J. Van De Kraats

Imaging retinal densitometry with a confocal Scanning Laser Ophthalmoscope.

Abstract We describe a novel use of the Scanning Laser Ophthalmoscope (SLO), viz. as an imaging retinal densitometer. In our SLO a helium-neon or an argon laser beam is moved in a raster pattern over the retina; the reflected light is descanned (confocal SLO) and collected by a photomultiplier. Images of the fundus subtending 22 by 18 deg are displayed on a TV monitor. Single frames taken with 514 nm light were stored in a computer in arrays of 256 by 256 pixels and density differences between dark adapted and bleached images were calculated. With a full bleach density differences of about 0.35 were found in the center of the fovea; at retinal eccentricities of 15–20 deg we found 0.15. After selective bleaching with 633 nm light substantial density differences were only seen in the foveal area. We conclude that the confocal SLO is a very suitable instrument for imaging fundus reflectometry.

Foveal cone mosaic and visual pigment density in dichromats.

1. Optical reflectance spectra of the fovea were measured in ten subjects with normal colour vision, ten protanopes and seven deuteranopes. Four conditions were used: perpendicular and oblique angle of incident and reflected light on the retina, both in a dark‐adapted and a fully bleached state. 2. The spectra were analysed to assess the effects of dichromacy on the cone mosaic. A replacement model, i.e. one where the total number of cones remains unchanged and all cones are filled with a single type of pigment, was found to fit our data best. 3. The analysis of the spectral fundus reflectance also provided estimates for densities of photo‐labile and photo‐stable retinal pigments and fraction of long wavelength‐sensitive (LWS) cones. Visual pigment density was 0.39 for protanopes and 0.42 for deuteranopes, significantly lower than the 0.57 found for colour normals. Macular pigment density was 0.54 for colour normals, 0.46 for protanopes and 0.42 for deuteranopes. 4. For colour normals the LWS cone fraction was 0.56, in agreement with psychophysical literature. The LWS cone fraction for protanopes was ‐0.04, and for deuteranopes 0.96, consistent with their Rayleigh matches.

Comparison of fluorescence of sodium fluorescein in retinal angiography with measurements in vitro

To quantify dye leakage in ocular fluorescein angiography, the arterial concentration of sodium fluorescein has to be determined. We investigated whether the nonlinear relationship between the fluorescein concentration and the fluorescence intensity obtained by in vitro measurements corresponds with that measured in vivo in a retinal artery. The time series of fluorescence in a retinal artery were recorded using an in-house-designed and -built confocal scanning laser ophthalmoscope in 11 healthy volunteers. Three different doses of sodium fluorescein were injected successively. About 10 min after the last injection a venous blood sample was drawn. The three in vivo peak intensities were fitted by least squares on the in vitro calibration curve using the first peak concentration and an intensity scaling factor as the two unknown parameters. The fit showed that the saturation of the three in vivo peak intensities corresponded well with the in vitro data. Calculation of the intensity scaling factor from the blood sampling data confirmed the result of the fit. The fitted concentration was verified by showing that the cardiac output necessary to obtain this concentration was within the physiological range. The fluorescence measured in our in vitro experimental setup corresponded well with the in vivo measurements. Therefore, the results from in vitro measurements can be applied in the analysis of fluorescein angiograms.

User friendly system for electrodiagnosis

A laboratory built computer system for clinical electrophysiology of vision is described to illustrate how modern technology may ease the task of handling otherwise rather complicated electronic equipment and of elaborating the data obtained. An example is given of a procedure for clinical electroretinography in which the task of the operator is virtually confined to using a single command letter, which is specified in continuously updated instructions displayed on a monitor. After the patient has left, the operator checks and if necessary corrects the automatically defined maxima of a and b waves of the electroretinogram. The print-out consists of graphs of amplitude and latency versus stimulus intensity in which the normal range is indicated. The task of the operator during an EOG measurement is confined to checking the collaboration of the patient, since the complete procedure including the elaboration of the data is automated.

Interactive Computer Program for Clinical Electrophysiology

When developing a new microprocessor based unit for clinical electrophysiology we made the demand that the system be as user-friendly as possible. This resulted in a set-up in which all visible equipment was reduced to two monitors and a keyboard. The operator receives instructions from one monitor and responds by pressing, in most instances, a single key on the keyboard. The second monitor displays the electrophysiological responses. The operator first chooses the type of measurement (ERG, EOG, VER research or standard procedure) from a menu. A standard ERG program starts with an automatic check of electrode resistances. Next a series of 10 flashes at low intensity is presented to the dark adapted subject. When the operator accepts the averaged results, a new series of flashes at a higher intensity is presented, and so on. The program features overload protection, calculation of oscillatory potentials from standard responses, and interactive assessment of a and b-wave latencies and amplitudes. Results of the measurements together with the patient record is stored on a mini-disk. After completion of all measurements the results are printed on a matrix printer.