Functional Imaging of the Ocular Fundus Using an 8-Band Retinal Multispectral Imaging System

Emanuel R De Carvalho,Reinier O Schlingemann,Cornelis J F Van Noorden,Richelle J M Hoveling,Maurice C G Aalders

doi:10.3390/instruments4020012

Abstract

Application of functional imaging in ophthalmology requires efficient imaging techniques that can detect and quantify chromophores to visualise processes in vivo. The aim of the present study was to develop and evaluate a fast and affordable imaging system. We describe an eight-band retinal multispectral imaging (MSI) system and compare it with a hyperspectral imaging (HSI) device. Determination of blood oxygen saturation was studied as proof of principle. Reflectance of incident light is measured as 1/absorbance at different wavelengths between 440 nm and 580 nm. Both devices have incorporated optical bandpass filters in a mydriatic fundus camera. The MSI system scans the retina at eight pre-defined wavelengths specific for the spectrum of haemoglobin. The HSI system acquires a full scan from 480 to 720 nm in 5 nm steps. A simple assessment of the ratio between the absorbance peaks of oxygenated haemoglobin (HbO2) and reduced haemoglobin (HbR) was not suitable for generating validated oxygenation maps of the retina. However, a correction algorithm that compares the measured reflectance with reflectance spectra of fully oxygenated and fully deoxygenated blood allowed our MSI setup to estimate relative oxygen saturation at higher levels, but underestimated relative oxygen saturation at lower levels. The MSI device generated better quality images than the HSI device. It allows customisation with filter sets optimised for other chromophores of interest, and augmented with extrinsic contrast imaging agents, it has the potential for a wider range of ophthalmic molecular imaging applications.

Highlights

The ability to follow molecular and cellular processes in the retina in vivo would be a powerful diagnostic aid
We have demonstrated that multispectral imaging (MSI) provides a practical method for estimating relative oxygen saturation in retinal blood, and that MSI offers several significant advantages over hyperspectral imaging (HSI), despite the fact that our setup suffered from the inherent difficulties common to retinal spectral imaging devices
By demonstrating the determination of oxygen saturation in retinal blood, we have shown that a newly-developed multispectral imaging system (MSI) has significant advantages over a hyperspectral imaging device (HSI)

Summary

Introduction

The ability to follow molecular and cellular processes in the retina in vivo would be a powerful diagnostic aid. For the recognition of chromophore molecules with characteristic light absorption properties, spectroscopy can be integrated into conventional ocular fundus cameras to provide retinal spectral imaging. Recent developments include hyperspectral imaging (HSI) and multispectral imaging (MSI). In HSI, sequential image acquisition is performed in small steps in a pre-defined wavelength range using tuneable filters [1], Fourier transform spectrometers [2], spectro-temporal scanners [3], or volume holographic methods [4]. MSI uses only a limited number of wavelengths—those with distinct absorbance peaks corresponding with those of the chromophore of interest Because the images are captured at only a small number of wavelengths rather than uniformly across a range of the electromagnetic spectrum, the acquisition time for MSI is shorter than for HSI

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