Evaluation of the Safety of Terahertz Scanning System on Corneas

Corneal disease is the second leading cause on irreversible blindness in the world. There are more than 250 million people living with visual acuity loss or blindness globally. The cornea is a transparent and avascular tissue that consists of 78% water by volume. With its high-water content, the homogeneity of corneal stromal tissue and the lack of physiological variations allows Terahertz (THz) imaging to be a promising method for sensing corneal hydration level as it is highly sensitive to water. The water content of the cornea is regulated by passive and active mechanism to ensure the preservation of essential transmission and focusing properties. When there is an abnormal elevation of water concentration in the cornea (>5% above the physiological level), it will lead to a significant change in the amount of scattered light and the gradual loss of corneal transparency, this is known as corneal opacification which can be due to corneal edema or corneal scarring. Corneal opacity is one of the leading causes of reversible blindness and visual impairment. When corneal opacity progresses to the end-stage disease, the current and only option would be undergoing corneal transplantation. However, there is a global shortage of donor corneas furthermore, patients who undergo corneal transplantation would require life-long topical immunosuppressant. Hence, monitoring the progression of corneal edema or corneal scarring is essential to help prevent the disease from reaching the end stage. However, a direct marker for the evaluation of corneal edema remains lacking.

THz waves are electromagnetic radiation, and have emerged as a novel, non-invasive and non-contact imaging modality in the biomedical field, with a fast-scanning speed (30ms per scan) to meet medical diagnostic needs. With its fast scanning, it provides a more accurate result for the estimation of corneal thickness. Its high sensitivity hydration absorption and characterization peaks of collagen will provide a useful tool to obtain information that are related to the corneal layers. Water has a high dielectric constant and is highly absorptive to THz spectrum. This allows THz wave to be highly sensitive to water content in tissue. Furthermore, THz scan has been applied to a variety of hydration-related diseases or conditions. With THz scan, early detection of disease and diagnosis can be carried out. Some examples would be the prediction of skin flap failure, diagnosis for the presence of caries lesion at its initial stage, identification of skin cancer in skin biopsy samples and to access the physiological dynamics of tear film in dry eye syndrome. Previous studies have demonstrated that THz scanning system could be used as an early diagnostic tool, to identify the cancerous area based on the lack of clear cellular order in the skin. It was able to detect disease conditions in deeper layers, allowing it to be an effective early diagnostic tool for basal cell carcinoma. Moreover, it had been confirmed that THz scanning system had the sensitivity to identify the difference in colon tissue composition. The findings showed that the properties of ex vivo tissue, had an increase in both absorption coefficient and refractive indices in disease tissue when compared to healthy tissue. A recent study showed that with THz scanning system, clinicians will be able to obtain different images, such as 3D imaging methods to provide multiple views of internal structures. Furthermore, with the THz scan, the process of water loss in an in vitro porcine corneal tissue could be determined using a THz reflection imaging system. It was found that THz wave was approximately linear to the water content concentration, and that the slope decreased monotonously with an increase in frequency. These results suggest that THz imaging can be used for the clinical diagnosis of corneal edema or scars, dry eye diseases and corneal hydration in refractive surgery. With this, investigators would like to conduct this study to ascertain the safety of THz imaging system on humans.