9 February 2012 Bone-demineralization diagnosis in a bone-tissue-skin matrix using the pulsed-chirped photothermal radar
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A chirped pulsed photothermal radiometric radar is introduced for the diagnosis of biological samples, especially bones with tissue and skin overlayers. The constraints imposed by the laser safety (maximum permissible exposure, MPE) ceiling on pump laser energy and the strong attenuation of thermal-wave signals in tissues significantly limit the photothermally active depth in most biological specimens to a level which is normally insufficient for practical applications (approx. 1 mm below the skin surface). A theoretical approach for improvement of signal-to-noise ratio (SNR), minimizing the static (dc) component of the photothermal signal and making use of the photothermal radiometric nonlinearity has been introduced and verified by comparing the SNR of four distinct excitation wave forms (sine-wave, square-wave, constant- width and constant duty-cycle pulses) for chirping the pump laser, under constant exposure energy. At low frequencies fixed-pulsewidth chirps of large peak power were found to be superior to all other equal-energy modalities, with an SNR improvement up to two orders of magnitude. Distinct thickness-dependent characteristic delay times in a goat bone were obtained, establishing an active depth resolution range of ca. 2.8 mm in a layered skin-fat- bone structure, a favorable result compared to the maximum reported pulsed photothermal radiometric depth resolution < 1 mm in turbid biological media. Compared to radar peak delay and amplitude, the long-delayed radar output amplitude is found to be more sensitive to subsurface conditions. Two-dimensional spatial plots of this parameter depicting the back surface conditions of bones with and without fat-tissue overlayers are presented.
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Sreekumar Kaiplavil, Sreekumar Kaiplavil, Andreas Mandelis, Andreas Mandelis, } "Bone-demineralization diagnosis in a bone-tissue-skin matrix using the pulsed-chirped photothermal radar", Proc. SPIE 8207, Photonic Therapeutics and Diagnostics VIII, 82076L (9 February 2012); doi: 10.1117/12.908659; https://doi.org/10.1117/12.908659

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