diabetic retinopathy ( dr ) is a common complication of diabetes mellitus ( dm ) and is a leading cause of blindness worldwide .
diabetic macular ischemia ( dmi ) is an important category of diabetic retinopathy ( dr ) [ 2 , 3 ] . during the imaging study of the normal macula ,
an important hallmark is the capillary - free region called foveal avascular zone ( faz ) .
it is recognized that the faz can enlarge and can become irregular in dr and seems to get larger as the stage of retinopathy advances [ 2 , 3 ] .
dmi is characterized by the occlusion and loss of the macular capillary network or capillary dropout [ 4 , 5 ] .
this condition results in upregulation of growth factors , which contributes to the development of diabetic macular edema ( dme ) , the most frequent sight - threatening disorder in individuals with dr .
dmi is an irreversible category of diabetic maculopathy , and its presence limits the potential benefits of treatments for dr [ 3 , 5 , 6 ] .
the early treatment of diabetic retinopathy study ( etdrs ) established dmi standards that were determined using fluorescein angiography ( fa ) . according to the etdrs report 11 , clinically , there is a correlation between dmi and poor prognosis that varies according to the severity of the macular ischemia .
the anatomy of the retina appears to be altered in dmi , with thinning of retinal nerve fibre layer ( rnfl ) and outer retina and thickening of outer choroid . in retinal microcirculation
, blood supply is divided mainly into superficial capillary plexus ( scp ) and deep capillary plexus ( dcp ) .
choroidal circulation appears to be the most important blood supply to the central macula , including photoreceptor inner segment ( is ) band , which is most likely the most important consumer of oxygen .
it is likely that the dcp is responsible for up to 15% of the blood supply to the photoreceptors , especially during dark adaptation [ 8 , 10 , 11 ] .
fa has been the gold standard imaging modality since it was introduced in 1961 [ 7 , 12 , 13 ] however , it requires venipuncture , and reports of anaphylaxis and death related to contrast injections , despite being rare , have been documented .
in addition , the technique is costly and time - consuming , requiring up to 10 minutes for framing acquisition [ 4 , 8 , 10 , 11 ] .
spectral domain ( sd ) oct has emerged as a potential alternative for detecting macular nonperfusion in diabetic patients , but results are contradictory [ 1 , 2 , 13 ] .
macular ischemia may disrupt the normal flow of nutrients to the outer retina , but photoreceptor status on sd - oct remains controversial . in diabetic patients , the disruption of photoreceptors in sd - oct can indicate a manifestation of underlying dcp nonperfusion in patients with a relatively healthy macula [ 1416 ] .
oct angiography ( octa ) has been used for 3d mapping at microcirculation level [ 9 , 1316 ] .
it allows detection of retinal and choroidal structures via motion contrast imaging and high speed scanning , which detect blood flow by analysing signal decorrelations between scans [ 7 , 11 , 14 , 15 ] .
both inner and outer retinal capillary plexuses are imaged in contradistinction in conventional angiography , which does not effectively image the outer plexus .
this study aimed to compare the use of fluorescein angiography and octa in the diagnosis and quantification of dmi when applying a split - spectrum amplitude - decorrelation angiography algorithm ( ssada )   to improve the detection of flow signals in angiography with the purpose of offering a suitable  sparing  contrast alternative using 3  mm  3  mm octa for clinical investigations of diabetic patients [ 8 , 1520 ] .
this was a retrospective cross - sectional comparative study analysis conducted at federal university of goias and approved by ethics committee of the same institution . signed informed consent was obtained from each subject prior to enrolment .
imaging data were collected from patients who underwent fa and octa on the same day in a tertiary referral retina center in the period between january 1 , 2015 , and july 30 , 2015 .
exclusion criteria included significant cataract ( without surgical indications at the time of examination ) , previous retinal arterial or venous occlusion , inherited macular dystrophy , posterior segment inflammation , and macular degeneration or scarring of any cause .
subjects that presented motion artifacts during octa or poor signal strength were also not included in this study .
all subjects underwent a comprehensive ophthalmologic examination , including best - corrected visual acuity ( bcva ) on etdrs charts , + 78  d noncontact lens slit - lamp fundoscopy , color fundus photography , and fa and oct angiography , on a single day .
color fundus photographs were obtained and fa was performed using a digital retinal camera ( topcon trc 50dx ; topcon , paramus , nj ) .
the central macular nonperfusion area , faz boundaries , and the maximum height and area were manually delineated .
the faz area was measured using a single fa photograph , such as the octa angiogram of the scp and then divided into 
( superior to 0.32  mm ) and  small  ( inferior to 0.32  mm ) groups .
group comprised both  moderate  and  severe  etdrs established grading groups [ 3 , 19 , 20 ] .
the quantification of macular nonperfusion in both fa and oct angiography captured images was completed using imagej software ( imagej , national institutes of health , bethesda , md ) .
octa instrument used was the rtvue xr avanti with angiovue software for octa ( optovue , inc . ,
fremont , ca ) , and imaging data were obtained using the split - spectrum amplitude - decorrelation angiography ( ssada )   software .
the algorithm was employed to improve the signal - to - noise ratio [ 8 , 1521 ] .
this instrument operates at ~840  nm wavelength , 70,000 a - scans per second , and a bandwidth of 50  nm .
the tissue resolution is 5  m axially and there is a 15  m beam width .
each b - scan consisted of 304 a - scans for a total of 2  304  304 a - scans per acquisition , with a total acquisition time of approximately 3 seconds .
the scanning area was captured in 3  mm  3  mm sections , and the acquired oct volumes were centered on the fovea [ 811 ] .
a 3  mm  3  mm octa image was used primarily because this size is the one that provides images with higher resolution in the used octa device [ 10 , 11 ] .
the ssada algorithm was used to generate a volumetric rendering of blood flow from the internal limiting membrane ( ilm ) to the choroid , and it allowed the direct visualization of normal and abnormal blood circulation [ 1821 ] . for the octa angiograms ,
it was used the automatic segmentation of the retinal layers at the level of the scp generated by the angiovue software in an orthogonal view . in order to correct for automated segmentation error and projection artifacts ,
the segmentation slab was manually adjusted , using corresponding structural oct b - scans as a guide for the placement of 2 segmentation lines : the inner located at 3  m beneath the internal limiting membrane and the outer boundary at 15  m beneath the inner plexiform layer .
this semiautomatic method permitted the readers to select images that pictured the largest extent of scp for subsequent quantitative analysis .
the dcp image was segmented with an inner boundary 15  m beneath the inner plexiform layer and an outer boundary at 70  m beneath the inner plexiform layer .
this section captured both layers of the outer capillary plexus , which sandwiched the inner nuclear layer .
all statistical analyses were performed using statistical package for social science 22.0 ( ibm spss ; ibm , armonk , ny ) .
fa and octa images were independently reviewed by two independent masked readers retina specialists ( jg and di ) who reached an agreement regarding the area ( mm ) of macular nonperfusion that was obtained using both methods according to etdrs report 11 .
intraclass correlation coefficient ( icc ) was used to estimate the agreement between individual measurements from both readers .
since the icc was consistently > 0.9 between the 2 readers , nonparametric wilcoxon signed - rank test was used to compare area measurements performed by 1 reader on fa and octa 3  mm  3  mm scans .
thirty - four eyes from 34 patients , including 20 ( 58.82% ) females and 14 ( 41.18% ) males , were enrolled and separated according macular status ( table 1 ) .
twenty - four eyes from 24 patients were placed in the group of patients with dmi , including 15 ( 62.5% ) females and 9 ( 37.5% ) males .
the mean ( sd ) age of the dmi population was 61.20  6.95 years .
the group without dmi comprised 10 patients , including 5 ( 50% ) females and 5 ( 50% ) males .
four eyes from 2 patients were excluded from the study due to a high quantity of motion artifacts .
patients with dmi presented a mean fa of 0.68  0.53  mm ( 95% ic , 0.460.90 ; p < 0.05 ) .
octa angiogram analysis demonstrated a mean of 0.58  0.35  mm ( 95% ic , 0.430.73 ; p < 0.05 ) .
the wilcoxon signed - rank test did not identify a significant difference between fa and octa in patients diagnosed with dmi ( p = 0.1374 ) .
patients without dmi presented a mean fa of 0.19  0.67  mm ( 95% ic , 0.140.24 ; p < 0.05 ) .
octa angiogram analysis demonstrated a mean of 0.20  0.79  mm ( 95% ic , 0.150.26 ; p < 0.05 ) .
the wilcoxon signed - rank test did not identify a significant difference between fa and octa in patients without dmi ( p = 0.9594 ) ( figure 1 ) .
the iccs for faz area measurements between 2 observers with respect to fa and octa were 0.96 ( ic : 0.360.71 ) and 0.92 ( ic : 0.350.79 ) , respectively , demonstrating the reproducibility and consistency of the methodology .
this result highlights the notion that the faz is most often clearly demarcated by a distinct foveal vascular ring and that its abnormalities , in addition to its capillary dropout , can be lucidly obtained from octa images .
etdrs research group connected the severity of macular nonperfusion to the potential for progression in dr .
in fact , in dr , the advanced deterioration of macular perfusion is the basis for macular ischemia , and developing a method to perceive perfusion maps may allow correlations between central ischemia and the different stages of dr .
however , it is an invasive method requiring venipuncture and contrast infusion ; it is a time - consuming test and provides only 2-dimensional images .
therefore , reports of anaphylaxis and death related to contrast injections have been documented , despite being rare .
octa is a noninvasive method , obtains highly detailed 3-dimensional images without requiring injection of a contrast dye , and allows faster acquisition of images ( figure 2 ) [ 9 , 10 , 1214 ] .
octa performed using a split - spectrum amplitude - decorrelation angiography ( ssada ) algorithm has already been shown to be useful for imaging microvascular changes in dr [ 811 , 14 , 20 ] .
cole et al . also observed macular nonperfusion in a diabetic patient in a 3  mm  3  mm octa that was centered on the fovea by applying a similar technology .
the 3  mm  3  mm octa central sections obtained using sd - oct allowed us to obtain a higher resolution over a small area ( figure 3 ) .
this area was sufficient for detecting central dmi , but it was not large enough to identify peripheral retinal nonperfusion .
high - resolution oct imaging allows measuring thickness of segmented retinal layers in angiographically apparent ischemic dr .
future octa devices improvements may provide clinicians the ability to obtain wider field images with better resolution . in the present study , statistical analysis
also did not indicate significant difference between area measurements obtained with fa and octa in patients diagnosed with dmi .
the same has occurred among patient without dmi regarding the measurement of normal faz area .
the icc for the zaf area between the 2 observers on fa and octa demonstrated the reproducibility and consistency of used methodology .
these results highlight the notion that faz is most frequently demarcated by a distinct foveal vascular ring and its abnormalities , in addition to its capillary dropout , can be lucidly obtained from octa imaging device .
first , all of the results were obtained during a single appointment , and there was no follow - up .
the present study demonstrates that fluorescein angiography and octa provide similar results when used to diagnose macular ischemia in diabetic patients . with further improvements
in summary , octa may provide images with higher details regarding macular status , becoming a novel imaging technique for the diagnosis of dmi , and may become an alternative to fa for this purpose .
the results also offer improved quantification of faz area in diabetic patients without dmi when compared to diabetic subjects with established macular ischemia .
 
 purpose . to compare fluorescein angiography ( fa ) and optical coherence tomography angiography ( octa ) images of foveal avascular zone ( faz ) in patients with diabetic retinopathy ( dr ) with and without diabetic macular ischemia ( dmi ) 
 . methods . the wilcoxon signed - rank test was used to compare area measurements and p values of < 0.05 were considered statistically significant . 
 fa and octa images were independently graded by 2 observers that reached agreement regarding quantitative dmi according established protocols . 
 the ischemic area was divided into  
 large  macular ischemia ( superior to 0.32  mm2 ) and  small  ( inferior to 0.32  mm2 ) groups . 
 quantitative analyses of the faz were performed using custom software . 
 results . 
 thirty - four eyes from 34 diabetic patients were enrolled . 
 subjects with dmi presented a mean area on fa and octa of 0.68  0.53  mm2 and 0.58  0.35  mm2 , respectively ( p = 0.1374 ) . 
 patients without dmi presented a mean area on fa and octa of 0.19  0.67  mm2 and 0.20  0.79  mm2 , respectively ( p = 0.9594 ) . 
 the icc for the faz measurements between the 2 observers on fa and octa was 0.96 and 0.92 , respectively . 
 conclusion . 
 octa represents a novel technique for the diagnosis of dmi and it may become an alternative to fa for this purpose . 