2011 prer elsevier_podoleanu_retina oct

7/30/2019 2011 PRER Elsevier_Podoleanu_retina OCT

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Combinations of techniques in imaging the retina with high resolution

Adrian Gh. Podoleanu a,, Richard B. Rosen b

aApplied Optics Group, University of Kent, Canterbury, UKb New York Eye and Ear Infirmary, NY, USA

a b s t r a c t

Developments in optical coherence tomography (OCT) have expanded its clinical applications for high-resolution imaging of the retina, as a standalonediagnostic 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.

& 2008 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465

2. High-resolution imaging technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466

2.1. Optical coherence tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466

2.2. Scanning laser ophthalmoscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466

2.3. Adaptive optics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468

2.4. Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468

3. Different scanning procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468

3.1. The one letter terminology of scanning, A, B, C, T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468

3.1.1. Longitudinal OCT or A-scan-based B-scan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

3.1.2. T-scan-based B-scan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

3.1.3. C-scan images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

4. Different OCT imaging methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

4.1. Time domain optical coherence tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

4.2. Spectral domain optical coherence tomography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

ARTICLE IN PRESS

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/prer

Progress in Retinal and Eye Research

1350-9462/$ - see front matter& 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.preteyeres.2008.03.002

Abbreviations: 3D, three-dimensional; AMD, age-related macular degeneration; AO, adaptive optics; APD, avalanche photodiode; cSLO, confocal scanning laser

ophthalmoscopy; dB, decibel; D-OCA, Doppler-optical coherence angiography; ERG, electroretinography; FA, fluorescein angiography; FD, Fourier domain; FF, full field; FFT,

fast Fourier transformation; I, imaging content units, or number of channels simultaneously working in a combined configuration; ICG, indocyanine-green; IS, inner

segments; LF, line field; L-SLO, line-scanning laser ophthalmoscopy; mfERG, multifocal electroretinography; Mv/s, megavoxels per second; NA, numerical aperture; n.a.,

non-applicable; OCA, optical coherence angiography; OCT/SLO, combined OCT and scanning laser ophthalmoscopy; OCT, optical coherence tomography; ODT, optical

Doppler tomography; OPD, optical path difference; OS, outer segment; PED, pigment epithelium detachment; RNFL, retinal nerve fiber layer; RPE, retinal pigment

epithelium; S-OCA, scattering-optical coherence angiography; SD, spectral domain; SLD, superluminescent diode; SLO, scanning laser ophthalmoscopy; SS, swept source;

TD, time domain; mm, micron; ms, millisecond; X, Y, Z, rectangular axes, with X- and Y-oriented lateral to the retina and Z along the depth. Corresponding author. Tel.: +441227823272; fax: +441227827558.

E-mail addresses: [email protected] (A.Gh. Podoleanu), [email protected] (R.B. Rosen).

Progress in Retinal and Eye Research 27 (2008) 464 499
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4.2.1. Fourier domain optical coherence tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470

4.2.2. Swept source optical coherence tomography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470

4.3. Full field or en-face non-scanning systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470

4.4. Line-field-SD-OCT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

4.5. Multiplexing in OCT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

4.5.1. Multiplexing in A-scan-based OCT imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

4.5.2. Multiplexing in T-scan-based OCT imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

4.6. Comparative assessment of the OCT methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472

5. Depth of focus range and dynamic focus in OCT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4736. Combining OCT with SLO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474

6.1. Simultaneous OCT/SLO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475

6.1.1. Recognizable patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

6.2. No splitter OCT/SLO configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

6.3. Sequential generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

6.3.1. Sequential OCT C-scan/SLO C-scan, sequential OCT B-scan/SLO B-scan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

6.3.2. OCT/line-SLO (sequential B-scan OCT with C-scan SLO). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478

6.4. Quasi-simultaneous operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

6.5. Generation of an SLO-like image using OCT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480

6.5.1. Real-time generation by using a low coherent source with sufficiently long coherence length. . . . . . . . . . . . . . . . . . . . . . . . 480

6.5.2. Generation of an SLO-like image from OCT stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483

7. Combination of high-resolution modalities with fluorescence imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485

7.1. SLO/fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485

7.2. OCT/fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485

7.3. OCT/SLO/fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485

7.4. No-dye fluorescence-based OCT imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486

8. Combinations of high-resolution imaging procedures with AO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487

8.1. SLO+AO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487

8.1.1. SLO/fluorescence+AO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488

8.2. OCT+AO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488

8.2.1. Trade-off between depth resolution in OCT and level of correction using AO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488

8.2.2. Incompatibility between A-scan-based OCT and AO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489

8.2.3. Small size imaging using TD-en-face OCT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489

8.2.4. Full field TD-OCT+AO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490

8.2.5. Longitudinal TD-OCT+AO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490

8.2.6. FD-OCT+AO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490

8.2.7. FD-OCT+AO equipped with two mirrors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490

8.3. OCT+SLO+AO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490

8.3.1. Simultaneous OCT/SLO+AO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490

8.3.2. Sequential OCT/SLO+AO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491

9. Combination of high-resolution imaging technologies with tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4929.1. SLO+AO+tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492

9.2. TD-OCT+lateral tracking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493

9.3. TD-OCT+AO+axial tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493

10. Combination of high-resolution imaging technologies with other techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493

10.1. Combinations with physiology methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493

10.1.1. Multifocal ERG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493

10.1.2. Combinations with optophysiology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493

10.2. Combinations with polarization information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494

10.3. Combinations with flow information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494

10.4. Combinations with spectroscopy imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495

10.5. Polarization/flow/optophysiology/spectroscopy imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495

11. Future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495

11.1. Potential of existing technologies and trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496

11.2. Synergies provided by the combination of techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497

1. 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 targetingand real-time focusing of the en-face optical coherence tomo-

graphy (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 thatophthalmologists are more familiar with the interpretation of

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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 require-

ments 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:

1. 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.

2. Tenface: The time to produce a two-dimensional (2D) OCT

image with the SLO orientation.

3. 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 ofinformation (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 informa-

tion 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. Suchdifferent 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.

2. High-resolution imaging technologies

2.1. Optical coherence tomography

OCT is a non-invasive high-resolution imaging modality, which

employs non-ionizing optical radiation. OCT derives from low-

coherence interferometry. This is an absolute measurement

technique that was developed for high-resolution ranging and

characterization of optoelectronic components (Al-Chalabi et al.,

1983, Youngquist et al., 1987). The first application of the low-

coherence interferometry in the biomedical optics field was for

the measurement of the eye length (Fercher et al., 1988). Adding

lateral scanning to a low-coherence interferometer, allows depth-

resolved acquisition of 3D information from the volume of

biologic material (Huang et al, 1991). The concept was initially

employed in heterodyne scanning microscopy (Sawatari, 1973).

OCT has the potential of achieving high depth resolution, which is

determined by the coherence length of the source. This is the

length over which a process or a wave maintains strict phase

relations; an ideal laser source for instance, emits light with more

than a few km coherence length while the coherence length of

light emitted by a tungsten lamp could be as short as 1 mm. More

intense optical sources, suitable for use in scanning the eye are

now available with coherence lengths below 1 mm (Drexler, 2004).

Using sources with extremely short coherence length, submicron

depth resolution is achievable even when the microscope

objective is far away from the investigated target, feature not

achievable with confocal microscopy. This is one of the most

important advantage of OCT, which explains the high level of

interest for OCT in ophthalmology. OCT delivers fast, non-contact

images of the cornea, lens and the retina with depth resolutionsbetter than 3mm (Drexler, 2004).

2.2. Scanning laser ophthalmoscopy

Confocal imaging was the first high-resolution imaging

technology applied to the eye (Webb, 1990). The influence of

scattered light from outside the focus point within the target is

suppressed by a pinhole in front of the photodetector and

conjugate to the focal plane (Elsner et al., 1996). 3D imaging

(Masters, 1998) is performed by acquiring en-face images (C-

scans) at different positions of the focusing element, each position

corresponding to a different depth. Increasing the beam diameter

of the beam sent to a lens leads to a better confinement of thelight in the focus of the lens and therefore to better transversal

and depth resolution. The key figure in following these changes is

the numerical aperture (NA), a quantity proportional with the

beam diameter and inversely proportional with the focal length of

the lens used in imaging. The transversal resolution varies inverse

proportional to NA while the depth resolution varies inverse

proportional to the square of the NA. Therefore, to improve the

resolutions, it will be desirable to work with a large eye opening.

However, in practice, increasing the eye pupil only leads to

cumulative addition of aberrations. Therefore, with or without the

pupil dilated, an SLO will provide approximately 15mm transversal

resolution and larger than 300mm axial resolution (Bartsch and

Freeman,1994; Woon et al., 1992). Similarly, a relatively low NA of

the anterior chamber limits the achievable resolution in imagingthe eye lens.

ARTICLE IN PRESS

A.Gh. Podoleanu, R.B. Rosen / Progress in Retinal and Eye Research 27 (2008) 464499466


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Table 1

Evolution of the OCT technology in imaging the retina in terms of number of Megavoxel/s and Tenface

Method employed Line rate (kHz) Frame rate (Hz) Pixels in the B-scan

(Nx, Nz)

Pixels in

the

C-scan

(Nx, Ny)

Tenface (s) (time to produce an

OCT C-scan image)

M

f

1 TD-OCT longitudinal OCT, first report on OCT,

in-vitro (Huang et al., 1991)

0.0008 0.0053a 150118b n.a. Not contemplated 0

2 TD-OCT longitudinal OCT, in-vivo (Swanson

et al., 1993)

0.042 0.42 100285c n.a. 6095d 0

3 TD-OCT longitudinal OCT with fast axial

scanning (Rollins et al., 1998)e8 16 (32 possible) 250250 n.a. 16 (8)c 1

4 En-face TD-OCT (Podoleanu et al, 1998b) 0.6 1.18 196100 196196 0.85 0

5 En-face TD-OCT (van Velthoven et al., 2006) 1 2 (8) 512250 512512 0.5 0

6 En-face TD-OCT using a resonant galvo-scanner

(Hitzenberger et al., 2003)

4 53.3 Not contemplated,

but possible

256128 0.019 1

7 FD-OCT (Wojtkowski et al., 2004) 16 6.7 (or 31) 30001024

(or 5121024)

n.a. 38.2c(or 8.25) 2

8 FD-OCT (Nassif et al., 2004) 29 29 (real-time

display 10)

1000320 n.a. 8.83c 9

9 FD-OCT Gotzinger et al., 2005) 20 20 1000292f n.a. 12.8d 5

s

10 FD-OCT/Line-field SLO (Iftimia et al., 2006) Not specified 15 1024512 n.a. 17c 7

1

11 Line-field FD-OCT (Nakamura et al., 2007) 51.5 (single

frame: 823.2)

201 128g108h n.a. 1.27 2

12 SS-OCT at 850nm (Lim et al., 2006) 43.2 84 512140i n.a. 3j 6

13 SS-OCT at 1050nm (Huber et al., 2007) 236 461 512512 n.a. 0.56c 1

14 Fastest SS-OCT (Moon and Kim, 2006) 5000 105 5069 n.a. 0.0026d 3

a Although potential to 5 Hz was also mentioned.b Evaluated using the values quoted in the paper of 2 mm depth range and 17mm depth resolution in air.c Evaluated using the values quoted in the paper of 3 mm depth range and 10.5mm depth resolution in air.d

Not contemplated and evaluated for 256 frames using the frame rate for B-scan imaging.e Although images from the anterior chamber of a murine eye are presented only and not from the retina, the fast scanning delay line method presents sufficient sensitivit

comparison of technologies.f Evaluated from 1.75mm depth range with 6mm resolution quoted.g Dividing the line of 2.1mm with the indicated value of transversal resolution, of 16.4mm.h Evaluated from 0.8 mm depth range and 7.4mm resolution quoted.i Evaluated from 1.4mm depth range and 10mm resolution in tissue.j Evaluated for 256 B-scan frames.


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2.3. Adaptive optics

AO uses a wavefront sensor, which instructs a corrector,

usually a deformable mirror, to alter the wavefront in order to

compensate for the aberrations in the eye and in the instrument

(Liang et al., 1997; Dreher et al., 1989; Roorda et al., 2002).

However, if aberrations are compensated using AO, the transversal

resolution of a confocal-SLO (cSLO) through a 6 mm pupil could beimproved to less then 3 mm (Zhang and Roorda, 2006) and the

axial resolution could be reduced to 40mm (Venkateswaran et al.,

2004). For an 8 mm pupil size, resolution is even more improved

(Miller et al., 2003). These levels of resolution pose compatibility

considerations when added to other imaging modalities, such as

OCT. After AO correction is applied, a fixed focus scanning

approach may adversely affect the signal strength of points not

far from the focus, due to the shrinkage of the confocal profile

produced by the correction of aberrations.

2.4. Tracking

Tracking is an assisting technology to high-resolution imaging,which has seen impressive evolution in the last 5 years. Tracking

is essential when information to be extracted needs a stationary

target. This is often the case in high-resolution or small size

imaging and especially when the signal returned from the retina

is weak. With weak signals, information from a single frame

may be insufficient, necessitating some form of aligned multiple

image collection and averaging. Tracking also enables psychophy-

sical and neurophysiological techniques, which require accurate

eye motion compensation, such as micro-perimetry and laser

surgery. Tracking is a complex technology which raises several

issues including: (i) selection of the reference region used for

tracking, either retina or cornea or both; (ii) wavelength of

operation; (iii) integration of the tracking system into the imaging

system, requiring modification of the interface optics tohandle both tracking and imaging beams; and (iv) adjustment of

power safety levels to compensate for at least two beams

launched into the eye.

3. Different scanning procedures

To obtain 3D information about the retina, any imaging system

is equipped with three scanning means, one to scan the object in

depth and two others to scan the object transversally. Depending

on the order these scanners are operated and on the scanning

direction associated with the line displayed in the raster of the

final image delivered, different possibilities exist. One-dimen-sional (1D) and 2D scans are known. 1D scans are labeled as: A-

and T-scans, while 2D scans are labeled as B- and C-scans and this

terminology will be explained below. A- and T-scans are 1D

reflectivity profiles while B and C are 2D reflectivity maps or

images. In terms of the strength, the brightness in the SLO image

is proportional to the reflectivity while in the OCT with the square

root of the reflectivity. While in the SLO, the display is generally

linear, in OCT, especially for B-scans, the display often represents

the logarithm of the reflectivity.

OCT systems, using CCD cameras or arrays of sensors or arrays

of emitters eliminate the need of scanning the beam. However, the

terminology below applies in such cases as well, where the ray

scanning has been replaced by different forms of electronic

scanning. The scanning terminology is illustrated in Fig. 1 and theutilization of the three scanners described in Fig. 2.

3.1. The one letter terminology of scanning, A, B, C, T

A-scan: represents a reflectivity profile in depth. This scanning

technology is used clinically for determining the eye length. In

principle, an A-scan can be provided by cSLO (Bartsch and

Freeman, 1994) as well. The depth scanning requires the axial

movement of a lens to alter the focus, as the lens is heavy, the

scanning cannot be fast. Electrically adjustable lenses may provide

a solution for fast axial scanning in cSLO, however, there is no

motivation for development in that direction in view of the much

better resolution obtained using OCT.

T-scans: represent the reflectivity (SLO) or square root of

reflectivity (OCT) obtained by scanning the beam transversally

across the target.

B-scan: represents a cross-section image, a (lateraldepth)

map. This could be obtained by grouping T-scans together fromdifferent depth values or A-scans together for different lateral

ARTICLE IN PRESS

B-scan

C-scan

A-scan

Y-Z

X

T-scan

Fig. 1. Relative orientation of the axial scan (A-scan), transverse scan (T-scan),

longitudinal slice (B-scan) and en-face or transverse slice (C-scan).

(iii)

C-SCAN IMAGES AND

3D SCANNING BASED

ON C-SCANS

(CONVENTIONAL

cSLO OPERATION

(ii)

B-SCAN IMAGE

GENERATED FROM T-SCANS

(METHOD COMPATIBLE WITH

C-SCANNING)

(i)

B-SCAN IMAGE GENERATED

FROM A-SCANS

(CONVENTIONAL)

LONGITUDINAL OCT

SCANNING)

-Z

XY

-Z

SLOW

SLOW

SLOWFAST

X

-Z

XSLOWEST

FAST

FAST

A-scans

T-scans

(iv)

3D SCANNING BASED ON

B-SCAN SLICES AT

DIFFERENT POSITIONS Y

CURRENTLY USED BYSD-OCT SYSTEMSZ

SLOWFAST

Y

X

SLOWEST

Fig. 2. Different modes of operation of the three scanners in a 3D imaging system.

Lateral scanning along the X- and Y-axes are implemented using an XY or 2D

transverse scanner in both cSLO and OCT systems. The scanning in depth, along the

axis Zdiffers, implemented using focus change in cSLO and OPD change in OCT. (i)

B-scan image generated from A-scans (conventional longitudinal OCT scanning).

(ii) B-scan image generated from T-scans (method compatible with C-scanning).

(iii) C-scan images and 3D scanning based on C-scans (conventional cSLO

operation. (iv) 3D scanning based on B-scan slices at different positions y

currently used by SD-OCT systems.



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positions, both in use in the OCT practice. Historically, the first

OCT image was a B-scan image of the retina (Huang et al., 1991)

made from A-scans, using flying spot longitudinal OCT technology

(see below).

Although possible in the cSLO, B-scans are not used in the

imaging of patients. cSLO can provide only T-scan-based cSLO

B-scan images. (An A-scan-based B-scan would require fast focus

change; see the comment above in connection to the A-scans incSLO). However, such a B-scan image is inferred in the practice of

glaucoma imaging in a post-acquisition process after collection of

C-scan images (Mikelberg et al., 1995).

B-scan images, analogous to ultrasound B-scan are generated

by collecting many A-scans (Fig. 1) for different and adjacent

transverse positions, as shown in Fig. 2(i). The lines in the raster

generated correspond to A-scans, i.e. the lines are oriented along

the depth coordinate. The transverse scanner (operating along X

or Y, or along the polar angle y in polar coordinates in Fig. 1 right,

with X shown in Fig. 2(i)) advances at a slower pace to build a B-

scan image. The majority of OCT reports in literature refer to this

mode of operation.

3.1.1. Longitudinal OCT or A-scan-based B-scanDevelopment of the longitudinal OCT based on A-scans was

facilitated by a technical advantage: in time domain OCT (TD-

OCT), when moving the mirror in the reference path of the

interferometer, not only is the depth scanned, but a carrier is also

generated (Huang et al., 1991; Swanson et al., 1993). The

reflectivity information is superposed on a carrier signal, having

a frequency equal to the Doppler shift produced by the long-

itudinal scanner itself (moving along the axis of the system, Z, to

explore the retina in depth). In longitudinal OCT, the axial scanner

is the fastest and its movement is synchronous with displaying

the pixels along the line in the raster, while the lateral scanning

determines the frame rate. Longitudinal OCT is performed as

explained here in TD and is also provided by SD-OCT methods (see

Section 4.2.)

3.1.2. T-scan-based B-scan

In this case, the transverse scanners (or scanner) determine(s)

the fast lines in the image (Podoleanu et al., 1996, 1998a,b). These

image lines represent T-scans (Fig.1). A T-scan can be produced by

controlling either the transverse scanner to scan along theX-coordinate, or the Y-scanner to scan along the Y-coordinate

with the other two scanners fixed, or controlling both transverse

scanners, along the polar angle y, with the axial scanner fixed. The

example in Fig. 2(ii) illustrates the generation of a T-scan-based

B-scan, where the X-scanner produces the T-scans and the axial

scanner advances slower in depth, along the Z-coordinate. This

procedure has a net advantage in comparison with the A-scan-

based B-scan procedure as it allows production of OCT transverse(or 2D en-face) images for a fixed reference path, images called

C-scans. In this way, the system can be easily switched from B to

C-scan, procedure incompatible with A-scan-based OCT imaging

mentioned above.

3.1.3. C-scan images

A C-scan represents a raster image, with the same orientation

as a TV image or image provided by microscopy, a (lateral lat-

eral) scanned map. Historically, the C-scan was the native

orientation for fundus cameras, SLOs and cSLOs. C-scans are

provided by the flying spot en-face OCT and the full-field (FF) OCT.

They can also be inferred post-acquisition in longitudinal OCT,

either TD or SD. C-scans are made from many T-scans along either

ofX, Y, r or y coordinates, repeated for different values of the othertransverse coordinate, Y, X, y or r, respectively in the transverse

plane (with the most used case, oriented along the horizontal axis,X). The repetition of T-scans along the other transverse coordinate

is performed at a slower rate than that of the T-scans (Fig. 2(iii)),

which determines the frame rate. In this way, a complete raster is

generated. For 3D imaging, different transversal slices can be

collected at different depths Z, either by advancing the optical

path difference in the OCT in steps after each complete transverse

(XY) or (r,y) scan, or continuously at a much slower speed than theframe rate. The depth scanning is the slowest in this case. In cSLO,

the focus is changed to select a C-can from a different depth

position and this is the typical procedure for 3D cSLO imaging

(Masters, 1998).

It is more difficult to generate en-face OCT images than

longitudinal OCT images as the reference mirror is fixed and no

carrier is produced. Therefore, in order to generate T-scans and

T-scan-based OCT images, a phase modulator is needed in order to

create a carrier for the image bandwidth (Hitzenberger et al.,

2003). This complicates the design and introduces dispersion.

Research has shown that the X or Y-scanning device itself

introduces a path modulation (Podoleanu et al., 1996, 1998a, b),

which plays a similar role to the path modulation created by the

longitudinal scanner employed to produce A-scans or A-scan-based B-scans.

4. Different OCT imaging methods

There are two main OCT methods, TD-OCT and SD-OCT. SD-

OCT can be implemented in two formats, FD-OCT and SS-OCT.

Their utility for retinal imaging has been presented in several

recent review articles in this journal (Costa et al., 2006; Drexler

and Fujimoto, 2008; van Velthoven et al., 2007). We will shortly

review them, to compare their performance and discuss how they

can be best combined with other retinal imaging modalities. Each

method has its own merits and deficits.

4.1. Time domain optical coherence tomography

An A-scan is produced by varying the optical path difference

(OPD) in the interferometer to output a square root of reflectivity

profile in depth. En-face flying spot OCT belongs to the same

category. A T-scan is produced by transversally scanning the beam

over the target maintaining the reference mirror fixed to output a

square root of reflectivity profile versus angle or lateral position.

In both cases, the envelope of the interferometric temporal signal

is processed in time.

4.2. Spectral domain optical coherence tomography

In the last 5 years, considerable research has been devoted by

different groups developing OCT for tissue imaging into the

spectral OCT method. SD-OCT is attractive because it eliminates

the need for depth scanning in TD-OCT, performed usually by

mechanical means. Recent studies (Choma et al., 2003) have

shown that SD-OCT can provide a sensitivity, which is more than

10 times higher than that of TD-OCT.

FD-OCT and SS-OCT output A-scans, therefore they do not

allow real-time C-scan imaging. En-face (C-scan) sections can only

be obtained in FD-OCT and SS-OCT by sectioning the 3D volume

generated from a series of B-scan images taken at different

transverse coordinates, i.e. as a post-acquisition process only.

Therefore, essential in comparing the different OCT technologies is

the time required to complete a volume acquisition of the retina,which is then sectioned to create C-scan slices

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4.2.1. Fourier domain optical coherence tomography

This refers to Fourier transformation of the optical spectrum of

a low-coherence interferometer (Hausler and Lindner, 1998). This

method is an extension of the work on white light interferometry

with initial applications in absolute ranging and sensing (Smith

and Dobson, 1981). The operation of FD-OCT is based on the

demodulation of the optical spectrum output of an interferometer.

The spectrum exhibits peaks and troughs (channeled spectrum)and the period of such a modulation is proportional to the OPD in

the interferometer (Jenkins and White 1957). If multi-layered

objects are imaged, such as retina, each layer imprints its own

modulation periodicity, depending on its depth. A linear CCD

camera can be used to transform the optical spectrum into an

electrical signal, which exhibits ripple of different frequencies. A

fast Fourier transform (FFT) of the spectrum of the CCD signal

translates the periodicity of the channeled spectrum into peaks of

different frequency, related to the path imbalance (Costa et al.,

2006). Such a profile is essentially the A-scan profile of the square

root of reflectivity in depth. Due to its sensitivity advantage, the

FD-OCT became the method of choice in current OCT investigation

of the retina (at least in the past 3 years) with video-rate images

from the retina demonstrated (Nassif et al., 2004; Cense et al.,2004; Wojtkowski et al., 2004; Gotzinger et al., 2005; Jiao et al.,

2005, 2006). The majority of FD-OCT reports employ linear

cameras at 29 kHz (and faster rates are already available), which

represents a line scan rate at least twice faster than en-face

imaging using a resonant scanner at 16 kHz and more than 30

times faster than line scan rates in en-face OCT using galvan-

ometer scanners. This also reflects in large values of the voxel

number Mv/s $10 to a few tens (Wojtkowski et al., 2004). Using

line field (LF) FD-OCT, where a line is projected to the retina,

eliminating the mechanical transverse scanner, frame rates as

high as hundred of Hz have been achieved (Nakamura et al.,

2007). However, even with such fast line scan rates, because

C-scans are perpendicular to the main scanning direction, which

is axial, the time to provide a C-scan, Tenface, is several seconds

(Table 1), much larger than the time achievable with en-face OCT

(tens of ms to sub-seconds, Table 1).

4.2.2. Swept source optical coherence tomography

Recent progress in the fast tuneability of laser sources has revived

the interest in SS-OCT. The achievable signal-to-noise (S/N) ratio is

similar to that of FD-OCT, i.e. at least 10 times better than TD-OCT

(Choma et al., 2003). The time required to tune the wavelength

determines the time to produce an A-scan. Tuning speeds in excess

of 10 MHz makes the SS-OCT the fastest scanning OCT method

(Moon and Kim, 2006) to date. Lower rates, of a few hundred kHz

have been reported in imaging the retina in-vivo. These values are

close to one order of magnitude higher than those achievable using a

CCD camera implementing FD-OCT. This method leads to muchlarger values of Mv/s$100 ((Huber et al., 2007) achieved a

Mv/s 122 for 400 frames of 512512 pixels cross sections

collected in 0.87s) and to a reduced value for the Tenface, of sub-

second to second. This value is comparable to that reported in typical

en-face OCT, but still larger than the ultrafast Tenface$20ms

achievable with resonant scanners (Hitzenberger et al., 2003).

4.3. Full field or en-face non-scanning systems

Full-field or coherence radar (Dresel et al., 1992) operates

according to the scanning operation described in Fig. 2(iii). The XY

scanning is provided in the process of reading a 2D charge-

coupled device (CCD) photodetector array. The eye is flood

illuminated, i.e. all pixels in transversal section are simulta-neously lit, in contrast to the flying spot method where each

transverse pixel in transversal section is independently lit at a

certain time only. The information acquired along a line of pixels is

equivalent to a T-scan in Fig. 1. Telecentric optics are used to transfer

the object beam from the target to the camera. Practically, every-

thing happens as for each pixel in the transversal section, an object

beam ray can be identified in the object beam, which interferes with

one ray within the cluster of reference rays. C-scans and B-scans can

be produced with no need for lateral scanning. The line rate is that ofreading 2D CCD cameras, which at video rate means more than

10kHz. The frame rate is in the range of tens to hundreds of Hz and

the depth scanning is the slowest, the OPD change rate being slower

than the CCD-frame rate. One of the main advantages of the method

is that it can use incoherent spatial optical sources, such as tungsten

lamps. These represent low-cost versions of low coherent sources

and easily exhibit bandwidths larger than 300 nm. Using such large

band spatial incoherent sources, and a Linnik interference micro-

scope (Dubois et al, 2002), images with submicron lateral and depth

resolutions have been reported (Grieve et al., 2004) from ocular

tissue in-vitro. Images from bovine retina in-vitro have also been

obtained using a superluminescent diode (SLD) (Qu et al., 2004). The

same system with AO enhancement (see Section 8.2.4. below) was

used to produce C- and B-scans of a living eye (Miller et al., 2003).Tremendous progress in sensitivity allowed the imaging of the

anterior chamber of rat eyes (Grieve et al., 2005) with a very high

value of Mv/s 33328 (depending on the frame averaging mode)

and a Tenface 4ms.

As a disadvantage of the FF-OCT, the amplitude of the

interference signal is recovered using phase-stepping algorithms.

Phase shifts are introduced by exact path difference steps, which in

total add up to a wavelength, or by a continuous change of the OPD

and comparing the sequences obtained. This means that real-time

processing is not possible, however, this is not important for fast

acquisition systems where this leads to a mere short delay in the

display, equal to the number of frames used for phase shifting

multiplied by the frame acquisition time. Using a fast acquisition

camera (Miller et al., 2003; Grieve et al., 2005), this amounts to some

tens of ms. As another disadvantage, the detection of reflective

interfaces in a multilayer object using the coherence radar method is

limited by the dynamic range of the analog to digital (A/D) converter

of the combination CCD-frame grabber system. The interference

signal sits on a large constant value, which consumes a large part of

the dynamic range of the CCD cameras.

The limited dynamic range of 16 bit CCD cameras, limits the

smallest signal to 1/65,365 of the digital value, which makes the

FF-OCT method less sensitive than the flying spot imaging

method. In principle, the flying spot can provide signals for

variations in the interference of less than 1013 in a 1 Hz electric

bandwidth (Takada et al., 1991).

A faster processing method uses an array of photodetectors in a

smart chip (Ducros et al., 2002). A photodetector is employed for

each pixel in the en-face image, followed by a processingelectronics channel (demodulation, rectifier, amplifier, condition-

ing). One pixel consists of a small silicon photodiode coupled to a

complementary metal-oxide semiconductor electronic circuit. A

maximum size chip of 5858 smart pixels was reported, which

limits the numbers of pixels in the images to similar values.

Because there is no transverse scanning to alter the OPD, a phase

modulator is used. The reading is sequential, similar to the

reading of a CCD camera in a coherence radar system but

the output is a fully demodulated OCT signal. The amplitude of

the signal provided by each channel is proportional to the

envelope of the OCT interference signal. The smart chip can also

operate in the C- and B-scan regimes. The technology still evolves;

for the moment there is no report on using the smart sensor on

ocular tissue. Medium values of Mv/s 2.5 have been achieved,however, with a record Tenface 1.33ms.

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4.4. Line-field-SD-OCT

This is a combination of the FF and SD-OCT, where instead of a

2D image, a lD transversal image is collected using principles of

SD-OCT. A recent report on combining the line imaging and FD-

OCT has been developed (Nakamura et al., 2007) for very high-

speed 3D retinal imaging. By this technique, the A-line rate

significantly improved to 823,200 A-lines/s for single frameimaging and 51,500 A-lines/ s for continues frame imaging. A

high-speed 2D CMOS area camera with effective pixels of 1104

(horizontal)256 (vertical) was used. The columns of 256 pixels

are oriented along the vertical lateral size of the image while the

horizontal arrays of pixels are used for spectral analysis of

the channeled spectrum, supplying the depth information in the

B-scan. With an image size (the length of the vertical line

projected on the retina) of 2.1 mm and a pixel size of 16.4mm, the

number of pixels along the vertical line is 128. Due to the decay of

sensitivity of the FD-OCT method with depth, a maximum depth

range of 2.88 mm was achieved and images with sufficient

contrast presented up to 0.8 mm in depth, which gives 108 pixels

of 7mm depth resolution (Table 1).

The frame rate at continues frame imaging is 201fps. This 3Dacquisition speed is more than two-fold higher than the

acquisition speed of standard flying spot SD-OCT. To enhance

the sensitivity, pulse illumination was used for the duration

of the frame, of 0.3 ms. The in-vivo 3D retinal imaging with

256 B-scan image frames was successfully performed in

1.27s, which gives a Mv/s 2.8 for a collection of

128140256 pixels. The time to produce a C-scan image,

Tenface, is quoted as 1.27 s in Table 1 considering the number of

lateral pixels of 128256 in the images reported. However, if

512512 C-scan images are required, Tenface scales eight times

larger to over 10 s.

4.5. Multiplexing in OCT

Multiplexing refers to acquisition of data from several points at

the same time. Extending this notion to OCT, would mean

acquisition of several OCT images simultaneously.

4.5.1. Multiplexing in A-scan-based OCT imaging

One reason for the recent success of SD-OCT method in the eye

imaging is the fact that the depth information is somehow

multiplexed, i.e. one FFT of the photodetected spectrum contains

the OPD of all resolved scattering points along the depth of the A-

scan. One spectrum contains all depth information, where each

depth is coded in the number of peaks in the channeled spectrum

at the interferometer output. This is not the case in TD-OCT, where

when scanning the depth, the same Doppler shift is obtained, for

any of the scattering points resolved.To a larger extent, multiplexing of images means different

procedures in A-scan-based OCT and T-scan-based OCT. 3D

complete information could be collected in different ways, either

acquiring many B-scan OCT images at different en-face positions,

as shown in Fig. 2(iv) or many C-scan OCT images at many depth

positions, as illustrated in Fig. 2(iii). Different possibilities exist

and the choice will depend on the best way to use the available

S/N ratio and the available bandwidth to collect simultaneous

images from the retina. Collecting several B-scan OCT images as in

Fig. 2(iv) would involve splitting the light in the sensing arm,

possible by using mirrors and beam-splitters. The optical

elements in the sensing arm have to be arranged in such a way

that in each new arm the optical path is the same. Consequently,

such a procedure would involve cumbersome optics before thetransverse scanner along with matching the optical paths of the

different sensing arms within a few wavelengths. Additionally, the

signal in each sensing arm and so in each channel decreases

proportionally with the number of channels.

4.5.2. Multiplexing in T-scan-based OCT imaging

In the case of T-scan-based imaging, the reference paths can be

split instead, thus avoiding any disturbances with the transversescanner in the object arm. Different optical paths can be employed

to obtain C-scan slices at different depths. This is why the en-face

OCT imaging is more tolerant to simultaneous collection of slices.

Although apparently it should be equivalent to build the 3D

profile from either longitudinal slices or en-face slices, the latter

procedure is less cumbersome in terms of technical implementa-

tion and less lossy in term of the object signal power. For the same

voxel volume (resolution) and number of voxels, the time taken

and the amount of memory required for storage is the same

irrespective of the method, Fig. 2(iii) or (iv).

Such a procedure, of dividing the power in the reference path

has been explored in two prior reports. Two OCT channels have

been demonstrated using a two splitter configuration (Podoleanu

et al., 1997). A different configuration employed an integrated

MachZehnder modulator, where two delays have been intro-

duced in the reference arm, each with its own RF modulation

(Podoleanu et al., 2001). The frequency modulation limit of the

first and the dispersion of the modulator of the second rendered

these approaches unsuitable for in-vivo applications.

Another possibility is to split both the object and reference

arms as shown in Fig. 3 (Podoleanu et al., 2004b). In this way, two

independent OCT imaging channels are assembled. The depth

scanning proceeds simultaneously in the two OCT channels and

from the same range, however, a differential optical path

difference can be introduced between the two channels. In this

way, two simultaneous images are generated where the depth

differs in each transversal pixel by the differential optical path

difference. A dual OCT system, OCT/OCT, working at 850 nm was

devised and its capability demonstrated by simultaneouslyacquiring images from the optic nerve and fovea of a volunteer.

The configuration ensures a strict pixel-to-pixel correspondence

between the two images irrespective of the axial eye movements,

while the depth difference between the corresponding pixels is

exactly the set differential optical path difference. The images are

collected by fast en-face scanning (T-scan), which allows both

B- and C-scan acquisitions. The reference light is passed via mirrors

M1 and M2 and the depth is selected in both channels at the same

time. A differential path difference between the two interferom-

eters is created by moving the mirrors M1 and M2. In this way, any

differential delay, d, of 1 to 1500 mm could be introduced.

ARTICLE IN PRESS

Low

coherence

source

INTERFEROMETER

1

INTERFEROMETER2

Depth 1

Depth 2

ImageDepth2

ImageDepth1

PC

Control of depth 1

Control of depth 2

M 1

M 2

Fig. 3. Schematic diagram of the OCT/OCT system to provide C-scan images at

different depths. The output beam from the low-coherence source is sent to two

interferometers,1 and 2, with independent adjustment of the imaging depth, using

mirrors M1 and M2, which are simultaneously and synchronously controlled by apersonal computer, PC.



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Fig. 4 presents images from the optic nerve of a volunteer. Pairs

of images have been collected at D 100mm OPD steps, obtained

by moving the two mirrors M1 and M2 simultaneously, whilemaintaining their differential separation d. The separation

between the pairs may be different from D in reality due to axial

eye movement. However, the images in each pair are from depths

separated by d 100mm exactly (the bottom image is deeper by

100mm), because d is independent to the eye movement. Within

each pair, the differential depth difference is certain because the

two images are collected simultaneously, while the depth

difference between the pairs is only approximate due to eye

movements. Therefore, the images should be interpreted strictly

within the pair, on each vertical in Fig. 4, as associated pairs of

pixels displaced in depth by 100 mm.

The problem for the current technology is how to produce at

the same time 10 or 100 of such images from different depths. If

such a technological difficulty can be overcome, en-face OCT could

reach Mv/s values similar to those obtained with the SD-OCT

method. So far, only two channels could be successfully demon-

strated.

4.6. Comparative assessment of the OCT methods

Each method has several advantages and disadvantages.

Advantages of SD-OCT: The main performance that dictates the

supremacy of SD-OCT in comparison with the TD-OCT is its

superior sensitivity or enhanced S/N ratio. This reflects in better

penetration and/or faster acquisition speed. Theoretical models

estimate that SD-OCT is better than TD-OCT by a factor of 10 at

least which will be considered for our comparative analysis in

what follows.Disadvantages of SD-OCT: Although the SD-OCT method is

currently favored for its speed, it has inherent limitations. In both

formats (FD-OCT and SS-OCT), SD-OCT has three main disadvan-

tages: (i) decay of sensitivity with the OPD, which means that the

relative intensity of layers along the depth in the retina is not real,

in fact their intensity depends on how far the retina is from the

value of OPD 0, which varies due to head position relative to the

chin rest; (ii) dynamic focus not possible (see below), i.e. ensuring

that each depth in the OCT depth is in focus when acquiring signal

from that depth; (iii) the optical spectrum of the interferometer

output consists of symmetric spectral terms, i.e. the same image

results for positive and negative OPDs. For the latter, an initial

adjustment of the OPD 0 outside the range of interest is

required. This is not possible all the time, especially whenimaging moving thick organs or tissue, and this is a problem for

imaging the eye too. Different methods have been devised to

attenuate the symmetric terms in order to obtain a correct image

such as phase-shifting interferometry, or complex signal proces-sing (Targowski et al, 2004), which are cancellation techniques

(and so sensitive to movement) requiring several images or steps

(at least 3). An independent method to the target movement was

also developed (Podoleanu and Woods, 2007).

Unexploited potential in multiplexing of en-face OCT images: This

refers to the possibility of improving the overall number of Mv/s.

In terms of line rate, TD-OCT could reach fast line-scanning rates

using resonant scanners (4, 8 and 16 kHz are available). This is 27

times less than the scanning rate of modern line scan cameras

used in FD-OCT and more than an order of magnitude smaller

than the rate achievable using SS-OCT. It looks unlikely that the

line rate in en-face OCT can be further increased. Polygon mirrors

may achieve faster line-scanning rates, but they introduce a non-

linear dependence of the OPD with scanning and the main

limitation is the S/N ratio in TD-OCT. However, for any given line

rate, en-face OCT has an unexploited potential in the possibility of

simultaneous acquisition of several C-scan images, at the

expense of power division in the reference path, where power is

normally attenuated to reduce the noise, as explained above in the

Section 4.5.2.

Table 1 presents the performance of the OCT technology in

imaging the retina in-vivo as presented in a selection of reports.

The first columns show the acquisition rate and number of pixels

in the image, depending whether B-scan or C-scan, while the last

two columns on the right are the most significant. They display

two main parameters: the number of Mv/s and the time required

to produce a C-scan image, Tenface. The table starts with the 1st

report of OCT from retina at MIT (Huang et al., 1991) and ends

with the highest OCT speed reported using chirped lasertechnology (Moon and Kim, 2006) to implement SS-OCT method

(although not on the eye). Where the authors have not specified

the numbers of pixels, such numbers were inferred from the data

available in each report, as detailed in the footnotes. The size of

the image varies from report to report, therefore the number of

Mv/s represents a suitable performance to compare the different

methods. The number of pixels in Table 1 are those specified in the

images presented in the reports. A rigorous comparison would

require an evaluation of the techniques mentioned based on a

similar size image, however, this was not possible. Anyway, the

important numbers are the order of magnitude of the Mv/s and

Tenface and not their exact value.

The time to produce an OCT cross-section (B-scan) image is not

used as a comparison criterion because on one hand is the inverseof the frame rate in A-scan-based systems and on the other,

ARTICLE IN PRESS

Fig. 4. Images from the optic nerve. Separation between pairs: D 100mm. Separation between images in the pair: d 100mm. Lateral size: 2.5mm2.5mm. Pairs

collected at 2Hz (Podoleanu et al., 2004b).



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a T-scan-based OCT system can be switched into B-scan

regime and produce a B-scan in the same time as for

generating a C-scan, Tenface. In other words, producing a B-scan

image is not a challenge for T-scan-based OCT systems while

producing a C-scan from A-scan-based OCT systems is. The

parameter Tenface also represents the time required to produce

an SLO-like image in the A-scan-based OCT systems, and this has

clinical relevance in the case of SD-OCT, which generates B-scans

and not C-scans.

The graph in Fig. 5 is assembled based on the exemplary reports

in Table 1 only, for simplicity. In reality, a large spread of points could

be placed in the graph based on the articles mentioned in the

reference list, however, when this is completed, they display

the same trend as shown using only the few points representing

the publications in Table 1. The graph shows extraordinary progress

in terms of the number of Megavoxels imaged in a time unit, due to

the progress in SD-OCT and especially in the case of SS-OCT imaging

using mode-locked SS technology (Huber et al., 2007). The SS-OCT

has further potential for improvement in data acquisition rate since

the chirped laser principle allows line rates in excess of tens of MHz.

However, the increase in the line rate is currently accompanied by

reduction in the signal and increase in noise due to large electronic

bandwidth required.

For A-scan-based OCT systems, the time to collect the wholevolume of voxels determines the time required to produce a

C-scan image, Tenface, since such an image is only available once

all data have been acquired. Where data were not available,Tenface was inferred considering a number of 256 frames. The

graph in Fig. 6 illustrates the progress over the years in reducing

Tenface. For A-scan-based TD-OCT systems, Tenface exceeds tens of

seconds, too long for imaging a moving eye (although the majority

of reports on OCT before 1998 required such large time intervals

for acquiring multivoxel data). To the right of the graph, progress

in mode-locked SS-OCT lead to a time of less than 1s, which

becomes useful in practice. However, even if the progress was

substantial in the last few years, this value is still more than an

order of magnitude larger than the time to produce a C-scan

image using an en-face OCT system equipped with resonantscanners (Hitzenberger et al., 2003).

In the past few years, progress has been reported not only

in the improvement of the two parameters, Mv/s and Tenface,

but in the depth resolution too. Excellent accounts on the

resolution improvement in OCT are presented elsewhere (Drexler,

2004). This is however implied in the number of pixels in depth in

the Table 1.

5. Depth of focus range and dynamic focus in OCT

In order to obtain images with high transverse resolution

throughout the whole depth of the retina, dynamic focus is

essential. Dynamic focus means maintaining the coherence gate

and the focus gate in synchrony in OCT. The confocal core of the

OCT channel is what determines the depth of focus in the OCT, and

this is an important issue in the progress towards high resolution,

sometimes ignored. A good S/N ratio requires that the confocal

core of the OCT channel focuses at the same depth where the

coherence gate of the OCT selects signal from. The procedure is

often utilized in the TD-OCT, where the focus and the coherence

gate can be synchronously scanned. A TD-OCT A-scan-based

system requires that the focus scanning be performed at the line

rate. In an A-scan-based TD-OCT system, dynamic focus is in

principle possible, but unachievable technically due to the high

speed required for focus change (it is difficult to move a lens at

kHz rate). A T-scan-based OCT system relaxes this demand, as the

focus needs to be changed at the frame rate, of the order of Hz or

tens of Hz, which is much smaller.

In opposition to the TD-OCT methods, dynamic focusis not applicable to FD-OCT and SS-OCT due to the very

principle employed. Therefore, for such systems, the interface

optics are devised with a large depth of focus, to accommodate

the entire range of the A-scan, usually, 12 mm. Not providing

focus change with depth is visible especially in imaging the

optic nerve, which extends for more than 2 mm, and where

the contrast in the image decreases at the image edges.

The constant focus in SD-OCT precludes the possibility of using

a high NA objective to enhance the transverse resolution and also

limits the efficiency of the combined SD-OCT/AO method as

detailed below.

Dynamic focus applied to a TD transversal (en-face) scanning

system for retinal imaging (Pircher et al., 2006b) demonstrated

that a transverse resolution of 4.4 mm can be achieved over anoptical depth of 1 mm in a model eye.

ARTICLE IN PRESS

Max SS-OCT

SS-OCT:

FD-OCT:

345:

TD-OCTResonantscannerTD-OCT

LF-FD-

OCT:0

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008

20

40

60

80

100

120

140

Megavoxel/s

Year

Fig. 5. Evolution of OCT technology including TD (time domain), FD (Fourier

domain), SS (swept source) and LF (line field) for imaging the retina in terms of

Megavoxel/s. The horizontal line at 345 Megavoxel/s represents the maximum

achieved today in terms of Megavoxel/s, using SS-OCT (Moon and Kim, 2006).

0

2

4

6

8

10

12

14

16

18

TimetoproduceaC-scan(s)

Year

Minimum, using transverse resonantscanner in en-face OCT

Mode LockedSS-OCT:

FD-OCT:

0.019

TD-OCTResonantscanner

SS-OCT:

SS-OCT

FD-OCT

1998 2000 2002 2004 2006 2008

Fig. 6. Evolution of Tenface, the time required to produce a C-scan image of the

retina, using longitudinal OCT (including TD and SD). The horizontal line at 19 ms

represents the time reported for en-face OCT using a resonant scanner

(Hitzenberger et al., 2003).



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6. Combining OCT with SLO

OCT has mainly evolved in the direction of producing cross-

sectional images, most commonly perpendicular to the plane

orientation of images delivered by a microscope or by an SLO.

Several groups have shown that the flying spot concept, utilized in

the SLO hardware can be combined with the OCT technology to

produce en-face OCT images from the anterior and posterior pole.The depth resolution in SLO is 30100 mm coarser than that in OCT

while the transversal resolution in OCT is affected by random

interference effects from different scattering centers (speckle),

inexistent in SLO images. Therefore, there is scope in combining

SLO with OCT. Once C-scan OCT images have had been demon-

strated, sharing the same natural orientation to that of SLO

systems, the next step was to combine OCT with SLO. Different

avenues have been evaluated, to provide SLO and en-face OCT

images simultaneously, quasi-simultaneously or sequentially.

The simultaneous OCT/confocal technology has been exten-

sively evaluated on more than 2000 eyes with pathology, using

what is now called the OCT/SLO or the OCT/Ophthalmoscope

instrument (Rosen et al., 2003). A variant of this instrument, the

OCT/SLO/indocyanine-green (ICG), has also been tested provingthe potential of multi-modal imaging. This allows collection of

simultaneous en-face OCT and ICG fluorescence images from the

retina. The sequential OCT-confocal imaging procedure is still in

laboratory phase. This concept provides better S/Nratio in the OCT

channel and better depth resolution in the confocal (SLO) channel

than the OCT/SLO system. The performances of the sequential

procedure will be discussed in comparison with the simultaneous

procedure.

Different solutions have been provided to assemble OCT/SLO

systems, depending on the scanning type and on the OCT regime

of operation. The main motivation for OCT/SLO combination is to

provide orientation to the OCTchannel. Crucial for the operation is

pixel-to-pixel correspondence between the two channels, OCT and

SLO, which can only be ensured if both channels share the same

transverse scanner to scan the beam across the eye. Different

possible configurations are shown in Fig. 7. Any OCT system is

constructed around an interferometer illuminated by a low-

coherence source. The interferometer is equipped with some

means to adjust the OPD to determine the axial (or depth)

scanning. To send the signal towards the eye, the OCT is equipped

with an interface optics, which contains lenses or curved mirrors

and the XY scanner. Light collected from the eye is received either

in a pinhole or via a single-mode fiber in order to ensure a high

visibility for the interference signal. This acts also as a confocal

core, which can be used for implementing an SLO channel,

however, this is not possible all the time as explained below,

therefore different possibilities exist.

Fig. 7(i) illustrates the principle of the OCT/SLO instrument

where the confocal core of the OCT is used to produce the SLOsignal. No splitting of the light from the eye is performed. Fig. 7(ii)

shows a different configuration, where light for the SLO channel is

produced using a separate optical source and light received from

the eye is diverted towards a separate confocal receiver. This

requires a splitter to divert light into the two paths, OCT and SLO.

Fig. 7(iii) is a simplification of Fig. 7(ii) where the same optical

source is shared by both channels. Again, this requires a splitter to

divert light into the two paths, OCT and SLO.

Historically, while the simplest configuration looks like

that in Fig. 7(i), the first OCT/SLO (Podoleanu and Jackson,

1998) implemented employed the configuration in Fig. 7(iii).

It was 7 years before a sequential version of Fig. 7(i) to be

described, and a simultaneous implementation has not yet been

reported, the main problem being the associated noise with thehigh reference power.

Fig. 7(ii) is useful in combining OCT with fluorescence. In this

case, the separate SLO source excites fluorescence, which is

processed in a separate SLO receiver. Such a configuration would

be useful in combining OCT with fluorescein angiography (FA),

where the OCT operates at 700900nm and the separate source

operates in greenblue to excite fluorescein. Such a OCT/

fluorescence configuration has been reported in endoscopy

(Barton et al., 2004).

The three generic configurations presented in Fig. 7 help to

reveal the diversity of possible combinations of OCT with SLO in

terms of the scanning regimes. SLO provides C-scan images.

Therefore the natural combination of the two channels would be

that where the two images generated are both C-scans. However,

solutions have been provided where the SLO channel maintains its

natural C-scan orientation while the OCT channel operates in

B-scan regime, in order to implement SD-OCT. In time, the two

images can be generated simultaneously or sequentially. Simulta-

neous generation could be implemented in C-scan orientation by

both channels. C-scan SLO and B-scan OCT, however, can onlybe sequential, if the same source and transverse scanner is to be

shared. (The operation in C-scan SLO and B-scan OCT can also be

achieved by combining an OCT channel and an SLO channel each

equipped with its own XY scanner, via a splitter, however, such a

configuration is not practical and will be difficult to ensure pixel-

to-pixel correspondence when using two different, independently

ran XY scanners).

The main advantage of the en-face imaging is that it allows

integration of SLO with OCT. This has proved useful in allowing

ophthalmologists and visions scientists to associate features seen

in cSLOs and SLOs with those highly fragmented due to enhanced

depth resolution in en-face OCT. Such a method allows a dual

presentation of high-resolution images (OCT and SLO) in different

regimes of operation, B- or C-scan, providing cross sections indepth or constant depth images, respectively. Other current

ARTICLE IN PRESS

OPD

control

SLO/OCT Aperture

Interferometer

Object

arm

Reference

armOptical

source

OPD

control

OCT Aperture

Interferometer

Object

arm

Referencearm

OCT

Optical

source

SLO channel

OPD

control

OCT Aperture

Interferometer

Object

arm

Reference

armOCT/SLO

Optical

source

SLO receiver

SLO

Optical

source

(i)

(ii)

(iii)

Fig. 7. Generic configurations used in the combination of OCT with SLO.



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developments include sequential OCT-confocal regime of opera-

tion, dual OCT/confocal fluorescence imaging and triple OCT/SLO/

confocal fluorescence imaging.

6.1. Simultaneous OCT/SLO

Simultaneous acquisition of images in two channels, OCT andSLO requires configurations using splitters, according to Fig. 7(iii).

In (Podoleanu and Jackson, 1998), a small fraction of the light

returned is diverted towards the SLO channel using a separate

splitter. There is an optimum splitting ratio which ensures

sufficient and similar S/N ratio in both channels (Podoleanu and

Jackson, 1999).

New imaging technology brings not only new information to

the clinician, but with it, the requirement of interpretation. En-

face OCT is no exception in this respect. The higher the depth

resolution of the OCT system, the more fragmented the en-face

OCT image looks like (Podoleanu et al., 1999). The fragmentation

is especially visible when the plane of the retina is tilted in

relation to the scanning plane. First, the en-face OCT image

appears fragmented, and on its own, such an image is difficult tointerpret. Second, variations in tissue inclination with respect to

the coherence wave surface alters the sampling of structures

within the depth in the retina, creating unbalanced distortions

among the elements being sampled (Podoleanu et al., 2004a). The

bright patches in the OCT image represent the intersection of

the surface of OPD 0 with the tissue. Due to the particular way

the retina is scanned, with the fan of rays converging on the eye

pupil, the surface of OPD 0 is an arc circle with the center in the

eye pupil. Depth exploration requires that the radius of the arc is

altered. If the arc has a small radius, it may just only intersect the

top of the optic nerve with the rest of the arc in the aqueous. The

radius of the arc is changed by changing the length of one of

the arms of the interferometer in the OCT channel to explore the

retina up to the RPE and choroid. The orientation of the retina

tissue at the back of the eye is not planar and this complicates theinterpretation of the image even further. Despite scanning images

in an en-face plane, the result is that the images may display the

structure in depth like in any B-scan OCT image. This is especially

visible in the high-resolution en-face OCT as shown in Fig. 8 (Cucu

et al., 2006), where the C-scan slice is very thin (3 mm). These two

effects, (i) fragmentation and (ii) multiple depths simultaneously

displayed in the C-scan images are present in a cSLO with high

depth resolution as well, however, at a scale where they are

regularly discarded. In a cSLO, the images do not look fragmented

and the depth structure is barely visible due to the coarse depth

resolution, 0.3 mm, comparable to the retina thickness. Going in

and out of focus results in a smooth transition from dark to bright

areas in the image. Both problems mentioned above are brought

about by the high depth resolution of OCT. Providing an SLO imagesimultaneously guides the user and addresses the fragmentation

problem.

In terms of data acquisition, the confocal image adds further

versatility. The design ensures a strict pixel-to-pixel correspon-

dence between the two C-scan images, OCT and SLO. This helps in

two respects: for small movements, the SLO image can be used to

track the eye movements between frames and for subsequent

ARTICLE IN PRESS

Fig. 8. Images acquired from the foveal region of the retina in-vivo using en-face technology and 120nm band SLD source. SLO images on the left and en-face high-resolution

OCT (3 mm depth resolution) images on the right for two different depths in the retina. The structure of layers normally encountered in B-scan OCT images is also visible

here: ILM, the inner limiting membrane; NFL, nerve fiber layer; GCL, ganglion cell layer; IPL, inner plexiform layer; INL, inner nuclear layer; OPL, outer plexiform layer; ELM,external limiting membrane; IS/OS, junction between the inner and outer photoreceptors; RPE, retinal pigment epithelium; CC, choriocapillaris; C, choroid; V, the vitreous.



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transversal alignment of the OCT image stacks; for large move-

ments and blinks, the SLO image gives a clear indication of the

OCT frames that need to be eliminated from the collected stack. As

a reference for the aligning procedure, the first artifact-free

confocal image in the set is used.

In the B-scan regime, movements of the eye are indicated by

lateral shifts of the confocal traces (Podoleanu et al., 2004a). Each

horizontal line in the SLO image when the OCT/SLO is switched to

B-scan, corresponds to a depth position. The relative lateral eye

movements lead to slight deviations of contours in the SLO image,

which can be employed to correct the lateral shift of the lines in

the B-scan OCT image (also illustrated here in left insets below theB-scan OCT images in Figs. 1014 below).

A bulk interferometer solution for simultaneous acquisition of an

OCT and an SLO image was also reported (Pircher et al., 2006a). The

bulk configuration allows placing the optical splitter close to the

optical source with the advantage of no signal lost towards the OCT

channel (however requiring to compensate for loss of power by

increasing the optical source power). Using such a system equipped

with dynamic focus, the cone mosaic was imaged simultaneously in

the SLO and OCT channels without AO elements.

Combination of techniques allows correlation of information in

orthogonal planes and facilitates more accurate diagnosis. For

instance, Fig. 9 top presents a B-scan OCT, which represents a

20mm thick slice through the central macula. Since the slice iscollected as the patient is instructed to fixate on a specific target,

ARTICLE IN PRESS

Fig. 9. Illustration of synergy between C-scans and B-scans.



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the image captures a macular region where the visual acuity is

best and suggests that the macular anatomy of this patient is

normal. Ho

2011 prer elsevier_podoleanu_retina oct

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