Determination of confocal parameters of OCT imaging for eliminating confocal effect on attenuation coefficient estimation

Optical coherence tomography (OCT) provides three-dimensional images of biological tissues based on low-coherence optical interference. Attenuation coefficient estimation is one of the important functional extensions of OCT and has received many research efforts as attenuation coefficient has been found to be associated with histopathological transformation of human tissues. However, attenuation coefficient estimation accuracy is deteriorated by the confocal effect on OCT A-scan. Thus, it is desired to eliminate the confocal effect, which requires accurate determination of the confocal parameters. In this study, we propose what we believe to be a novel method for confocal parameter extraction, called dual NA ratio fitting (DNRF). DNRF requires a repetition of B-scan with varied numerical apertures (NA), altering the Rayleigh length of B-scan but keeping the focal depth fixed. Then, the focal depth and Rayleigh length can be determined by fitting an A-scan ratio function from the two repeated B-scans. The NA tuning was achieved by adding a beam expansion module into the sample arm. The method was evaluated on intralipid samples and multi-layer phantoms. The results demonstrate that our method is capable of determine the confocal parameters with good accuracy. With the extracted confocal parameters, the confocal effect was removed effectively, upon which attenuation coefficient estimation using traditional depth-resolved method appeared to be more accurate than the confocal parameter extraction based on the A-scan ratio function of two repeated B-scans with varied focal depths. Last, our method was tested on human skin <i>in vivo</i>, yielding attenuation coefficients consistent with literature. This indicates good potential of our method to be used for clinical applications.