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Michele D Lee, Raksha Urs, Leejee Han Suh, Ronald H Silverman; Thermal Index in Ex Vivo Bovine Lenses from Acoustic-Radiation-Force Impulse (ARFI) Pulse Imaging. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5863.
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Non-invasive characterization of corneal elastic properties is of potential value in keratoconus screening, evaluation of stromal crosslinking, and accurate measurement of intraocular pressure. Acoustic-radiation-force-impulse (ARFI) imaging represents one modality for such evaluation. ARFI imaging allows assessment of tissue stiffness by producing stress by acoustic radiation force while measuring displacement via pulse-echo ultrasound. While ARFI exposures at levels within FDA guidelines can generate sufficient tissue displacement for stiffness estimation, actual thermal measurements under these conditions has not been attempted in ocular tissue. Our aim was to make sure determinations.
We chose to utilize the lens rather than cornea due to its greater thickness and high attenuation coefficient, both of which would tend to cause greater absorption and temperature rise than in the cornea itself. We used a 25 MHz single element transducer with a 6 mm aperture and an 18 mm focal length. We exposed 8 fresh bovine lenses to ARFI at a series of increasing intensities while measuring temperature rise using a 23-gauge needle thermocouple probe (time constant 0.15 sec), placing it inside the lens with the tip 1 mm beneath the surface. Exposures consisted of two bursts separated by a 0.25 second interval each consisting of twenty 400-µsec pulses at an 80% duty cycle. We repeated this three times for each lens. Temperatures were recorded at 0.2 sec intervals and averaged 10 seconds before and 10 seconds after the ARFI pulse was triggered.
There was a steady, linear increase in temperature with increases in pulse intensity (see Figure 1). Temperature increases within the lens did not exceed 0.056°C.
The thermal increases within the lens were minimal and well within safety limits. These results support the hypothesis that ARFI imaging will not cause harmful thermal increases within the lens or cornea.
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