Lasers, optical systems and safety in ophthalmology: a review

Abstract

This is a review of optical methods related to biomicroscopy and laser treatment of the posterior segment of the eye. Contact lenses can be used to observe optical structures and couple laser radiation into the eye for a vast range of conditions and techniques. A small laser spot size is indispensable for photodisruptive work, though this requires a large beam diameter at the pupil and therefore optical systems and techniques such as scleral indentation which maximize the pupillary beam diameter are preferred. For coagulation work the choice of beam focusing optics is crucial for optimum safety. Vitreous replacement can be used to change the refractive power of the eye and permit new combinations of treatment and optical systems. This review covers many aspects of laser irradiation of the eye. It should be clear that, with the multitude of different procedures and optical systems involved, the laser surgeon is faced with a daunting task in assessing and meeting safety limits.

Magnification-corrected photodynamic therapy

“…Alternatively, most modern digital imaging systems can correct the patient_s refractive error when measuring lesion size with sufficient accuracy, and reproducibility [4,6]. Alternatively, the Goldmann-type fundus lens could be used in conjunction with the true GLD of the lesion without changing the magnification factor [4], because its magnification is independent of axial length [8].…”

Transpupillar laser phototherapy for retinal and choroidal tumors: A rational approach

“…An additional factor when using positive optics is the dependence of the radiation field on the refraction of the eye considered. Negative lenses of -60 D, such as the Goldmann contact lens do not suffer from this dependence [26], which greatly simplifies the calculation of the spatial irradiance in the radiation field and therefore enhances safety.…”

“…Irradiation by means of indirect ophthalmoscopy [29][30][31] or when using biconvex contact lenses is not without risk [10,21,26]. The dangers inherent in using the indirect ophthalmoscope are sufficiently well known [22,29,52,82,88,94].…”

The effect of digital measurement software on photodynamic therapy

“…As a result, the refraction-corrected digital measurement software could be considered for determining of the GLD when a Goldmann-type fundus lens is used for performing PDT treatment, because the latter has a magnification independent of axial length of the eye [5].…”

“…The laser spot is inversely related to the axial length and varies between 3.2 and 5 mm with the Mainster Standard, 2.9-4.4 mm with the Mainster Wide Field, 2.7-4.3 mm with the Volk Transequator, and 2.8-4.3 mm using the Volk PDT Laser lens.The 95% confidence intervals for repeated measurements were from +2.25 to −1.76% (refraction-corrected measurement software), +3.35/−2.87% (manual measurement), +2.97/−2.32% (Mainster Standard), +2.45/−2.88% (Mainster Wide Field), +2.64/−2.94% (Volk Transequator), and +2.78/−2.56% (Volk PDT Laser). These are expressed as percentage of the spot size and are within acceptable limits[1].Figures 4,5 show the differences between laser and calculated spot sizes for the 4-mm spot in relation to axial length. The spot size differences calculated with the refraction-corrected measurement software varied between −0.76 and +0.57 mm with the Mainster Standard, −0.16 and +0.92 mm with the Mainster Wide Field, −0.06 and +1.07 mm with the Volk Transequator, and −0.06 to +1.02 mm using the Volk PDT Laser lens…”