Yuen Y. Chen, Andrew W. Wood. Application of a temperature-dependent fluorescent dye (Rhodamine B) to the measurement of radiofrequency radiation-induced temperature changes in biological samples. Bioelectromagnetics. June 8, 2009. Ahead of print.

Temperature change can affect cellular metabolism, proliferation, and regulation. In relation to radiofrequency (RF) heating, most studies estimate average or maximum specific absorption rate (SAR) within the biological tissue as an indicator of temperature changes that might be expected.  Many approaches use biomarkers to measure whether cells are under environmental stress from RF exposure, though this is an indirect method. 

This paper examines a non-contact method for studying the temperature changes produced by RF radiation, especially in small biological samples.

The non-contact method provided two-dimensional and three-dimensional images of temperature changes and distributions in biological samples, which were taken from tendons collected from the tails of Sprague–Dawley rats.  The fluorescent dye, Rhodamine B, was imaged by laser scanning confocal microscopy.  The method had a spatial resolution of a few micrometers, with an estimated absolute precision of around 1.5C and a differential precision of 0.4C.  Rho-B was dissolved in phosphate buffered saline (PBS) at a final concentration of 0.1 mmol/L. 

Temperature rise within tissue was found to be non-uniform.  Estimates of specific SAR from absorbed power measurements were greater than those estimated from the rate of temperature rise, measured at 1 min intervals.  When the signal generator was turned off, the temperature decreased to room temperature in 14 min, with an exponential time constant of approximately 4.5 min.  This is of the same order of magnitude as the estimated 5 min for the rising (heating) portion of the curve.

Discussion and conclusion
The paper presents a proof-of-concept study, which showed that direct measurement of the temperature distribution in biological tissue is possible.  This provides data that can be correlated with modeling studies.  The authors also identified a number of parameters requiring further investigation, such as the use of live tissue.  The authors conclude that the application of the Rho-B is a simple and novel method to detect temperature change in small biological samples. According to the study, Rho-B fluorescent intensity decreases at approximately 3% per degree Celsius increase.

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