The choroid is a highly vascular tissue, necessitating in vivo
imaging to accurately determine its true structure and thickness. Until recently, information regarding choroidal thickness in normal eyes was based primarily on histological results, which do not necessary reflect the true measurements of this dynamic tissue. OCT has been well established as an accurate imaging study of known retinal pathology through good correlation between histology of animals and humans in vivo17
; however exact correlation between OCT and histology is limited since histological fixation produces artifacts due to processing, tissue deformation, and shrinkage.18
Recent studies by Spaide et al demonstrated the potential to image choroidal structure and thickness using new OCT imaging techniques with one commercially available spectral domain OCT instrument, the Heidelberg Spectralis. Already, this OCT imaging technique has revealed novel findings of choroidal thickness in known diseases such as central serous chorioretinopathy5
and myopic degeneration4
, and helped define new entities such as age-related choroidal atrophy.6
Since 2006, a variety of spectral domain OCT instruments have become commercially available, therefore determining if choroidal thickness measurements were possible using another one of the commonly used spectral domain OCT devices is of interest. This report confirms that another simple, reproducible method for choroidal measurement using the Zeiss Cirrus HD-OCT device is possible in the majority of eyes. Reliable measurements of choroidal thickness were obtainable in nearly three quarters of examined eyes (74%). The results of this study were very similar to those reported previously using the Heidelberg OCT device. The mean subfoveal choroidal thickness was found to be 272 μm (SD, +/- 81 μm), compared to 287 μm (SD, +/- 76 μm) found in the study of normal eyes by Spaide et al.3
As reported previously3
, similar variability of choroidal thickness across the macula in normal eyes was observed, with the choroid thinnest proximal to the disc. Additionally, a negative correlation between both area and age and thickness and age was noted, suggesting that progressive choroidal thinning occurs over time in normal eyes, which is a similar finding to previous reports. In addition, this study suggests that area measurements might be another value useful to track changes in choroidal thickness.
The most apparent limitation to this technique is the inability to measure choroidal thickness in all eyes due to an unclear posterior boundary of the choroid (choroid/sclera junction) in approximately ¼ of eyes (). Reasons for this may be a suboptimal number of averaged OCT B scans, the lack of eye tracking software, and the potential for eye movement during imaging with subsequent degradation of the image. Moreover, densely pigmented RPE and even mild cataracts, which were noted in 60% of dilated eyes that could not be measured, can attenuate light penetration, particularly at 800 nm wavelength, which further contributes to obscuration of the choroid/sclera boundary. The frequency at which normal choroidal thickness can be measured in this study and the previously mentioned study of normal choroidal thickness3
cannot be compared as data regarding the percentage of eyes meeting inclusion criteria which could not be successfully measured was not reported.
Cirrus HD-OCT 1 line raster scans demonstrating poor visualization of posterior choroid/sclera junction
In the future, segmentation software that could define choroidal thickness and volume might prove to be a valuable measure in tracking choroidal changes. Currently, both Cirrus and Heidelberg OCT devices are limited by the number of lines that can be scanned at one time due to the large numbers of scans that need to be processed or averaged. Therefore, two dimensional mapping of the choroid thickness across the fundus becomes difficult due to the large number of scans that are required, as well, as lengthy measurement times. Potential new software may also improve image inversion, image averaging, and signal-to-noise ratio, thus further improving visualization of choroidal structures. Studies have demonstrated that imaging at 1050 nm wavelength provides superior choroidal penetration13-15
; however, the present software enhanced the capabilities of an 800 nm based system to provide good visualization of the choroid.
In summary, choroidal thickness can be measured using Cirrus HD-OCT high definition raster scans in the majority of eyes. Our findings were similar to those previously described using the Heidelberg Spectralis device, including a thinner choroid nasally, thickest choroid sub-foveally, and again thinner temporally, and a trend toward decreasing choroidal thickness with age. Improved in vivo visualization of the choroid and measurement of choroidal thickness using OCT is likely to improve our understanding of a variety of ophthalmic diseases in the future.