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HYPERSPECTRAL IMAGING OF THE HUMAN RETINA:

OXIMETRIC STUDIES

D.J. Mordant1, I. Al-Abboud2, A.R. Harvey2*, A.I. McNaught1**

PURPOSE

Hyperspectral imaging of the human retina is a relatively new concept that has the potential to determine the metabolic status of the retina. Oximetric studies have been the main focus of previous research as the differential spectral characteristics of the two functional haemoglobin derivatives may be exploited to determine the oxygen saturation in the blood vessels.1,2,3

This study aims to demonstrate the ability to detect oximetric variations in the retinal circulation amongst normal subjects and in patients with retinal arteriopathy and glaucoma using hyperspectral imaging and spectral analysis techniques.

METHODS

A hyperspectral retinal imaging system consisting of a modified commercial fundus camera, a liquid crystal tuneable filter and a low-noise CCD detector (figure 1) was used to capture sequential hyperspectral images of the human retina. A hyperspectral data cube with a spectral bandwidth of 500nm to 700nm and a spectral resolution of 10nm at wavelength steps of 2nm were obtained for each subject. Normal subjects (n = 11) were examined and compared to subjects with retinal arterial occlusion (n = 3) and advanced primary open angle glaucoma (POAG)(n = 1).

Pre-processing algorithms were used to dark calibrate and co-register the raw retinal images. A further image processing algorithm produced a reflectance optical density map of the retina for each wavelength (figure 2).

Linear spectral unmixing is used in spectral imaging to determine the relative abundance of materials (endmembers) in each pixel of a scene through the analysis of its spectral characteristics. An average spectral profile of the arteries in the optic disc were calculated from all normal eyes (figure 3). This spectral profile was used to represent a pure endmember of arterial blood. In addition, a region within the optic disc cup was included into the analysis and represents a relatively non-oxygenated and spectrally inert endmember. Linear spectral unmixing, performed in ENVI 4.1 (ITT Visual Information Solutions), incorporating these two endmembers was used to produce a qualitative abundance map of oxygenated blood in the retina.

Figure 1.Schematic arrangement of the Hyperspectral Retinal Camera.

RESULTS

Linear spectral unmixing produced consistent oximetric maps of the retina in normal subjects (figure 4) where oxygenated blood (red) has been identified within the arteries and arterioles. In subjects with arterial occlusions and advanced POAG, this technique was able to detect changes in the oximetric status of the retinal circulation. Figure 5 illustrates one of these subjects with arteritic retinal vasculopathy caused by giant cell arteritis. The oximetry maps demonstrate an improvement in oxygenation of the retinal vasculature and retina following treatment with intravenous methylprednisolone. Figure 6 illustrates the oximetric variation in the retinal circulation of a patient with advanced POAG which corresponds with the severity of visual field loss.

Figure 2. Pre-processing methods of the hyperspectral retinal images.

Figure 3. Linear spectral unmixing methods

Figure 4. Oximetric retinal maps from a selection of normal subjects. Oxygenated blood (red) has been identified in the retinal arteries and arterioles. Venous blood, containing less oxygenated haemoglobin resulting in a different spectral characteristic to oxygenated arterial blood, is shown in black.

Figure 5. Oximetric maps of an 81 year old female with a right retinal vasculitic arteriopathy secondary to giant cell arteritis.

(Left) Oximetric map of the right retina at presentation. Linear spectral unmixing reveals a reduction in oxygenated blood in the retinal arteries (pale red/white) and an increase in oxygenated blood in the superior retinal veins. This indicates a reduced metabolic activity in the retina consistent a visual acuity of perceiving hand movements.

(Middle) Colour photograph of the right retina 3 days after presentation whilst on intravenous methylprednisolone.

(Right) Oximetric map of the right retina 8 days after presentation. Linear spectral unmixing reveals an improvement in the amount of oxygenated blood within the retinal arteries associated with a change in the oxygenation of the blood in the veins. This indicates an improved metabolic activity in the retina which corresponds with an improvement in the visual acuity to 6/24.

CONCLUSIONS

Hyperspectral imaging is capable of detecting oximetric changes in the retina and monitoring its response to treatment. However, the sequential technique of capturing retinal images described here is heavily dependent on other factors such as accurate co-registration of the images This limitation will be addressed in poster 2582/B659 where we will describe the development of a “snapshot” spectral retinal camera.4

Linear spectral unmixing offers a powerful and visually useful method of producing semi-quantitative oximetric maps of the retina, but to increase the effectiveness in detecting changes caused by diabetic retinopathy and early glaucoma increased and absolute accuracy is required. This requires the incorporation of a physical model for light propagation in the retina into the calculation of oxygenation – as is described in poster 2581/B658. 5

1Ophthalmology Department, Cheltenham General Hospital, Cheltenham. United Kingdom.

2School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh. United Kingdom.

Superior temporal artery

Superior temporal vein

Inferior temporal artery

Inferior temporal vein

REFERENCES

1. Schweitzer D, Hammer M, Kraft J, Thamm E, Konigsdorffer E, Strobel J. In vivo measurement of the oxygen saturation of retinal vessels in healthy volunteers. IEEE Tram Biomed Eng. 1999; 46:1454-1465.

2. Khoobehi B, Beach JM, Kawano H. Hyperspectral imaging for measurement of oxygen saturation in the optic nerve head. Invest Ophthalmol Vis Sci. 2004; 45(5):1464-72.

3. Lawlor J, Fletcher-Holmes DW, Harvey AR and McNaught AI. In vivo hyperspectral imaging of human retina and optic disc. ARVO Annual Meeting Fort Lauderdale, Florida 2002. (4350-B319).

4. G.Muyo, A.Gorman, I.Al Abboud, D.J. Mordant, A.I. McNaught, A.R. Harvey. En Face Snapshot Spectral Imaging of the Retina. ARVO Annual Meeting Fort Lauderdale, Florida 2007. Tuesday, May 8 2007 8:30 AM - 10:15 AM. (2582/B659).

5. I.Alabboud, III, A.McNaught, D.Mordant, A.R. Harvey. Quantitative Spectral Imaging of the Retina. ARVO Annual Meeting Fort Lauderdale, Florida 2007. Tuesday, May 8 2007 8:30 AM - 10:15 AM. (2581/B658).

COMMERCIAL RELATIONSHIPS

* QinetiQ, AstraZeneca. **Alcon; Allergan; MSD; Pfizer.

Figure 6. A 76 year old male with advanced POAG.

(Left) Colour photographs of the optic discs showing advanced cupping (cup-disc ratios 0.9).

(Middle) Humphrey visual field examination demonstrates asymmetrical visual field loss affecting the right eye more than the left eye.

(Right) Oximetric maps of the retina shows an increased oxygenation in the venous circulation of the right retina compared to the left retina. This may suggest a reduced metabolic activity in the right eye.

Left eye: Snellen acuity 6/9, intraocular pressure 12.

Right eye: Snellen acuity 6/9, intraocular pressure 13.