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Submillimeter wave imaging: system development for stratum corneum water content mapping
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School of Electrical Engineering |
Doctoral thesis (article-based)
| Defence date: 2025-12-12
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en
Pages
73 + app. 44
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Aalto University publication series Doctoral Theses, 250/2025
Abstract
Terahertz (THz) imaging has emerged as a promising modality in biomedical diagnostics due to its non-ionizing nature and sensitivity to water content – a critical parameter in biological tissues. Despite advancements in source and detector technologies, challenges persist in achieving highresolution, wide-field-of-view, and polarization-controlled THz imaging, particularly for in vivo applications. This dissertation presents the design, implementation, and validation of a novel reflective imaging platform: the Telecentric Offset Reflective Imaging System (TORIS).
The research begins with a theoretical and experimental investigation into cross-polarization effects arising from the use of off-axis parabolic mirrors (OAPs) in conventional THz systems. It demonstrates how cross-polarized fields generated at the sample plane are rotated back into the co-polarization axis at the detector, masking the polarization response of the sample. To address this, a comparative analysis of different reflective geometries, including Dragonian dual-reflector configurations, is performed to optimize polarization fidelity and beam quality.
Building on these findings, the TORIS platform introduces a dual-mirror scanning geometry with an offset rotational axis, combined with a telecentric ! − # focusing lens, ensuring large field-of-view imaging while maintaining near-normal incidence angle with maximum deviation of 2.7° from normal incidence, and minimizing beam aberrations with beam waist of 2.1l at the edge of the field-of-view. The system is further integrated with frequency-domain reflectometry to enable high-speed, broadband THz measurements.
The practical capabilities of the TORIS system are demonstrated through in vivo skin measurements, focusing on real-time monitoring of hydration changes and epidermal thickness. A spectral perturbation analysis method is developed to extract physiological parameters from reflectivity spectra with sensitivity beyond the spatial resolution limit. Overall, this work establishes a comprehensive optical, mechanical, and analytical framework for THz imaging and spectroscopy of skin and other biological tissues, offering potential for future applications in hydration monitoring, wound assessment, and dermatological diagnostics.
Description
Supervising professor
Taylor, Zachary, Prof., Aalto University, Department of Electronics and Nanoengineering, FinlandThesis advisor
Tamminen, Aleksi, Dr., Aalto University, Department of Electronics and Nanoengineering, FinlandAla-Laurinaho, Juha, Dr., Aalto University, Department of Electronics and Nanoengineering, Finland
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Parts
- [Publication 1]: Rezapoor, P., Tamminen, A., Nefedova, I., Ala-Laurinaho, J., Llombart, N., Rodilla, H., Stake, J., and Taylor, Z.. Cross Polarization in Swept Beam THz Imaging Systems Using Off-Axis Parabolic Mirrors. In Proceedings of the 2022 16th European Conference on Antennas and Propagation (EuCAP), pp. 1–4, 2022 IEEE.
Full text in Acris/Aaltodoc: https://urn.fi/URN:NBN:fi:aalto-202301251490DOI: 10.23919/EuCAP53622.2022.9769500 View at publisher
- [Publication 2]: Rezapoor, P., Tamminen, A., Ala-Laurinaho, J., Dabironezare, S. O., Llombart, N., and Taylor, Z.. Cross Polarization and Aberrations with Dragonian and Equivalent Off-Axis Parabolic Mirrors for Beam Collimation in THz Imaging Systems. In Proceedings of SPIE: Passive and Active Millimeter-Wave Imaging XXV, Vol. 12111, pp. 124–130, 2022 SPIE.
Full text in Acris/Aaltodoc: https://urn.fi/URN:NBN:fi:aalto-202211306693DOI: 10.1117/12.2627020 View at publisher
- [Publication 3]: Rezapoor, P., Tamminen, A., Ala-Laurinaho, J., Ruan, D., and Taylor, Z.. A Telecentric Offset Reflective Imaging System (TORIS) for Terahertz Imaging and Spectroscopy. IEEE Transactions on Terahertz Science and Technology, Early Access, pp. 1–14, 2025 IEEE.
Full text in Acris/Aaltodoc: https://urn.fi/URN:NBN:fi:aalto-202510158205DOI: 10.1109/TTHZ.2025.3579299 View at publisher
- [Publication 4]: Rezapoor, P., Tamminen, A., Ala-Laurinaho, J., Ruan, D., and Taylor, Z.. Perturbed Resonance Frequency Analysis for Imaging Epidermal Thickness and Water Content Using Terahertz Spectroscopy. Submitted to IEEE Transactions on Terahertz Science and Technology, 2025.
DOI: 10.1109/TTHZ.2025.3637149 View at publisher