Assessment of corneal hydration sensing in the terahertz band:in vivoresults at 100 GHz

Abstract

Abstract. Terahertz corneal hydration sensing has shown promise in ophthalmology applications and was recently shown to be capable of detecting water concentration changes of about two parts in a thousand in ex vivo corneal tissues. This technology may be effective in patient monitoring during refractive surgery and for early diagnosis and treatment monitoring in diseases of the cornea. In this work, Fuchs dystrophy, cornea transplant rejection, and keratoconus are discussed, and a hydration sensitivity of about one part in a hundred is predicted to be needed to successfully distinguish between diseased and healthy tissues in these applications. Stratified models of corneal tissue reflectivity are developed and validated using ex vivo spectroscopy of harvested porcine corneas that are hydrated using polyethylene glycol solutions. Simulation of the cornea's depth-dependent hydration profile, from 0.01 to 100 THz, identifies a peak in intrinsic reflectivity contrast for sensing at 100 GHz. A 100 GHz hydration sensing system is evaluated alongside the current standard ultrasound pachymetry technique to measure corneal hydration in vivo in four rabbits. A hydration sensitivity, of three parts per thousand or better, was measured in all four rabbits under study. This work presents the first in vivo demonstration of remote corneal hydration sensing.

Recent advances in terahertz technology for biomedical applications

“…Given the permittivity of the sample and the dehydrated tissue, also called the biological background, the volume fraction (14,15) and stratified media theory can be used to calculate the reflectivity of samples resembling multi-layer structures (16). This approach has been used successfully in vivo on both rat skin (17) and rabbit cornea (18)(19)(20). The Bruggeman model is also used to estimate the safety thresholds in the THz range of skin and cornea (21).…”

Millimeter-Wave Tissue Diagnosis: The Most Promising Fields for Medical Applications

“…The exact characteristics for a corneal hydration sensor depend on the targeted application; e.g., the critical resolution for diagnosing Fuchs' dystrophy and keratoconus is 0.99% and 1.6%, respectively [54]. Some general parameters to account for are as follows: the nominal thickness of the (human) cornea is between 500 and 600 nm [102], and its water concentration is depth dependent and around 76%, on average [104].…”

“…high illumination fluencies for human exposure [54], limiting this technology to in vitro or animal models. An in vivo measurement of the hydration itself could allow for higher sensitivity and specificity in the diagnosis.…”

“…Spectroscopic information is calculated from the comparison of the transmitted and the received radiation. The illuminating radiation can either be monochromatic [54], [56], [57] or a wideband pulse [52], [53], [55]. Since it is a contactless measurement, it is easier to scan over the sample for imaging or for reference measurements.…”

THz Imaging of Skin Burn: Seeing the Unseen—An Overview

“…Pertaining to these undisruptive characteristics, THz imaging systems have been invariably proposed for various medical applications, and cancer detection, 38,39 corneal hydration monitoring, 40,41 and skin burn detection are among them.…”