The relaxed confocal scanning laser ophthalmoscope.

The development of the Scanning Laser Ophthalmoscope is reviewed from a historical perspective. Since a flying-spot scanning principle for an electro-optical ophthalmoscope was first disclosed in 1950, enabling milestones have included the introduction of the laser and inversion of the usual Gullstrand's configuration of optical pupils in 1977, and the application of the optical principle of confocality by means of double or de-scanning in 1983. As a result, high resolution and high contrast confocal infra-red ophthalmoscopy with a 790 nm diode laser, at video rates, is a major novel imaging modality when compared to traditional optical techniques. This imaging mode is ideal to provide the necessary fiducial landmarks for microperimetry, therapeutic laser and SD-OCT based optical sectioning of the retina. DPSS or He-Ne lasers emitting at 532, 543, 561 or 575 nm are used for complimentary red-free fundus imaging. The diode 790 nm and DPSS 490 nm lasers are also used for fluorescence excitation.

Microperimetry.

In SLO microperimetry a multi-channel overlay graphics frame grabber synchronizes the scanning of an infra-red laser with the modulation of a visible laser source. Graphics are created in the laser raster with a fast 25 ns sub-pixel rise-time 80 MHz acousto-optic modulator, Bragg angle optimized for 532 nm, 543.5 nm, 632.8 nm or 660 nm wavelengths over a 40 dB attenuation range. Our software kernel comprises 4 alternative forced choice (4AFC), parameter estimation by sequential testing (PEST) and manual or automated tracking algorithms based on two-dimensional normalized gray-scale correlation. They enable fast and accurate registration of fixation patterns, precise measurements of potential acuities and retinal sensitivity using a range of background illumination levels. Opto-electronic characteristics of physiologic significance are discussed. Clinical examples are given.

Topographical analysis of peripheral vs central retina with pattern reversal visual evoked response and the scanning laser ophthalmoscope.

The contribution of the central and the surrounding peripheral retina to the pattern reversal visual evoked response (PVER) was analyzed simultaneously using double frequency stimulation and the Fast Fourier transform (FFT) method. The Scanning Laser Ophthalmoscope (SLO) was used to project the pattern stimulus, a square subtending 10 degrees on a side, on a specific location in the fundus and to monitor accurately the stimulus' position during PVER testing. When the stimulus area on the central retina was less than or equal to 50 min of arc on a side, or 0.69% of the total area was stimulated (a square of 10 degrees on a side), the contribution from the central retina was negligible, and the PVER was dominated by surrounding peripheral retinal activity. When the stimulus area of the central retina was greater than or equal to 100 min of arc on a side, or 2.78% of the whole area was stimulated, activity from the central retina became more evident. When the stimulus area projected on the central retina became 150 min on a side (6.25% of the area or greater), activity from the central retina showed an increase and exceeded that of the peripheral retina. When the central area became larger, its activity dominated the PVER, and the contribution from the peripheral retina became negligible at this stage. The PVER was highly dependent upon the activity of the central retina, and activity levels from the central and surrounding peripheral retina were correlated.

[Static microperimetry with the laser scanning ophthalmoscope]. / La micropérimétrie statique avec l'ophtalmoscope à balayage laser.

We have used the scanning laser opthalmoscope (SLO) with a personal computer to develop static microperimetry techniques. They allow to see in real-time on a television monitor the precise retinal localization of the stimulus and fixation. The testing is performed under strict conditions. The size of stimuli can vary between 6 and 30 minarc on a side. 255 different intensity levels are possible with the instrument. We have selected 12 of them, representing a logarithmic scale. Stimulus duration can vary between 50 and 500 ms. Examples of macular pathology including subretinal neovascularization, drusen and macular edema are given.

Recording pattern reversal visual evoked response with the scanning laser ophthalmoscope.

We recorded visual evoked responses (VERs) to alternating, checkerboard pattern stimuli using the scanning laser ophthalmoscope (SLO). Retinal position and focus of checkerboard stimuli were monitored on the SLO video monitor throughout testing. Checkerboard size, check size, and retinal positions were varied. Consistent with other, well-established pattern reversal techniques, the SLO method produced: 1) reliable VERs with amplitudes of 2 to 10 microvolts, 2) maximum amplitudes at an intermediate check size for a fixed overall pattern size, and 3) variations in VER amplitude depending on stimulus retinal position relative to the fovea. Hence, the SLO-VER technique would be useful for clinical VER measurements when precise retinal stimulus position and focus are desired.