Speckle Reduction in OCT using Massively-Parallel Detection and Frequency-Domain Ranging

Abstract

Speckle noise significantly limits the information content provided by coherent optical imaging methods such as optical coherence tomography and its recent derivative, optical frequency-domain imaging (OFDI). In this paper, we demonstrate a novel OFDI system that simultaneously acquires hundreds of angularly resolved images, which can be compounded to reduce speckle noise. The system comprises an InGaAs line-scan camera and an interferometer, configured so that the elements of the detector array simultaneously capture light spanning a backscattering angular range of 32 degrees. On successive read-outs of the array, the wavelength of the laser source was stepped through a range of 130 nm centered at 1295 nm to concurrently generate 400 angle-resolved OFDI images. A theory of angle-resolved OFDI and the design equations of the system are presented. Incoherent averaging of the angle-resolved data is shown to yield substantial speckle reduction (as high as an 8 dB SNR improvement) in images of a tissue phantom and esophageal tissue ex vivo.

Fig. 1

Angle-resolved OFDI system. C: collimator; L1, L2, L3: cylindrical lenses; L4: aspheric doublet lens; PC: polarization controller; BS: beam splitter: P: polarizer; M1: stationary mirror; M2: galvanometer mirror. The gray-dashed region is oriented perpendicular to the plane of the interferometer.

Fig. 2

Frequency-swept source. SOA: semiconductor optical amplifier; Circ: circulator: PC: polarization controller; C: collimator; L1, L2: aspheric doublet lenses; DG: diffraction grating; M: galvanometer mirror; Isol: isolator.

Fig. 3

Images of a two-layer tissue phantom obtained from 1 angular sample (180° backreflection) (a) and from compounding 400 angular samples (b). The arrow points to the boundary between the two layers. Top layer μ_s = 12 cm^-1; bottom layer μ_s = 24 cm^-1. The scale bar corresponds to 500 μm in depth; the transverse extension of the images is 4 mm.

Fig. 4

Angular distribution obtained from one resolution element within the tissue phantom (a) with corresponding normalized cross-correlation function (b).

Fig. 5

SNR as a function of the number of angular averages, N_A, for signals acquired from a depth of 500 μm within the tissue phantom.

Fig. 6

Images of porcine esophageal tissue obtained from a conventional OFDI system (a) and from the angle-resolved OFDI system by compounding 1 (b), 4 (c), 16 (d), 64 (e), and 256 (e) angular samples. The scale bar corresponds to 500 μm in depth; the transverse extension of the images is 6 mm. The arrow points to the top surface of the coverslip overlying the tissue.