Measuring Volume of Subretinal Injections with Commercial Intraoperative OCT

Abstract

Purpose : To accurately measure volume of subretinal (SR) injections using OCT scans from a commercial intraoperative OCT (iOCT) system. To propose the use of this quantitative OCT based method to advance SR gene therapy delivery.

Methods : Calibration: Ceramic spheres of known volume (0.52ul) were inserted into ex vivo porcine eyes and placed near the retina. 3D OCT scans (nominally 16x16mm in human eyes, 5.8mm depth) were acquired with a Zeiss RESCAN 700 iOCT system (Oberkochen, Germany). The boundary of the sphere was manually marked in each B-scan using ImageJ (Fig. 1). With the axial pitch and scan dimensions of the OCT system (from manufacture specifications) we were able to derive a calibration factor (CF) that allowed us to convert the manual markings (in voxels) to volumes (ul).
Application: Enucleated porcine eyes underwent trans-vitreal SR injections of 100ul of BSS using a calibrated syringe, followed by iOCT scan acquisition. The iOCT bleb images were manually segmented (Fig. 2) and the preliminary volumes calculated using MATLAB. The CF was then applied to obtain the calibrated subretinal volumes.

Results : The iOCT system yielded preliminary volumes greater than the sphere’s real volume. For spheres in the periphery (S_P) mean volume (0.95uL, n=26) was significantly greater than for spheres in the central posterior pole (0.85uL, n=36) (t-test, p<0.01). Volumes of centrally located spheres (S_C) were not associated with axial or lateral sphere location (Pearson correlation, r= 0.23 and 0.24 respectively) and ranged from 0.70-1.02uL (SD= 0.066). Mean CF for S_C was 0.62±0.05. Volumes for S_P had a moderate positive correlation with lateral distance from the posterior pole (Pearson correlation, r= 0.71). Mean CF for S_P was 0.56±0.06 and significantly different from the CF for S_C (t-test, p<0.01).
Ten subretinal 100uL injections were calibrated using the mean peripheral CF. Calibrated volumes ranged from 47.0 to 100.1 uL with mean (SD) volume of 71.2uL (17.0). Mean absolute difference from expected volume was 28.8uL and mean percent error was 28.8±17%.

Conclusions : Subretinal injections can be accurately measured from intraoperative OCT scans and allow for more reliable studies of retinal gene therapy efficacy and toxicity.

This is a 2020 ARVO Annual Meeting abstract.

 

Figure 1. Spheres in porcine eye a) microscope b) OCT en-face c) OCT B-scan view.

 

Figure 2. OCT B-scans of a bleb with segmentation lines outlining subretinal fluid.