Spatially resolved, diffuse reflectance imaging for subsurface pattern visualization toward development of a lensless imaging platform: phantom experiments

Experimental Validation of Shifted Position-Diffuse Reflectance Imaging (SP-DRI) on Optical Phantoms

Numerous diseases such as hemorrhage, sepsis or cardiogenic shock induce a heterogeneous perfusion of the capillaries. To detect such alterations in the human blood flow pattern, diagnostic devices must provide an appropriately high spatial resolution. Shifted position-diffuse reflectance imaging (SP-DRI) has the potential to do so; it is an all-optical diagnostic technique. So far, SP-DRI has mainly been developed using Monte Carlo simulations. The present study is therefore validating this algorithm experimentally on realistic optical phantoms with thread structures down to 10 μm in diameter; a SP-DRI sensor prototype was developed and realized by means of additive manufacturing. SP-DRI turned out to be functional within this experimental framework. The position of the structures within the optical phantoms become clearly visible using SP-DRI, and the structure thickness is reflected as modulation in the SP-DRI signal amplitude; this performed well for a shift along the x axis as well as along the y axis. Moreover, SP-DRI successfully masked the pronounced influence of the illumination cone on the data. The algorithm showed significantly superior to a mere raw data inspection. Within the scope of the study, the constructive design of the SP-DRI sensor prototype is discussed and potential for improvement is explored.

Contact, high-resolution spatial diffuse reflectance imaging system for skin condition diagnosis

Spatially resolved diffuse reflectance spectroscopy (srDRS) is a well-established technique for noninvasive, in vivo characterization of tissue optical properties toward diagnostic applications. srDRS has a potential for depth-resolved analysis of tissue, which is desired in various clinical situations. However, current fiber-based and photodiode-based systems have difficulties achieving this goal due to challenges in sampling the reflectance with a high enough resolution. We introduce a compact, low-cost architecture for srDRS based on the use of a multipixel imaging sensor and light-emitting diodes to achieve lensless diffuse reflectance imaging in contact with the tissue with high spatial resolution. For proof-of-concept, a prototype device, involving a commercially available complementary metal-oxide semiconductor coupled with a fiber-optic plate, was fabricated. Diffuse reflectance profiles were acquired at 645 nm at source-to-detector separations ranging from 480  μm to 4 mm with a resolution of 16.7  μm. Absorption coefficients (μa) and reduced scattering coefficients (μs') of homogeneous tissue-mimicking phantoms were measured with 4.2  ±  3.5  %   and 7.0  ±  4.6  %   error, respectively. The results obtained confirm the potential of our approach for quantitative characterization of tissue optical properties in contact imaging modality. This study is a first step toward the development of low-cost, wearable devices for skin condition diagnosis in vivo.

Contrast Ratio Quantification During Visualization of Microvasculature

INTRODUCTION

Visualization and monitoring of capillary loops in dermis and mucosa are of interest for a number of clinical applications, such as capillaroscopy, early cancer, or shock detection. For historical reasons, an unaided eye is still a primary aide to diagnostics in visual examinations for many medical specializations. However, the ability to make an early diagnosis using the unaided eye has remained poor. New optical modalities can significantly improve the accuracy of anomaly detection. To compare the image quality of various optical schemes, a systematic way to quantify it is required. The goal of this work is to develop an analytical approach for assessment of a contrast ratio as a single number quantitative metric image quality during optical imaging of capillary network.

METHODS

Based on skin layers geometry, we developed a two-layer optical tissue model. Then, we extended a two-layer Kubelka-Munk model to calculate the contrast ratio of a subsurface defect (absorption or scattering) imaging.

RESULTS

We have obtained an explicit expression for the contrast ratio in the two-layer model. Then, we investigated how the contrast ratio is affected by the tissue optical parameters and depth of the inhomogeneity. Based on this analysis we identified two important cases: (a) the top layer with negligible absorption, and (b) the 'optically thick' top layer. The contrast ratio deteriorates differently with the inhomogeneity depth in these two cases.

CONCLUSIONS

The contrast ratio can be used for quantification of image quality of subsurface inhomogeneities in the skin. The developed approach can be employed for estimation of interrogating depth of various tissue inhomogeneities and optimization of imaging techniques.