Extraction of thickness and water content gradients in hydrogel-based, water-backed corneal phantoms via submillimeter wave reflectometry

Aleksi Tamminen, Mariangela Baggio, Irina Nefedova, Qiushuo Sun, Semyon Presnyakov, Juha Ala-laurinaho, Elliot Brown, Vincent Wallace, Emma Macpherson, Thaddeus Maloney, Natalia Kravchenko, Mika Salkola, Sophie Deng, Zachary Taylor

Research output: Contribution to journalArticleScientificpeer-review

6 Citations (Scopus)
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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°, 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 μm (∼λ/9) with an average of 65 μm (∼λ/17) and standard deviation of 44 μm (∼λ/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 μm, 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.

Original languageEnglish
Pages (from-to)647-659
Number of pages13
JournalIEEE Transactions on Terahertz Science and Technology
Issue number6
Early online dateJul 2021
Publication statusPublished - 1 Nov 2021
MoE publication typeA1 Journal article-refereed


  • Cornea
  • Corneal phantom
  • gelatin hydrogel
  • Optical device fabrication
  • Optical imaging
  • Optical reflection
  • optical-coherence tomography
  • Phantoms
  • Reflectivity
  • submillimeter-wave spectroscopy
  • Thickness measurement


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