the piezoelectric transducer were also explored.