Optical modelling and image dewarping procedures for quantitative measurements of subretinal microinjection volumes using intraoperative OCT

Abstract

Purpose : New genetic therapies are an increasingly relevant and effective form of treatment for a variety of retinal diseases, but these procedures are limited by the complexity of delivering the correct volume of therapeutics to the target location. Intraoperative OCT (iOCT) can show delivery location, but quantitative measurements are challenging because of distortions due to intraocular structures. We present a novel method to make quantitative measurements of subretinal features and dewarp retinal OCT scans.

Methods : An optical model (Fig 1) of the OCT system and novel experimental procedure allows us to characterize the behavior of an OCT scan in the eye and correct distorted retinal OCT images. This was tested using a custom swept-source iOCT system. The technique was calibrated by computing the volume of 1 mm diameter alumina ceramic spheres inserted into ex vivo porcine eyes.

To test microinjection quantification, a small entry bleb was injected into the subretinal space of 37 ex vivo porcine eyes using a suprachoroidal delivery system from Orbit Biomedical, an OCT volume was captured, the full therapeutic dose was delivered, and a final OCT volume was captured. An expert grader manually segmented the bleb regions. The volume of the entry and final blebs was computed, and the difference was the therapeutic volume. The delivery procedure was repeated outside the eye to establish a control volume.

Results : The measured sphere volumes were 0.847±0.041 mm³ (n=15) compared to the actual 0.524 mm³, resulting in a calibration factor of 0.619. The calculated therapeutic volume of subretinal injections was 54.8±12.3 μL (n=10) (Fig 2). The control volume was 66.4±2.46 μL (n=20).

Conclusions : OCT imaging is suitable for visualizing subsurface features and extracting quantitative measurements.

This is a 2020 ARVO Annual Meeting abstract.

 

Fig 1: OCT optical model. Distance microscope moves to image the retina (irrd), distance from IR card to OCT pivot point (irpd), reference arm length change (refd), and pivot-to-retina distance (prd) allow us to find scan lengths in the eye.

 

Fig 2: Uncorrected (a,f) and corrected B-scan (c,h), uncorrected (b,g) and corrected volume render (d,i) and maximum intensity projection (MIP) (e,j) showing the initial entry (left column) and final bleb (right column). Microinjection needle (blue arrow), manual segmentation (red outlines) and B-scan position (green lines).