Bahram Khoobehi and James M. Beach

Fig. 4.8. Graphic depiction of the RSI algorithm, showing reference spectral curves for oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) and components of the algorithm. Isosbestic points (solid circles) where curves intersect divide the curves into three bands. Short line segments (gray lines) connect isosbestic points. Within each band, areas between the curve and line segment are sensitive to saturation. Polygonal areas under the line segments (A1, A2, and A3) compensate for total light intensity (see text). HbO2 and Hb curve segments are plotted below after subtracting the polygonal areas, showing the limits of the oxygen-sensitive areas (a1, a2, and a3) for complete saturation and desaturation. The oxygen-sensitive component (OSC) is determined from ratios of the oxygen-sensitive areas, a, and total light areas, A (see text). From the area between the hemoglobin curve and the long gray line segment connecting the first and last isosbestic points (b), and the area under the long gray line segment (B), the blood volume component (BVC) is determined (see text). Reprinted with permission from Khoobehi, B., Beach, J., and Kawano, H. Hyperspectral imaging for measurement of oxygen saturation in the optic nerve head. Invest Ophthalmol Vis Sci 45:1464–1472, 2004. ©2004 Association for Research in Vision and Ophthalmology.

RSIs from retinal vessels and ONH for IOP experiment are shown in Fig. 4.8 and Table 4.2.

4.2.2. Results

Figure 4.7 shows the area of the ONH obtained from the 570-nm band in the hyperspectral image for the oxygen concentration images (Fig. 4.7, topleft) and the variable IOP images (Fig. 4.7, bottom-left). Confirmation of vessel type was done by fluorescein angiography (images at right from the venous phase).

4.2.2.1. Spectral signatures

Increased oxygen saturation is indicated in Figs. 4.9 and 4.10 when the experimental spectrum changes to match more closely the HbO2 signature of

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Table 4.2. RSIs from the IOP experiment.

Unpaired samples of equal variance.

Average over three-time points at normal IOP.

†Average over last two-time points at high IOP.

‡All differences are significant (P < 0.05). Data are the means ± S.D.

Note: Reprinted with permission from Khoobehi, B., Beach, J., and Kawano, H. Hyperspectral imaging for measurement of oxygen saturation in the optic nerve head. Invest Ophthalmol Vis Sci 45:1464–1472, 2004. ©2004 Association for Research in Vision and Ophthalmology.

Fig. 4.6, with stronger minima at 542 and 577 nm. Desaturation is indicated when the curve more closely resembles the Hb signature having a single spectral minimum.

4.2.2.2. Oxygen breathing

The artery (top-left of Fig. 4.10) showed a small increase in the HbO2 signature with pure O2, relative to room air. In the vein (top-right), this increase was markedly larger. In the nasal and temporal ONH (bottom-left and -right), pure O2 increased the HbO2 signatures, but not to the degree observed in the vessels.

4.2.2.3. Intraocular pressure

In the artery (top-left of Fig. 4.10), high IOP sustained for five minutes gradually converted the HbO2 signature to an Hb signature. In the vein (top-right), the normal IOP curve showed a weak HbO2 signature. Within one minute after the onset of high IOP, however, the curve was converted to a strong Hb signature. These results suggest that high IOP causes desaturation of the retinal blood supply in both arteries and veins. Increased IOP resulted in only modest increases in total reflectance. High IOP reduced saturation in the ONH microcirculation but to a lesser degree than in the retinal circulation and suggested that saturation was partially restored in some regions.

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Fig. 4.9. Spectral curves during oxygen breathing experiment. Top row: artery (left) and vein (right). Bottom row: nasal ONH (left) and temporal ONH (right). Bottom curves: room air breathing; top curves: pure oxygen breathing. Reprinted with permission from Khoobehi, B., Beach, J., and Kawano, H. Hyperspectral imaging for measurement of oxygen saturation in the optic nerve head. Invest Ophthalmol Vis Sci 45:1464–1472, 2004. ©2004 Association for Research in Vision and Ophthalmology.

4.2.2.4. Responses to oxygen breathing

Figure 4.11 shows spatial changes in the relative saturation of ONH structures during room air breathing (left) and two minutes after switching to pure oxygen (right). The partial-signature maps (Fig. 4.11A) reveal saturation differences; however, structures such as the large vein are more clearly delineated during high saturation in the full-signature maps (Fig. 4.11B, right). Under normal conditions, high saturation areas included outlines of

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Fig. 4.10. Spectral curves from ONH during the IOP experiment. Top-left: artery at normal IOP and at 1, 3, and 5 minutes after IOP was increased to 60 mmHg; top-right: vein at normal IOP and 1 minute after pressure was increased to 60 mmHg. Bottom-left: nasal ONH; bottom-right: temporal ONH. For the high-IOP curves, the dotted line represents 1 minute and the solid line represents five minutes after IOP was increased to 60 mmHg. Reprinted with permission from Khoobehi, B., Beach, J., and Kawano, H. Hyperspectral imaging for measurement of oxygen saturation in the optic nerve head. Invest Ophthalmol Vis Sci 45:1464–1472, 2004. ©2004 Association for Research in Vision and Ophthalmology. .

arteries out to the ONH boundary. During pure oxygen breathing, saturation increased in the arteries, and new areas of high saturation appeared where veins were located.All structures showed significant increases (P < 0.05) in the RSI during pure oxygen breathing (Table 4.1). The increase in the veins was nearly twice (factor of 1.9) that was found in the artery, whereas smaller increases in the ONH (averaged over the cup and rim) were approximately

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