Kaiplavil Sreekumar, Mandelis Andreas
Center for Advanced Diffusion-Wave Technologies, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, Toronto, Ontario M5S 3G8, Canada.
Rev Sci Instrum. 2011 Jul;82(7):074906. doi: 10.1063/1.3616140.
A novel chirped pulse photothermal (PT) radiometric radar with improved sensitivity over the conventional harmonically modulated thermal-wave radar technique and alternative pulsed laser photothermal radiometry 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) 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 (a few 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 ~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.
一种新型啁啾脉冲光热（PT）辐射雷达被引入用于生物样本的诊断，特别是对于带有组织和皮肤覆盖层的骨骼，它比传统的谐波调制热波雷达技术和替代的脉冲激光光热辐射测量具有更高的灵敏度。激光安全（最大允许暴露）上限对泵浦激光能量的限制以及热波信号在组织中的强烈衰减，显著限制了大多数生物样本中的光热作用深度，使其通常处于不足以用于实际应用的水平（皮肤表面以下几毫米）。通过比较在恒定曝光能量下对泵浦激光进行啁啾的四种不同激发波形（正弦波、方波、定宽脉冲和恒定占空比脉冲）的信噪比，引入并验证了一种提高信噪比（SNR）、最小化光热信号的静态（直流）分量以及利用光热辐射非线性的理论方法。在低频下，发现大峰值功率的固定脉宽啁啾优于所有其他等能量模式，信噪比提高了两个数量级。在山羊骨中获得了与厚度相关的明显特征延迟时间，在分层的皮肤 - 脂肪 - 骨骼结构中建立了约2.8毫米的有效深度分辨率范围，与浑浊生物介质中报道的最大脉冲光热辐射深度分辨率<1毫米相比，这是一个有利的结果。
