Indian Journal of Ophthalmology

: 2011  |  Volume : 59  |  Issue : 2  |  Page : 175--179

Spectral domain optical coherence tomography

Rajesh Sinha, Sumit Khanduja, Neha Khanduja, Prafulla K Maharana, Satpal Garg 
 Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India

Correspondence Address:
Rajesh Sinha
S-7, R. P. Centre, AIIMS, New Delhi

How to cite this article:
Sinha R, Khanduja S, Khanduja N, Maharana PK, Garg S. Spectral domain optical coherence tomography.Indian J Ophthalmol 2011;59:175-179

How to cite this URL:
Sinha R, Khanduja S, Khanduja N, Maharana PK, Garg S. Spectral domain optical coherence tomography. Indian J Ophthalmol [serial online] 2011 [cited 2020 Sep 30 ];59:175-179
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Full Text

Over the last 20 years, imaging of the eye has significantly grown in importance for the diagnosis of ocular diseases. Out of a variety of techniques such as scanning laser polarimetry (SLP) and confocal scanning laser ophthalmoscopy (SLO), optical coherence tomography (OCT) has emerged at the forefront of ocular imaging because of the wide variety of information it can provide, its high resolution, and the complex data it is able to gather. The Spectral Domain OCT (SDOCT) introduced recently has come as an advantage over the previous technology of Time Domain OCT (TDOCT) in being able to handle and analyze complex data for three-dimensional image reconstruction, better resolution which should result in better delineation of the pathology of interest, better retinal coverage and increased imaging depth. In addition, motion artifacts can be eliminated as the result of the speed with which the data are acquired. In the present article we have made an attempt to analyze the information regarding SDOCT published in current literature.

 Tool for in vivo Histopathology and Disease Evolution

Mojana et al. (Retina 2010; Dec 13. Epub ahead of print) performed long-term SDOCT/ SLO imaging of neuroretina and retinal pigment epithelium (RPE) after sub-threshold infrared laser treatment of drusen and observed that sub-threshold diode laser treatment causes long-term disruption of the retinal photoreceptor layer, and the concept that sub-threshold laser treatment can achieve a selected RPE effect without damage to rods and cones may be flawed.

Zweifel et al. (Retina 2010 Oct 20. Epub ahead of print) performed SDOCT of vitelliform detachment in patients with reticular pseudodrusen and found vitelliform material to be hyperautofluorescent and located in the subretinal space between the inner segment/ outer segment (IS/OS) junction and the RPE.

Sikorski et al. (Ophthalmology 2010; 10 (8) Epub ahead of print) reported large, confluent drusen to be accompanied by subretinal spaces that appeared to be filled with fluid which was limited to the depression between the adjacent drusen and concluded that the appearance of subretinal fluid (SRF) between coalescent drusen may not be a reliable marker for the presence of choroidal neovascular (CNV) membrane.

Querques et al. (Retina 2010 Dec 9. Epub ahead of print) performed integrated imaging of reticular pseudodrusen in Age-related Macular Degeneration (ARMD) using infrared reflectance, fundus autofluorescence, fluorescein angiography (FA) and eye-tracked Spectralis high-resolution SDOCT and noted that they are central lipofuscin-like retinal deposits localized above the RPE, giving insights into other possible aspects of age-related retinal changes.

Spaide et al. (Retina 2010;30(9):1441-54) studied various drusen subtypes and saw cuticular drusen appear as numerous, uniform, round, yellow-white punctate accumulations under the RPE; soft drusen as larger, yellow-white dome-shaped mounds of deposit under the RPE; subretinal drusenoid deposits as polymorphous light-gray interconnected accumulations above the RPE.

Schmitz-Valckenberg et al. (Ophthalmology 2010;117(6):1169-76) studied reticular pseudodrusen using SDOCT and noted that they corresponded to changes at a level anterior to the RPE and Bruch's membrane complex up to the interface of inner and outer photoreceptor segment layer.

Schuman et al. (Ophthalmology 2009; 116(3):488-96) reported photoreceptor layer thinning over drusen in eyes with ARMD imaged in vivo with SDOCT and also noted that hyper-reflective foci overlying drusen are likely to represent progression of diseased RPE cell migration into the retina and possible photoreceptor degeneration or glial scar formation.

Cohen et al. (Am J Ophthalmol 2010; 150(8):211-17) reported retinal pseudocysts in age-related geographic atrophy, and postulated to correspond to Muller cell degeneration. They concluded that their presence should not be considered as a manifestation of neovascular ARMD that requires prompt treatment.

Horii et al. (Am J Ophthalmol 2010; 150(6):840-8) observed OCT characteristics of microaneurysms in diabetic retinopathy as delineated capsular structure, and hyper-reflective spots. The location of microaneurysms, and microaneurysms with the ring sign were positively correlated with cystoid spaces.

Kang et al. (Graefes Arch Clin Exp Ophthalmol. 2010; 248(12):1705-11) performed SDOCT following photocoagulation treatment in diabetic retinopathy and found alterations of retinal thickness at the location of the laser burns which mainly occurred within the retinal layers that extend from the outer plexiform layer (OPL) to the outer highly reflective layer (HRL).

Agoumi et al. (Ophthalmology 2010; 23(6). Epub ahead of print) studied laminar and prelaminar tissue displacement due to intraocular pressure (IOP) elevation in glaucoma patients and healthy controls and found that in glaucoma patients and controls, the anterior laminar surface is noncompliant to acute IOP elevation.

Lima et al. (Retina 2010; 30(4):542-7) reported intraretinal crystalline deposits in neovascular ARMD localized to the outer nuclear layer (ONL) and OPL often after treatment with a variety of different modalities. The potential etiologies of these deposits include residual lipid material from choroidal neovascularization leakage and degenerated Muller cell elements.

Querques et al. (Retina 2010; 30(2):222-34) compared angiographic features of retinal-choroidal anastomosis (RCA) by confocal SLO and SDOCT and noted an early neovascularization (a discrete focal hyperfluorescence) arising from the choroid that initially simply erodes the basement membrane/RPE (erosion sign; Phase 1) and later breaks the basement membrane/RPE (flap sign), infiltrating first into the outer retina forming an early RCA (Phase 2, a typical hot spot without a serosanguineous pigment epithelial detachment [PED]) and later into the inner retina (kissing sign) forming an established RCA (Phase 3, a typical hot spot with a serosanguineous PED).

Brar et al. (Retina 2010; 30(9):1262-6) evaluated vitreoretinal interface abnormalities in healed cytomegalovirus (CMV) retinitis and found the presence of epiretinal membranes, vitreoretinal gliosis, and traction that may help explain the higher incidence of retinal elevation, retinal breaks, and retinal detachment in these eyes.

Manjunath et al. (Retina 2010; Nov 22. Epub ahead of print) evaluated posterior lattice degeneration by SDOCT. Four characteristic changes of the retina and vitreous were seen in 13 eyes with lattice degeneration, which included anterior/posterior U-shaped vitreous traction, retinal breaks, focal retinal thinning, and vitreous membrane formation.

Nomura et al. (Ophthalmology 2010; Nov 20. Epub ahead of print) studied vitreomacular interface in typical exudative ARMD and Polypoidal Choroidal Vasculopathy (PCV) and noted that in eyes with ARMD, the frequency of complete Postenior vitneous Detachment (PVD) was significantly lower than in the controls, however the frequency of vitreomacular traction (VMT) tended to be higher in eyes with ARMD. This association was not evident in PCV.

Chang et al. (Am J Ophthalmol 2008; 146(7):121-7) correlated ultrastructural and SDOCT findings in VMT syndrome and found that adhesion between the vitreous and fovea in VMT was accompanied by fibrocellular proliferation along the exposed surfaces of the inner retina and the posterior surface of the vitreous which accounts for the prominent OCT signal seen along the posterior surface of the vitreous in these cases.

Michalewski et al. (Graefes Arch Clin Exp Ophthalmol.2010; 12(8). Epub ahead of print) using SDOCT proved that macular pseudohole may progress to lamellar macular hole without any vitreous traction and progressive contraction of epiretinal membrane (ERM) may be a cause of both being an advanced stage of the same non-full-thickness macular disorder.

Takahashi et al. (Ophthalmology2010;117(4):806-10) reported foveal anatomic changes in a progressive Stage-1 macular hole and found that the cone-shaped structure was pulled from the outer retina by anteroposterior vitreous traction causing a progressively enlarging pseudocyst in some cases of Stage-1 macular holes.

Murakami et al. (Ophthalmology 2010; 23(7)) compared foveal cystoid spaces on SDOCT and microaneursyms in perifoveal capillary network (PCN) and foveal avascular zone (FAZ) in FA images and found that eyes with cystoid spaces at the foveal center delineated by OCT had more mass in the PCN and larger FAZ in FA images.

Brar et al. (Ophthalmology 2010; Jul 23) evaluated the correlation between morphologic features on SDOCT and angiographic leakage patterns in macular edema and found a significantly higher incidence of cyst formation on SDOCT than FA. Diffuse edema is associated with thickening or distortion of the retinal layers without cyst formation in most cases and cystoid leakage on FA is always associated with cystic changes on SDOCT.

Van Dijk et al. (Invest Ophthalmol Vis Sci. 2010; 51(7):3660-5) demonstrated ganglion cell layer (GCL) thinning in the pericentral area and corresponding loss of retinal nerve fibre layer (RNFL) thickness in the peripheral macula in patients of Type-1 diabetes with no or minimal diabetic retinopathy (DR) compared with control subjects.

Imamura et al. (Retina 2009; 29(10):1469-73) reported very thick choroid in patients with central serous retinopathy (CSR) providing additional evidence that CSR may be caused by increased hydrostatic pressure in the choroid.

Fujiwara et al. (Retina 2010; 30(9):1206-16) reported fundus auto-fluorescence abnormalities in acute zonal occult outer retinopathy (AZOOR) as distinct patterns of RPE involvement, which may be progressive. Thinning of photoreceptor cell layer with loss of the outer segments and abnormal inner retinal lamination as detected in SDOCT in the context of a normal choroid are commonly found in AZOOR.

Vasconcelos et al. (Retina 2010; 30(1):33-41) reported retinal pigment epithelial changes in chronic Vogt-Koyanagi-Harada (VKH) disease and delineated RPE/outer retinal changes in patients with chronic VKH, which were consistent with previous histopathology reports.

Hangai et al. (Arch Ophthalmol 2009; 127(10):1307-13) reported SDOCT features of multiple evanescent white dot syndrome as moderately reflective focal lesions within the outer photoreceptor layer, where the inner and outer segment junction was disrupted, that corresponded to hypofluorescent areas in the late phase of indocyanine green (ICG) angiogaphy.

Puche et al. (Br J Ophthalmol 2010; 94(9):1190-6) using SDOCT hypothesized that in adult onset foveomacular vitelliform dystrophy early changes involve the layer between RPE and the IS/OS interface; first with vitelliform material accumulation beneath the sensory retina, and then with IS/OS alterations, pigment migration towards inner layers and fluid accumulation along with RPE alterations such as hypertrophy or sub-RPE deposits.

Zweifel et al. (Retina 2010 Oct 20.Epub ahead of print) reported that SRF in patients with pseudoxanthoma elasticum detected on SDOCT is not always indicative of active leakage from underlying CNV and can be resistant to antivascular endothelial growth factor injections. This fluid is associated with pattern dystrophy-like findings and may indicate abnormal RPE function.

Thiadens et al. (Invest Ophthalmol Vis Sci. 2010; 51(11):5952-7) studied progressive cone loss in patients of achromatopsia. The initial feature of cone cell decay was loss of inner and outer segments with disruption of the ciliary layer on OCT, which was observed as early as at eight years of age and progressed further with age.

Lee et al. (Invest Ophthalmol Vis Sci. 2010; 51(2):666-71) studied cross-sectional anatomic configurations of peripapillary atrophy (PPA) and found that the ganglion cell layer and the inner and outer plexiform layers ended in a tapering fashion at the edge of the optic disc, whereas the RNFL continued into the optic cup. The external limiting membrane (ELM), IS/OS, and RPE/ Bruch's membrane complex were significantly more commonly absent before the optic disc edge within the PPA-beta compared with areas outside the PPA-beta.

Baumuller et al. (Ophthalmology 2010; 117(11):2162-8) reported hyper-reflective spots in the outer retina of macular telangiectasia Type 2 patients which were detected in all stages of disease and were confined to the foveolar and parafoveolar region. It was suggested that this phenomenon represents an early sign of a neurodegenerative process.

 Tool for Determining Visual Prognosis

Maheshwary et al. (Am J Ophthalmol. 2010; 150(7):63-67) evaluated the association between percent disruption of the photoreceptor IS/OS junction and visual acuity (VA) in diabetic macular edema (DME) and concluded that there was a statistically significant correlation between the two.

Otani et al. (Retina 2010; 30(5):774-80) studied the correlation between VA and foveal micro-structural changes in DME and reported that the integrity of the ELM and IS/OS of the photoreceptors was more strongly correlated with best-corrected visual acuity (BCVA) when compared with central subfield thickness.

Oishi et al. (Am J Ophthalmol. 2010; 150(7):27-32) studied the significance of ELM status for VA in ARMD and found a strong correlation between ELM status and VA (r = -0.75, P < .001), and that was higher than that of the IS/OS interface (r = -0.69, P < .001).

Hayashi et al. (Am J Ophthalmol. 2009; 148(1):83-9) reported that eyes with a continuous or discontinuous IS/OS line beneath the fovea had better final VA than did eyes without an IS/OS line in patients of neovascular ARMD.

Inoue et al. (Am J Ophthalmol. 2010; 150(12):834-9) reported that the IS/OS junction can recover in eyes with preoperative intact IS/OS junction as assessed over a follow-up period of one year after surgery for idiopathic ERM. They observed that the preoperative integrity of the IS/OS junction was an important prognostic factor for better postoperative BCVA after ERM surgery.

Sayegh et al. (Ophthalmology 2010; 117(10):2010-7) found that SDOCT was a valuable method for evaluating retinal changes after surgery for VMT. Reconstitution of neurosensory layers was identified as the most relevant parameter for visual improvement compared to central retinal thickness (CRT), retinal volume, presence of retinal surface folds, and foveal contour.

Oster et al. (Retina. 2010; 30(5):713-8) reported that disruption of the photoreceptor IS/OS junction is a statistically significant predictor of poor VA among patients with ERM and is most useful when combined with CRT measurement.

Watanabe et al. (Ophthalmology 2009; 116(9):1788-93) reported that metamorphopsia induced by ERM may be related to the edema of the inner nuclear layer (INL) detected with SDOCT. The classification of ERM based on INL thickness is a potentially useful indicator for surgery.

Shin et al. (Acta Ophthalmol. 2010; 14(12) Epub ahead of print) reported the association between the integrity of the foveal photoreceptor layer and visual outcome in retinal vein occlusion. They concluded that better VA and the smaller length of disrupted IS/OS on SDOCT at initial visit are indicators of better visual outcome in patients with retinal vein occlusion.

Oh et al. (Invest Ophthalmol Vis Sci. 2010;51(3):1651-8) reported that the postoperative area of the IS/OS defect, when directly measured, correlates more strongly with BCVA than do linear-based measurements, perhaps because of the irregular shape of the IS/OS defect after surgery.

Inoue et al. (Graefes Arch Clin Exp Ophthalmol. 2009; 247(3):325-30) used SDOCT for quantitatively measuring IS/OS junction defects and attributed postoperative IS/OS junction to play an important role in visual recovery after macular hole surgery.

 Choroidal Imaging and SDOCT

Rahman (Invest Ophthalmol Vis Sci. 2010; Nov 18.Epub ahead of print) reported that a change of more than 32 μm is likely to exceed inter-observer variability in SDOCT in patients of chorioretinal pathology.

Yeoh (Graefes Arch Clin Exp Ophthalmol. 2010; 248(12):1719-28) evaluated patients with Stargardt macular dystrophy, macular dystrophies, Best disease, Bifocal chorioretinal atrophy, Bietti crystalline retinal dystrophy and Choroideraemia. Ten patients showed no choroidal thinning, five had focal mild to moderate choroidal thinning, three had focal severe choroidal thinning, and two patients had diffuse severe choroidal thinning with no association between choroidal thinning and VA.

Manjunath et al. (Am J Ophthalmol. 2010; 150(3):325-329) reported mean subfoveal choroidal thickness (CT) to be 272 μm (SD, ± 81μm) with Cirrus HD-OCT. The thickness across the macula demonstrated a thin choroid nasally, thickest subfoveally, and again thinner temporally, and a trend towards decreasing choroidal thickness with age.

Imamura et al. (Retina2009; 29(11):1469-73) reported thick choroid in patients of CSR providing additional evidence that CSR may be caused by increased hydrostatic pressure in choroid.

Fujiwara et al. (Am J Ophthalmol. 2009; 148(3):445-50) reported that in highly myopic eyes mean subfoveal CT was 93.2μm (±62.5μm) and was correlated negatively with age (P= .006), refractive error (P< .001), and history of choroidal neovascularization (P= .013), and subfoveal CT decreased by 12.7μm for each decade of life and by 8.7μm for each diopter of myopia. They concluded that choroidal atrophy affects older individuals in whom posterior pole abnormalities develop that may mimic and also be associated with findings typical of ARMD, and patients with age-related choroidal atrophy may be at higher risk for glaucoma.

Margolis et al. (Am J Ophthalmol. 2009;147(5):811-5) reported that choroid was thickest underneath the fovea (287±76 μm) and choroidal thickness decreased rapidly in the nasal direction and averaged 145±57 μm at 3 mm nasal to the fovea.

 SDOCT as a Guide to Monitor and Evaluate Treatment Response

Golbaz et al. (Invest Ophthalmol Vis Sci. 2010;4(11).Epub ahead of print) used manual segmentation on SDOCT to assess intraretinal, subretinal and sub-RPE compartments and their response to different treatment regimens of anti-vascular endothelial growth factor (anti-VEGFs) and found that in contrast to the sub-RPE compartment, intraretinal and subretinal fluid accumulation demonstrated an immediate response to ranibizumab therapy.

Ishihara et al. (Ophthalmology 2009; 9(9):1799-807) visualized a membranous structure on SDOCT in cases of VKH which was continuous with the junction of the photoreceptor IS/OS and hypothesized that the membranous form of the outer segment is bound with the action of inflammatory products, such as fibrin, and this membranous structure changes to a granular structure as steroid therapy "dissolves" the fibrin.

 Glaucoma Diagnosis and SDOCT

Mwanza et al. (Ophthalmology 2010; Oct 27. Epub ahead of print) reported that Cirrus HD-OCT ONH parameters are able to discriminate between normal eyes and eyes with glaucoma or even mild glaucoma. There is no difference in the ability of ONH parameters and RNFL thickness measurement, as measured with Cirrus OCT, to distinguish between normal and glaucomatous eyes.

Shoji et al. (Invest Ophthalmol Vis Sci. 2010 Nov 4. Epub ahead of print) reported that ganglion cell complex thickness offered the best parameter for the clinical diagnosis of glaucoma in patients with high myopia and concomitant pre-perimetric glaucoma.

Rao et al. (Invest Ophthalmol Vis Sci. 2010; 51(12):6401-7) reported that diagnostic accuracies of RTVue scanning protocols for glaucoma were significantly influenced by disease severity. Sensitivity of rim area increased in large optic discs at the expense of specificity.

Kotera et al. (Invest Ophthalmol Vis Sci. 2010; Nov 18. Epub ahead of print) reported that macular inner retinal layer thickness was less in eyes with suspected and pre-perimetric glaucoma compared to healthy eyes but the mean total retinal and macular nerve fiber layer thicknesses were not affected.

Rao et al. (Ophthalmology 2010; 117(9):1692-9) reported that RTVue RNFL and inner retinal macular thickness measurements had good ability to detect eyes with glaucomatous visual field loss and performed significantly better than ONH parameters.

Seong et al. (Invest Ophthalmol Vis Sci. 2010; 51(3):1446-52) reported that macular RNFL thickness showed a strong correlation with peripapillary retinal nerve fiber layer thickness in patients with normal tension glaucoma (NTG) at an early stage.

 Detecting Occult Macular Pathologies

Chavala et al. (Ophthalmology 2009; 116(12):2448-56) reported preretinal structures (ranging from 409 to 2700 μm in width and 212 to 440 μm in height), retinoschisis, and retinal detachment in the posterior pole of patients with advanced retinopathy of prematurity (ROP) which were not identified on conventional examination or imaging by expert pediatric ophthalmologists.

Muni et al. (Retina 2010; 30(4):S45-50) found vitreoretinal abnormalities not detected on clinical examination, including, for example, multilayered retinoschisis in patients of shaken-baby syndrome.

Fujimoto et al. (Am J Ophthalmol. 2010; 150(12):863-70) evaluated features associated with myopic macular retinoschisis and found that ILM detachment in the superior and/or inferior peripheral macula was associated with a foveal retinal detachment. The retinoschisis in the outer retina was accompanied by splitting in the IPL in 50% and a detachment of ILM in 50% in the superior and/or inferior peripheral macula.

Koleva-Georgieva et al. (Graefes Arch Clin Exp Ophthalmol. 2009; 247(11):1461-9) studied cases of serous macular detachment (SMD) and found SDOCT particularly useful in diagnosing sub-clinical SMD in eyes with DME. It disclosed the presence and strength of macular traction either by partially detached posterior hyaloid or by ERM.

 Advantage of SDOCT over TDOCT

Kim et al. (Arch Ophthalmol. 2010; 128(9):1121-8) reported that deviation map from Cirrus OCT was more sensitive in detecting RNFL defects in patients with open angle glaucoma (OAG) than the clock-hour and quadrant maps derived from cross-sectional peripapillary RNFL measurements by Stratus OCT.

Lee et al. (Invest Ophthalmol Vis Sci. 2010; 51(12):6424-30) reported that, in mild-to-moderate glaucoma, SDOCT offers an improved structure-function correlation compared with TD-OCT, when applied to the detection of inferior and superior RNFL defects.

Keane et al. (Am J Ophthalmol. 2009; 148(8):242-8) compared clinically relevant findings from high-speed Fourier-domain (FD) and conventional TDOCT and found that the average sensitivity for detection of all features in this study was 94% for FDOCT and 60% for TDOCT. FDOCT detected features that were not visible on conventional OCT scans in 78% (39/50) of cases. FDOCT was more sensitive than TDOCT for the detection of multiple findings, including diffuse intraretinal edema (87% vs. 60.9%), SRF (100% vs. 46.2%), large PED (100% vs. 81%), and subretinal tissue (100% vs. 61.5%).

Sayanagi et al. (Ophthalmology 2009; 116(5):947-55) reported that SDOCT devices were superior in their ability to delineate sub-RPE, intraretinal, subretinal fluid and intraretinal cysts compared with TDOCT.

Khurana et al. (Ophthalmology 2010; 117(7):1376-80) reported that SDOCT seems more likely than TDOCT to detect abnormalities when fluorescein leakage from CNV is detected after anti-VEGF therapy.

 Reproducibility and Repeatability of SDOCT

Serbecic et al. (Br J Ophthalmol.2010; Nov 19. Epub ahead of print) reported high reproducibility of measurements of the nerve fiber layer with the new Heidelberg SDOCT and concluded that this may enable clinicians to detect discrete levels of retinal thickness change over time.

 Limitations and Shortcomings of SDOCT

The power to image better, farther and deeper tissues comes with a few disadvantages. These three-dimensional high-resolution systems are more expensive than time-domain systems by anywhere from 25-150%. The quality of the images may decrease in the presence of a patient with small pupils or media opacities. With numerous companies manufacturing various SDOCT instruments, conducting multicenter clinical trials may be problematic. The machines differ not only in hardware but also in segmentation algorithms and analyses software. This makes comparison of values between the systems difficult.

Giani et al. (Am J Ophthalmol 2010; 150(12):815-24) evaluated the reproducibility of retinal thickness measurements on normal and pathologic eyes by different SDOCT instruments and suggested that retinal thickness measurements obtained with various OCT devices are different beyond clinical practice tolerance, according to the Bland-Altman analysis.

Bengtsson et al. (Acta Ophthalmol 2010; Oct 14. Epub ahead of print) reported that specificity and positive predictive values were generally better with Stratus than Cirrus, and sensitivity was generally better with Cirrus when used as a tool for glaucoma screening.

Seibold et al. (Acta Ophthalmol. 2010; Oct 14. Epub ahead of print) reported that RNFL measurements taken with Spectralis, RTVue, and Cirrus differ significantly between instruments and should not be used interchangeably.

Grover et al. (Invest Ophthalmol Vis Sci. 2010; 51(5):2644-7) reported an increased measurement in retinal thickness of approximately 65 to 70 microns, as measured by SDOCT compared with Stratus OCT, and is consistent with the extent of axial retinal thickness measured by the two instruments.

Han et al. (Am J Ophthalmol.2009;147(5):847-5) reported that retinal thickness measurements obtained by SDOCT are consistently greater than those obtained by TDOCT in healthy and diseased eyes.

Schlanitz et al. (Invest Ophthalmol Vis Sci.2010;51(12):6715-21) evaluated performance of drusen detection by SDOCT which was proved in visualizing drusen-related RPE disease but the available automated segmentation algorithms showed distinct limitations to reliable identification of the amount of drusen, particularly smaller drusen, and the actual size.

Forooghian et al. (Retina 2010; 30(1):63-70) studied the relationship between photoreceptor outer segment length and VA in DME and found that it correlated better than CMT though had the shortcoming of being less repeatable.

Watson et al. (J Neuroophthalmol. 2010; Dec 3. Epub ahead of print) compared RNFL and CMT measurements among five different OCT instruments in patients with Multiple Sclerosis and Optic Neuritis and found statistically significant difference among them due to differences in data acquisition and segmentation algorithm software among OCT instruments.

Smretschnig et al. (Graefes Arch Clin Exp Ophthalmol. 2010; 248(12):1693-8) reported that there are significant differences in CRT values in patients with vascular PED measured by Cirrus versus Spectralis OCT due to different segmentation algorithms and a high error rate in automatically set threshold lines.

Sull et al. (Retina 2010; 30(2):235-45) compared retinal thickness measurements of three commercially available spectral domain OCT instruments Cirrus HD-OCT, RTVue-100, 3D OCT-1000 and noted that macular thickness measurements had high reproducibility across all instruments, with intraclass correlation coefficients values ranging from 92.6-97.3% for Cirrus Cube, 76.4-93.7% for RTVue MM5, 61.1-96.8% for MM6, 93.1-97.9% for 3D OCT-1000 radial, and 31.5-97.5% for 3D macular scans.

Mylonas et al. (Br J Ophthalmol.2009; 93(11):1453-60) compared retinal thickness measurements and segmentation performance of four different spectral and time domain OCT devices in neovascular ARMD and observed that SDOCT demonstrated the highest values of all OCT devices in central millimeter thickness (CMMT), and Topcon OCT raster scans showed the lowest values. Significant correlations could be found between the CMMT measurements of Cirrus and Spectralis OCT (r = 0.87). The analyses showed best segmentation for Cirrus and SDOCTs.

Smretschnig et al. (Graefes Arch Clin Exp Ophthalmol. 2010; 248(12):1693-8) compared Cirrus OCT and Spectralis OCT as regards to differences in segmentation in fibrovascular PED and noted that mean central retinal thickness (CRT) was 262.38 ± 133.18 μm (176-507) in Cirrus and 337.82 ± 137.75 μm (277-790) in Spectralis scans, mainly caused by different software approaches in positioning the posterior threshold line. There were failures in positioning the outer retinal boundary line in 50% of Cirrus scans and in 73.52% of Spectralis scans.