Abstract
Measurement of optical properties is critical for understanding light-tissue interaction, properly interpreting measurement data, and gaining better knowledge of tissue physicochemical properties. However, conventional optical measuring techniques are limited in point measurement, which partly hinders the applications on characterizing spatial distribution and inhomogeneity of optical properties of biological tissues. Spatial-frequency domain imaging (SFDI), as an emerging non-contact, depth-varying and wide-field optical imaging technique, is capable of measuring the optical properties in a wide field-of-view on a pixel-by-pixel basis. This review first describes the typical SFDI system and the principle for estimating optical properties using the SFDI technique. Then, the applications of SFDI in the fields of biomedicine, as well as food and agriculture, are reviewed, including burn assessment, skin tissue evaluation, tumor tissue detection, brain tissue monitoring, and quality evaluation of agro-products. Finally, a discussion on the challenges and future perspectives of SFDI for optical property estimation is presented.
Highlights
The results showed that Spatial-frequency domain imaging (SFDI) could provide quantitative maps of optical and vascular parameters of precancerous lesions like human actinic keratosis, and feedback on the process of precancerous lesions transforming into malignant lesions
The results showed that SFDI could maintain the sensitivity to local scattering contrast in a wide range, which indicated that SFDI is suitable for the edge assessment of breast surgery
The results demonstrated that SFDI provided mappings of microscopic structural biomarkers that cannot be obtained with diffuse imaging, as well as characterized spatial variations not resolved by point-based optical sampling
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. These optical techniques have been widely used for measuring optical properties of different biological materials, such as human skin, brain, and tumor tissues [16,17,18] Most of these techniques (i.e., IS, TR, FD, and SR) employ a point light source for illuminating the target samples, which only enables one estimation of optical properties through single measurement. In the SFDI technique, special patterns of 2-D illumination, usually sinusoidal patterns, with different spatial frequencies are projected onto the surface of a target sample, and the remitted diffuse reflectance is captured by using an imaging device (e.g., high-performance camera) Demodulation algorithms, such as three-phase demodulation [20], Gram–Schmidt orthonormalization [21], and spiral phase transform [22], are applied to obtain the direct component (DC) image and alternating component (AC) image. The SFDI technique provides potential for decoupling the absorption property from scattering property of biological tissues
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