Ординатура / Офтальмология / Английские материалы / Optical Coherence Tomography in Age-Related Macular Degeneration_Coscas_2009

Overlay with Angiography

The high quality fundus image obtained by the confocal channel can be used as a reliable reference to make software-inferred overlay images from more conventional diagnostic techniques as fluorescein angiography (FA) onto the OCT part of the C-scans.

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With appropriate stand-alone software, conventional angiographic images can be spatially transformed and superimposed over the confocal image (van Velthoven 200515 ).

The pixel-to-pixel correspondence between confocal and OCT images allows image overlay.

The late FA image of a Birdshot patient (Figure 12) shows leakage along the vascular arcades and in the fovea (Figure 15)

An overlay image of this FA and the C-scan is shown (Figure 16)

This image shows that the temporal area of retinal thickening on the OCT exactly matches the temporal, parafoveal area of leakage on the angiographic image.

Closer examination of these overlay images shows the correlation between the smaller areas of leakage at the nasal side of the fovea, along the vascular arcades on the angiographic image, and the irregular retinal contour on the C-scans.15

In the case of unexplained leakage on an angiographic image or an unexplained patchy surface on OCT, this overlay feature can be used to combine the functional and morphological information provided separately by the two imaging techniques and help elucidate the findings in either one or both.

A similar feature as described above is available in the system’s acquisition program.

Conclusion

The pixel-to-pixel correspondence between OCT and SLO images allows more accurate follow-up over time.

Accurate overlays between angiographic images and OCT can also be obtained. SLO combined with this OCT technology provides new anatomical data to understand the pathophysiology of retinal diseases.

The only major difficulty, is learning to interpret coronal images and evaluation of the new features displayed by this technology.

This new approach will soon become self-evident

References

1.Podoleanu AG, Dobre GM, Webb DJ et al Simultaneous enface imaging of two layers in the human retina by low-coher- ence reflectometry. Optics Letters 1997;22:1039-41.

2.Podoleanu AG, Seeger M, Dobre GM et al Transversal and longitudinal images from the retina of the living eye using low coherence reflectometry. J Biomed Opt 1998;3:12-20.

3.Podoleanu AG, Rogers JA, Webb DJ et al Compatibility of transversal OCT imaging with confocal imaging of the retina in vivo. SPIE 1999;3598:61-7.

4.Podoleanu AG, Rogers JA, Jackson DA. 3D OCT Images from retina and skin. Optics Express 2000;7:292-8.

5.Podoleanu AG, Dobre GM, Cucu RG et al Combined multiplanar optical coherence tomography and confocal scanning ophthalmoscopy. J Biomed.Opt 2004;9:86-93.

6.van Velthoven ME, Verbraak FD, Yannuzzi LA et al. Imaging the Retina by en-face Optical Coherence Tomography. Retina 2006;26:129-36.

7.Schuman JS, Puliafito CA, Fujimoto JG. Optical Coherence Tomography of Ocular Diseases (2nd edition). 2 ed. Thorofare (USA):

SLACK Inc., 2004.

8.Podoleanu AG, Jackson DA. Noise analysis of a combined optical coherence tomograph and a confocal scanning ophthalmoscope. Applied Optics 1999;38:2116-27.

9.Podoleanu AG, Jackson DA. Combined optical coherence tomography and scanning laser ophthalmoscopy. Electronics Letters 2002;34:1088-90.

10. Rogers JA, Podoleanu AG, Dobre GM et al Topography and volume measurements of the optic nerve using en-face optical coherence tomography. Optics Express 2001;9:533-45.

11. Podoleanu AG, Dobre GM, Seeger M et al. Low coherence interferometry for en-face imaging of the retina. Lasers and Light 1998;8:187-92.

12. Rogers JA, Podoleanu AG, Fitzke FW et al Visualisation and measurement methods using transversal OCT images of the eye fundus. SPIE 2000;4160:16-23.

The advantage of the OCT/SLO is based on its ability to provide a real-time overview of the area of interest.

This allows rapid localization of abnormal areas and assessment of the extent of any pathology in the central retinal area.

13. Hee MR, Izatt JA, Swanson EA et al Optical coherence tomography of the human retina. Arch.Ophthalmol. 1995;113 :325-32.

14. Huang D, Swanson EA, Lin CP et al Optical coherence tomography. Science 1991;254:1178-81.

15. van Velthoven ME, de Vos K, Verbraak FD et al Overlay of conventional angiographic and en-face OCT images enhances their interpretation. BMC.Ophthalmol 2005;5:12.

Optical coherence tomography (OCT) is a noninvasive imaging technology that provides high-resolution crosssectional images of tissue microstructures in vivo

OCT imaging is useful for the evaluation of retinal anatomy (ie, to elucidate subtle microstructural abnormalities of the retina and to explore the adhesion of the vitreoretinal interface).

OCT is now one of the most useful modalities for evaluating and managing a variety of vitreoretinal diseases, such as idiopathic macular hole macular edema, and agerelated macular degeneration (AMD).

The principle of OCT was reported for the first time in 1991 (Huang 19911). The first commercially available OCT ( OCT 1 TM or System 2000 Humphrey Instruments, Inc.) was introduced in the mid-1990s.

First Generation of OCT

This first generation of OCT had an axial resolution of about 15 μm and a transverse resolution of 20 μm in tissue. The data acquisition rate was 100 A-scans per second.

The Stratus* OCT (OCT 3000™, Carl Zeiss Meditec, Dublin, CA), a Time-Domain OCT (TD-OCT) apparatus, was introduced in 2002 and is now the most widely used OCT system.

This system provides axial resolution of up to 10 μm and a higher acquisition rate of 400 A-scans per second by using a high-speed scanning system, thus allowing a 4-fold increase in imaging speed compared with OCT-1.

The increased axial and transverse pixel counts of the Stratus* OCT provides improved imaging quality.

Intraretinal as well as subretinal and subretinal pigment epithelium lesions can be clearly analyzed.

The Stratus* OCT has therefore become a standard imaging modality for the diagnosis and assessment of AMD (Salinas-Alamán 2005 2, Coscas 2007 3 ).

Retinal thickness analysis by OCT, which allows quantitative assessment of changes in retinal morphology, has

become one of the critical parameters for various clinical trials (Avery 2006 4 Michels 2005 5 Fung 2007 6).

Recent Advancement in OCT Technology

Spectral-Domain OCT (SD-OCT), also known as FourierDomain OCT (FD-OCT), is a recent advancement in OCT technology.

This technology has considerably improved imaging quality with axial resolution of less than 5 μm and a 50fold higher acquisition speed compared to conventional OCT (Iam 2006 7).

The enhanced imaging capabilities of FD-OCT have the potential to improve the visualization of retinal morphology and provides additional information compared to Stratus* to elucidate the pathogenesis of macular disease.

The increased scan speed also allows comprehensive 3D information, not previously available with Stratus* (Hangai 2007 8 ).

This chapter presents an overview of the principles of FDOCT and demonstrates the high definition of this new generation of OCT on retinal morphology in normal subjects.

Basic Concepts: TD-OCT and SD-OCT

OCT performs cross-sectional imaging by directing a beam of light onto tissue and measuring the echo time delay and intensity of backscattered light from different tissue structures at varying axial distances in the eye.

Since OCT imaging uses light rather than sound, it is not possible to directly measure echo time delays.

Correlation techniques, known as low-coherence interferometry, are required (Podoleanu 1997 9 Schuman 2004 10).

To measure echoes of light by low-coherence interferometry, a beam of broadband, low-coherence light is generated by a superluminescent diode.

The interferometer has a superluminescent diode (SLD) light source, sample (eye), reference mirror, and detector arms.

The output of the SLD light source is launched into the source arm and split by the coupler into the beam focused on the eye and then directed at a reference mirror.

The reflected light beam from the eye, consisting of multiple echoes of different intraocular structures, and the reference beam reflected from a reference mirror and consisting of a single echo at a known delay, is recombined by the interferometer and produces interference.

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Time-Domain OCT

In TD-OCT, the reference mirror is moved to match the delay in various layers of the sample because interference is only observed when the reference delay matches the echo delay in the signal light (Figure 1).

The resulting interference signal is processed to give the axial scan. The reference mirror must move one cycle for each axial scan.

The need for this mechanical movement limits the image acquisition speed.

The single detector detects the output signal from the interferometer, which is then processed and displayed on a computer.

Ultrahigh-Resolution OCT (UHR-OCT)

A new generation of Time-Domain OCT has been developed, known as ultrahigh-resolution OCT (UHR-OCT).

This ultrahigh-resolution OCT will provide axial image resolution of 3μm

UHR-OCT is not widely used in clinical practice, however, due to the need for a special light source (femto-second ti- tanium-sapphire laser) and the long data acquisition time.

The spectral interferogram is Fourier-transformed to provide an axial scan. The absence of moving parts allows the image to be acquired very rapidly.

The comparison of TD-OCT and SD-OCT is summarized (Table 1). Several types of SD-OCT are now commercially available (Table 2).

Imaging Advancements in SD-OCT

High Resolution

The axial resolution of OCT is essentially determined by the coherence length of the light source, which is inversely proportional to the bandwidth (Wojtkowski 200711

Huang 200512).

Time-Domain OCT uses SLD sources emitting a wavelength of about 820 nm over a bandwidth of 20 nm with an axial resolution of about 10 μm in tissue.

The first FDA-approved SD-OCT (RTVue™ system), uses SLD light sources with an 840 nm center wavelength over a bandwidth of 45 nm with an axial resolution of less than 5 μm, (ie, 2-fold higher than that of Time-Domain OCT).

High-Speed Acquisition

RTVue* can capture 2048 pixels (entire A-scan) simultaneously without any mechanical motion, while TimeDomain OCT captures one pixel at a time.

In the time it takes Time-Domain OCT to form a single axial scan, SD-OCT can capture an entire image

The RTVue* provides scan speed of 26,000 A-scans per second, 65 times faster than the conventional TimeDomain OCT system.

Spectral-Domain OCT or SD-OCT

In SD-OCT, the reference mirror is kept stationary (Figure 2).

The interference between the subject and reference reflections is split into a spectrum and captured by a spectrometer instead of a single detector.

Improved Imaging Quality

Interpretation of the normal macula as a result of improved imaging quality

The shortened image acquisition time minimizes motion artifacts. In addition, the higher acquisition speed of SD-OCT can increase the number of transverse pixels per image to yield high-definition images with further improved image quality.