optical coherence tomography ( oct ) , since its introduction in the late 90s , has become an indispensible imaging technique for evaluation and management of retinal and optic nerve disorders . as a non - invasive test ,
enhanced depth imaging ( edi)-oct enables cross sectional imaging of the retina with resolution approaching histologic sections . current spectral domain ( sd)-oct devices are empowered by software capable of measuring retinal thickness and constructing retinal maps ; these features are particularly useful in evaluating and comparing retinal structures in pathologic conditions such as macular edema and vitreoretinal interface disorders .
the choroid is a vascular compartment which provides oxygen and nourishment to the outer retina ; it seems to play a pathophysiologic role in many disorders affecting the retina , such as age - related macular degeneration ( amd ) , central serous chorioretinopathy ( csc ) , polypoidal choroidal vasculopathy and vogt- koyanagi - harada ( vkh ) disease .
light scatter by the retinal pigment epithelium ( rpe ) and choroidal vasculature degrades the quality of images obtained from the choroid in conventional oct methods.1 nevertheless , in vivo cross - sectional visualization of deeper choroidal layers is now achievable with the recent introduction of edi - oct.2,3
we obtained edi - oct images in eyes with dry and wet type amd and compared them to that of normal eyes ( fig .
1 ) . the novelty of our approach is to provide choroidal mapping and choroidal volume measurement in retinochoroidal disorders . to obtain the choroidal image
, we positioned the combined heidelberg spectralis device ( spectralis hra+oct , heidelberg engineering , heidelberg , germany ) sufficiently close to the eye , obtaining an inverted image of the fundus ; an alternative method was employing the automatic edi mode of the apparatus . by applying these methods
, the choroid approximates the zero - delay line , positioning the most sharply focused portion of the oct scan at the level of the choroid .
we selected a high resolution 19 line raster scan protocol to encompass the macula within a 2020 area ( fig .
the automatically plotted boundary lines of the retina were adjusted to choroidal boundaries ; internal limiting membrane ( ilm ) to rpe / choroid junction , and rpe to choroid/ sclera junction ) .
the resultant images were viewed and measured with the heidelberg eye explorer software ( version 5.3 ) to produce a color coded topographic map of choroidal thickness and volume ( fig .
the subfoveal area had the largest choroidal thickness ( ct ) in normal eyes ( 248.9350.92 m ; mean subject age , 65.81 years ; sample size , 32 ) .
mean total choroidal volume ( cv ) in the central 3.45 mm macular zone was 2.32 l in the normal group .
, subfoveal ct was found to decrease 17.39 m per each decade of age in normal subjects .
mean ct and cv measurements within the etdrs layout profile showed gradual nasal thinning of the choroid .
these observations are similar to the results of previous edi - oct studies in normal eyes which have revealed highest ct at the fovea decreasing on both temporal and nasal sides , but more so nasally ( fig .
we observed that mean subfoveal ct in wet amd eyes was greater than that in dry amd eyes ( 233.1372.95 m versus 217.8274.52 m ) despite similar mean age in both groups .
this pattern of difference was demonstrated at all measured locations across the macula , but was only statistically significant 1 mm temporal to the fovea .
similarly , measured cv values in wet amd eyes were higher than dry amd eyes ; the differences were statistically significant at all locations except nasal sectors .
mean total cv in the central 3.45 mm macular zone in dry amd eyes was 1.8 l , the corresponding value was 2.32 l in wet amd eyes which is still larger than normal eyes . in another subset of patients
, we employed the 31 raster line pattern to generate a choroidal map in csc .
subfoveal choroidal thickness in these subjects was 45479  which is significantly larger than normal .
comparing the area of increased ct on choroidal maps with the area of hyper - permeability on indocyanine green angiography , we demonstrated that in 91% of patients the hyper - permeability region resides within the area of increased ct on choroidal maps ( fig .
ocular and systemic disorders associated with vascular changes can affect the choroid which itself contains rich vascular networks .
previous studies employing sd - oct for choroidal evaluation measured ct at limited points.2 - 8 however , choroidal mapping can provide topographic and volumetric information in the entire macular area and in each subfield as defined by the etdrs layout pattern .
these measurements may more accurately exhibit choroidal structure . in a study including 80 eyes of 40 healthy volunteers , shin et al used sd - oct applying a high resolution 6 radial scan protocol to obtain choroidal images.9 each radial line averaged 8 b - scan images .
the authors failed to reconstruct maps in 16.2 % of cases because of obscure choroidal boundaries limiting accurate segmentation .
we were able to create topographic maps in all cases even in wet amd eyes in which posterior shadowing of retinal lesions made the segmentation process somehow difficult in some cases . because each section was composed of an average of 100 scans , image clarity was enhanced .
the eye - tracking technology , which is a specific feature of heidelberg devices , also improved image quality . moreover
, a 6-radial scan protocol can not represent the entire macular area and encounters a higher probability of interpolation errors .
we used the 19 line raster protocol , a denser scan in the macula which is more representative and valid for choroidal measurements , however , this required more effort and time for the segmentation process .
it seems that choroidal vascular changes play certain roles in the development of chorioretinal disorders .
figure 4 demonstrates increased ct in an eye with csc , which is evident by the presence of hot colors in the choroidal map .
quantification of choroidal thickness and volume by edi - oct can help clinicians evaluate the efficacy of therapeutic intervention for pathologic features of the choroid such as choroidal hemangiomas ( fig .
5).10 normal reference values for ct and cv can be established by various sd - oct devices .
this normative database can be used to analyze choroidal changes due to various pathologic states .
we detected an increase in ct and cv in wet amd eyes in comparison to dry amd eyes , which approximated values in our normal control group .
subfoveal ct has been reported to be increased in csc and vkh disease , but reduced in high myopic chorioretinal atrophy and idiopathic macular hole.11 - 16 the importance of choroidal measurements may emphasize the necessity of creating automated segmentation algorithms similar to the retinal map analysis protocols for faster and more precise evaluation of choroidal structure . in conclusion ,
choroidal mapping and quantitative analysis of the choroid seem to be a valuable method for evaluating chorioretinal disorders and monitoring the effect of therapeutic interventions .
further investigations are warranted to better establish the use of topographic ct and cv mapping in various chorioretinal disorders and to provide a normative database .
 there are a limited number of non - invasive imaging techniques available for assessing the choroid , a structure that may be affected by a variety of retinal disorders or become primarily involved in conditions such as polypoidal choroidal vasculopathy and choroidal tumors . 
 the introduction of enhanced depth imaging optical coherence tomography ( edi - oct ) has provided the advantage of in vivo cross - sectional imaging of the choroid , similar to the retina , with standard commercially available spectral - domain oct machines . in this article , we review this imaging technique and introduce choroidal mapping as a novel approach for obtaining accurate topographic and volumetric information on the choroid in normal and diseased states . 