Comparison of optical coherence tomography models OCT1 and Stratus OCT for macular retinal thickness measurement

Comparison of optical coherence tomography models OCT1 and Stratus OCT for macular retinal thickness measurement
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  SCIENTIFIC REPORT Comparison of optical coherence tomography modelsOCT1 and Stratus OCT for macular retinal thicknessmeasurement   V Pierre-Kahn, R Tadayoni, B Haouchine, P Massin, A Gaudric ............................................................................................................................... Br J Ophthalmol   2005; 89 :1581–1585. doi: 10.1136/bjo.2005.069815  Aims:  To compare the values measured for retinal macular thickness with the first and last generations of the opticalcoherence tomograph (OCT1 and Stratus OCT, Zeiss,Humphrey Division). Methods:  This was a cohort study. 59 eyes were examined:17 had a normal macula and 42 had a diabetic macular oedema. In each eye, mean retinal thickness (RT) wasmeasured automatically in the nine macular Early Treatment Diabetic Retinopathy Study areas and at the foveal centre,using OCT1 and Stratus OCT. The paired mean RT values for each area and the type and proportion of artefacts werecompared. Results:  Of the 590 automatic measurements, 505 had noartefact, either with OCT1 or Stratus OCT. The meandifference between the OCT1 and Stratus OCT measure-ments was 25 (SD 26.2)  m m (p , 0.0001). With Stratus OCT,RT values were significantly higher, by 8.1% (7.8%), than with OCT1. Artefacts were only observed in cases of diabeticmacular oedema and were significantly more frequent withOCT1 than Stratus OCT (10.5% versus 4.4, p , 0.0001). Conclusion:  The macular retinal thickness values measured with Stratus OCT were significantly higher than thosemeasured with OCT1. Stratus OCT has the advantage of producing fewer artefacts than OCT1 in pathological cases. T he introduction of optical coherence tomography (OCT)in clinical practice in 1996 made it possible to measureretinal thickness (RT) routinely. The first version, OCT1(Carl Zeiss Meditec, Humphrey Division, Dublin CA. USA),had an axial resolution of about 15  m m. In 2002, the newStratus OCT came into use and reduced axial resolution by upto 7  m m. The repeatability and reliability of RT measurements with OCT1 have been demonstrated in several studies. 1 2 It was therefore of interest to test the new Stratus OCTinstrument, to see if it gave the same retinal thickness valuesas the OCT1, and whether artefacts were less frequent withStratus OCT than with OCT1. We then compared the macularretinal thickness values obtained with OCT1 and Stratus OCTfor normal and diabetic patients. PATIENTS AND METHODS Patients Retinal macular thickness was measured in 59 eyes of 37patients using OCT1 and Stratus OCT. Seventeen eyes of 13healthy emmetropic volunteers (six men and seven women,mean age 37.5 (SD 8.6) years) as well as 42 eyes of 24diabetic patients with untreated focal or diffuse macularoedema (15 men and nine women, mean age 56.5 (11.2) years) were included in the study. The volunteers gaveinformed consent to participate. For diabetic patients, OCTexamination was part of the usual assessment of the fundus.Diabetic patients with the following characteristics wereexcluded: opaque media, treated macular oedema, or loss of central fixation. Optical coherence tomography  OCT1 and Stratus OCT were performed on each eye on thesame day and by the same examiners (BH or VPK). Radialline scan protocol was used for both OCT instruments to mapthe macular areas. With OCT1, RT was measured automati-cally using the latest software version (A6.2, Carl ZeissMeditec, Dublin CA, USA), which enabled us to obtain, in1 second, cross sectional tomographic images 6 mm long(radial B-scan), comprising 100 axial measurements (A-scans). Axial resolution was about 13  m m. The retinal map was calculated by integrating the results of the six radialscans (fig 1A). Mean macular RT was displayed for the nineEarly Treatment Diabetic Retinopathy Study (ETDRS)-typeareas, 3 including a central 1000  m m disc and inner and outerrings of 3000  m m and 6000  m m, respectively. Each ring wasdivided into four quadrants (fig 1B). Average RT wascalculated automatically for each of the nine quadrants(A1–A9). Central foveal RT was also calculated at the point of intersection of the six radial lines. A paired comparison wasperformed of the two sets of 590 values.With Stratus OCT, the latest software version (2.0, CarlZeiss Meditec, Dublin, CA, USA) was used for RT measure-ments. Tomographic images 6 mm long were obtained in1.2 seconds and integrated 512 A-scans with consequenthigher longitudinal resolutions than those of OCT1. Axialresolution was also better, at about 7  m m. Mean macular RT was displayed on the same radial spoke pattern grid as withOCT1 (fig 2).  Artefacts The proportion of artefacts on radial B-scans was alsocompared for OCT 1 and Stratus OCT. Artefacts were definedas the discordance between the automatically detectedanterior and posterior retinal boundaries and the boundariesdetected by the examiner. Such artefacts can either increaseor reduce the RT values measured manually in the quadrantsconcerned (fig 3). Data analysis Comparison of OCT1 and Stratus OCT macular RT estima-tions was based on quadrant by quadrant paired comparison.The coefficient of correlation and the mean differencebetween RT measurements with OCT1 and Stratus OCT werecalculated. Results are expressed as means (SD). Fisher’s  Abbreviations:  IS, inner segment; OCT, optical coherence tomography;OS, outer segment; RPE, retinal pigment epithelium; RT, retinal thickness1581 www.bjophthalmol.com  exact test was used for the correlation studies, and pairedStudent t test, for the comparative studies. Statistical analysis was performed with Apple iMac StatView software (SASInstitute Inc, Version 5.0). All analyses were performed onthe two complete sets of 590 values and also on the sets of  values that remained after the exclusion of quadrants withartefactual measurements. The type and proportion of theartefacts obtained with OCT1 and Stratus OCT were alsocompared ( x 2 test). Retinal outer boundary reference line Careful examination of the outer boundary reference line,automatically aligned by the mapping software of OCT,showed that, in Stratus OCT, it was constantly located on theinner segment/outer segment (IS/OS) photoreceptor linerather than on the retinal pigment epithelium (RPE) line.This misalignment was not noted in OCT1, which did notdistinguish between these two lines because of its lower axialresolution. To evaluate and quantify this error moreaccurately, we measured the mean distance between the IS/ OS and the RPE lines, at the foveal centre of healthy maculasmapped with Stratus OCT, using the ‘‘Scan Profile’’ protocolof the software. The outer boundary of the retina wascharacterised by two peaks, the outermost being the signal of the RPE. Callipers easily allowed measurement of thedistance between the two peaks. This measurement was 2 mm AB 01 A1A2A4A8A6A5A9A3A7 Figure 1  Retinal mapping protocol.(A) Radial spoke pattern of six 6 mmlong scans centred on the subject’sfixation point in a healthy left eye, (B)ETDRS-type areas of the macula in a left eye. A1, central zone 1000  m m indiameter; A2–A5, superior, temporal,inferior and nasal areas of a disc3000  m m in diameter; and A6–A9 of adisc 6000  m m in diameter. Figure 2  (Top) 6 mm long verticalscan line on (left) OCT1 and (right)Stratus OCT, with the automatically positioned anterior and posterior retinalboundaries (white lines). (Bottom) Right eye of a diabetic patient with macular oedema, as seen with (left) OCT1 and(right) Stratus OCT macular mapping with colour coded map and numericalRT. Figure 3  Stratus OCT scan line 6 mmlong centred on the fovea, at an angleof 30 ˚ from the horizontal line, in adiabetic right eye with focal temporalmacular oedema. Two intraretinalhyper-reflective exudates (white arrows)generate (left) a lower automatically positioned inner retinal boundary with(right) two subsequent artefacts (black arrows) in the retinal thickness mapping(quadrants A7 and A9).1582 Pierre-Kahn, Tadayoni, Haouchine, et al www.bjophthalmol.com  repeated at the five contiguous pixels on each side. Theaverage of the 11 measurements performed was consideredas the distance between the two lines at the foveal centre. RESULTS  Artefacts Of the two sets of 590 paired values, 85 exhibited artefacts with either OCT1 or Stratus OCT or both. With OCT1,artefacts were present in 19 eyes, and with Stratus OCT, innine. They were only observed in eyes with diabetic macularoedema (DMO). Of the two sets of 420 values obtained withOCT1 and Stratus OCT in the 42 eyes with DMO, 62 (10.5%)and 26 (4.4%), respectively, exhibited artefacts (p , 0.0001). Artefacts resulting in lower RT values than with manualmeasurement were observed in 44/62 quadrants (71%) in theOCT1 scans, and in 16/26 quadrants (61.5%) in the StratusOCT scans. These underestimations were mainly the result of the presence of intraretinal hard exudates, or by erroneousanalysis of cystoid macular oedema (fig 4A, B, and C). There were a few overestimated artefacts, the result of erroneouspositioning of the inner boundary of the retina. The detachedposterior hyaloid was, for example, misinterpreted as theinternal limiting membrane (figu 4D and E). Retinal thickness values On the basis of the two complete series of 590 values—that is,including the artefacts, the mean RT in the 6 mm diameterarea was 322.3 (126.3)  m m with OCT1 and 358.9 (156)  m m with Stratus OCT. There was good agreement between thetwo instruments ( r  =0.899, p , 0.0001). However, the RT values obtained with Stratus OCT were always higher thanthose obtained with OCT1, by 11.6% (23.1%) (meandifference: 36.6 (70.1)  m m p , 0.0001); the greater the retinalthickness, the greater the difference.On the basis of the two series of 505 values—that is,excluding the artefacts, the mean RT in the 6 mm diameterarea, was 308.8 (116.8)  m m with OCT1 and 333.8 (126.7)  m m with Stratus OCT. The agreement between the OCT1 andStratus OCT measurements was better than when theartefactual quadrants were included ( r  =0.98, p , 0.0001).However, these measurements still remained unequal(fig 5A). The mean difference between Stratus OCT and Figure 4  Examples of artefactsresulting in lower RT values due to (A)the presence of hard exudates withOCT1, (B) erroneous analyses of acystoid macular oedema with bothOCT1 and (C) Stratus OCT. Examplesof artefacts resulting in increased RT values due to misinterpretation of adetached posterior hyaloid as theinternal limiting membrane by both (D)OCT1, and (E) Stratus OCT. Table 1  Distribution of (1) the retinal thickness (RT) measured with OCT1 and with Stratus OCT, (2) the difference between theRT values measured with Stratus OCT and OCT1, and (3) the RT ratio (Stratus OCT – OCT1)/OCT1 based on the 505 non-artefactual RT measurements for 42 eyes with diabetic macular oedema and 17 eyes with healthy maculae OCT 1( m m)Stratus OCT( m m) Stratus OCT  2  OCT 1 ( m m) (Stratus OCT  2  OCT 1)/OCT 1 Number of values 505 505 505 505Mean 308.8 333.8 25.0 0.08SD 116.8 126.7 26.2 0.078Error of mean 5.2 5.6 1.2 0.003Minima 124 126  2 68  2 0.192Maxima 761 824  + 216  + 0.557  Comparison of OCT1 and Stratus OCT for macular RT measurement 1583 www.bjophthalmol.com  OCT1 RT values was 25 (26.2)  m m (p , 0.0001). The RTmeasurements obtained with Stratus OCT always exceededthose obtained with OCT1, by 8.1% (7.8%) (fig 5B andtable 1).In the 17 healthy eyes, the mean RT in the central area1 mm in diameter, according to the mapping software, was178.4 (11.5)  m m with OCT1, and 191.4 (17.6)  m m with StratusOCT. These measurements were also correlated ( r  =0.76,p=0.0002). In this area, mean difference between StratusOCT and OCT1 RT measurements was 13 (11.8)  m m,(p=0.0004). Distance between the two outer hyper-reflective lineson stratus OCT The distance between these two outer lines, measured at thefoveal centre on Stratus OCT A-Scan as the mean value of 11contiguous measurements, was 46.6 (9)  m m. DISCUSSION  Axial resolution is twice as good with Stratus OCT as withOCT1 (about 7  m m with 1000 pixels for each A-scan versus13  m m with 500 pixels for OCT1). Longitudinal resolution isalso better with Stratus OCT (512 axial profiles pertomographic line, instead of 100 with OCT1). In the presentstudy, RT was therefore measured in each 6 mm diameterarea at a total of 600 points using OCT1 and 3072 pointsusing Stratus OCT. In both OCT instruments, computerimage processors measure RT from retinal tomograms as thedistance between the highly reflective inner and outerboundaries of the retina, which are located by a thresholdingalgorithm. 5 The mapping software of OCT1 has been shown to havegood reproducibility for RT measurements in healthysubjects, and in patients with DMO. 1 2 It also appeared tobe a sensitive tool for detecting early retinal thickening indiabetic patients. 6 7 We therefore compared the RT measure-ments obtained with OCT1 (software A6.2) and Stratus OCT(software 2.0) and showed that both give measurements thatare highly correlated but nevertheless slightly different. Wedo not know which OCT gives RT values closest to the in vivoreality, and can only compare the macular RT measured withboth instruments. Stratus OCT tended to overestimate the RTmeasured with OCT1, by 25 (26.2)  m m (p , 0.0001). As statedin Results, the greater the RT, the greater the differencebetween OCT1 and Stratus OCT values. The equation:RT(Stratus OCT)=1.064 RT(OCT1)  +  5.43 wasextrapolated from the regression graph ‘‘ RT(StratusOCT)=f(RT(OCT1))’’ (fig 5A) obtained with StatView soft- ware. The mean percentage by which Stratus OCT exceededOCT1 was 8.1% (7.8%) (fig 5B). Despite the wide rangeobserved for this ratio (–19% to  + 56%), the standard error of the mean was only 0.3%—that is, in 95% of the RT measures,Stratus OCT exceeded OCT1 by 7.5% to 8.7%. This means thatby adding 8% to our OCT1 values, we can estimate the StratusOCT values with an error of less than 0.6% in 95% of cases.Conversely, by reducing the Stratus OCT values by 7%, theOCT1 values can be estimated with an error of less than 0.6%in 95% of cases.Because of its better axial resolution, Stratus OCT displaystwo different outer hyper-reflective lines. The innermost isgenerated by the IS/OS junction, as shown by Drexler  et al 4  with an ultra high resolution OCT prototype, and theoutermost, by the RPE. Stratus OCT takes the IS/OS line asthe outer boundary of the retina, thus underestimating RT.The difference between the RT measurements obtained withthe two OCT instruments would have been greater if theouter boundary line of the Stratus OCT had been correctlylocated at the RPE level.The significantly lower artefact rate with Stratus OCTthan with OCT1 for DMO (4.4%  v  10.5%) may be the resultof the better definition of A-scans with the formerinstrument. Artefact locations were also different. Most of the artefacts encountered in OCT1 were not seen in StratusOCT and vice versa. Maculopathies with hard exudatesare the most likely to generate artefacts. However, even inthese eyes, artefacts were fewer with Stratus OCT than withOCT 1.In conclusion, retinal thickness measured with StratusOCT (version 2.0) was significantly greater than with OCT1(version A6.2). Therefore, extrapolation of retinal thicknessmeasurements from OCT1 to Stratus OCT should take intoaccount a correcting value. This value would be even higher,by up to 46  m m, if the outer reference line for macularthickness measurement were correctly located on the RPEand not on the IS/OS line. Although Stratus OCT has theadvantage of being more accurate and producing fewerartefacts than OCT1 in pathological cases, the retinalthickness values provided by its mapping software shouldbe carefully reappraised.  Authors’ affiliations .....................  V Pierre-Kahn, R Tadayoni, B Haouchine, P Massin, A Gaudric, Department of Ophthalmology, Hospital Lariboisiere, AssistancePublique-Hoˆpitaux de Paris, Universite´ Paris 7, Paris, France 900700800600400500300200100800 OCT 1 ( µ m)A r   = 0.98 r   = 1    O   C   T   3   (      µ   m   ) 100700600500400300200–0.1–0.3800 OCT 1 ( µ m)B    (   O   C   T   3 –   O   C   T   1   )   /   O   C   T   1 100–0.2700600500400300200 Figure 5  Non-artefactual measurements of macular retinal thicknessobtained with OCT1 compared to those obtained with Stratus OCT. (A)Graph showing the correlation between RT measurements with OCT1and Stratus OCT (solid line: observed correlation  r  =0.98; broken line:equal correlation  r  =1). (B) The values for macular retinal thicknessmeasured with Stratus OCT were higher than those measured withOCT1. The excess ranged from 7.5% to 8.7% (mean 8.1) in 95% of themeasures.1584 Pierre-Kahn, Tadayoni, Haouchine, et al www.bjophthalmol.com  Correspondence to: Ramin Tadayoni, MD, Service d’Ophtalmologie,Hoˆpital Lariboisie`re 2, rue Ambroise Pare´, 75475 Paris Cedex 10,France; ramin.tadayoni@lrb.ap-hop-paris.fr  Accepted for publication 1 August 2005 REFERENCES 1  Massin P , Vicaut E, Haouchine B,  et al.  Reproducibility of retinal thicknessmeasurements in healthy and diabetic subjects using optical coherencetomography.  Arch Ophthalmol   2001; 119 :1135–42.2  Baumann M , Gentile R, Liebmann J,  et al.  Reproducibility of retinal thicknessmeasurements in normal eyes using optical coherence tomography. Ophthalmic Surg Lasers  1998; 29 :280–85.3  Early Treatment Diabetic Retinopathy Study Research Group .Photocoagulation for diabetic macular edema. ETDRS report number 1.  ArchOphthalmol   1985; 103 :1796–806.4  Drexler W  , Sattmann H, Hermann B,  et al.  Enhanced visualization of macular pathology with the use of ultrahigh-resolution optical coherence tomography.  Arch Ophthalmol   2003; 121 :695–706.5  Hee MR , Puliafito CA, Duker JS,  et al.  Topography of diabeticmacular edema with optical coherence tomography.  Ophthalmology  1998; 105 :360–70.6  Massin , Erginay A, Haouchine B,  et al.  Retinal thickness in healthy anddiabetic subjects measured using optical coherence tomography mappingsoftware.  Eur J Ophthalmol   2002; 12 :102–8.7   Otani T , Kishi Sh, Maruyama Y. Patterns of diabetic macular edema with optical coherence tomography.  Am J Ophthalmol  1999; 127  :688–93.  Video reports To view the video reports in full visit our website www.bjophthalmol.com and click on the link to the video reports. Video Suite: Subconjunctival dirofilariasis N  Removal of Dirofilarial worm from the subconjunctival space.  D Malik, S Alexander  N  Subconjunctival Dirofilariasis.  G Singh, K Myint, P Sathyain, S Mon, R Manikandan,B Dhillon N  Magnet-assisted pars plana vitrectomy for giant metallic intraocular foreign body. R Jorge, RA Costa, JC Castro, RC Siqueira  N  Suture of a Subluxated Posterior Chamber Lens within the Capsular Bag.  LE Ferna´ndez de Castro, KD Solomon N  Ocular Onchocerciasis: Anterior Chamber Microfilariae.  WJ Flynn, HD Dillon N  Zero Phaco Microincision Cataract Surgey: The Hacc Technique.  K Vaitheeswaran,S Gars, R Grover, M Nadar, S Sharma  N  The presenting features of multiple sclerosis.  VJM Barrett, J Walker, JS Elton N  Removal of INTACS: Stepped surgical complexity demonstrated with three cases. L Ilari, J C McAlister, D S Gartry  N  The Nuclear Slide: A novel approach for unleashing the potential of the hydrodissection wave.  A Naseri  N  Giant pleomorphic adenoma of the lacrimal gland: pre- and post-operative function.  A Jain, V I Nehru, U N Saikia, C E E Reddy  N  Limbal-sparing lamellar keratoplasty.  S L Watson, S Rauz, J Dart  Comparison of OCT1 and Stratus OCT for macular RT measurement 1585 www.bjophthalmol.com
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