Millimetre Wave and THz Techniques

The cornea is the outermost optic of your eye is responsible for s the majority of the eye’s focusing power. The cornea lies on the surface of water volume called the aqueous humor. The corneal layers adjacent to the aqueous humor (endothelium) are responsible for maintaining the water volume of the cornea by actively pumping water across the barriers between the endothelium and aqueous humor. Many diseases, such as endothelial dystrophies, perturb the active pumping mechanisms and the cornea hyperhydrates due to diffusion. Late detection of cornea hyperhydration almost always leads to corneal graft surgery where the diseased endothelium is replaced with a donor endothelium. Donor endothelia can be rejected by the body and late detection requires the removal and the donor tissue with additional donor tissue. Currently, no methods to directly measure corneal water content exits thus early detection of endothelial dystrophies and corneal graft rejection are not possible. The geometry and ordered nature of cornea enable THz frequency thin film metrology (i.e. frequency domain ellipsometry) to extract corneal thickness and corneal water content simultaneously thus providing the first ever direct measurement of disease dependent corneal hydration.

Skin burns severity is generally divided into three, increasing severities: 1st, 2nd, and 3rddegree. 3rd degree burns often require surgical excision to remove dead tissue. It is often difficult to delineate 2nddegree from 3rddegree severities immediately following burn injury due to the buildup and redistribution of significant tissue edema. This further delays appropriate treatment and leads to poor outcomes. THz imaging generates sensitive a specific maps of injury edema and these maps provide information that enables accurate assessment of burn severity.

Patients often require the transplantation of donor skin from a healthy area of the body to close wounds or surgical excisions. These sections of tissue are called “flaps” and they can become unviable if the blood supply is not successfully reestablished. Flap rejection significantly complicates wound healing and leads to substantial loss of function and is a detriment to aesthetics. Flap failure is hard to predict and most optical techniques are confounded by the presence of massive tissue edema. THz imaging generates sensitive a specific maps of injury edema and these maps provide information that enables early and accurate detection of flap failure.

Traditional THz quasi-optical techniques are often applied to the design of systems whose target of interest is located a significant distance from the THz transceiver (e.g. radio astronomy, personnel screening). Further, these systems typically employ apertures that are on the order of 102 to 103 wavelengths in diameter. This orientation often produces broad depths of focus and some tolerable sensitivity to misalignment. In contrast, our medical imaging is more similar to microscopy where the tissue of interest is very close to the THz transceiver and practical constraints limit apertures to < 102 free space wavelengths. This setup results in very sharp depths of focus and extremely high sensitivity to misalignment. Since patients are always moving around, the performance optical system under misalignment is often more important than during alignment. Thus, our quasioptical system design and evaluation efforts are directed towards the understudied problem of tracking beam under when nothing is lying where it should.

We have pioneered the use of mixed technology, THz imaging systems. Our most commonly used architecture is a photoconductive switch source and a Schottky diode detector. The goal of this system is incoherent radiation detection (diode) with a low coherence source (switch) where the coherence effects, typical of swept source or time domain detection, are suppressed. This combination of components produces high resolution, low clutter images but creates interesting operational situations that are difficult to characterize. For example (1) The peak electric field coupled to the detector is many orders of magnitude beyond the breakdown voltage of the Schottky diode thus the component operates in the large signal, series resistance regime. (2) The self-complementary, spiral antenna generates a circularly polarized beam with ~ 200% bandwidth while the diode is mounted in a rectangular waveguide with a probe designed to couple to the TE10 mode. (3) The large signal operation and complex coupling combine to produce a signal whose spectral mapping is nearly intractable.

Sensitivity and specificity to water in tissue has never been characterized. These parameters require a gold standard, in situ method of tissue water content of which there is only one: magnetic resonance imaging. Significant research is still needed to correlate our THz imaging data with MRI measurements. This task requires high magnetic fields and resolution of surface features; a combination that is rarely employed clinically. Basic research is ongoing to refine these correlative experiments.