| Terahertz wave is the electromagnetic wave with a frequency band of0.1THz?10THz.It is in a special position in the electromagnetic spectrum transition from electronics to photonics.It has high resolution,safety,penetration and high sensitivity to moisture,and shown great application potential in biomedical detection.THz Time Domain Spectroscopy(THz-TDs)technology is a coherent detection technology that can simultaneously obtain the amplitude information and phase information of terahertz pulses,and is expected to become a biomedical imaging technology with unique advantages.However,the current speed of terahertz THz-TDs imaging is still the bottleneck restricting its application in clinical biomedical testing,and further research is required on THz-TDs systems and methods with lower cost,faster imaging speed and higher imaging quality.Aiming at accelerating the speed of terahertz time-domain spectral imaging and improving the imaging contrast and taking skin burn detection as the application object,this thesis carried out research on THz-TDs rapid imaging technology and related problems about its application on skin burn.For the first time THz-TDs imaging is used for skin burn detection,and is used to explore related issues from four aspects: skin THz reflective characteristic,fast imaging algorithm,rapid imaging system and skin burn detection experiment,which provides new methods for rapid THz-TDs imaging and its application on skin burn auxiliary detection.The main contents and innovative work of this thesis are as follows:(1)Based on the physical structure of skin,the distribution of different skin tissue moisture gradients is defined,and an effective medium model is used to calculate the dielectric constant of skin.Then the two-dimensional multi-layer media analytical model and three-dimensional numerical calculation model of skin tissue are established for fast calculation of the reflection coefficient of skin tissue and analysis of related influencing factors.Based on these models,effects of changes in moisture content caused by different edema inside the skin,rough skin structure and sweat glands on the skin reflection coefficient in the 0.1THz?1THz frequency band are analyzed,and the main influencing factors and affecting rules are revealed.Finally,the sensitivity of THz sensing on skin moisture under different measurement mode was compared and analyzed,providing theoretical basis for the building and signal processing of THz-TDs system for skin burn detection.(2)A fast construction method of Hadamard measurement matrix based on total variation ordering is proposed.The influence of image type,measurement noise and reconstruction algorithm on the sampling matrix and the mechanism of improving imaging quality are analyzed by comparative analysis.At the same time,a one-step THz image reconstruction algorithm based on Fourier domain regularization inversion(FDRI)is proposed,forming a TV-FDRI imaging framework for rapid compressive image measurement and reconstruction,and simulation experiments are used to verify the effectiveness and efficiency of the proposed framework.Finally,the imaging framework is verified based on the near-field THz fast imaging experiment,the sampling rate is compressed to 10% or less,and high-quality image reconstruction is achieved in dozons of milliseconds for a 32×32 image.(3)A fast scanning THz-TDs imaging system for ex vivo pig skin and in vivo skin measurement is built and their measurement errors are analyzed.Consequently,methods for signal calibration,sample parameter extraction and image processing are proposed.Afterwards,combining the proposed TV-FDRI imaging framework with a rapid THz-TDs single pixel imaging system,a transmission and reflection compressive measurement THz-TDs single-pixel fast imaging system are constructed and imaging verification experiments are carried out to obtain the relationship between the sampling rate and imaging quality of both transmission and reflection measurment,achieving an imaging speed of 6 frames per second for 32×32 pixel images at the sampling ratio of40%,which is very close to real-time imaging.(4)The skin moisture content evaluation method and GA-LM hybrid optimization algorithm with the skin surface reflectance as the optimization target are proposed,and the fitting error is less than 2%.A single-point high-precision and rapid measurement method is used to carry out skin edema simulative experiments,and the changes of THz spectral parameters under different skin water contents and the applicability of the evaluation method are obtained.Subsequently,the THz-TDs imaging experiment of burned pig skin is carried out,and the shape and location of the burned area of the pig skin samples at the first degree burn and the second-degree burn are obtained,and the burn depth can be estimated quantitatively by means of deconvolution in frequency domain.Finally,compressive rapid single-pixel THz-TDs imaging technology is used to image the burned pig skin.From the imaging results,the burned area and shape can be seen,and the total imaging time can be saved to 1.5min.The images corresponding to the entire THz time-domain waveform can be obtained within 30 seconds through in vivo imaging of the thumb skin,and the moisture content of the thumb skin can be preliminarily evaluated,which verifies the effectiveness of the evaluation method and rapid imaging technology. |
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