Combinations of techniques in imaging the retina with high resolution

Abstract

Developments in optical coherence tomography (OCT) have expanded its clinical applications for high-resolution imaging of the retina, as a standalone diagnostic and in combination with other optical imaging modalities. This review presents currently explored combinations of OCT technology with a variety of complementary imaging modalities along with augmentational technologies such as adaptive optics (AO) and tracking. Some emphasis is on the combination of OCT technology with scanning laser ophthalmoscopy (SLO) as well as on using OCT to produce an SLO-like image. Different OCT modalities such as time domain and spectral domain are discussed in terms of their performance and suitability for imaging the retina. Each modality admits several implementations, such as flying spot or using an area or line illumination. Flying spot has taken two principle forms, en-face and longitudinal OCT. The review presents the advantages and disadvantages of different possible combinations of OCT and SLO with AO, evaluating criteria in choosing the best OCT method to fit a specific combination of techniques. Some of these combinations of techniques evolved from bench systems into the clinic, their merit can be judged on images showing different pathologies of the retina. Other potential combinations of techniques are still in their infancy, in which case the discussion will be limited to their technical principles. The potential of any combined implementation to provide clinical relevant data is described by three parameters, which take into account the number of voxels acquired in unit time, the minimum time required to produce or infer an en-face OCT image (or an SLO-like image) and the number of different types of information provided. The current clinically used technologies as well as those under research are comparatively evaluated based on these three parameters. As the technology has matured over the years, their evolution is discussed as well with their potential for further improvements.

Introduction

Today, ocular imaging technology has reached high heights of sophistication, building on the tremendous progress in the last 5 years. However, none of the current imaging methods available fulfill all the ideal requirements of the ophthalmologist faced with the need for rapid and accurate diagnosis. This has led to exploration of combinations of imaging and assistive techniques by groups attempting to solve these deficiencies. The goals driving the combination of different imaging technologies are diverse, including the need for precise targeting and real-time focusing of the en-face optical coherence tomography (OCT) (which lead to the addition of a scanning laser ophthalmoscopy (SLO) channel to the OCT channel), the need for correlation of retinal blood flow with changes in morphology (such as in the combination of OCT with fluorescence imaging) or the need for enhancing the imaging performance (such as the addition of adaptive optics (AO) and tracking to SLO or OCT or combined OCT/SLO).

Expansion of a familiar perspective found in one type of instrument may stimulate interest in combining it with an additional modality, which shares the same viewpoint. For example, en-face imaging (C-scan) has the advantage that ophthalmologists are more familiar with the interpretation of transversal images since they are of similar orientation as those found in ophthalmoscopes, fundus cameras and SLOs. This was an important factor that stimulated research into combining the OCT with SLO technology.

This review will focus primarily on combinations of techniques where the core technology is OCT. Initially, OCT technology advanced towards enhancing the acquisition rate along a line in depth in the image. Nowadays, OCT imaging is found into an ever-growing collection of combinations, which pair OCT with techniques such as SLO, flow imaging, polarization, multifocal electroretinography (mfERG), optophysiology, oximetry, micro-perimetry, etc.

A major goal of current research is scanning a target volume of the retina as fast as possible. Significant progress has been achieved in terms of line-scanning rate, which increased from tens of Hz in the first OCT implementation (Huang et al., 1991) to tens of kHz (Nassif et al., 2004) to using the channelled spectrum or the Fourier domain OCT (FDOCT), and to hundreds of kHz (Huber et al., 2007) using the swept source OCT (SS-OCT) method (see Table 1). However, a fast line-scanning rate may not be sufficient to guarantee superiority in respect of all performances required by an accurate diagnosis.

For instance, imaging methods that are recognized as very fast in the modern OCT imaging today, such as spectral domain OCT (SD-OCT), operate under fixed focus, limiting the accuracy of three-dimensional (3D) acquisition. If the same sensitivity is required within the whole 3D volume, then repetition of acquisitions under several different positions of the focus could lead to longer acquisition times for high-density, high-resolution volumes than the time required by slower line-scanning methods, which allow focus change. Therefore, specific imaging requirements may take precedence over the raw line-scanning rate in order to respond to the need of good sensitivity and sufficient sampling data.

In the discussion, which follows, different imaging modalities and specific implementations are compared in their performance taking into consideration three parameters:

• Mv/s: The number of pixels along three rectangular directions (two lateral and one in depth) acquired in the unit of time. Sometimes, in order to shorten the overall scanning time required to capture a given retina volume, a coarse sampling size is chosen for one of the axes, leading to enlargement of the pixel size along that particular direction, trade-off best described by the parameter Mv/s.

• T_en−face: The time to produce a two-dimensional (2D) OCT image with the SLO orientation.

• Imaging content units (I): The number of different types of information provided in a system by a specific configuration. As combination of techniques compound different types of information (OCT, SLO, fluorescence), the performance Mv/s is multiplied by I to obtain an overall performance of a given combination of techniques, as IMv. For instance for a combined system incorporating OCT, SLO and fluorescence channels which can deliver images simultaneously, I=3.

The combination of techniques is evolving in two principal directions: combination of channels providing multiple information and combination of imaging techniques with assisting technologies, such as AO and tracking. Both lines of development will be presented here.

The quest for faster and more complete acquisition of information from the eye demands evaluation of several trade-offs in the performance of the technologies combined. Such different demands and trade-offs will be discussed, presenting the problems raised by the hardware combination as well as the challenges in developing synergistic interpretations of composite images collected from several imaging channels.