Imaging the eye fundus with real-time en-face spectral domain optical coherence tomography

Abstract

Real-time display of processed en-face spectral domain optical coherence tomography (SD-OCT) images is important for diagnosis. However, due to many steps of data processing requirements, such as Fast Fourier transformation (FFT), data re-sampling, spectral shaping, apodization, zero padding, followed by software cut of the 3D volume acquired to produce an en-face slice, conventional high-speed SD-OCT cannot render an en-face OCT image in real time. Recently we demonstrated a Master/Slave (MS)-OCT method that is highly parallelizable, as it provides reflectivity values of points at depth within an A-scan in parallel. This allows direct production of en-face images. In addition, the MS-OCT method does not require data linearization, which further simplifies the processing. The computation in our previous paper was however time consuming. In this paper we present an optimized algorithm that can be used to provide en-face MS-OCT images much quicker. Using such an algorithm we demonstrate around 10 times faster production of sets of en-face OCT images than previously obtained as well as simultaneous real-time display of up to 4 en-face OCT images of 200 × 200 pixels 2 from the fovea and the optic nerve of a volunteer. We also demonstrate 3D and B-scan OCT images obtained from sets of MS-OCT Cscans, i.e. with no FFT and no intermediate step of generation of A-scans.

Complex master slave interferometry

“…Initially, for the comparison operation required by the MSI method, correlation was used [15][16][17][18][19]. To improve on the calculation speed, a modified correlation operation was proposed [20][21], reduced to the correlation calculation in lag Ñ = 0. This delivers the value of an A-scan at depth z, the MSI signal, according to…”

“…An improvement in the practice of MSI would be to eliminate the shape of A from the masks in Eq. (10) as suggested by the theoretical model in [19] but not done in [15][16][17][18]20,21]. Equation (17) also shows that combination of phase in the complex r with the random phase impedes the recovery of the complex r. In order to address this problem, we propose to use complex masks, as we describe in the next section.…”

“…In a second stage (Slave), the object replaces the mirror and the channeled spectrum is compared with every CS exp saved in the memory block and used as a mask. The comparison operation of each mask with the channeled spectrum was implemented via correlation [15][16][17][18][19] or by simplified dot product procedures for faster implementation of correlation for argument zero [20,21]. The maximum value of each comparison (correlation) is selected to provide the depth information profile (A-scan) at each OPD value selected during the Master stage.…”

“…This method, Complex Master Slave Interferometry (CMSI), does not directly use CS exp as masks, contrary to [15][16][17][18][19][20][21], but develops a procedure to infer any number of masks from a reduced number of CS exp acquired. Additionally, the masks generated are complex in order to conserve the phase information.…”

“…This paper is organized into three theoretical sections and an experimental section. The theoretical sections describe the interferometric signal in a SDI set-up unbalanced from the point of view of dispersion and equipped with a chirped decoder, obtained in conventional MSI [15][16][17][18][19][20][21] and in the novel CMSI presented here. The chirp affecting the channeled spectrum at the interferometer output is modeled by two functions g and h. Based on these functions, a procedure of inferring a number of Q channeled spectra (masks) at the Slave stage from a variable number P of CS exp , with Q>>P is then presented.…”