Hyperspectral Image Mapping Spectrometry for Retinal Oximetry Measurements in Four Diseased Eyes

Abstract

We describe the hyperspectral Image Mapping Spectrometer (IMS) which is able to acquire spectral images of the retina with no scanning components. We then utilize this technology to obtain absolute oxygen saturation measurements in four patients with retinal diseases. The IMS acquires spatial and spectral information of a scene in a single camera exposure time. The IMS is attached to the port of a Topcon TRC-50EX funduscopic camera in order to acquire images. The IMS has high spatial (350×350) and spectral sampling (40), and avoids motion artifacts associated with eye movements. Oximetry values are obtained by measuring the absorption of retinal blood. Four patients with different retinal diseases were imaged via the IMS in this study. Oxygen saturation is calculated using a least squares fit applied to the whole blood oximetry equation. Oxygen saturation was found to be 88.6% (±1.9) and 34.1% (±7.0) for the artery and vein, respectively, in a patient with non-exudative age-related macular degeneration (AMD) with borderline glaucoma, 100.0% (±0.0) and 50.4% (±8.2) in a patient with exudative AMD, 100.0% (±0.0) and 45.2% (±7.0) in a patient with retinitis pigmentosa, and 94.1% (±2.5) and 44.9% (±5.6) in a patient with chronic iridocyclitis. These values are consistent with previously published data. Based on spatial and spectral data, we created an oxygen saturation map with oxygenation data overlying retinal vasculature. Hyperspectral imaging may be a promising way to measure retinal oxygen saturation. IMS technology is advantageous in its ability to obtain images free of motion artifacts.

Hyperspectral Imaging and the Retina: Worth the Wave?

“…To extract OS levels, saturation is calculated after preprocessing of the hyperspectral cube by (1) taking the ratio between vessel optical density at two wavelengths (one sensitive to saturation, the other isosbestic) 30,32,36,54,90 ; (2) using a formula that combines several features defined to describe the shape of the spectra, decreasing the risk of incidental outliers [70][71][72][73] ; or (3) by decomposing optical density into a product including saturation, using the Beert-Lambert law for several wavelengths and solving the resulting set of equations. 23,25,26,28,63,82,86,87 Of note, although using hyperspectral hardware and measuring broad spectra, some of these articles solely rely on the two-wavelength method-(1) in previous paragraph-to produce their main findings and one LCTF study that makes use of more than two spectral bands, though with low spectral resolution (20 nm), failed to demonstrate superiority over the older twowavelength method. 54,60,89 In moderate and severe primary open-angle glaucoma higher mean venular OS was found compared with healthy eyes.…”

“…Such information is valuable to diagnose and monitor various ocular diseases including agerelated macular degeneration (ARMD), glaucoma, and diabetic retinopathy. [23][24][25][26][27][28][29][30][31][32][33] For further adoption of HSI technology in ophthalmologic research, it is important to explain the underlying imaging principles and describe the current relevant techniques, allowing for the identification of gaps and directions for future research and practice. Retinal HSI for ophthalmology has been documented before, but only included in reviews that analyzed the entire medical field or in manuscripts that date from several years ago.…”

Optimization of wavelength selection for multispectral image acquisition: a case study of atrial ablation lesions

“…Other examples include custom fitted systems for real-time spectral imaging, such as the image mapping spectrometry (IMS). The system is capable of simultaneously acquiring 40 channels at a resolution of 350x350 pixels [17]. More recently, the same group reported using Lenslet array based tunable snapshot imaging spectrometry (LATIS) for high resolution fluorescence imaging with reported resolutions of 200x200 at 27 spectral channels and integration time of less than a second [13].…”

“…It is important to note that unlike absolute oxygen saturation measurements previously reported [17,[34][35][36], our measurements rely on detecting the differences between the two states of the tissue under study (ablated vs. unablated), both visible in the field of view. As such, the need to perform calibration measurements would have less of an effect on the unmixing protocol since detected changes are relative differences between these two states of the tissue in the same subject.…”

“…Finding combinations of few spectral bands that are sufficient for lesion identification, would allow for simultaneous acquisition using the latest advances in fast scanning and high spatial resolution non-scanning imaging [13][14][15][16][17]. Overall, our studies offer several unbiased quantitative strategies as to how to limit the number of acquisition wavelengths while recognizing the target tissue with high accuracy.…”