Browsing by Author "Ala-laurinaho, Juha"

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    Extraction of thickness and water content gradients in hydrogel-based, water-backed corneal phantoms via submillimeter wave reflectometry
    (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2021-11-01) Tamminen, Aleksi; Baggio, Mariangela; Nefedova, Irina; Sun, Qiushuo; Presnyakov, Semyon; Ala-laurinaho, Juha; Brown, Elliot; Wallace, Vincent; Macpherson, Emma; Maloney, Thaddeus; Kravchenko, Natalia; Salkola, Mika; Deng, Sophie; Taylor, Zachary; Department of Electronics and Nanoengineering; Department of Bioproducts and Biosystems; Zachary Taylor Group; Ville Viikari Group; Bio-based Materials; University of Birmingham; Higher School of Economics; Wright State University; University of Western Australia; University of Warwick; Revenio Oy; University of California, Los Angeles
    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.