Response: Letter to the editor: Assessment of choroidal vascularity index during the menstrual cycle

Dear Editor,

I would like to thank Fioretto et al. ¹ for their interest in the study. In their letter, the authors proposed that measurement of choroidal vascularity index (CVI) has a limitation due to the blooming effect that may appear in the OCT imaging. ²

In the study, Niblack's autolocal threshold technique was used as the binarization method. This is because it takes into consideration the mean and standard deviation of all the pixels in the region of interest. In addition, given that binarization could be also affected by the direction of light and focussing issues, these were taken into account by using a distinct binarization threshold for individual subject. ² The binarization methods have led to precise and reproducible measurements of choroidal parameters.

The blooming effect appears when the illumination is strong enough to saturate pixels which is proportional to the intensity of the illumination. Providing an adequate light exposure is especially important because blooming effect can change the image details. This requires a careful balancing of the OCT settings, combined with evaluation of image quality during capture. ³

Optical coherence tomography (OCT) is probably the most common technique used in clinical practice, and even if it is considered a highly sensitive tool in the diagnosis and evaluation of patients, it has some limitations such as image quality, motion artifact, and segmentation error etc. ⁴ One of the commonly used measure related to the quality of OCT image is the signal strength. Although different OCT devices can use different values, signal strength commonly falls between 0 and 10. All OCT devices including the SPECTRALIS® OCT (Heidelberg Engineering, Germany) have different recommended thresholds according to the manufacturer signal index, and unless it is set to proper values, the automatic algorithms can not work reliably. In the study, all OCT images were captured and analyzed according to the manufacturer's recommended settings. ²

In a study by Sonoda et al. binarization of the the OCT image was done by Niblack method. According to their study, the conversion of EDI-OCT images to binary images with publicly accessible software can be used to quantify the luminal and interstitial areas of the choroid. The interclass, intraclass, and intersession agreements were high; and the procedures were valid so that this method could be widely used. ⁵

The signal amplification or blooming effect may have an impact on the measurement of binarized OCT images if the device settings are changed or a neutral density filter is used. However, it is not well known to what extent a change in signal strength can impact the measurements of binarized areas. This issue needs further investigation and clarification.