A method to non-invasively and quantitatively characterize thick biological tissues by combining both experimental and computational approaches in tissue optical spectroscopy was developed and validated on fifteen porcine articular cartilage (AC) tissue samples. To the best of our knowledge, this study is the first to couple non-invasive reflectance and fluorescence spectroscopic measurements on freshly harvested tissues with Monte Carlo computational modeling of time-resolved propagation of both excitation light and multi-fluorophore emission. For reflectance, quantitative agreement between simulation and experiment was achieved to better than 11%. Fluorescence data and simulations were used to extract the ratio of the absorption coefficients of constituent fluorophores for each measured AC tissue sample. This ratio could be used to monitor relative changes in concentration of the constituent fluorophores over time. The samples studied possessed the complexity and variability not found in artificial tissue-simulating phantoms and serve as a model for future optical molecular sensing studies on tissue engineered constructs intended for use in human therapeutics. An optical technique that could non-invasively and quantitatively assess soft tissue composition or physiologic status would represent a significant advance in tissue engineering. Moreover, the general approach described here for optical characterization should be broadly applicable to quantitative, non-invasive molecular sensing applications in complex, three-dimensional biological tissues.

Terahertz (THz) imaging is being adopted for non-destructive evaluation (NDE) applications in aerospace and other government and industrial settings [1-3]. NASA is currently employing THz reflection NDE to examine the space shuttle external tank sprayed on foam insulation (SOFI) for voids and disbonds. Homeland security applications such as the inspection of personnel[2], the detection of concealed explosives[2], biological agents, chemical weapons, flammables, metallic and non-metallic weapons, and other potentially dangerous items are the subject of active investigation. Advancement of many of these application beyond small table top experimentation had been limited by slow imaging speed (tens of minutes or hours), small scan areas (<10 square cm) and in many cases the requirement that the sample itself be mechanically raster scanned. We report the development and applications of a high speed large area time domain terahertz non destructive evaluation imaging system.

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We demonstrate a large area time domain terahertz (THz) imaging system capable of scanning 1 meter square area in less than 20-100 minutes for several security applications. The detection of concealed explosives; metallic and non-metallic weapons (such as ceramic, plastic or composite guns and knives); and flammables in luggage, packages and personnel has been demonstrated. Transmission mode images of luggage containing threat items are discussed. Reflection mode images of luggage and personnel are discussed. Time domain THz images can be analyzed for 3 dimensional and volumetric information. Time domain THz images have advantages over coherent narrow band imaging methods, with freedom from interference artifacts and with greater ability to discard irrelevant or intervening reflections through time discrimination.

Time domain terahertz (TD-THz) reflection imaging tomography can be used to investigate the laminar structure of objects. In a monostatic configuration, a sequence of pulses is generated by reflection from each discontinuity in index of refraction. Through analysis of the return pulses, the material absorption and index of refraction properties of each layer can be determined. TD-THz reflection tomography can be used to precisely measure the thickness of coatings such as yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) on aircraft engine turbine blades; paint on aircraft, ships, and cars; and other thin film measurement applications. In each of these cases, precise determination of the optical delay of the TD-THz pulses is required with as little as sub-10 femtosecond precision for pulses which can be greater than 500 fs in duration. We present a method to accurately measure optical delay between layers where the pulses are fit to a reference template. These are demonstrated to achieve micron scale accuracy in coating thickness. As an example, TD-THz non destructive evaluation (NDE) imaging is used to two-dimensionally map the thickness of YSZ TBCs on aircraft engine turbine blades. Indications of thermal degradation can be seen. The method is non-contact, rapid, and requires no special preparation of the blade.