A Novel Signal Enhancement Method Based on High Dynamic Range (HDR) Processing Concept for Optical Coherence Tomography (OCT) Images

Abstract

 

Purpose:

 

To develop and test a novel signal enhancement method for OCT images based on the HDR processing concept. A set of OCT images obtained under varying corneal dryness conditions was processed to see the effect on retinal nerve fiber layer (RNFL) thickness measurements between good and poor signal strength (SS) images scanned on the same eye.

 

Methods:

 

Signal normalization was performed pre-processing so that each A-scan had a nearly identical intensity histogram. Following this, three copies of OCT signal datasets, which represent low, medium, and high signal components, were produced for each OCT image. The dynamic range of each of the divided signal components was normalized to the full gray scale range (0 to 255). Finally, the three components were recombined into one image using various weights, which were functions of the signal quality index. Fourteen eyes of 14 healthy volunteers were scanned multiple times using Stratus OCT (Carl Zeiss Meditec, Dublin, CA; Fast RNFL scan) before and while preventing blinking in order to produce a wide variety of signal strength images on the same eye. A pair of scans, with the highest and lowest SS but no RNFL segmentation failures, were selected for each eye to test the signal enhancement effect. In addition, three poor signal quality spectral-domain (SD-) OCT images (RTVue; Optovue, Fremont, CA) were also processed.

 

Results:

 

Mean SS of good and poor quality scans were 9.0±1.1 and 4.4±0.9, respectively. Prior to signal enhancement, Stratus RNFL thickness showed significant differences between good and poor quality scans on the same eye (mean difference 11.9±6.0 μm, p<0.0001, paired t-test), while there was no significant difference after signal enhancement (mean difference 1.7±6.2 μm, p=0.33). The absolute difference of RNFL thickness between good and poor quality images was statistically significantly reduced by signal enhancement (p=0.002). Successful signal enhancement was also confirmed on SD-OCT images (Figure).

 

Conclusions:

 

The novel signal enhancement presented herein successfully restored OCT signal and image quality such that RNFL thickness measurement differences between good and poor quality images were reduced to the expected measurement variability.  

 

Keywords: image processing • imaging/image analysis: clinical • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)