Multi-contrast imaging with computational refocusing in polarization-sensitive optical coherence tomography

Optical coherence tomography (OCT) has been widely used for imaging biological sample due to its capability of three-dimensional (3-D) reconstruction. Recently, polarization-sensitive optical coherence tomography (PSOCT) has been used to investigate polarization properties of samples such as retina and muscle [1,2]. Such extensions provide additional contrasts other than traditional reflected intensity and offer a 3-D multi-functional reconstruction of materials and biological tissue. PS-OCT uses A-scan wise computation to obtain the polarization properties from a set of OCT images. In this computation, the lateral resolution is implicitly assumed as infinitely high. However, the birefringence measurement itself will be affected by the lateral optical resolution, defocus, and aberrations. One simple way to obtain high-resolution birefringent data is to use an objective with higher NA. However, OCT also suffers from the trade-off between the lateral resolution and the depth-of-focus (DOF), which might limit its application in thick samples. There have been various methods to overcome this issue. Hardware solutions such as mechanical depth scanning [3] have been reported, but the additional configurations would increase the complexity of system. On the other hand, computational methods do not require additional hardware set-up and thus can be easily adopted, such as interferometric synthetic aperture microscopy (ISAM) [4] and forward model based computational refocusing [5]. However, to our acknowledgement, there are only few reports of long imaging depth for 3-D birefringence imaging. One presented work combined the ISAM with PS-OCT [6]. But the impact of refocusing on multi-contrast imaging, such as artifact in birefringence measurement, has not been thoroughly investigated. In this paper, we present a DOF extended polarization-sensitive imaging by applying computational refocusing to Jones-matrix based PS-OCT (JM-OCT). Computational refocusing is applied to each of four polarization channels (images) of JM-OCT, and the birefringence and degree-of-polarization-uniformity (DOPU) images are computed from the refocused OCT images. Enhanced lateral resolution away from the focus plane and hence extended DOF are demonstrated through phantom and ex vivo porcine muscle measurements. The ex vivo porcine muscle measurement also suggests that the refocusing may reduce the birefringence artifacts.