Quantitative evaluation of Spectralis optical coherence tomography image quality with systematic variation of B-scan averaging before and after cataract surgery

Abstract

Purpose: To determine optimal B-scan averaging settings for Spectralis optical coherence tomography (OCT) in patients with and without optical media opacities using quantitative analysis of image quality.

Methods: In a prospective study, patients with senile cataract were imaged 7 times each with Spectralis OCT employing a logarithmically escalating scale of B-scan averaging using active eye tracking. Measurements were performed one week before and one week after uncomplicated cataract surgery. The degree of cataract was graded using the lens opacity classification system (LOCS) III. Image quality was quantitatively evaluated in MatLab using four different measures as follows: 1) Signal-to-noise ratio; 2) Distinction of individual retinal layers using classification; 3) Distinction of individual retinal layers using histogram overlap; 4) Image smoothness to evaluate potential blurring effects of B-scan averaging. The four quantitative measures were compared among different degrees of B-scan averaging, and before versus after surgery.

Results: 13 eyes of 13 subjects with a mean age of 68.3±8.7 years were included. Cataracts were graded as predominantly nuclear in 8 eyes, predominantly cortical in 4 eyes and predominantly posterior in 1 eye. Before cataract surgery, the logarithmic increase in averaging degree led to a linear increment in image quality in all measures. After cataract surgery, a logarithmic increase in averaging resulted in saturation in all image quality measures with a cut-off point at 16 B-scans. Likewise, averaging using 16 B-scans after surgery provided comparable image quality to a setting of 96 B-scans before surgery. Both before and after surgery, image smoothness continuously increased together with higher degrees of averaging, leading to progressive image blurring and loss of detail.

Conclusions: In patients with clear optical media, Spectralis OCT scan acquisition using averaging of 16 B-scans delivered excellent signal quality and contrast between retinal layers without the risk of excessive smoothing and loss of detail. Further increase in the averaging degree did not deliver corresponding image quality benefits. However, in patients with media opacities such as cataract, maximum B-scan averaging resulted in further increases in image quality.

Keywords: 550 imaging/image analysis: clinical • 549 image processing • 445 cataract