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Sunday / April 14.
HomemiequipmentIt’s not only about the image

It’s not only about the image

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Optical Coherence Tomography (OCT) technology provides a powerful non-invasive diagnostic tool for non-mydriatic cross-sectional imaging of the retina and anterior segment.

Recent advances in OCT technology have resulted in a generation leap with the advent of Fourier/Spectral Domain OCT technology, delivering significantly higher definition images and scanning speed over the previous time-domain technology.

Significant differences were found for the various ethnicities, each having less variability than the combined database and allowing for increased diagnostic accuracy.

Spectral/Fourier Domain OCT systems, such as the Optovue RTVue, employ an interferometer and spectrometer, simultaneously capturing all light echo time delays to capture an A-Scan comprising up to 2048 pixels.

Thousands of tangential A-Scans are acquired within a fraction of a second to reconstruct cross sectional B-Scan images and create 3D images of tissue at a microscopic level, without significant eye motion errors.

Unparalleled Clinical Care

OCT systems are unparalleled and routinely used in daily clinical care for clear observation, analysis and monitoring of macular disorders. Typical conditions where OCT scans are most beneficial include Vitreomacular Traction, Epiretinal Membranes, Macular Holes, Macular Pucker, RPE Tears and Detachments, Wet and Dry ARMD, Cystoid Macular Oedema, Central Serous Chorioretinopathy and Choroidal Neovascularisation.

The RTVue provides a wide variety of scan protocols for multiple ways to view and analyse the retinal layers, optic nerve, cornea and anterior segment.

Ultra high scan speed allows for multiple individual B-Scans to be acquired virtually simultaneously and viewed as an averaged composite image with noise reduction and enhanced image detail. All scan presentations are user selectable for fine grayscale or colour viewing.

Retinal analysis functions include layer segmentation, elevation maps, normative comparison, symmetry and progression.

The RTVue software utilises vessel tracing to register the primary macular mapping scan to the ‘en face’ 3D reference image generated by the 3D SLO scan, allowing for tracking of the same location over time, even in cases of fixation drift.

Glaucoma is a progressive optic neuropathy characterised by loss of Retinal Ganglion cells, thinning of the Retinal Nerve Fiber Layer (RNFL), thinning of the neuroretinal rim, and enlarged cupping.

The macular region contains over 50 per cent of all retinal ganglion cells and is an ideal region to detect early cell loss and changes over time for earliest detection and monitoring of Glaucoma.

The RTVue segments and directly measures the Ganglion Cell Complex (GCC), comprising of the Nerve Fiber Layer (Ganglion Cell Axons), Ganglion Cell Layer (Ganglion Cell Bodies) and the Inner-plexiform Layer (Ganglion Cell Dendrites).

GCC scan data is analysed, compared to an age-matched normative database, and displayed as a colour coded Thickness Map, percentage Deviation Map and Significance Map.

The RTVue’s nerve head map provides quantitative information and mapping of the Peripapillary RNFL, Cup and Neuroretinal Rim, compared to an age-adjusted normative database, and statistically classified as within normal limits, borderline, and outside normal limits.

The normal range of ganglion cells in a normal eye can be very large, varying from approximately 750,000 to 1.5 million, requiring substantial ganglion cell loss for many patients before the value falls outside the normal range.

Glaucoma Detection

To detect progression in glaucoma and determine the rate of change, measurements must be reproducible, accurately registered to each other, and statistical testing must differentiate true biological change from normal measurement variability.

The 5μ resolution scanning of the RTVue has been shown by several independent investigators to have excellent reproducibility.

Reproducibility of RNFL data and consistency of scan placement is achieved with the RTVue software aligning the large diameter ONH scan pattern concentrically around the centre of the disc.

A series of scans are automatically aligned and registered to each other based on matching blood vessel patterns. Vertical, horizontal and rotational alignment ensures the same location is compared over time for the most accurate change analysis possible.

Meticulous Statistic Image Mapping is employed to determine significant change and the rate of change.

A normative database provides a benchmark against which a scanned eye is compared and statistically flagged if the values fall outside the normal range.

The latest RTVue Version 4.0 software incorporates expanded ethnic specific databases. Data accumulated from fifteen global clinical sites, all following approved protocols, has been consolidated to provide the largest database of any OCT device. Data collected for glaucoma evaluation from an age range of 19-82 years include Cup Area, Rim Area, Optic Disc Size, Vert. Cup/Disc ratio, and RNFL thickness.

Significant differences were found for the various ethnicities, each having less variability than the combined database and allowing for increased diagnostic accuracy.

The analysed data revealed several correlations:

• Signal strength reduces with age due to reduced media clarity. However it was found that the reduced signal strength has only a weak effect on measured RNFL, signifying accuracy of measured RNFL is maintained with reduction of scan strength in older individuals.

• Strong correlations were found for optic disc size and various optic disc structures, demonstrating the need to factor in optic disc size when statistically evaluating if a particular cup to disc ratio is within the normal range.

• As predicted, increased age strongly correlated with a reduction in RNFL thickness, and also correlated with increased cupping and cup/disc ratio, and decreased rim area. Inclusion of age is therefore an important factor for increased sensitivity and specificity.

The Cornea Anterior Module

The RTVue Cornea Anterior Module (CAM), consisting of a wide angle and a high magnification attachment lenses, plus scanning and analysis software, provides fine detailed corneal and anterior images.

High speed scanning provides real time on screen viewing. A series of frames, each acquired in 0.04 seconds and comprising of 1024 axial line scans, are captured essentially freezing out motion errors.

Automated composite frame averaging further improves image quality for clear distinguishing of the epithelium, front and back of Bowman’s layer, LASIK flaps, Descement’s layer and endothelium.

Fine detail in the anterior segment such as Schlemm’s canal, trabecular meshwork, aqueous collector channels and veins, and iris vessels can be visualised.

Visualisation and measurement functions of the anterior chamber angle are useful for evaluation of closed angle glaucoma.

The CAM pachymetry map with statistical parameters provides essential information to determine eccentric focal corneal thinning characteristic of even mild keratoconus and the sub clinical form, forme fruste keratoconus.

It is important to rule out these conditions and to calculate the expected residual stromal bed thickness in preoperative evaluation of prospective LASIK patients to preclude high-risk corneas and development of post LASIK ectasia.

The CAM corneal power function provides accurate measurements of both anterior and posterior corneal curvature and power for determination of IOL power with eyes having undergone laser vision correction.

Clinical benefits of the CAM module also include observation of IOLs, Phakic IOL Vault measurement, Shunt and Implant imaging, evaluation of contact lens fit and tear film.

Implementation of modern OCT technology allows for enhanced patient care by providing significant improvements for earlier diagnosis, a better understanding of pathogenesis, monitoring of disease and response to therapy. Furthermore, OCT systems can now deliver multi-functional uses offering much greater versatility and value.

Robin Lanesman is a qualified optometrist. He has worked in ophthalmic instrumentation/equipment as a product specialist and sales consultant for 14 years and is currently with BOC Instruments. He is a specialist in Retinal and OCT imaging.
References:
1. Chen R, Duker J, Srinivasan V, Fujimoto J. Speed and Resolution Improve in Newest OCT. Review of Ophthalmology July 2007.
2. Tano Y. Using Fourier Domain OCT to Improve Clinical Outcomes. Uncovering the Dynamic Capabilities of Fourier Domain OCT. Ophthalmology Management March 2008.
3. Chopra V. Improving Glaucoma Diagnosis. Uncovering the Dynamic Capabilities of Fourier Domain OCT. Ophthalmology Management March 2008.
4. Duker J, Ho J, Castro L, Gomi F, Lumbroso B, Rispoli M, Tano Y. RTVue Fourier-Domain Optical Coherence Tomography Primer Series: Vol. I Retina. 2008.
5. Huang D, Binder P, Huang J, Li Y, Liu X, Mahdaviani S, Memarzadeh F, Ramos S, Salaroli C, Tang M, Trokel S, Yiu S. RTVue Fourier-Domain Optical Coherence Tomography Primer Series: Vol. II Cornea and Anterior Segment. 2008.
6. Lewandowski J. A Fresh Take on Optical Coherence Tomography. Glaucoma Today Jan/Feb 2008.
7. Huang D. Improving Cornea Care with Fourier Domain OCT. Uncovering the Dynamic Capabilities of Fourier Domain OCT. Ophthalmology Management. March 2008.
8. Sinai M, PhD. The Normative Database for the RTVue. Optovue white paper Dec 2008.
9. Sinai M, PhD. RNFL Progression Analysis with the RTVue. Optovue white paper Dec 2008.
10. Sinai M, PhD. Direct Ganglion Cell Assessment with the RTVue: The Ganglion Cell Complex Analysis. Optovue white paper Dec 2008.
11. Nordan Lee. Form Fruste Ecatasia. Cataract and Refractive Surgery Today Sept 07.