Year : 1979 | Volume
: 27 | Issue : 4 | Page : 119--122
Photo-microdensitometric method of investigating the oxygenation of blood in the disc vessels
Institute of Ophthalmology, Aligarh Muslim University, Aligarh, India
B S Goel
Institute of Ophthalmology, Aligarh 202 001
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Goel B S. Photo-microdensitometric method of investigating the oxygenation of blood in the disc vessels.Indian J Ophthalmol 1979;27:119-122
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Goel B S. Photo-microdensitometric method of investigating the oxygenation of blood in the disc vessels. Indian J Ophthalmol [serial online] 1979 [cited 2021 Jan 27 ];27:119-122
Available from: https://www.ijo.in/text.asp?1979/27/4/119/32597
Hickam and Frayer described a photographic method of determining retinal venous oxygen saturation by measuring the difference in optical density between the images of the vein and of the surronding optic disc. Photographs were taken simultaneously at two wave lenghts on two films by means of a split beam device and using a different colour filter for each film. The same principle has later been used by Laying et al.
In a photograph of the optic disc image of a retinal vessel, especially an artery, often shows a light central stripe along its length, this being produced by specular reflection from the convexity of the vessel. It would seem that this effect could well introduce an error into any densitometric procedure on the image of the vessel. This difficulty could perhaps be overcome by arranging the illumination of the optic disc by the fundus camera in such a way that no light fell directly upon the vessel. It is known that the tissues of the optic nerve head are capable of diffusing light strongly Gloster and that it is possible to photograph the whole of the optic disc by illuminating only part or it Gloster. This paper deals particularly with this problem and associated difficulties.
(a) Fundus photography-The following types of photographs were obtained on volunteers with a Zeiss fundus camera with a Nikon motor drive. i) "Plain" photographs with introduction of an orange filter (llford 201 or 607), or an interference filter with a peak transmission at 598 mm) in front of the film. [Figure 1].
(ii) Photographs with above filters, out with the [Figure 3] in the illuminating pathway of the fundus camera as described by Gloster .
Calibration of differences in optical density was done by placing a step wedge containing neutral density filters from 0.2 to 0.8 log units, in steps of 0.2 in the illuminating pathway, and taking a photograph of a plain background on the same strip of film at the end of the fundus photography.
Various photographic films were used including Kodak (400 A.S.A.) processed in acuspeed for 6 minutes, and Kodak plus Xpan (125 A. S.A.) processed in acutol for 8 minutes.
(b) Microdensitometric procedure-All measurements were made on a Wooster scanning microdensitometer. The measurements were made with a circular aperture smaller than the vessel under observation. The tracing was obtained at right angles to the vessel recording the dens ty of the disc tissue surrounding the vessel and the vessel itself. Pairs of vessels, artery and vein, were selected, lying near to each other but sufficiently far away so as not to interfere with the density measurements on each other. The density difference between the artery and the vein was then measured on the chart. The density differences between adjoining steps of neutral density wedge were also measured on the film and an average was obtained for a density difference of 0.2, this calibration was used to find out the difference in the density of the artery and the vein in question.
The observations were made on the plain photographs at various levels of intensity of the camera flash, and on photographs with either a slit or a bar in the illuminating pathway. In some instances the subject was breathing air while in others he was breathing pure oxygen.
Plain photographs consistently showed bright linear reflexes along the vessels especially the arteries [Figure 1]. The tracing on the densitometer had small notches corresponding to these light reflexes [Figure 2], and the depth of the notches varied with different aperture sizes of the densitometer. This was thus expected to introduce a small error in the densitometer measurements, Attempts were then made to obtain photographs of the vessels chosen for densitometric measurement by intro ) ducing a slit in the illuminating pathway so as to illuminate only a strip of the disc, the rest of the vessels being photographed from light transmitted through the disc. The reflexes on the vessels were obliterated to a large extent [Figure 3] and the densitometric trace did not show notches [Figure 4].
A difference in the heights of the densitometric tracings for the artery and vein was always seen and it was fairly constant in one individual from frame to frame. The densities of the images of the vein and the artery were determined from the densitometric tracing by comparing them with the standard neutral density wedge photographed in the same strip of film with similar conditions of exposure and developing.
[Table 1] shows the densitometric differences between the veins and arteries in log units. In the photographs, which gave better quality images by eliminating the reflex, the greatest density differences was obtained with Ilford spectrum orange filter (607). The quality of the photographs suffered when an interference filter was used together with a slit and thus the density difference could not be accurately determined. Subsequent observations with regard to air and oxygen breathing were made with Ilford (607) filter together with a slit.
[Table 2] shows the density difference between the vein and the artery in log units on slit photographs while breathing air, pure oxygen for 6-8 minutes and later air. The difference between breathing air and oxygen was highly significant (P 0.005), while the difference between breathing air and later air after oxygen was not significant.
The density of the veins and arteries was also measured while breathing air or oxygen [Table 3]. It was observed that the mean density of the vein during air breathing was 0.18+0 . 03 which changed to 0.13±0.03 on breathing oxygen. This difference was statistically significant (P 0.005). The density of the arteries was the same during air and oxygen breathing. This shows that there was essentially a change of about 27% in the density of the veins during oxygen breathing as compared with air breathing.
By using the principle described by Gloster,, the quality of the disc photographs could be improved by introduction of a slit in the illuminating pathway. In this way, photographing the vessels with light transmitted from the adjacent disc tissue completely eliminated light reflexes consistently noted with plain photographs. The density difference between the vein and the artery was best discernible in photographs with Ilford (607) filter, which gave the optimum results. Thus the method can be said to detect differences in oxygen saturation at the disc between the vein and the artery and by further elaboration of the method it would be possible to study this arteriovenous difference in diseases like glaucoma, hypertension, diabetes, etc. where a slowing of the retinal vasicular circulation may be expected.
A photographic and densitometric procedure is described for estimating the difference in the oxygen concentration of the disc vein and artery. The quality of photographs has been improved by introduction of a slit in the illuminating pathway of the fundus camera. An orange filter (Ilford 607) with peak transmission in the range of 598 mm has been used to differentiate the oxygen concentration in the vein and the artery. There was 27% change in density of veins and 5% change in density of the arteries by breathing pure oxygen for 6-8 minutes, and these differences were statistically significant.
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