Invasive Vs Non-Invasive Imaging

Fluorescein angiography (FFA) has been the gold standard to understand, diagnose and treat retinal disorders. However, being an invasive procedure, it has several limitations including adverse drug reactions. Hence, non-invasive tests that can be repeated during the course of the disease are the need of the hour. The aim of our study was to compare images of patients with retinal microvasculature pathology taken from three different imaging modalities (invasive vs. non-invasive). Lesions were detected more easily and with a greater resolution of morphology on retinal function imaging (RFI) and optical coherence tomography angiography (angio-OCT). Functional integrity of the vessels was better delineated on FFA. RFI and angio-OCT are non-invasive rapid and efficient methods to image vascular conditions with easy repeatability and negligible adverse effects.

Discussion

Fundus FFA is a sensitive tool to investigate patients with retinal vascular morphologies. However, it necessitates intravenous injection of a contrast dye and entails adverse reactions of varying severity, including anaphylaxis. It is also relatively contraindicated in various clinical conditions such as renal failure, ischemic heart disease, and pregnancy. To overcome these limitations, non-invasive modalities can be used to study vascular morphology. Angio-OCT is a relatively new technology based on two technologies split spectrum amplitude-decorrelation angiography and motion correction technology. It works by collating the decorrelation signals (differences in backscattered OCT signal intensity or amplitude) between sequential OCT B-scans taken at the same cross-section to build a map of blood flow. RFI is equipped with a standard fundus camera, a customized stroboscopic flash lamp system and a fast-digital camera. A 60 Hz and 1024 × 1024 pixels digital imaging system captures images of the fundus at high rates to reduce retinal motion in between subsequent frames and follow erythrocytes moving at up to 20 mm/s. A single click acquires a “series” of eight monochrome standard fundus camera images. This sequence of eight frames can be presented in the form of a movie. The RFI system has the capability of distinguishing the direction of blood flow within the retinal blood vessel and thereby distinguishes between a retinal artery and vein.

These non-invasive modalities have their advantages and limitations. Salient ones include rapidity of performing the imaging, repeatability and an absence of any adverse effects. Morphological patterns as well as CNP areas can be very well delineated with the RFI and angio-OCT. Leaks on FFA can obscure these details. Only angio-OCT allows segmentation; hence, abnormalities in different retinal layers can be detected. A higher resolution and field of view is obtained with the RFI when compared to angio-OCT. Measurement of blood velocity and flow, oximetry and metabolic mapping are other features of the

Disadvantages of the non-invasive modalities include a smaller field of view, a maximum of 50° compared to 200° in ultra-wide field FFA. This limits the indication to macular disease currently. In addition, the angio-OCT and RFI requires that the patient to fixate for several seconds, whereas a single FFA frame can be obtained in seconds. Interpretation and image processing is time consuming and at times, a difficult aspect of the newer modalities. Vascular leakage, which is of diagnostic importance in several retinal diseases, is not seen in the non-invasive modalities. Cost of instrumentations and further upgrades is a deterrent with these devices when compared to the simpler FFA.

Despite some limitations, we see in the above three case that vascular frond morphology of neovascularization and delineation of areas of CNP is satisfactorily achieved with the non-invasive imaging modalities. We have seen that the outcomes are comparable between the RFI and angio-OCT. The added advantage over FFA of being non-invasive and repeatable make them excellent tools for macular imaging.