Source: http://aoot.osa.org/boe/abstract.cfm?uri=boe-5-9-2963
Timestamp: 2019-04-22 00:06:26+00:00

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We present a 1300 nm OCT system for volumetric real-time live OCT acquisition and visualization at 1 billion volume elements per second. All technological challenges and problems associated with such high scanning speed are discussed in detail as well as the solutions. In one configuration, the system acquires, processes and visualizes 26 volumes per second where each volume consists of 320 x 320 depth scans and each depth scan has 400 usable pixels. This is the fastest real-time OCT to date in terms of voxel rate. A 51 Hz volume rate is realized with half the frame number. In both configurations the speed can be sustained indefinitely. The OCT system uses a 1310 nm Fourier domain mode locked (FDML) laser operated at 3.2 MHz sweep rate. Data acquisition is performed with two dedicated digitizer cards, each running at 2.5 GS/s, hosted in a single desktop computer. Live real-time data processing and visualization are realized with custom developed software on an NVidia GTX 690 dual graphics processing unit (GPU) card. To evaluate potential future applications of such a system, we present volumetric videos captured at 26 and 51 Hz of planktonic crustaceans and skin.
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Fig. 1 Schematic of the high speed FDML laser followed by an 8x buffer stage and a booster optical amplifier. The filter is driven at 400 kHz resulting in a 3.2 MHz scan rate after the buffer stage. SOA: semiconductor optical amplifier, FBG: fiber Bragg grating, PC: polarization controller, LDC: laser diode controller, FRM: Faraday rotation mirror, FP-TF: Fabry-Pérot tunable filter, ISO: isolator, CTRL: Driver electronics and arbitrary waveform generator, OSA: optical spectrum analyzer.
Fig. 2 Schematic of the interferometer and the data acquisition. After the photo diode, the radio frequency (RF) signal is split and subsequently fed into two digitizer cards. These two digitizers operate at 2.57 GS/s in volume interleaving mode. Sampled data is streamed into computer RAM. For real-time visualization, RAM only serves as a temporary buffer for a few volumes. Bidirectional scanning allows an 85% scan duty cycle along the fast axis resulting in a sustained average total data transfer rate of ~2.1 GBytes/s.
Fig. 3 Scanning protocol used by the video rate 3D OCT setup. The fast axis uses a 4.29 kHz resonant scanner. Depth scans near the turning points of the sinusoidal galvo motion are not used leaving a usable frame size of 320 out of 375 depth scans. 8 frames are lost during fly back of the slow axis. The net frame rate is 8.37 kHz and the volume rate is 26.1 Hz resulting in usable volumes of size 320 x 320 x 400 voxels. The overall acquisition duty cycle is 83%.
Fig. 4 Left: Data processing on the host computer. After the acquisition, the data is transferred into GPU RAM. The NVidia GTX690 is a dual GPU card. The first GPU performs OCT data processing while the second GPU is dedicated to 3D visualization using a ray caster. Right: Screen shots from NVidia visual profiler showing the timing sequence of OCT data processing. The 4 steps in the most zoomed-in view are resampling and apodizing (violet), FFT (cyan), magnitude compression (red) and copying into the output texture memory (yellow).
Fig. 5 Examples from live display: Top left: Convection at wire spiral - (Media 1). Top right: Daphniae - (Media 2). Bottom left: Dynamics of filter legs, Triops - (Media 3). Bottom right: Syringe over finger - (Media 4).
Table 1 Previous work on real-time 4D OCT presenting full OCT systems including light source, data acquisition and real-time display. Columns 2, 3 specify the raw line and frame rates. The raw line rate is the swept source sweep rate or camera line readout rate. The raw frame rate is the galvo scanning rate. Column 5 shows the calculated “true” net depth scan size in image voxels along the depth axis, i.e. without any zero-padding or interpolation which would artificially create voxels and with complex conjugate frequency samples removed. (‘F’ denotes full-range OCT without removal of negative frequency samples.) Columns 6, 7 are the usable frame and volume sizes in depth scans / frames. Raw values before removing back-scan/turning point parts are included in brackets. Column 8 is the raw data rate in 106 samples per second at the acquisition device. The voxel rate cannot be larger than 0.5x (1x) this value for usual (full-range) OCT, respectively. Column 9 is the effective depth scan rate after removing all those scans not displayed (e.g. during galvo turning, back-scan,…). Finally, column 11 specifies true OCT speed in million voxels/second and column 10 the efficiency which is the effective scan rate divided by the raw scan rate or in other words the fraction of non-wasted raw A-scans. * denotes numbers where due to missing information, an estimate close to the optimum value was used resulting in potentially over-estimated efficiencies close to 100%.
Previous work on real-time 4D OCT presenting full OCT systems including light source, data acquisition and real-time display. Columns 2, 3 specify the raw line and frame rates. The raw line rate is the swept source sweep rate or camera line readout rate. The raw frame rate is the galvo scanning rate. Column 5 shows the calculated “true” net depth scan size in image voxels along the depth axis, i.e. without any zero-padding or interpolation which would artificially create voxels and with complex conjugate frequency samples removed. (‘F’ denotes full-range OCT without removal of negative frequency samples.) Columns 6, 7 are the usable frame and volume sizes in depth scans / frames. Raw values before removing back-scan/turning point parts are included in brackets. Column 8 is the raw data rate in 106 samples per second at the acquisition device. The voxel rate cannot be larger than 0.5x (1x) this value for usual (full-range) OCT, respectively. Column 9 is the effective depth scan rate after removing all those scans not displayed (e.g. during galvo turning, back-scan,…). Finally, column 11 specifies true OCT speed in million voxels/second and column 10 the efficiency which is the effective scan rate divided by the raw scan rate or in other words the fraction of non-wasted raw A-scans. * denotes numbers where due to missing information, an estimate close to the optimum value was used resulting in potentially over-estimated efficiencies close to 100%.

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