The Confocal Tonal Shift

Timothy Bennett, CRA, OCT-C, FOPS

Penn State Eye Center, Hershey, Pennsylvania, USA

Purpose: A review of the cSLO confocal tonal shift and its effect on different pathologies.
Methods: The Heidelberg Spectralis confocal scanning laser ophthalmoscope (cSLO) is a commonly used diagnostic imaging device that uses monochromatic laser illumination to image the eye. The cSLO utilizes a focused laser to scan the subject and then captures the reflected light after it passes through a confocal pinhole. The pinhole suppresses out-of-focus light from reaching the image detector resulting in high-contrast, finely detailed images. The cSLO is most light efficient (brightest) at the plane of focus.

Results: A secondary result of the confocal aperture is how it affects the appearance of elevated or out-of-focus retinal structures. Some ocular findings may appear dark simply because they are slightly out of focus. Elevated pathologies such as serous detachments, macular edema, or papilledema are examples of the tonal shift that occurs due to change in focus. In some cases, the confocal pinhole may also suppress reflectivity in lesions that are slightly blurred because of scattering in tissue that is normally clear. IR imaging with the cSLO can help identify retinal whitening in artery occlusions or paracentral acute middle maculopathy (PAMM).

Discussion: Although these lesions aren’t elevated, light scatter from the slight thickening in the inner or middle retinal layers are suppressed by the confocal pinhole making the lesion appear dark. The confocal tonal shift can enhance diagnostic information by clearly outlining the borders of an elevated area or lesion. It is important to note that elevated lesions can appear dark regardless of the pathologic location in the fundus. The confocal tonal shift also has the potential to create tonal “artifacts” which can confound the appearance of findings like blood or hemorrhage that inherently appear dark. Vitreous opacities will appear dark when out of focus and blocked by the confocal pinhole. Blood and vitreous debris will appear the same even though one is translucent and one is more opaque when viewed ophthalmoscopically or with a fundus camera.

Conclusion: It is important to understand the confocal tonal shift to recognize structures that truly are dark from those that are simply out-of-focus. In some cases the tonal shift enhances areas of interest that may not be easily identified by other means. In others it may confound the documentation of blood or hemorrhage. A second imaging modality such as color fundus photography, OCT or angiography is often needed to present a more complete diagnostic imaging study.

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ICOP 2023
Maaike van Zuilen

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