this cross - sectional , observational study was done from december 2007 to august 2008 after approval from the institutional review board and ethics committee of the kangbuk samsung hospital in seoul , korea . this prospective study was performed after approval from the subjects were recruited from the outpatient glaucoma service of the kangbuk samsung hospital . the detailed ocular examination included visual acuity , intraocular pressure measurement using the goldman applanation tonometer , and slit lamp and fundus examinations . a digital fundus camera was used to take red - free fundus photographs of the retina and stereo disc photographs ( visucam ; carl zeiss meditec , dublin , ca , usa ) . in addition , other basic ocular exams were performed without the participants being under mydriasis . the study classified participants as normal if they met the extensive criteria of having a highest documented iop of 20 mmhg bilaterally , normal bilateral examination findings - including fundus examination under mydriasis , normal bilateral visual field results defined as mean deviation within 95% of normal limits , and glaucoma hemifield test results within 99% of normal limits . included among subjects with glaucoma were those with diagnoses of open angle glaucoma defined as optic disc abnormalities , and rnfl defects with or without visual field loss . glaucomatous optic disc abnormality was defined as having one of several features , including neuroretinal rim thinning , notching , or excavation . other inclusion criteria for both normal and glaucomatous subjects were , 1 ) the subject was 18 years or older , 2 ) the subject had best - corrected visual acuity of 20 / 40 or better , 3 ) the subject had no history of ocular or neurological disease or surgery that might produce test results or vision changes confounding recognition of test results due solely to glaucoma , 4 ) the subject had no history of amblyopia , 5 ) the subject had the mental and physical capacity to perform the tests , and 6 ) willingness to participate as a subject in the study . twice in the same day the oct scans were performed on each subject by a single technician through a dilated pupil with fast rnfl thickness protocols using internal fixation . an operator applied artificial tears ( hyalein ; santen pharmaceuticals , osaka , japan ) to each patient before scanning . a scan was successfully obtained if the fundus image was sufficiently visible to distinguish the optic disc and the scanning circle , and if there were no obvious movement artifacts with missing data at the acquired scan pattern . images with poor scan quality ( error messages and obvious segmentation errors ) , decentration , poor focus or low analysis confidence were excluded . for subjects who had both eyes scanned , one eye was randomly selected for analysis . in addition , cases having the same signal strength in repeated oct scans were selected for analysis . when the signal strength was different in repeated oct scans despite the scans being of good quality , we evaluated one additional oct scan and selected scans with the same signal strength for analysis . the rnfl thickness measurements from initial and repeat scans were compared and subsequently correlated with the differences in ss . mean rnfl thickness around the entire circumference , average thickness within the four quadrants ( temporal , superior , nasal , and inferior ) , and average thickness in each of 12 clock - hour sectors , where clock hours 1 to 5 represent the nasal clock hours , and clock hours 7 to 11 represent the temporal ones . left optic disc areas were considered to be the mirror images of right optic disc areas . in all tables , the measured areas of the nasal region were named clockwise from 12 to 6 . statistical analyses were performed using pasw statistics ver . 17.0 ( spss inc . , chicago , il , usa ) . parameters , including average rnfl thickness and mean clock hour rnfl thicknesses , were automatically generated in the analysis report . the superior clock hour was designated as 12 o'clock , and the others were assigned accordingly . the clock hours were assigned clockwise in the right eye and counterclockwise in the left eye . the four quadrants , clock hours , and average rnfl measurements obtained at different ss were compared with repeated - measures of anova . the false positive ratio was defined as values below the fifth percentile of rnfl thickness for normal eyes . the false positive ratio was analyzed to compare diagnostic ability according to varying ss . the value of p < 0.05 was considered statistically significant . twice in the same day , subjects were scanned with the stratus oct ( software ver . the oct scans were performed on each subject by a single technician through a dilated pupil with fast rnfl thickness protocols using internal fixation . an operator applied artificial tears ( hyalein ; santen pharmaceuticals , osaka , japan ) to each patient before scanning . a scan was successfully obtained if the fundus image was sufficiently visible to distinguish the optic disc and the scanning circle , and if there were no obvious movement artifacts with missing data at the acquired scan pattern . images with poor scan quality ( error messages and obvious segmentation errors ) , decentration , poor focus or low analysis confidence were excluded . for subjects who had both eyes scanned , one eye was randomly selected for analysis . in addition , cases having the same signal strength in repeated oct scans were selected for analysis . when the signal strength was different in repeated oct scans despite the scans being of good quality , we evaluated one additional oct scan and selected scans with the same signal strength for analysis . the rnfl thickness measurements from initial and repeat scans were compared and subsequently correlated with the differences in ss . mean rnfl thickness around the entire circumference , average thickness within the four quadrants ( temporal , superior , nasal , and inferior ) , and average thickness in each of 12 clock - hour sectors , where clock hours 1 to 5 represent the nasal clock hours , and clock hours 7 to 11 represent the temporal ones . left optic disc areas were considered to be the mirror images of right optic disc areas . in all tables , the measured areas of the nasal region were named clockwise from 12 to 6 . statistical analyses were performed using pasw statistics ver . 17.0 ( spss inc . , chicago , il , usa ) . parameters , including average rnfl thickness and mean clock hour rnfl thicknesses , were automatically generated in the analysis report . the superior clock hour was designated as 12 o'clock , and the others were assigned accordingly . the clock hours were assigned clockwise in the right eye and counterclockwise in the left eye . the four quadrants , clock hours , and average rnfl measurements obtained at different ss were compared with repeated - measures of anova . the false positive ratio was defined as values below the fifth percentile of rnfl thickness for normal eyes . the intraclass correlation coefficient and variability ( standard deviation ) of all included subjects are shown in table 2 . 1 . when ss was 3 , the variability of rnfl thickness significantly increased , as opposed to when signal strength was 4 or greater . correlations between ss and variability in average rnfl thickness , rnfl thickness of the four quadrants , and the 12 o'clock hour rnfl thickness are shown in fig . significant correlations were observed between ss and variability in average rnfl thickness , rnfl thickness of the four quadrants , and the 12 o'clock hour rnfl thickness . significant negative correlations were observed between variability in rnfl thickness and signal strength , which indicates that a decrease in the ss value is associated with increased variability in rnfl thickness . furthermore , when ss was 3 , the variability in rnfl thickness significantly increased , as opposed to when ss was 4 or greater . dependent variables were the variability in average rnfl thickness , the rnfl thickness of four quadrants , and 12 o'clock hour rnfl thickness ( table 3 ) . ss was an independent predictor for variability in the average rnfl thickness , superior quadrant rnfl thickness , and inferior quadrant rnfl thickness . ss was also an independent predictor for variability in the 1 o'clock hour rnfl thickness and in the rnfl thicknesses from the hours of 5 to 12 o'clock . the false positive ratio was 28% when ss was 4 , and it was 19.1% when ss was 6 . when ss was 6 or greater , the false positive ratio was 16.4% . an important finding of this study is that when signal strength increased , variability decreased significantly ( fig . 2 ) . the method of test - retest is very good for investigating the reproducibility of a device because the method systematically maintains the same environment , same time , and same technician . the results show that when signal strength is 3 , the variability is too high for accuracy or reliability ( fig . when signal strength is 4 or greater , the variability decreases significantly . by the time that signal strength reaches 6 or greater the difference of variability of average rnfl thickness between signal strength 4 ( 4.94 m ) and signal strength 6 ( 4.41 m ) was 0.53 m . the variation between signal strength 4 and signal strength 7 ( 3.49 m ) was 1.45 m . test - retest reliability as low as 3.5 microns has been reported in normal patients . glaucoma is an optic neuropathy characterized by retinal ganglion cell death and corresponding nerve fiber layer loss . this significant structural damage in the rnfl may precede visual field loss . to estimate the extent of the damage and evaluate the rate of disease progression , it is important to understand the relationship between structural injury and functional loss . one of the most useful technologies for the structural evaluation of glaucomatous damage is oct . in comparison to other diagnostic technologies , but in some cases , glaucomatous rnfl defects identified in stratus oct images are classified as normal . several studies have postulated that signal strength is related to the value of rnfl thickness as determined by oct imaging if ss impacts diagnostic accuracy , false positives may increase in cases where low ss is imperative due to uncorrectable patient factors . in light of this point , this research compares the false positive ratio according to varying ss . oct imaging lacks reliable diagnostic ability in low ss 4 . because the detection of progression is essential for the diagnosis of glaucoma , however , image reproducibility is a very important factor for patient care . at the low level of ss 4 , the incidence of glaucoma increases with age , and the disease occurs most frequently in patients over 40 years of age . ten percent of people over 80 years of age have glaucoma . the incidence of cataracts , vitreous opacity , and corneal opacity increase with age as well . many oct images of elderly glaucoma patients show low signal strength and low image accuracy . moreover , poor images are more frequently acquired from advanced glaucomatous eyes than from other groups . this is likely because rnfl is thinner and has lower internal reflectivity as a result of glaucomatous damage in the eyes of patients with advanced retinal disease . results are not reliable because of the low signal strength in these cases , however , even though oct images are measured with accurate centering . furthermore , there are no guidelines for interpretation when oct images show low signal intensity . some studies select images for analysis only when the signal strength is at least 8 , while other studies consider a signal strength of 5 to be of good image quality for analysis . signal strengths of 6 [ 21 - 23 ] and 7 [ 24 - 26 ] have also been used as the minimal inclusion criteria in various studies . although signal strength is a useful index of oct image quality , opinions vary on the level of signal strength required for good quality images . regardless , there is no recommendation on a minimal acceptance value of signal strength in poor quality scans . a study by wu et al . has suggested that , when measuring rnfl thickness with the stratus oct , it is important to aim for a signal strength of at least 7 . in the current study 's cases that show signal strengths of 3 or 4 , the current study used clinical techniques of mydriatics and artificial tears to improve the signal strength for analysis . regardless , among this patient population , it is often difficult to improve the quality of images with clinical techniques . therefore , the field of oct needs another way to solve the problem of poor imaging due to uncorrectable patient factors . accordingly , this study aims to determine the minimal acceptance level of signal strength for poor quality scans . even small differences can be significant , particularly in patients with moderate to advanced glaucomatous optic nerve damage and markedly reduced rnfl . in a scan with higher signal strength , light penetrates deeper into the retina , which increases the reflectivity of all layers and results in more retinal tissue having higher reflectivity than the quality threshold defined by oct . in cases where low signal strength is an unavoidable consequence of certain patient factors , the low signal strength compromises image quality specifically , scans with lower signal strengths tend to underestimate the thickness of the rnfl . this variability is especially pronounced in patients with advanced optic nerve damage , even in scans without explicit error messages . one study has shown rnfl thickness measurements that are more variable among glaucoma patients than normal subjects ( 13.8 microns ) . accordingly , it is important to obtain scans of equal and high signal strengths to facilitate the longitudinal follow - up for treatment of glaucoma . the results of the current study show that high ss ( above 6 ) is recommendable . however , when signal strength is low due to uncorrectable patient factors - despite clinical efforts to maximize signal strength - our results suggest that signal strength of 4 is acceptable to obtain reproducible scanning images among patients with ocular media opacities . our results offer a standard reference for analysis of oct results in patients with ocular media opacities when ss is below 6 despite the correction of all modifiable factors with clinical techniques . establishing a common low limit of acceptable signal strength may make our results more broadly applicable in clinical settings . regardless of whether or not our results are applicable to establishing new standards for spectral domain oct , because the stratus oct is used all over the world , the findings of this study are useful for diagnosis and follow - up treatment of glaucoma .