A Magnetic Resonance-Compatible Fiberoptic Temperature Sensor for Measuring Focused Ultrasound-Induced Heating
Sara L Johnson1, Henrik Odéen1, Harry Vine2
1Radiology & Imaging Sciences, University of Utah, Salt Lake City, UT, USA.
Objective:
This work evaluated a fiberoptic probe that meets all features and qualities that render it highly suitable for validating magnetic resonance temperature imaging measurements for magnetic resonance-guided focused ultrasound (MRgFUS) applications. The fiberoptic probe performance was compared with other probes with viscous heating artifact (VHA) effects in both a free-field and a controlled tissue-mimicking phantom environment with MRgFUS heating applied.
Methods:
Three fiberoptic probes with different coatings (nylon, ethylene tetrafluoroethylene and polyimide) and tip diameters (1, 0.75 and 0.14 mm) were evaluated in free-field and tissue-mimicking phantom environments at 1 and 3 MHz under a variety of acoustic intensity and probe incidence angles with respect to the acoustic field. Concurrently acquired temperature measurements from the fiberoptic probes and magnetic resonance temperature imaging were compared, with Bland-Altman analysis also performed.
Results:
The 0.14 mm diameter fiberoptic probe composed of a glass optical fiber and ethylene tetrafluoroethylene coating had negligible VHAs under all acoustic exposure conditions. Comparison of the magnetic resonance temperature imaging and fiberoptic temperature data resulted in a root mean squared error of 0.52°C and 0.47°C and a limit of agreement of 1.2°C and 1.0°C when tested in a 1 and 3 MHz acoustic field, respectively. This contrasts with the results from a 1 mm-diameter fiberoptic probe composed of a polymer optical fiber with a nylon coating that demonstrated a root mean squared error and limit of agreement of 15°C and 3.5°C, and 37°C and 7.5°C at 1 and 3 MHz, respectively.
Conclusion:
The thin-diameter (0.14 mm) glass fiberoptic probe monitored focused ultrasound-induced heating in the acoustic field without evidence of a VHA. The ability to perform calibration and accuracy studies in an active acoustic field will broaden the range of testing allowed, improving the calibration range of magnetic resonance imaging temperature sequences.
