Image splitting--a technique for measuring retinal vascular reactivity.

Abstract

This report describes the application of this technique to the eye, where it provides, as in the brain, a rapid and accurate method of measuring changes in arterial calibre. Materials and methodsThe system consists of a standard Zeiss fundus camera through which the fundus is visualised by means of a low-light-level television camera (Ikegami). This camera contains a silicon intensifier target (SIT) tube. The image of the retinal arterioles is passed from the fundus camera through an image splitting eyepiece (Vickers), and the split image received by the television camera is displayed on a monitor. The shearing screw of the eyepiece which controls the degree of image splitting is connected to a 10-turn potentiometer and in turn to a digital readout of resistance. In this way frequent measurements of shear (which is directly proportional to vessel calibre) were obtained in arbitrary units. The fundus is illuminated with the standard observation light of the fundus camera on position 2.Pupils were dilated with 2 drops of tropicamide 1% instilled 20 and 10 minutes before measurement. The accuracy of the method (intraobserver error) was first assessed by making 50 measurements of shear on a first-order arteriole near the optic disc in a human volunteer. One observer determined the end point by observing the point between overlap and separation of the optically split image on the television monitor. A second observer recorded each measurement from the digital meter. Interobserver error was assessed by 3 trained observers making 10 measurements of a single retinal arteriole in a second volunteer.A further 7 volunteers were used in 11 experiments in which 5 measurements were made at resting intraocular pressure followed by another 5 measurements after increasing the intraocular pressure to 30-50 mmHg. Initial attempts to increase and maintain the intraocular pressures with a suction cup were not reliable. With this method the rise in pressure was unstable while the suction resulted in distortion of the globe, which degraded the quality of the image. The method finally employed used a Mackay-Marg tonometer mounted on a jig which could be adjusted by a micrometer screw to compress the temporal sclera of the eye and so increase the intraocular pressure.4 The tonometer was connected to an analogue meter from which continuous readings of increase in intraocular pressure were monitored, and the intraocular pressure was kept at a sustained level. Arterioles were selected at random, but all were near the disc and either first-or secondorder retinal arterioles. Results INTRAOBSERVER ERRORThe results of 50 measurements made on a secondorder arteriole by one observer gave a mean of 76 291

Reproducible estimation of retinal vessel width by computerized microdensitometry

“…Previous techniques for estimating retinal vessel widths were based on an observer attempting to define the borders of a vessel from the image projected from a slide on a negative or a profile based on density measurements [6][7][8][9][10][11][12][13][14]. Since all the decisions in these procedures are subjective, and since the vessel borders are usually vague and with edges difficult to place, the reliability of the quantification of vessel width and blood flow measurements based on these is low.…”

Retinal blood vessel width measured on color fundus photographs by image analysis

“…They chose a ten fold projection of ne-gatives and concluded that a trained observer on consecutive days obtained relative width measurements reproducible to 9 microns on the retina with a 95% probability. Another direct observation method was to use image splitting in an image obtained by a lowlight-level television camera attached to a fundus camera [13]. Among 3 different observers the percent coefficient of variation (standard deviation/ mean X 100) for 10 repeated measurements of a retinal arteriole width ranged from 3.7 to 6.0%.…”

“…Delori et al have given a detailed review and comparison of various micrometric and microdensitormetric methods for measuring retinal blood vessel width [14]. Both methods involve obtaining measurements of the width of the vessel at a specific point, rather than obtaining the average width of the vessel from measurements of a segment of the vessel [12,13]. Micrometric methods involve an observer estimating visually the location of the vessel wedge on a projected image of the vessel.…”