optical coherence tomography ( oct ) , which provides b - mode retinal images , has become essential for diagnosing retinal disease and glaucoma [ 19 ] since the technology was first reported by huang et al . in 1991 . oct also provides quantitative retinal thickness data , which are useful to monitor retinal changes in clinical and research settings . time - domain oct ( td - oct ) includes an interferometer that measures the echo delay time of light that is reflected and backscattered from various retinal microstructures . the echo time delays of light beam reflected from the retinal microstructure are compared with the echo time delays of the same light beam reflected from a reference mirror at known distances . therefore , it takes a relatively long time to obtain a - scan and b - mode retinal images , and it is almost impossible to obtain a three - dimensional retinal image . in spectral - domain oct ( sd - oct ) , light beams returning from the sample and reference paths are combined at the detector , a spectrometer that resolves the interference signals throughout the depth of each a - scan without varying the length of the reference path . this is possible because the spectrometer resolves the relative amplitudes and phases of the spectral components backscattered from all depths of each a - scan simultaneously using the fourier transformation . this allows sd - oct to acquire retinal images about 50 times faster compared with td - oct . a few studies have compared the macular thicknesses of patients with diabetic macular edema ( dme ) obtained with td - oct and sd - oct [ 12 , 13 ] . we compared the retinal thickness measurements obtained with the two oct devices in subjects with dme to understand the differences in measurements between the two octs . we already reported the difference in mean retinal thickness between td - oct and sd - oct in normal eyes . to the best of our knowledge , the current study is the first to compare the macular thickness measurements from the two oct devices between normal subjects and patients with dme . the macular thickness was measured in 50 eyes of 29 patients with dme using td - oct ( stratus oct , carl zeiss meditec , dublin , ca , usa ) and sd - oct ( cirrus hd - oct , carl zeiss meditec , dublin , ca , usa ) to determine a correlation between the devices . the mean patient age was 68.0  9.0 years ( range , 4585 years ) . with td - oct , the macular thickness data were obtained using the fast macular thickness scan pattern ( figure 1 ) . this scan pattern acquires six linear b - scans in a continuous , automated sequence . the scans are centered at the fovea in a radial pattern and separated by 30-degree increments . the axial resolution of td - oct is less than 10  m according to the manufacturer 's data . the macular cube 200  200 scan pattern in sd - oct generates a data cube through a 6  mm square grid by acquiring a series of 200 horizontal scan lines , each comprised of 200 a - scans ( figure 2 ) with an axial resolution of 5  m . the average retinal thickness at the central 1  mm area was analyzed with both oct machines . about 128 points were measured within a 1  mm circle with td - oct and about 872 points with sd - oct . three operators measured the macular thickness of each subject using the two oct instruments on the same day . apparent segmentation failures in td - oct and sd - oct were excluded from this study . the best - corrected visual acuities ( bcvas ) were obtained in decimal va and converted to logarithm of the minimum angle of resolution ( log  mar ) for statistical analysis . the mean macular thicknesses in patients with dme measured with td - oct was 401.5  117.8  m ( mean + sd ; range , 203712  m ) and with sd - oct 446.2  123.5  m ; range , 245775  m ) . the mean macular thickness with sd - oct was 44.7  m thicker than that with td - oct , which was a significant difference ( p < 0.001 , paired t - test ) . the macular thickness measured with td - oct correlated well with that measured with sd - oct ( pearson 's product moment correlation ,  r = 0.977 , td - oct traced the ilm and is / os line automatically , and the central retinal thickness was 543  m ( figure 4(a ) ) . sd - oct traced the ilm and rpe automatically , and the central retinal thickness was 567  m ( figure 4(b ) ) , which was 24  m thicker than the td - oct measurement . compared with the five - line mode of the sd - oct ( figure 4(c ) ) , the true is / os line of the td - oct may be the pale line that is defined by the arrowheads ( figure 4(a ) ) . if td - oct automatically traced the true is / os line , the central retinal thickness would be thinner than 543  m . we assumed that the retinal thickness using td - oct was thicker than the actual retinal thickness , and as a result , the difference in the retinal thicknesses between the two machines was small . this is one reason that the macular thickness in normal subjects is about 15  m thicker than in patients with dme . the relationship between the bcva and the retinal thickness also was evaluated , and a significant correlation was found between the two devices ( pearson 's product moment correlation , td - oct , r = 0.34 ; p < 0.05 ; sd - oct , r = 0.32 ; p < 0.05 ) ( figure 5 ) . in the current study , the average retinal thickness measured with td - oct was 401.5  117.8  m and with sd - oct 446.2  123.5  m , a difference of about 45  m . the difference in the average retinal thicknesses between the devices appears to have resulted from the different definitions of the retinal thicknesses . td - oct defines retinal thickness as the distance from the surface of the inner limiting membrane ( ilm ) to the boundary between the inner and outer segments of the photoreceptors ( is / os ) . sd - oct defines the retinal thickness as the distance from the surface of the ilm to the surface of the retinal pigment epithelium ( rpe ) . lammer et al . compared the retinal thickness of dme between td - oct and sd - oct and reported mean difference was 58.5  m . forooghian et al . also reported that the mean difference of the two machines was 53.0  m . these results were almost same as our result , and they suggested that different algorisms make the different results . attention should be paid to the retinal thickness when data from different machines are compared , although the retinal thickness in patients with dme measured by sd - oct correlated strongly with that measured by td - oct . we reported previously that when using sd - oct in normal subjects , the macula was 60  m thicker than when measured with td - oct . in the current study , when using sd - oct in subjects with dme , the macula was 45  m thicker than when measured with td - oct . the difference in the macular thickness between td - oct and sd - oct in normal subjects was about 15  m thicker than that in patients with dme . with td - oct , the fast macular thickness scan pattern acquires six linear b - scans in a continuous automated sequence . the scans are centered at the fovea in a radial pattern and separated by 30-degree increments . with sd - oct , the macular cube 200  200 scan pattern generates a data cube through a 6  mm square grid by acquiring a series of 200 horizontal scan lines , the average retinal thickness at the central 1  mm circle was about 128 points with td - oct and about 872 points with sd - oct . compared with td - oct , the higher reliability of sd - oct is based on the uniformity and the larger number of scan points . the scan pattern and the scan accuracy may explain the difference in the macular thickness between normal subjects and dme . using td - oct , the is / os line disappeared in some areas or was not clearly detected in some cases . the measurement line was not traced on is / os and traced on the rpe line . this may result in a thicker measurement than the actual thickness on td - oct . as in previous studies , we also found a correlation between the macular thickness and bcva in subjects with dme [ 16 , 17 ] . the diabetic retinopathy clinical research network reported that the relationship between the va and central retinal thickness measured by oct was linear . however , koleva - georgieva and sivkova reported no correlation between the bcva and retinal thickness , which may have resulted from the small number of subjects and that six of nine eyes had macular ischemia . the low va could be due to the serous macular detachment with large cystoid spaces and the presence of macular ischemia . in conclusion , the mean retinal thickness in patients with dme measured with sd - oct was about 45  m thicker than that with td - oct .
purpose . to compare the macular thicknesses in diabetic macular edema ( dme ) measured with spectral - domain optical coherence tomography ( sd - oct ) and time - domain ( td ) oct . 
 patients and methods . 
 the average macular thicknesses of 50 eyes of 29 patients with dme were measured using sd - oct and td - oct . 
 results . 
 the mean macular thicknesses measured with td - oct and sd - oct were 401.5  117.8  m ( mean  sd ) and 446.2  123.5  m , respectively . 
 the macular thicknesses measured with the two devices were well correlated ( pearson 's product moment correlation , r = 0.977 , p < 0.001 ) . a significant correlation was found between the best - corrected visual acuity and the retinal thickness measured by td - oct and sd - oct ( pearson 's product moment correlation , td - oct , r = 0.34 ; p < 0.05 ; sd - oct , r = 0.32 ; p < 0.05 ) . 
 discussion . the mean macular thickness measured with sd - oct was about 45  m thicker than that measured with td - oct . 
 attention should be paid when comparing data obtained using different oct machines .