Multiple scattering suppression by multi-focus averaging in Jones-matrix optical coherence tomography

Optical coherence tomography (OCT) has been utilized to investigate the optical properties of tissue in animal model studies [1]. Polarization sensitive OCT (PS-OCT) is one of the functional extensions of OCT, which provides additional contrasts such as birefringence and degree-of-polarization uniformity (DOPU). It enables multi-contrast imaging for discrimination of tissue properties in biological samples [2,3]. However, biological samples often exhibit multiple scattering (MS). It severely hampers the penetration depth and degrades the optical property measurement in deep regions for both OCT and PS-OCT [4,5]. In a recently presented work, it was found that all of the scattering OCT value, birefringence value, and DOPU value were altered if they were measured at the deep region. This alteration is suspected to be artifacts caused by MS [6]. So far, various methods have been proposed to tackle the MS issue. One is averaging several images with decorrelated MS signals. The decorrelation of MS signals was realized by wavefront shaping with complex devices such as spatial light modulators or deformable mirrors [7-9]. However, these optics increase the cost and system complexity, and the wavefront shaping is still challenging. Simple hardware, such as multimode fiber, was only demonstrated in full-field OCT [10]. In this work, we present a method termed as "multi-focus averaging" (MFA) to suppress the MS effect and improve the image contrast in deep regions of scattering samples [11]. It uses focus modulation by a tunable lens to decorrelate MS signals among multiple acquisitions, and computational refocusing is used to cancel the different defocus during focus modulation. The refocused OCT signals are averaged in complex to reduce MS signals. It is demonstrated in standard OCT scheme, and a scattering phantom and a post-mortem zebrafish are used to validate the intensity contrast improvement. Results show that MFA images can provide better intensity contrast than standard complex averaging in scattering samples. In addition, we further adapt this method to PS-OCT to suppress the MS and to improve the accuracy of polarization sensitive measurement. We found MFA mitigated low-DOPU artifact in polarization sensitive measurement.