3-D Fourier domain OCT offers more scans over larger area

Results provide better delineation of pathology and accurate retinal registration.

In our clinical experience of axonal loss through retinal nerve fiber layer and macular thickness assessment, optical coherence tomography has shown its versatility as an effective method of screening for epidemic causes of blindness such as diabetic retinopathy and age-related macular degeneration, as well as its potential to replace current glaucoma evaluations.

Our experience suggests that having access to 3-D Fourier domain OCT technology is important to remain competitive, particularly as a retinal specialist.

Versatility and potential

Fourier domain OCT overcomes several problems associated with conventional time domain OCT systems. Because time domain OCT relies on mechanical movements of internal components to take thickness measurements, image acquisition speed is limited. This restricts the number of scans that can be acquired before a patient looks away from the fixation point, making eye movement artifacts common.

Current time domain OCT instruments construct a retinal thickness map using six radially oriented B-scans captured in 8 to 10 seconds, measuring less than 5% of the total macular area and approximating more than 95% of the output data. Therefore, the device may incorrectly estimate retinal thickness (error of interpolation) and may miss small lesions (error of detection) that fall between the radial-line scans. By offering a greater number of scans over a large area, 3-D Fourier domain OCT data provide better delineation of pathology and accurate retinal registration.

Current time domain OCT software has been reported to incorrectly identify the outer retinal boundary and be incapable of distinguishing features under the retina, such as subretinal fluid and retinal pigment epithelial detachments. This can lead to the inaccurate reporting of true retinal thickness in diseases such as AMD.

Three-dimensional Fourier domain OCT is 50 times faster and 25% more accurate; the faster speed, higher resolution and improved signal-to-noise ratio may ultimately improve the detection of the true outer retinal boundary and retinal layers, and enable the detection and quantification of clinically relevant disease features. Furthermore, the 3-D OCT is fast and requires less expertise to operate, which makes it easy to train new technicians. Because it is fast, non-invasive and sometimes does not require pupil dilatation, it is more comfortable for patients.

High-definition 3-D imaging improves image quality, retinal coverage and registration. This technology has the potential to become a useful tool for elucidating disease pathogenesis and improving disease diagnosis and management. The 3-D OCT generates realistic images of the vitreofoveal interface and intraretinal microstructures, which is critical in diseases such as macular hole, diabetic retinopathy and epiretinal membranes. In cases of proliferative membrane in diabetic retinopathy, this system is useful for training and for planning of the surgical procedures.

The 3-D overview of the vitreofoveal interface helps the clinician gain an immediate understanding of the dynamic interactions of the vitreous and fovea. The 3-D OCT also provides consecutive orthogonal images that allow much more precise observation of three-dimensionally extending intraretinal structural changes associated with a macular hole, especially in the photoreceptor inner and outer segments.

Three-dimensional OCT imaging delineates the microstructural changes that occur within the photoreceptor layers and demonstrates the spatial relationship between the laterally spreading or scattering microstructures and the fovea. The 3-D visualization of the relationship between the external limiting membrane and each photoreceptor layer before and after macular reattachment facilitates understanding of anatomic and vision changes.

The 3-D OCT also works as a fundus camera, and its software simultaneously displays the 3-D image and 2-D scan with the fundus image. Fundus color images allow the practitioner to map the exact location of retinal abnormalities, enabling a more precise comparison of OCT data. Alignment of OCT data with other imaging modalities enables point-to-point comparisons that may assist in disease localization and allow a better understanding of the underlying pathophysiology. The tridimensional reconstruction imaging is also useful in educating patients because these visuals make it easier for them to understand their pathology.

All of these benefits of 3-D OCT are illustrated in a recent study that my colleagues from the Department of Ophthalmology at the University of São Paulo Medical School and I performed quantifying axonal loss through retinal nerve fiber layer (RNFL) and macular thickness assessment.

RNFL loss study

The purpose of our study was to evaluate the ability of Fourier domain OCT to differentiate eyes with band atrophy of the optic nerve from normal eyes and to evaluate the relationship between RNFL and macular thickness parameters and the severity of visual field damage in patients with band atrophy.

The ability of Fourier domain OCT RNFL and macular thickness parameters to detect patients with band atrophy was compared with that of Stratus OCT using the same sample of subjects. A total of 36 eyes from 36 patients (22 men) with temporal hemianopia from chiasmal compression and 36 eyes from 36 (15 men) normal age-matched controls were studied.

Fundus retinography of the left eye showing the rectangular map (6 mm x 6 mm of area) of macular thickness measurements and the corresponding scanned area (above); note the reduction of macular thickness in the area of axonal loss (below).

Tridimensional topography retinal map protocols of the macular area showing the localized defect corresponding with the area of axonal loss. Images: Cunha LVFC, Cunha LP

The study findings indicate that the 3-D OCT RNFL and macular thickness parameters were able to differentiate normal eyes from eyes with band atrophy of the optic nerve, as did the Stratus OCT. The study shows that, although the 3-D OCT was able to detect the abnormalities, the software can still be improved, as far as the analysis of the RNFL is concerned.

With regard to the macula thickness measurements for quantifying neural loss, the 3-D OCT’s current software already provides improvement over time domain OCT. Despite that, we believe that the segmentation of the ganglion cell layer should be implemented, and that it will certainly help in clinical use. We concluded that although the device performed very well in these patients, future studies using other RNFL and macular thickness measurements analysis protocols are necessary to better explore the diagnostic potentialities of 3-D OCT.

Case study

A particularly salient case study highlights the value of 3-D OCT. A 22-year-old man presented to us, and his history revealed that he had developed a central scotoma in the left eye during an episode of severe headache, 6 months previously. He had a previous diagnosis of migraine headaches with aura since the age of 15 years. Ophthalmic examination showed RNFL thickness reduction on the papillomacular bundle in the left eye and unremarkable findings in the right eye.

Topcon’s 3-D OCT documented severe macular thickness reduction and a mild and localized RNFL loss on the temporal side of the optic disc in the left eye. Systemic investigation, neuroimaging and cardiovascular studies were non-revealing. The cause of his visual loss was possibly related to retinal migraine, although an ischemic episode of other origin could not be excluded.

Our case draws attention to the fact that 3-D OCT may be able to identify localized neural loss, not only by measuring peripapillary RNFL but also by means of macular thickness measurements. Awareness of this is crucial, particularly when estimation of neural loss is intended in patients with optic neuropathies associated with optic disc edema.

References:

• Costa RA, Calucci D, Skaf M, et al. Optical coherence tomography 3: Automatic delineation of the outer neural retinal boundary and its influence on retinal thickness measurements. Invest Ophthalmol Vis Sci. 2004;45(7):2399-2406.
• Costa Cunha LV, Cunha LP, Malta RF, Monteiro ML. Comparison of Fourier-domain and time-domain optical coherence tomography in the detection of band atrophy of the optic nerve. Am J Ophthalmol. 2009;147(1):56-63.
• Cunha LP, Vessani RM, Monteiro ML. Localized neural loss detected by macular thickness reduction using optical coherence tomography: case report. Arq Bras Oftalmol. 2008;71(5):743-746.
• Pons ME, Garcia-Valenzuela E. Redefining the limit of the outer retina in optical coherence tomography scans. Ophthalmology. 2005;112(6):1079-1085.

• Luciana Virgínia Ferreira Costa Cunha, MD, and Leonardo Provetti Cunha, MD, can be reached University of São Paulo, Hospital de Olhos Juiz de Fora - Clínica de Olhos Dr. Antônio Gabriel, Barão do Rio Branco Avenue 4051 - Bom Pastor 36021630; +55-32-3215-5104; e-mails: luciana.costa.cunha@terra.com.br, leonardo_provetti@yahoo.com.br; Web site: www.hojf.com.br. They have no direct financial interest in the products discussed in this article, nor are they paid consultants for any companies mentioned.