Fred Reinholz

Simultaneous three wavelength imaging with a scanning laser ophthalmoscope.

BACKGROUND Various imaging properties of scanning laser ophthalmoscopes (SLO) such as contrast or depth discrimination, are superior to those of the traditional photographic fundus camera. However, most SLO are monochromatic whereas photographic systems produce colour images, which inherently contain information over a broad wavelength range. METHODS An SLO system has been modified to allow simultaneous three channel imaging. Laser light sources in the visible and infrared spectrum were concurrently launched into the system. Using different wavelength triads, digital fundus images were acquired at high frame rates. RESULTS Favourable wavelengths combinations were established and high contrast, true (red, green, blue) or false (red, green, infrared) colour images of the retina were recorded. The monochromatic frames which form the colour image exhibit improved distinctness of different retinal structures such as the nerve fibre layer, the blood vessels, and the choroid. CONCLUSIONS A multi-channel SLO combines the advantageous imaging properties of a tunable, monochrome SLO with the benefits and convenience of colour ophthalmoscopy. The options to modify parameters such as wavelength, intensity, gain, beam profile, aperture sizes, independently for every channel assign a high degree of versatility to the system.

Oximetry with a multiple wavelength SLO

Oximetry may be useful for understanding pathologies of the eye. We use a prototypescanning laser ophthalmoscope capable of simultaneous multiple wavelength imaging to record fundus images. The system is run under non-confocal conditions in using slitapertures with a width of up to 600 μm. The laser lines launched into the SLO were the 633 nm line of a HeNe-laser, and the 815 nm line from a tunable (700 to 900 nm) cw Ti:Sapphire-laser. These wavelengths were selected because of their availability and absorption characteristics. The difference in absorption at these two wavelengths is used to assess blood oxygen content. Images were averaged to improve the signal to noise ratio.This simultaneous method of measuring oxygen content may be preferred to othertechniques such as sequential SLO imaging at different wavelengths, or spot analysisusing a modified fundus camera. The advantages of this technique are that imageregistration is not required, and a large area of the retina can be assessed concurrently.

Improvements in colour fundus imaging using scanning laser ophthalmoscopy

Abstract. The objective of this study was to examine the properties of two laser lines 514 nm and 532 nm when used to image the retina with a scanning laser ophthalmoscope (SLO), and to compare the images taken with a simultaneous multiple wavelength SLO, to those taken with a fundus camera. From this we concluded that the 514 nm line is the preferred line for visualising the nerve fibre layer whereas the 532 nm line is preferred for visualising retinal vessels. Based on these results the 532 nm laser light source was selected as the green line for imaging with the simultaneous colour SLO. Cases are presented where the colour SLO images contain more information than traditional digitised fundus photographs.

General purpose control system for scanning laser ophthalmoscopes

A flexible control system for scanning laser ophthalmoscopes is described that is quick and simple to configure, easily modified or adapted, and containing many useful features. The system facilitates adjustment of several parameters to account for changes to the scan position, ambient light and temperature, including both optical and electronic components, which is otherwise difficult and time‐consuming to perform. The system is portable and uses custom‐designed printed circuit boards. All system parameters, such as focus, scan rate, scan depth and stereo control can be digitally controlled from a computer via a single serial port. Custom software allows changes to any system parameters by simply sending the required control data to the rack. The circuit boards in the system are multilayer, incorporating good ground‐plane techniques to minimize noise, programmable logic and semicustom logic for low cost and compact size, and microcontrollers with embedded firmware for flexible operation. Retinal images demonstrate that the system performs well.

Development of a versatile stereo scanning laser ophthalmoscope

The scanning laser ophthalmoscope (SLO) is a modern tool which is now widely used to image the fundus of the eye, particularly for assessment of the optic nerve head. We describe a modified SLO capable of producing stereo pairs of the optic disk in real time. A pair of toggling mirrors is used to switch between entry positions of the scanned laser beam into the pupil of the eye thereby creating a stereo base for capturing the two different views required for the pair. Our laboratory prototype is constructed from reflective optics only in the bi-directional part of the beam path, including the focusing and beam shaping unit. Thus, we avoid unwanted back reflections and chromatic aberrations. Light from different laser sources (458 to 1100 nm) can be launched into the SLO, also simultaneously. Collimated beams in beam splitting locations allow for easy modifications. Imaging in fluorescence mode or polarization dependent imaging is also possible. High quality multi-wavelengths stereo pairs of both model and real optic disks were obtained. a lateral resolution of up to 6 micrometer and an axial resolution of up to 65 micrometer was established.

Differential imaging in scanning laser ophthalmoscopy

Differential imaging may be useful for understanding pathologies of the choroid. We use a prototype scanning laser ophthalmoscope capable of simultaneous multiple wavelength imaging to record fundus images. The advantages of simultaneous acquisition compared to serial acquisition are, reduced image capture times, and image registration is not required.The system is run under non-confocal conditions in using slit apertures with a width of up to 600 μm. The laser lines launched into the SLO were the 633 nm line of a HeNe- laser, and the 815 nm line from a tunable (740 nm to 840 nm) cw Ti:Sapphire-laser. The difference in absorption at these two wavelengths is used to produce a differential image. We compare conventional imaging with differential spectral imaging for viewing the choroidal vessels. Qualitative results show the contrast of choroidal vessels in differential imaging is improved compared with normal imaging in the region of the optic disk and for the level of pigmentation in the subjects examined.

Comparison of different imaging modes for scanning laser ophthalmoscopes

We describe a bench top system for digital scanning laser ophthalmoscopy. This system is used for both regular patient screening and experimental imaging studies. The complete set- up is assembled from a number of modules (e.g. launching, detection, scanning, focusing unit) which may be altered readily to offer a high degree of flexibility in the imaging conditions. Both the launching and the detection unit can be used in a simultaneous, multiple channel configuration. This allows the acquisition of true color and false color images of the back of the eye. In particular, the use of infrared lines permits the investigation of deeper retinal structures. Digital image processing methods can then be used to generate differential images of frames taken with different wavelengths, such as red and infrared. Furthermore, the separate detection channels can also be used to obtain recordings in other imaging modes, such as tightly confocal, loosely confocal, indirect or polarization dependent contrast. We discuss the merits and problems of different imaging modes. Cases are presented where the differential imaging shows clear advantages over the standard (monochromatic) confocal method in the perceptibility of deeper laying structures (choriod).

Multi-Spectral Imaging in Scanning Laser Ophthalmoscopy

Fundus imaging is an important diagnostic tool in ophthalmology. The most common instrument used for recording fundus images is the fundus camera.