Optical method for direct temperature measurement of high intensity focused ultrasound heating

Abstract

The acoustic pressure field or heat deposition from the passage of an ultrasonic wave can produce a perceptible alteration in the medium’s optical refractive index. With proper placement of the light source and imaging camera, the optical effect can be recorded. This talk describes an optical temperature measurement methodology dubbed laser-ray-bundle (LRB) of the heat deposition from a high intensity focused ultrasound (HIFU) transducer in a transparent tissue phantom. The method does not require calibration, and directly converts the thermo-optic signal to temperature without recourse to any iterative computations. LRB boasts of excellent temporal and spatial resolutions and high temperature sensitivity better than any fine wire thermocouple. Furthermore, it is completely non-invasive. The LRB method is demonstrated with the heat deposition from single millisecond long HIFU pulses in a PDMS tissue phantom or from a train of single pulses with very good performance. The method represents a new capability for quickly assess a HIFU transducer output, and for laboratory and preclinical studies of heating near tissue interfaces including bone, air, or fat. A preliminary experimental setup for measuring the acoustic pressure field is presented.