Terahertz imaging and sensing for ophthalmic evaluation of corneal diseases

In the span of electromagnetic wave band from microwave to X-rays, Interactions of biological tissue and light at the terahertz (THz) wavelength band (λ ~ 30m - 3mm) are particularly unique for two reasons. First, the THz band retains a large dielectric constant for water from the microwave region while its shorter wavelength enables imaging applications with < 1mm resolution. Second, the effect of scattering from typical soft tissue structures (i.e. cells, collagen matrix, etc.) is less in the THz band compared to that in higher-frequency bands (IR band and up). Using this balance of properties, our study applies non-invasive THz sensing to study diseases and conditions that compromise our body’s ability to balance water in tissues.This study focuses on developing a novel medical imaging technology using THz frequency waves to accurately assess and image the tissue water content of the cornea, which is a critical refractive and protective component of the eye. In ophthalmology, most corneal disorders such as Fuchs endothelial dystrophy (failure of stromal hydration regulation), Keratoconus (corneal ectasia), pseudophakic bullous keratopathy (unintentionally damaged endothelial layer), and corneal graft rejection result in corneal edema. Corneal edema is a contributing factor leading to corneal opacity, and if left untreated, can lead to chronic vision impairment. Accurate corneal tissue water content (CTWC) measurement, therefore can help with early diagnosis and intervention for corneal diseases and further our understanding of the formation and progression of corneal disorders. This work details the development of a THz remote-sensing technique based on rigorous electromagnetic model of cornea-THz interaction, and an imaging system that can be practically implemented in the clinical setting. A novel ophthalmic THz imaging system is designed and implemented to perform completely non-contact, all normal-incidence imaging of the corneal surface. The imaging system uses a wavelength independent quasioptical design that achieves a <1.4 mm spatial resolution at 650 GHz. The corneal hydration sensing capability of this imager is demonstrated in in vivo corneas. For the first time, THz images of a living cornea showed the onset of acute corneal edema from endothelial damage. The THz imaging system is currently being used in the first phase of the clinical study to evaluate healthy and diseased corneas of patients with corneal graft and corneal dystrophies