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Development of an Ultrasonic Nonlinear Frequency Compounding Method for Use in Noninvasive Tissue Thermometry

In order for a prescribed thermal dose to be delivered accurately during thermotherapy, the availability of a reliable and robust thermometry technique is of the utmost importance. To this end, an ultrasound based nonlinear single frequency method (NLSF) was originally proposed by our group, based on the temperature sensitive change in acoustic harmonics backscattered energy. The NLSF, however, suffers from limitations in that it produced high standard deviation and low signal-to-noise ratio (SNR) when performing temperature calibrations was applied to tissue thermometry. In this work a novel nonlinear frequency compounding (NLFC) method is introduced, which utilizes the advantages of frequency compounding to improve SNR. All NLFC input parameters were optimized to maximize SNR of both B-mode and backscattered energy images in a tissue mimicking phantom. The NLFC method was then used to produce 2D temperature maps for heating and cooling of ex vivo porcine tissue samples with a focused ultrasound transducer. The new NLFC method was able to produce superior temperature maps over the previously developed NLSF with percent improvements of up to 64.1%.