Flexible A-scan rate MHz-OCT: efficient computational downscaling by coherent averaging

Abstract

In order to realize adjustable A-scan rates of fast optical coherence tomography (OCT) systems, we investigate averaging of OCT image data acquired with a MHz-OCT system based on a Fourier Domain Mode Locked (FDML) laser. Increased system sensitivity and image quality can be achieved with the same system at the cost of lower imaging speed. Effectively, the A-scan rate can be reduced in software by a freely selectable factor. We demonstrate a detailed technical layout of the strategies necessary to achieve efficient coherent averaging. Since there are many new challenges specific to coherent averaging in swept source MHz-OCT, we analyze them point by point and describe the appropriate solutions. We prove that coherent averaging is possible at MHz OCT-speed without special interferometer designs or digital phase stabilization. We find, that in our system up to ∼100x coherent averaging is possible while achieving a sensitivity increase close to the ideal values. This corresponds to a speed reduction from 3.3 MHz to 33 kHz and a sensitivity gain of 20 dB. We show an imaging comparison between coherent and magnitude averaging of a human finger knuckle joint in vivo with 121 dB sensitivity for the coherent case. Further, the benefits of computational downscaling in low sensitivity MHz-OCT systems are analyzed.

Resolution-enhanced OCT and expanded framework of information capacity and resolution in coherent imaging

“…2 a,b). Our results are consistent with previous work in OCT and theoretical trends 51 , 52 (see Supplementary Section III for caveats of experimentally achieving the theoretical performance). Namely, noise intensity was reduced by 20 dB (a factor of 100) after a coherent average over N = 100 volumes (Fig.…”

“…However, deconvolution inherently amplifies noise and degrades the SNR of the image. To address this problem, we also conjecture that the SNR penalty associated with computational BE can be compensated by first suppressing the system noise—one efficient way in OCT is by coherently averaging multiple successively acquired complex tomograms 50 – 52 . The underlying principle of RE-OCT is illustrated in Fig.…”

“…Others in OCT have also developed image registration and phase correction methods that successfully enabled phase-sensitive processing in biological samples, including for in vivo settings 27 , 70 – 72 . Furthermore, advances in high-speed imaging have enabled OCT imaging at MHz-rate 73 , combatting motion artefacts and supporting phase-sensitive imaging in vivo 29 – 31 , 52 , 74 – 76 . RE-OCT may be particularly attractive for high-speed systems, some of which already incorporate coherent or incoherent averaging 52 , 74 , 75 , where the RE-OCT procedure can be easily integrated into the existing imaging workflow with minimal additional effort and imaging time.…”

“…Bilenca et al derived the scattering limit to the information capacity of depth-resolved coherent imaging (using OCT as a case study) through turbid media as a function of SNR 49 . Furthermore, coherent averaging of complex tomograms provides a more efficient method for noise suppression than incoherent (magnitude-only) averaging in OCT, due to the decorrelation of random phase noise 50 – 52 . Coherent averaging has also been used for multiple scattering suppression 53 – 55 .…”

“…Then, we demonstrate noise suppression via coherent averaging and analyse its impact on the OCT signal in not only the space, but also the spatial-frequency domain. Our analysis of the impact of coherent averaging in the transverse spatial-frequency domain provides a new perspective compared to prior work in OCT (which has only investigated the impact of coherent and incoherent averaging in the space domain 50 – 52 ). Next, we demonstrate resolution enhancement by RE-OCT in both resolution phantom and biological samples (collagen gel and ex vivo mouse brain).…”