In-vivoreflection terahertz imaging for non-invasive skin diagnostics: a topical review

Medical imaging has revolutionized disease detection and patient care; however, conventional modalities such as Magnetic Resonance Imaging (MRI), Computed Tomography (CT), ultrasound, and optical imaging have inherent limitations in sensitivity, penetration depth, and safety. Terahertz (THz) imaging is an emerging, non-ionizing technique that offers high sensitivity to water content and dielectric property variations, which indirectly reflect changes in bulk tissue composition, making it particularly suitable for skin diagnostics. This review provides a comparative analysis of transmission and reflection-mode THz imaging, with a detailed focus on the two primary reflection techniques- Terahertz Pulsed Imaging (THz-PI) and Continuous-Wave Terahertz Imaging (CW-THz). Their working principles, benefits, limitations, and clinical relevance are critically evaluated. Reflection-mode THz imaging shows strong potential for biological tissue analysis, offering high contrast for detecting skin malignancies, assessing hydration levels, monitoring wound healing, and evaluating transdermal drug delivery. Despite ongoing challenges in penetration depth and real-time imaging, advancements in AI-based analysis, multimodal integration, and system miniaturization are progressively enhancing its clinical applicability. This review serves as a comprehensive resource for researchers and clinicians aiming to integrate THz imaging into skin diagnostics. It highlights the transformative potential of THz technology in facilitating early disease detection, enabling personalized treatment strategies, and advancing the future of biomedical imaging.