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Interferometry for Detection of Temperature Rise in a Water Phantom

R Tosh

R Tosh1*, (1) NIST, Gaithersburg, MD

SU-C-105-1 Sunday 1:00PM - 1:55PM Room: 105

Purpose: Imaging of absorbed dose fields directly in a water phantom has been done using ultrasonic phase-sensitive detection. The question arises as to whether similar or better sensitivity could be obtained via phase-sensitive detection of optical probe radiation traversing a water phantom. The present study was undertaken to compare the relative thermal sensitivity of optical and ultrasonic probes of water heated by a source capable of elevating the water temperature over a range (~100 mK) that typifies heating in a radiotherapy beam.

Methods: To obtain a direct comparison of the optical and ultrasonic approaches, a dual interferometer was constructed in which both types of probe radiations could sample the same thermal field simultaneously. For the ultrasonic measurements, two 5 MHz contact transducers were placed on opposite sides of a rectangular water phantom (10 ltr), and the phase change along the ultrasonic flight path was measured using an rf lock-in amplifier. For the optical measurements, a Michelson interferometer was constructed using a HeNe laser as light source, and it was configured so that one of the two arms passed through the water phantom adjacent to the path followed by the ultrasound radiation. A string of power resistors placed in the water, configured to lie parallel to both ultrasonic and laser beam paths, provided heat, and a calibrated thermistor probe provided temperature reference.

Results: Both ultrasonic (above) and optical (below) results were found to track temperature changes registered by the thermistor probe, and, moreover, gave results that agreed very well with theory. Preliminary findings indicate that the optical system was ~25x more sensitive than the ultrasonic system.

Conclusion: The preliminary findings suggest that optical interferometry may outperform both ultrasound and thermistor detectors for resolving radiation-induced changes in water temperature. Testing in radiotherapy beams is planned for the near future.

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