Multi-MHz retinal OCT

Abstract

We analyze the benefits and problems of in vivo optical coherence tomography (OCT) imaging of the human retina at A-scan rates in excess of 1 MHz, using a 1050 nm Fourier-domain mode-locked (FDML) laser. Different scanning strategies enabled by MHz OCT line rates are investigated, and a simple multi-volume data processing approach is presented. In-vivo OCT of the human ocular fundus is performed at different axial scan rates of up to 6.7 MHz. High quality non-mydriatic retinal imaging over an ultra-wide field is achieved by a combination of several key improvements compared to previous setups. For the FDML laser, long coherence lengths and 72 nm wavelength tuning range are achieved using a chirped fiber Bragg grating in a laser cavity at 419.1 kHz fundamental tuning rate. Very large data sets can be acquired with sustained data transfer from the data acquisition card to host computer memory, enabling high-quality averaging of many frames and of multiple aligned data sets. Three imaging modes are investigated: Alignment and averaging of 24 data sets at 1.68 MHz axial line rate, ultra-dense transverse sampling at 3.35 MHz line rate, and dual-beam imaging with two laser spots on the retina at an effective line rate of 6.7 MHz.

Keywords: (120.3890) Medical optics instrumentation; (140.3510) Lasers, fiber; (170.3880) Medical and biological imaging; (170.4460) Ophthalmic optics and devices; (170.4500) Optical coherence tomography.

Fig. 1

Sensitivity vs. axial line rate for retinal imaging at 1050 nm (0.7A/W photodetector responsivity) for a) 1.4 mW and b) 3.5 mW sample arm power. Blue shaded regions: Shot noise limited sensitivity with 100% collection efficiency. Red shaded regions: sensitivity with a 3 dB penalty (for instance, 65% coupling efficiency and additional 1.13 dB penalty). Horizontal black lines indicate the limits for 95 dB (“clinical imaging”) and 90 dB (“research”) sensitivity.

Fig. 2

FDML laser cavity and buffer stage layout (4x buffering version shown). AWG: arbitrary waveform generator; LDC: laser diode controller; SOA: semiconductor optical amplifier; ISO: isolator; PC: polarization controller; Yb: ytterbium doped fiber; BFP-TF: bulk fiber Fabry-Perot tunable filter; Pol: polarizer; pump: 978nm pump diode; cfBG: chirped fiber Bragg grating (Teraxion, Inc.), OSA: optical spectrum analyzer; MZI: Mach-Zehnder interferometer (used for the sensitivity roll-off measurements).

Fig. 3

Laser spectra and sensitivity roll-off at axial line rates of a) 1.68 MHz and b) 3.35 MHz. Note that for long imaging depths, sensitivity decay is dominated by the detection system’s 1 GHz electronic bandwidth.

Fig. 4

Dual-beam OCT imaging setup. BPD: Balanced photo-receiver; PC: polarization controller; DC: dispersion compensating glass blocks; Recal.: recalibration arm; Ref.: reference arm; Galvo: galvanometer mirrors (x and y), L1/L2: relay lenses; BS: beam-splitter; TG: target; DM: Dichroic mirror. All fiber couplers (blue) have a 50/50 coupling ratio.

Fig. 5

Retinal imaging at 1.68 MHz axial line rate. More than one million axial scans were acquired in 0.83 s. a) En-face fundus view, reconstructed from the OCT data set. b) Unaveraged B-frame, at the position indicated by the red bar in the en-face view. Media 1 shows an unaveraged flight-through of the entire data set at a playback speed 50 frames per second, i.e. much slower than the OCT acquisition frame rate of 1310 frames per second.

Fig. 6

a) Multi-data set averaging at 1.68 MHz with the acquire-align-average (AAA) scheme. After acquisition of N data sets, the data sets are aligned and averaged. Thus, distortion per frame is “averaged out” with increasing N. b) Result of AAA approach to imaging at 1.68 MHz. 4 frames from each of the 24 data sets were averaged (i.e. a total of 96 frames) to yield strong speckle reduction. c) Enlarged region from the region indicated by the white frame in b). All retinal layers including the ELM are clearly visible, indicating that alignment worked well. Note that image displayed in the right column of a) is the averaged en-face image after AAA processing of the 24 en-face images. Media 2 shows the en-face reconstructions of all acquired data sets before registration at a playback speed of 2 volumes per second. Slight “zipper” artifacts can be observed due to instabilities in the galvanometer scanners operating at very high speed. A blinking artifact is clearly visible in the first data set.

Fig. 7

3.35 MHz imaging. a) Reconstructed fundus view, with red horizontal bars indicating the position of the unaveraged B-frames in b,c). As can be seen in c), field of view is not limited by sample arm optics or laser coherence length, but by the sampling rate of the A/D card (and the resulting small imaging range). Media 3 shows only every fifth frame of the data set to reduce the size of the movie. For the movie, frame rate was set to 50/s and aspect ratio was adjusted by 2x downsampling in the horizontal direction. No further averaging was applied.

Fig. 8

a) 24x and b) 48x average of adjacent B-frames 3.35 MHz axial line rate. Note that, even though the speckle pattern is still partially developed in the 48x averaged image, some anatomical features are already distorted. For instance, the red circle in b) highlights an image detail, which is not visible in the 24x averaged frame. c,d): Enlarged view at the position indicated by the white box in a).

Fig. 9

Two-beam imaging at an effective axial line rate of 6.7 MHz, using only 0.85 mW optical power for each beam. a) Reconstructed en-face view: White dotted outline shows the area scanned by beam 1. There is around 10% overlap between the areas scanned by the two beams. b,c) B-frames from beams 1 and 2, as indicated by the red bars in the en-face view. The frames were 2x decimated in the transverse direction. d) 6x B-frame average at the same position as the frame shown in c), with enlarged region e).