Study On Terahertz Attenuated Total Reflection Imaging And Its Application In Biomedicine

Terahertz(THz)wave with the frequency between microwave and the infrared,has the characteristics of low photon energy and special penetrability.Owing to the fact that the vibrational and rotational energy levels of many biomacromolecules are located in this frequency range which enables the extraction of their fingerprint spectra,the bioapplication has become the research focus of THz science and technology.To meet the requirement of THz spectroscopy and imaging in biomedical diagnosis furtherly,the dissertation studies theoretically and experimentally on the THz attenuated total reflection(ATR)imaging about its geometry,technology,and detection of different bacteria and tissues.The main works are summarized below:1.The interaction between THz waves and samples under the ATR geometry is studied in theory.The penetration depth and effective thickness of the evanescent wave are analyzed theoretically with their influencing factors.The approximate value of the effective thickness is given and compared with the accurate to investigate the suitable condition of a weakly absorptive sample.The solutions to the refractive index and absorption coefficient of a sample,in the system either with a continuous or with a pulsed source,are studied respectively.For the samples in the continuous source based systems,a calculation method of the above parameters is proposed based on the approximate effective thickness.For the samples in the pulsed source based systems,the glucose solution is exemplified to assess the detection limit and sensitivity of the system quantitatively.2.THz-ATR imaging technology is theoretically and experimentally studied.Two imaging methods are proposed based on the vertical and horizontal scans of a moving prism.The related two systems are optimized from the aspects of prism design and polarization selection.The effects of the beam size and focal length on the signal collection are simulated.The methods to enhance the data accuracy and image resolution are proposed and discussed from modifying the signal-collecting path and compensating the incident optical path.For the vertically scanning method,the ppolarization incident onto a prism with the base angle between 21? and 31? leads to a higher accuracy and repeatability.For the horizontally scanning method,the signal collection is simplified by fixing the position of the output beams,the data accuracy is enhanced by freeing the sampling spot from the influence of multiple reflections,and the resolution degradation effect is mitigated by reducing the beam focal deviation.3.In-situ bacterial detection is realized by the proposed THz-ATR imaging.Given the difference of the THz absorption of different bacteria and the advantage of the ATR geometry without the need for complicated pre-processing of samples,the same and different bacterial species are identified and differentiated in situ.To enhance the contrast of the bacteria and agar culture medium in the original in-situ detected images,a series of digital image processing techniques are employed to realize the automatic location,coloring and differentiation of bacterial colonies.A quantitative parameter defined by the ratio of the THz absorptions of the bacteria and agar is proposed to characterize the bacterial species,as well as improving the repeatability and stability of the detection.4.Fast label-free detection of various biotissues are realized by the proposed THzATR imaging.Based on the different dielectric responses of the tissues of different characteristics and states to the THz waves,three kinds of samples are studied.For porcine tissues,the differentiation of their adipose and lean parts are compared between different incident angles,frequencies,temperatures,and imaging setups,so as to explore the best condition of the high-resolution and high-sensitivity detection of big tissues.For traumatic brain injury,the rat samples with different coronal planes and different acquiring methods are studied and compared to prove the reliability of the THz-ATR imaging.Different grades of the injury are identified according to the edema size.For brain glioma,the tumors of mouse samples are localized through digital image processing,and quantitatively recognized through studying the ATR-reflectivity ratio of the tumorous and normal regions.By normalizing the original image with the average grayscale of the normal tissue,the detection repeatability is improved,and the tumorous region is demarcated.