Optimization of the US Probe Design and the RF Signal for Focused US Tissue Heating

Abstract

Focused ultrasound (FUS) hyperthermia is a non-invasive therapeutic technique applied to the selectively and targeted ablation of cancerous lesions in the breast. This is done by the thermal effect (bio-mechanical heating) and non-thermal effect (e.g. cavitation), with minimal deleterious effects to the surrounding healthy tissues. The prime objective of this work is to optimize an ultrasound transducer design and the radiofrequency signal for focused ultrasound that will result in the creation of a very high acoustic pressure at a focal point inside the normal breast tissues by FUS energy to favor a potential in improving both oncologic and cosmetic results in breast cancer therapy. To attain this objective, an Ultrasound (US) probe which increases the targeted volume of cancer and that preserves neighboring healthy tissues from being ablated in a modeled realistic breast is designed using a finite element analysis (FEA) tool in this work. The acoustic pressure field and focal point of the FUS were characterized by optimizing the transducer frequency and initial water temperature values at 1.3 MHz and 20°C respectively. This optimal condition yielded qualitative properties of the Focused Ultrasound (FUS) at the focal point which are extendable to wider areas. This enable the achievement of quantitative results by displacing the probe at reduced time intervals of less than 60s. An elliptical focal volume of 10.2 mm (along beam axis) × 4 mm (in the transverse direction) at 50s of dosing for a tumor diameter of 10mm, corresponding to a fractional healthy tissue damage of 2.1%, a temperature rise of 99°C above the ablation threshold temperature of 42°C, at 100s of sonication and an increased pressure oscillation at the focus that favors tumor ablation by cavitation are obtained. The optimal condition of the setup is therefore found to be mostly sensitive to variation of the transducer frequency on which depend the attenuation coefficient and thermal conductivity of the tissue as well as the surface intensity of the transducer