Journal
IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY
Volume 11, Issue 6, Pages 647-659Publisher
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
DOI: 10.1109/TTHZ.2021.3099058
Keywords
Phantoms; Cornea; Reflectivity; Thickness measurement; Optical reflection; Optical imaging; Optical device fabrication; Corneal phantom; gelatin hydrogel; optical-coherence tomography; submillimeter-wave spectroscopy
Funding
- Research Project AGRUM through the Academy of Finland Programme Radiation Detectors for Health, Safety, and Security (RADDESS)
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A new submillimeter-wave measurement technique has been developed to extract both thickness and water-content gradients from a soft-tissue phantom, without the need for ancillary measurements. The results show that the free-water volume fraction in the phantoms varies minimally, and the thickness accuracy is high in physiologically relevant geometry.
Absolute thickness and free-water-content gradients in gelatin-based corneal phantoms with physiologically accurate radii of curvature and aqueous backing were extracted via coherent submillimeter-wave reflectometry at 220-330 GHz. Fourier-domain-based calibration methods, utilizing temporal and spatial gating, were developed and yielded peak-to-peak amplitude and phase clutter of 10(-3) and 0.1 degrees, respectively, for signal-to-noise ratios (SNRs) between 40 and 50 dB. Total 12 phantoms were fabricated. Calibration methods enabled quantification of target sphericity that strongly correlated with optical-coherence tomography-based sphericity metrics via image segmentation. The extracted free-water volume fraction varied less than 5% in the five phantoms whose fabrication yielded the most spherical geometry. Submillimeter-wave-based thickness accuracy was better than 111 mu m (similar to lambda/9) with an average of 65 mu m (similar to lambda/17) and standard deviation of 44 mu m (similar to lambda/25) for phantoms with physiologically relevant geometry. Monte-Carlo simulations of measurement noise and uncertainty limits agree with the experimental data analysis and indicate a lower thickness accuracy limit of 33 mu m, and water-content sensitivities of 0.5% and 11.8% for the anterior and posterior segments, respectively. Numerical analysis suggests that the measurement fidelity was SNR limited and identified optical path length ambiguities within the cornea where a continuum of thickness/water gradient pairs produces statistically insignificant differences in complex reflection coefficient for finite SNR. This is the first known submillimeter-wave measurement technique, which is able to extract both the thickness and water-content gradients from a soft-tissue phantom, with a water backing, without the need for ancillary measurements.
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