Active THz imaging for medical applications

THz radiation is typically defined as electromagnetic radiation falling the 300 GHz to 3 THz band. THz technology grew out of radio astronomy research and was initially lumped in with the far-infrared and millimeter wave fields of research. However, in the last couple decades THz has emerged as its own field with unique potential applications including defense and security imaging, bio-agent spectroscopy, process control, and medical imaging. While security imaging is currently the most popular imaging application, medical imaging has been gaining ground with recent research into skin burn and cancer imaging.;This thesis details the design, construction, and characterization of 2 active THz imaging systems and their associated components. First the free space power spectral density of the photoconductive switch is measured using a Michelson interferometer and golay cell total power detector. Then the beam pattern is measured using a 0-bias schottky diode detector and x-y translation stage. The switch outputs ∼ 110 muW average power into 1 THz of bandwidth with a FWHM of ∼ 10o.;Next a single pixel, whiskbroom scanning imaging system is built and characterized using the photoconductive switch as the source and a 0-bias schottky diode detector. The imaging head portion of the system consumes a volume of <1/3 ft3 and uses 4 off axis parabolic mirrors to focus the fixed THz beam onto the sample mounted on an x-y translation stage. The system utilizes a novel THz objective assembly which allows the user to change spot size and working distance without realigning the THz optics. The system is capable of sub-mm spot size as measured with knife edge targets and post detection SNR's exceeding 50 dB at a video rate of 60 Hz as measured with a low frequency spectrum analyzer. The system achieves a Noise Equivalent Refractive Index Difference (NERID) of 0.0253 and achieves a Noise Equivalent Delta Water Concentration (NEDeltaWC) of 0.054% by volume.;Biological sample results are presented displaying good spatial discrimination of water gradients. 2nd degree partial thickness burns on porcine skin models are imaged bare and beneath up to 10 layers of gauze. The results suggest good contrast at 600 GHz and the possible identification of burn injury extent not visible with infrared or the naked eye.;A fast scanning system is then presented as the next logical step towards clinically viable THz imaging with faster image acquisition times and stationary targets. The fast scanning system uses a spinning polygonal mirror and translating dielectric lens to raster scan a THz beam across the target at normal incidence. The optics are configured such that the beam is retrodirective allowing the source and detector to remain completely stationary during imaging. The system achieves a spot size of ∼ 5 mm and a scan speed of 5 cm 2/sec with a 1 mm2 pixel size.