Complex conjugate resolved retinal SDOCT using integrating buckets

Yuankai K. Tao, Mingtao Zhao, Joseph A. Izatt Department of Biomedical Engineeri ng, Duke University, Durham NC 27708 ABSTRACT The complex conjugate artifact intrinsic to spectral domain optical coherence tomography (SDOCT) complicates retinal image acquisition in patients with poor fixation or head control, imaging of vitreal and choroidal structures, and imaging of extended retinal pathologies. We demonstrate high speed complex conjugate artifact-resolved imaging of human retina using simple and inexpensive sinusoidal reference mirror modulation in combination with 4-step inte grating-bucket acquisition a nd a quadrature projection reconstruction algorithm. The method uses a sinusoidal PZT driving signal to modulate the reference delay continuously during N integration buckets per modulation. The spectral interferometric signal measured by the CCD is phase shifted as a function of the amplitude and phase offset of the driving signal. We show that the amplitude and phase can be optimized for DC and complex conjugate artifact removal and minimal fringe washout. This method is illu strated experimentally us ing a four bucket phase modulating signal and quadrature projection algorithm for complex conjugate suppression. DC suppression of 53dB and complex conjugate suppression of 30dB is demonstrated for sets of four A-scans, each acquired at 17kHz. Densely sampled ( 3000 A-scans/image, acquired at 1.46 images/sec) in vivo complex conjugate artifact-resolved images of fovea and optic nerve-head acquired show complex conjugate artifact suppression for most image reflections to the noise floor. Keywords: optical coherence tomography, integrating buckets, in vivo complex conjugate artifact-resolved 1. INTRODUCTION Rapid recent progress in Fourier domain OCT (FDOCT), including spectrometer-based (SDOCT) and swept source (SSOCT or OFDI) approaches, have dramatically altered the technological landscape. The potential clinical impact of these developments is particularly strong in retinal imaging, where state-of-the-art SDOCT systems enable high-resolution, motion-artifact-free cross-sectional imaging and rapid accumulation of volumetric macular datasets. Both F DOCT approaches suffer from the so-called complex conjugate or mirror image artifact, in which positive a nd negative distances in the sample (relative to the reference path length) cannot be uniquely resolved. In current retinal imaging practice, this artifact is avoided by locating the sample entirely within the positive or negative displacement range, thus utilizing only one half of the total potential imaging depth. Such one sided imaging is tolerable for imaging of normal retina and most pathologies which fit within the ~1-2mm imaging depth range of current-generation SDOCT systems, however the presence of the artifact complicates image acquisition in patients with poor fixation or head control, and imaging of extended pathologies (such as vitreous strands, deep optic nerve head cups, and chor oidal structures) is limited by the characteristic roll-off in sensitivity associated with the finite spectral resolution of SDOCT systems. Several approaches for removing the complex conjugate artifact in SDOCT have been demonstrated, many of which borrow from established techniques of phase shift interferometry. These include discretely stepped reference phase shifting using PZT-mounted reference reflectors [1], electro-optic [2] or acousto-optic modulators [3]; instantaneous phase-separated spectral interferogram acquisition by use of 3x3 interferometers [4] or polar ization encoding [5]; and harmonic lock-in detection