Inconsistent distortion in ultra-widefield fundus image.

Abstract

While the advantages of modern ultra-widefield imaging based on ellipsoidal-mirror-SLO-techniques are evident, these images emphasize the need for raising awareness in Optos® image interpretation, particularly when precision and reproducibility of the content is critical, for example, in choroidal naevus follow-up. It is well known from cartography that projecting the surface of a 3D globe to a 2D image causes stretching of the projected image, for example, Greenland and Canada appear larger on the world maps. Similar findings may be assumed in ultra-widefield fundus imaging. There is only one study regarding this entity, and it demonstrates horizontal stretching in Optos® images compared to conventional images (Nicholson et al. 2016). However, in their study, the stretching of the image appears to be in the horizontal plane and evenly distributed. We here report a case that demonstrates a nonlinear distortion within the Optos® image. A 74-year-old woman presents for choroidal naevus follow-up. It appeared as the choroidal naevus was shrunken at the Optos® Image (Fig. 1A) compared to the previous visit (Fig. 1B). Subsequently, traditional fundus colour image (Zeiss FF450plus, Zeiss, Germany) and OCT-image (Heidelberg Spectralis OCT: Heidelberg Engineering, Heidelberg, Germany) were taken. Size and location of the choroidal naevus in the images were compared. In contrast to Optos® images, traditional fundus image and OCT scans showed no change in naevus size or location. More detailed examination of the Optos® images 1A/1B showed similar optic disk diameters (ODD) (ODDA/ODDB = 1.00). Image 1A and 1B were further compared by calculating the index-ratio between the distance from the optic disk in image 1A and 1B (ODA and ODB) to the point of venous branching (PVB A1–A6, B1–B6), respectively, in image 1A/1B and by calculating the index-ratio between the PVB (A1–A6, B1–B6) in image 1A/1B. This revealed a highly inconsistent distortion pattern with indexing varying from 1.00 (no difference) to 0.74 (upto 26% difference in|A4A5→|/|B4B5→|). The index-ratio is presented in Table 1. These Optos® Images (1A and 1B) highlight there is no simple correction factor, as the distortion is inconsistent within the image. This warrants a need for further evaluation and a better understanding of the algorithm in the ultra-widefield imaging system converting the 3-D globe as a 2D image. Finally, we recommend that Optos® ultra-wide imaging is not used as the only imaging modality in evaluation and follow-up of intraocular tumours.