Infrared (IR) thermography is a technique that has the potential to rapidly and noninvasively determine the intensity fields of ultrasound transducers. In the work described here, IR temperature measurements were made in a tissue phantom sonicated with a high-intensity focused ultrasound (HIFU) transducer, and the intensity fields were determined using a previously published mathematical formulation relating intensity to temperature rise at a tissue/air interface. Intensity fields determined from the IR technique were compared with those derived from hydrophone measurements. Focal intensities and beam widths determined via the IR approach agreed with values derived from hydrophone measurements to within a relative difference of less than 10%, for a transducer with a gain of 30, and about 13% for a transducer with a gain of 60. At axial locations roughly 1 cm in front (pre-focal) and behind (post-focal) the focus, the agreement with hydrophones for the lower-gain transducer remained comparable to that in the focal plane. For the higher-gain transducer, the agreement with hydrophones at the pre-focal and post-focal locations was around 40%.
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