Characterization of Medical Ultrasound Transducers using Infrared Imaging Open Access
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Characterization of high-intensity focused ultrasound (HIFU) transducers operating at clinical power levels (1000 - 10000 W/cm2) can be difficult, owing to the possibility of hydrophone damage or sensor interference with the focused beam. This study presents a noninvasive method (no sensors are present in the HIFU field eliminating the issue of interference caused by objects present in the field) for determining the intensity field of a HIFU beam using infrared (IR) imaging. A tissue phantom was used and sonicated with a 1 MHz HIFU transducer. The temperature rise occurring due to attenuation of the ultrasound energy was measured using an infrared camera separated from the surface of the phantom by an air gap.A commercially available software (PZFlex, Weidlinger Associates Inc., CA) was used to arrive at a finite-element solution to the wave propagation and heat equations within the phantom. By using PZFlex with an iterative inverse technique, the acoustic field giving rise to the IR temperature measurements was computed. From the iterative process, the acoustic power was determined. Power levels of two HIFU transducers were tested and the predicted power values were within about 25% of Radiation force balance measurements. The inverse method based upon infrared thermography shows promise as a tool for quantitatively measuring radiated power for HIFU beams. The temperature measurements taken using the system may also be used to infer the acoustic intensity field and estimate in-vivo temperatures.