THz and mm-Wave Sensing of Corneal Tissue Water Content: Electromagnetic Modeling and Analysis

Abstract

Terahertz (THz) spectral properties of human cornea are explored as a function of central corneal thickness (CCT) and corneal water content, and the clinical utility of THz-based corneal water content sensing is discussed. Three candidate corneal tissue water content (CTWC) perturbations, based on corneal physiology, are investigated that affect the axial water distribution and total thickness. The THz frequency reflectivity properties of the three CTWC perturbations were simulated and explored with varying system center frequency and bandwidths (Q-factors). The modeling showed that at effective optical path lengths on the order of a wavelength the cornea presents a lossy etalon bordered by air at the anterior and the aqueous humor at the posterior. The simulated standing wave peak-to-valley ratio is pronounced at lower frequencies and its effect on acquired data can be modulated by adjusting the bandwidth of the sensing system. These observations are supported with experimental spectroscopic data. The results suggest that a priori knowledge of corneal thickness can be utilized for accurate assessments of corneal tissue water content. The physiologic variation of corneal thickness with respect to the wavelengths spanned by the THz band is extremely limited compared to all other structures in the body making CTWC sensing unique amongst all proposed applications of THz medical imaging.

Keywords: Biological and medical imaging; clinical instruments; hydration interactions; medical diagnostics.

Fig. 1

Corneal structure. The bulk of the cornea is composed of the Stroma. All other layers are between 5–15 µm (approximately optically thin at THz wavelengths). The cornea sits atop a body of water called the Aqueous humor.

Fig. 2

Plot of (1) with physiologic relevant thickness denoted by the shaded region. A linear fit to (1) within the shaded region is represented by the dotted line.

Fig. 3

Effective media model for cornea. The axial water content curve is representative of healthy cornea and has been adapted from [52]. The first layer in the stack represents the epithelium and the final layer is the aqueous humor (body of water).

Fig. 4

Dielectric properties of cornea. (left) real and imaginary parts of the corneal tissue at 79.4% water by volume. (right) computed absorption coefficient and depth of penetration.

Fig. 5

Candidate CTWC gradients for CTWC sensitivity simulations. (left) pinned front, (middle) pinned back, and (right) global.

Fig. 6

System power spectral densities (PSD): center frequencies and quality factors with Q = 5, 50 at 100 and 525 GHz.

Fig. 7

Dependence of corneal reflectivity on CTWC and thickness computed at 100 GHz for the cases of (top row) pinned front CTWC change (middle row), global CTWC change, and (bottom row) pinned back CTWC change. Each case was simulated with a source/detector Q of 5 and 50. All figures are displayed with a common colormap, with pixel intensities representing percent reflectivity.

Fig. 8

Millimeter-wave corneal reflectivity profiles. (top row) constant thickness, varying tissue water content. (right) constant thickness, varying CTWC. These profiles are plotted in solid lines for the three different variation types; pinned front, pinned back, and global. The dotted lines are the projection of (1) onto each axis.

Fig. 9

Dependence of corneal reflectivity on CTWC and thickness computed at 525 GHz for the cases of (top row) pinned front CTWC change (middle row), global CTWC change, and (bottom row) pinned back CTWC change. Each case was simulated with a source/detector Q of 5 and 50. All figures are displayed with a common colormap, with pixel intensities representing percent reflectivity.

Fig. 10

525 GHz corneal reflectivity profiles. (top row) constant thickness, varying CTWC and (bottom row) constant CTWC, varying thickness.

Fig. 11

CTWC sensitivity fit to data confirming the validity of the Bruggeman model. Note the effective frequency invariance of corneal tissue constituents. Data obtained from [15].