Quantitative assessment of laser beam displacement during photocoagulation on patient retina using high-speed optical coherence tomography (OCT)

Abstract

Purpose : Reliable doseage of retinal photocoagulation depends on exact lesion application. Precision is increasingly important in non-damaging treatments where the therapeutic window shrinks. Patient and physician unsteadiness, however, blur the laser beam image on the fundus, increasing lesion diameter and decreasing irradiance in an uncontrolled manner. In this study, we quantified such movements.

Methods : We recorded B-scan videos using a slit-lamp adapted OCT at a frame rate of 3 ms. OCT videos were acquired prospectively during photocoagulation of 220 study lesions in 5 patients (irradiation diameter 300 µm, exposure time 50 or 200 ms, moderate whitening). Image displacement within the field of view was calculated to quantify two-dimensional movements in axial and horizontal directions. We determined maximal decentrations in both axis and the total distance covered in the plane.

Results : OCT recording started about 30 ms prior to photocoagulation and was continued for about 2 s. Sufficient video quality was achieved in 20 lesions with 50 ms exposure time and 25 lesions with 200 ms. Lateral/axial decentration amplitudes and total distances were [mean±SD] 46±22/26±21/75±66 µm during 50 ms irradiations and 70±30/68±49/199±78 µm during 200 ms irradiations. The corresponding values assessed during the total OCT recording time of 2157±151 ms were 246±142/259±189/3155±2375 µm.

Conclusions : Considering that visible changes in OCT of a moderate retinal lesion measure about twice the irradiation spot diameter, lateral and axial decentrations of 45-70 µm are well acceptable in our 300 µm study irradiations. Motion becomes more critical if lesion intensity decreases, irradiation spot diameter shrinks or exposure time increases. It will induce significant variation in small (50 µm) sub-visible macular lesions. While the image is relatively stationary during the irradiation itself, maximal decentrations before and afterwards are in the order of 250 µm and add up to a total distance of > 3mm. Study limitations include ignorance of vertical movements. While only OCT videos with good quality could be assessed, greater movement is expected in lesions with lower video quality.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.