New spectral imaging techniques for blood oximetry in the retina

Abstract

Hyperspectral imaging of the retina presents a unique opportunity for direct and quantitative mapping of retinal biochemistry -particularly of the vasculature where blood oximetry is enabled by the strong variation of absorption spectra with oxygenation. This is particularly pertinent both to research and to clinical investigation and diagnosis of retinal diseases such as diabetes, glaucoma and age-related macular degeneration. The optimal exploitation of hyperspectral imaging however, presents a set of challenging problems, including; the poorly characterised and controlled optical environment of structures within the retina to be imaged; the erratic motion of the eye ball; and the compounding effects of the optical sensitivity of the retina and the low numerical aperture of the eye. We have developed two spectral imaging techniques to address these issues. We describe first a system in which a liquid crystal tuneable filter is integrated into the illumination system of a conventional fundus camera to enable time-sequential, random access recording of narrow-band spectral images. Image processing techniques are described to eradicate the artefacts that may be introduced by time-sequential imaging. In addition we describe a unique snapshot spectral imaging technique dubbed IRIS that employs polarising interferometry and Wollaston prism beam splitters to simultaneously replicate and spectrally filter images of the retina into multiple spectral bands onto a single detector array. Results of early clinical trials acquired with these two techniques together with a physical model which enables oximetry map are reported.

Review of spectral imaging technology in biomedical engineering: achievements and challenges

“…On the other hand, LCTF is a solid-state instrument that utilizes electronically controlled liquid crystal cells to transmit light with a specific wavelength and the elimination of all other wavelengths. The LCTF approach also has been used in coupling with the microscope, 65,[89][90][91][92] fundus microscope, 114,[117][118][119] and camera lens [56][57][58]60,62,64,[68][69][70]77 to create spectral image cubes for biological imaging in vivo and in vitro.…”

“…This kind of imaging system can capture a scene of multispectral images faster than most of the spatial scanning and spectral scanning imagers. In recent years, researchers have proposed some methods for biomedical snapshot spectral imaging, such as the CTIS, 120 image mapping spectrometer, 125 coded aperture snapshot spectral imaging, 153,154 image replicating imaging spectrometer (IRIS), [114][115][116] imaging slicing spectrometer, 12 and Fourier transform-based snapshot imaging systems. 155,156 A major advantage of this kind of imager is speed.…”

“…Harvey et al developed an image replicating imaging spectrometer to capture spectral images of the retina and proposed a physical model that enables the oximetry map of retina. 114,116 Johnson et al and Ramella-Roman et al obtained oxygen saturation maps of retina with a threewavelength algorithm and found that the oxygen saturation were ∼95% for healthy subjects' arteries and 30 to 35% less for the veins. 18,122 Although the snapshot mode can capture the retinal spectral imaging quickly, the spatial and spectral resolutions are limited.…”

“…The reason is that the optical depth of HbO 2 changes from being less than to being greater than the thickness of the larger retinal arteries from 500 to 650 nm, and the reflectance spectrum of different locations across the retina is dominated by HbO 2 and Hb in both choroidal and retinal vessels, and by melanin components that vary depending on subjects and ethnicity. 114,117 Another example is tooth diseases' diagnosis using the spectral imaging method in the NIR spectral range. Studies show that dental enamel manifests high transparency (the mean free path of the photons being ∼3 mm at 1310 and 1550 nm, while being <0.1 mm for visible wavelengths) for the light in the NIR spectral region and the tooth caries process usually increases absorption and scattering of the incident light.…”

Molecular simulations for the spectroscopic detection of atmospheric gases

“…Infrared spectroscopy can be used in a wide-range of industrial applications such as climate monitoring [56,24], combustion analysis [61,23], fire detections [47], biomedical imaging [2], pharmacology [31], pollution tracking [46], atmospheric surveys [7], geochemical mapping [6], catalytic cracking [36], and crop maintenance [14]. Spectral interpretation is also indispensable to astrochemistry, such as for studies of galaxies [43,55,16], stars [32], comets [10], brown dwarfs [11,3], Solar System bodies [8,26,30,15] and exoplanet atmospheres [59,38,5,60].…”