Weitz , Rishard ( 2019 ) Progress in Multimodal En Face Imaging : feature introduction

This feature issue contains papers that report on the most recent advances in the field of en face optical coherence tomography (OCT) and of combinations of modalities facilitated by the en face view. Hardware configurations for delivery of en face OCT images are described as well as specific signal and image processing techniques tailored to deliver relevant clinical diagnoses. The value of the en face perspective for enabling multimodality is illustrated by several combination modalities. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement Multimodal optical imaging vastly enriches the amount of information that can be retrieved from biological tissue. Towards this purpose, many different approaches have been reported to date, which include combining optical coherence tomography (OCT), microscopy, scanning laser ophthalmoscopy (SLO), fluorescence imaging, multi-photon imaging, Raman spectroscopy, microperimetry, electrophysiology, oximetry, photo-acoustics, and others. Depending on the underlying imaging technology, images with different preferential orientations have been combined with varying degrees of success. In OCT, for example, onedimensional depth profiles (A-scans) are typically acquired and are then combined into a cross sectional image (B-scan). This presents a specific set of challenges for a combination of this technique with e.g. confocal microscopy (CM) or scanning laser ophthalmoscopy (SLO), as the image acquisition of both latter methods lies in the en face plane, that is orthogonal to that of the more conventional cross sectioning OCT imaging. The year 2018, when the initiative of creating this special issue was taken, marked 20 years since the publication of a first report [1] on a combination of the original SLO [2], dating back from 1980 and OCT [3], a newer imaging technique, introduced in 1991. What initially appeared challenging to achieve, fusion of the two different modalities to simultaneously display a contiguous SLO image and a thin fragmented en face OCT image, was demonstrated successfully in 1998. The en face OCT [4,5] display of a thin section of the eye [6] together with its SLO counterpart [1] opened new diagnostic avenues. Apart from providing complementary anatomic data, the combination of these two methods additionally affords the opportunity for motion correction since different imaging planes are usually recorded with different speeds. Of particular advantage is the fact that many structures of interest are best viewed in the en face plane. Cone and rod photoreceptor mosaics of the retina recorded with adaptive optics (AO) assisted imaging methods, are a good example. The regularity of the mosaic is best revealed in the en face imaging plane. Vascular imaging is another example where only the en face display reveals tissue specific patterns and disease related loss of perfusion. Most strikingly, OCT angiography (OCTA), a product of OCT acquisition speed progress, which has attracted widespread interest for its non-invasive, Vol. 10, No. 4 | 1 Apr 2019 | BIOMEDICAL OPTICS EXPRESS 2135 #359230 https://doi.org/10.1364/BOE.10. 002135 Journal © 2019 Received 30 Jan 2019; published 1 Apr 2019 quantitative imaging of retinal vasculature, could only have come into being as a product of the en face perspective. These are some of the reasons why en face display is so intimately linked to multimodality, hence the theme of this feature issue. The multimodality seed launched by the OCT/SLO combination is reflected in the more than 5 million web search hits generated by the acronym. This feature issue contains reports on the most recent advances in the field of en face OCT and of combination of modalities facilitated by its perspective. The special issue celebrates the past 20 years of engineering creativity in the field of multimodal and en face imaging and the spark of clinical necessity that inspired the creation of combinations of diverse imaging modalities. A comprehensive review by Rainer Leitgeb [7] from Medical University Vienna, presents the hardware evolution of technology, as well as the adoption of terminology over the years. As OCT addressed a problem in imaging the eye and more than 50% of reports on OCT are on the eye, this review focuses on ophthalmology. The review concludes by outlining exciting technological prospects of en face OCT based both on time domain as well as on spectral (Fourier) domain OCT. Wes M. Allen et al. [8] describe an automated segmentation algorithm to fuse en face micro-elastograms with OCT images to provide dual contrast images. The procedure enables rapid visualization of en face images required for intraoperative assessment. In order to validate the contrast in micro-elastograms, a method is presented that enables co-registration of en face images with histology of wide-local excision specimens, sectioned in the orthogonal plane, without any modification to the standard clinical workflow. The results presented demonstrate the potential of optical coherence micro-elastography for imaging tumor margins. Acner Camino, et al. [9] present an algorithm to identify shadowed areas in OCTA due to obstacles to the propagation of light such as vitreous floaters or the pupil boundary. This algorithm is applied to healthy subjects as well as patients with diabetic retinopathy, uveitis and age-related macular degeneration. The aim is to exclude the shadowed areas from analysis so that the overall OCTA parameters are minimally affected by such artefacts. Michaal Chlebiej et al. [10] present a method of OCT angiography (OCTA) data filtering for noise suppression and improved visualization of the retinal vascular networks in en face projection images. The core of the method is based on application of directional filtering to the C-scans, i.e. one-pixel thick sections of the 3-D data set, perpendicular to the direction of the scanning OCT beam. The method uses a concept of structuring, where directional kernels of shapes match the geometry of the image features. Rotating ellipses are employed to find the most likely local orientation of the vessels and use the best matching ellipses for median filtering of the C-scans. Arash Dadkhah et al. [11] present a multimodal microscopic optical imaging system with the capability of providing comprehensive structural, functional and molecular information of living tissues. This imaging system integrated photoacoustic microscopy, OCT, optical Doppler tomography and confocal fluorescence microscopy in one platform. Focusing adjustment is achieved using an OCT-guided surface contour scanning methodology. Mayank Goswami et al. [12] present a novel mouse ocular xenograft model of glioblastoma, where the resultant xenografts are imaged in vivo with combined fluorescence SLO and volumetric OCT for a period up to several months. The ocular model shows potential for examining the relationships between neoplastic growth, neovascularization and other features of the immune microenvironment, and for evaluating treatment response longitudinally in vivo. The paper demonstrates quantitative tracking of tumor development, of Vol. 10, No. 4 | 1 Apr 2019 | BIOMEDICAL OPTICS EXPRESS 2136