Megahertz-rate optical coherence tomography angiography improves the contrast of the choriocapillaris and choroid in human retinal imaging

: Angiographic imaging of the human eye with optical coherence tomography (OCT) is becoming an increasingly important tool in the scientific investigation and clinical management of several blinding diseases, including age-related macular degeneration and diabetic retinopathy. We have observed that OCT angiography (OCTA) of the human choriocapillaris and choroid with a 1.64 MHz A-scan rate swept-source laser yields higher contrast images as compared to a slower rate system operating at 100 kHz. This result is unexpected because signal sensitivity is reduced when acquisition rates are increased, and the incident illumination power is kept constant. The contrast of angiography images generated by acquiring multiple sequential frames and calculating the variation caused by blood flow, however, appears to be improved significantly when lower-contrast images are taken more rapidly. To demonstrate that the acquisition rate plays a role in the quality improvement, we have imaged five healthy subjects with a narrow field of view (1.2 mm) OCTA imaging system using two separate swept-source lasers of different A-line rates and compared the results quantitatively using the radially-averaged power spectrum. The average improvement in the contrast is 23.0% (+/ − 7.6%). Although the underlying cause of this enhancement is not explicitly determined here, we speculate that the higher-speed system suppresses the noise contribution from eye motion in subjects and operates with an inter-scan time that better discriminates the flow velocities present in the choroid and choriocapillaris. Our result informs OCT system developers on the merits of ultrahigh-speed acquisition in functional imaging applications.


Introduction
Since its initial demonstration more than two decades ago [1], optical coherence tomography (OCT) has become a standard tool for visualizing the laminar structure of the retina. The rapid development of OCT performance has led to its widespread clinical adoption in ophthalmology and promising growth in dermatology and cardiology [2]. This continued growth and expansion is largely due to improvements in imaging speed, enabling the observation of faster dynamics in tissue and reduced motion artifacts [3]. One of the most exciting emerging applications of OCT is in the examination of retinal vasculature, widely seen as a way to observe a functional aspect of the visual system: the perfusion of blood. Though it has been recently termed OCT angiography (OCTA), research groups have been developing vascular imaging since the early years of OCT [4][5][6]. Currently, this capability is being transferred to the clinic through commercial development. With this increased accessibility, several journal special issues, review articles, and international conferences now exist, highlighting studies done with both research and commercial OCTA systems [7][8][9].
The morphology of vasculature has important implications in some retinal diseases. The inner retinal vessels nourish the neural tissue other than the photoreceptors and retinal pigment epithelium. The choroidal vessels provide the significant amount of oxygen and nutrients that these outer two layers require and are a complex network of high-flow vasculature. Gross changes in retinal vasculature and large vessels of the choroid are clinically observable using fluorescein and indocyanine green angiography (FA and ICGA, respectively), and current OCTA methods offer a non-invasive alternative to these techniques that clinicians use to manage retinal disease. One of the most prevalent diseases that affects vision in a large fraction of the elderly is age-related macular degeneration (AMD). It is believed that defects of the choroid, particularly of the choriocapillaris layer, may play a significant role in the initial stages of the disease [10]. Detailed imaging of the choriocapillaris, therefore, may lead to a better understanding of the etiology and treatment of AMD.
Structural OCT images do not normally provide significant contrast between the retinal neurons and the vasculature. To detect the flow, an area of the retina is imaged multiple times in rapid succession to reveal the presence of moving scatterers, presumably blood cells moving in the vessels. OCT Doppler studies use phase changes to determine the velocity of objects traversing the imaging beam. In OCTA, the direction and velocity are ignored, and the brightness of the signal is determined by the variance of the OCT signal. Most angiography techniques do not quantify the blood velocity, but there is growing interest in quantifying flow in OCTA imagery, albeit imprecisely or in a relative sense [11][12][13][14][15]. Advancement towards these goals of better angiography have been marked by the utilization of progressively faster OCT systems.
A clear benefit of imaging with a rapid OCT acquisition system is that a wider region of the tissue can be captured within a conventional recording time [16,17]. Using a simultaneous 2-D retinal scanner, such as a scanning light ophthalmoscope, can track and correct eye motions artifacts and increase the achievable field of view of OCTA imaging [18][19][20]. As an alternative to capturing a larger area, one can record images of the same area more quickly, reducing the deleterious effect of object motion. Studies conducted with high-speed OCTA systems have shown that a rapid frame rate can significantly improve the visualization of the high-speed flows present in the choroid [21,22]. These results encouraged us to build an ultrahigh-speed system in which the B-scan frame rate is exceptionally fast, and many smallarea images can be acquired in a similar timespan, yielding a much more temporally dense data set. Our main findings were that, despite the reduced signal sensitivity of such a highspeed system, the quality of choroidal and choriocapillaris images were unexpectedly good. Here, we demonstrate the improvement in image quality, which we believe is afforded by the rapid frame rate.

Design and methods
This study compares two OCT imaging systems of significantly different acquisition speeds by performing similar imaging on the same group of subjects. The optical systems were assembled from commercially-sourced components, and the acquired data were processed using software developed by the authors. The list of components, lens prescription, and postprocessing scripts are included in the supplementary materials, which include Dataset 1, Dataset 2, and Dataset 3 [23][24][25]. In this section, we describe the general system, scan parameters, image processing, and image quality analysis.
The most significant component of our ultrahigh speed OCT system was the rapid wavelength-tuning laser source. It was a Fourier-domain mode-locked (FDML) laser (FDML-1060-750-4B-APC, OptoRes GmbH, Munich, Germany) operating at an A-scan rate of 1.644 MHz. The FDML device consists of a ring laser with a high-speed tunable filter within the loop and two buffering stages which create three replicates of the original wavelengthsweeping pulse group [26,27]. The other, slower, swept-source laser was also a commercial device (1060 nm SSOCT, Axsun Inc., Billerica, Massachusetts, USA) operating at a 100 kHz sweep rate, which is of a more similar speed to many OCTA systems in research groups and clinics. Table 1 lists the significant characteristics of the two OCT systems for comparison. The fiber couplers (OCT-060A42H10, Gooch & Housego Ltd, Torquay, Devon, UK) were connected in an arrangement such that the reference light is transmitted from one fiber tip to a second by way of a retroreflector on a sliding translation stage. Dispersion mismatch in the system is compensated using a small fiber patch cord, the length of which was determined by iteratively trying different cord lengths until the best correction was achieved. Any remaining dispersion mismatch was corrected numerically in post-processing. This two-couplers topology, diagramed in Fig. 1(A), allows for balanced detection, an essential feature when using swept-source lasers which have significant relative intensity noise [28]. The topology allowed the operator to easily adjust the reference power to maximize the sensitivity [29,30]. The exposure level of the imaging beam was set to 1.8 mW, below the limit determined by the ANSI standard for the safe use of lasers at this wavelength [31]. The measured sensitivity of the instrument was −95.3 dB when using the fast laser, and −107.4 dB for the slow one. The sensitivity was optimized by characterizing all the major noise sources and choosing the reference power level (3.2 mW on the photoreceiver) to ensure that the system was shot-noise limited. The phase noise was also measured for both systems by imaging both a mirror and paper sample near the zero-distance position, and the results are listed in Table 1. The signallimited phase noise is the estimated by measuring the standard deviation of the noise in the OCT images, and dividing it by the magnitude of the OCT signal in the region of interest [21]. In this study, this region is the choriocapillaris layer, and the signal is averaged over all subjects. 1.96 sec. † Phase noise calculated with the bulk-phase correction applied to the image sequence. * Theoretical phase noise value determined by the ratio of the image noise over the signal value in an area of interest, which is the choriocapillaris layer in this case [21].
‡Based on an assumed 16.67 mm effective focal length of the human eye.
The sample arm was assembled from commercially available lenses, scanners, and optomechanics. The first lens after the fiber output was a 0.5 inch diameter achromat with a focal length of 18 mm. For the fast system, the first scanner in the beam path was a galvanometer scanner (8310KM40B, Cambridge Technologies Inc., Bedford, MA, USA) which scanned the beam vertically. The nearby ( Generating an angiographic image requires that motion of the blood cells within the vasculature be detected. As in most OCTA methods, sequential B-scans were acquired at the same position multiple times such that temporal changes in the cross-section could be captured. In both systems the fast and slow scanning directions were the horizontal and vertical directions, respectively. For the fast system (~1.64 MHz), which operated with an approximately 2 kHz B-scan rate, scans were repeated at every vertical position 10 times before moving to the next position. A total of 392 separate vertical positions were captured in each volume, which amounted to 3920 total B-scans. This resulted in a volume acquisition time of 1.96 seconds. For the slow system (100 kHz), which had a B-scan rate of 200 Hz, only 5 repetitions were performed at each position, and 180 vertical positions were collected for a total of 900 B-scans in every volume. If the slower system were configured to acquire the same number of B-scans as the faster system, and over the same field of view, the time required would be 19.6 seconds. This duration is not practical in most human retinal imaging because of the relatively short drying time of the tear film, the consequent blinks and loss of fixation.
The scanning range for images taken with the fast system was 4°x4° for the fast system and 4°x1.6° for the slow one, which corresponds to approximately 1.2x1.2 mm and 1.2x0.55 mm on the retina, respectively. The resonant scanner of the fast system has sinusoidal distortion which is corrected after the intensity and angiogram images are processed. To determine the appropriate interpolation, a grid-pattern test target was imaged and analyzed every few months. The average pixel pitch of both systems was approximately 3.1 µm. These sampling densities are approximately 3 times smaller than the full-width at half-maximum (FWHM) spot size diameter which was estimated to be 9.1 µm from the line-spread function shown in Fig. 1(E), and most OCTA studies do not exploit oversampling to such a degree [17].
The acquired data were post-processed in separate software to generate the vascular maps of the tissue. The angiography method used here is the phase-variance method, which computes the variance of the phase component of each pixel [32][33][34][35]. The unwanted effect of bulk motion of the tissue is compensated using the method described by Makita et al. [32]. The magnitude value of each pixel was not considered except in computing a threshold on the image. This threshold determined which phase pixels would be used in the final angiogram. We have found that adjusting the threshold value can have a significant impact on the appearance of the vascular network that the user chooses to target. In all images taken for both systems, the threshold was chosen iteratively by the user to best elucidate the vasculature of interest. Angiogram volumes were then flattened with respect to the vascular bed of interest to create en face representations of vascular networks. We used the graph-cut method to segment the retinal pigment epithelium (RPE) in each image and flatten the volume according to the segmented trace [36].
We quantified the image quality of the choriocapillaris vessels by using the radial average of the spatial 2-D discrete Fourier transform (DFT). We refer to this as the radially-averaged power spectrum (RAPS) curve, although one could similarly call it a concentrically-averaged power spectrum. RAPS curves were presented in a logarithmic magnitude scale. Because most images had differing brightness ranges, the RAPS curves were normalized by pinning the maximum and minimum values between unity and zero. These curves allowed us to examine the contrast of the spatial frequencies present in the image. Other studies have used  While the OCT angiograms computed from low-speed images revealed significant detail in retinal and choroidal vascular layers, increasing B-scan rate improved the apparent contrast. Figure 3 shows a direct comparison of angiograms taken with the slow (100 kHz) and fast (1.64 MHz) systems, shown in the left and right halves of the figure, respectively. These images were centered on the fovea of subject 4, a 36-year-old emmetropic male. Crosssectional intensity images (3-A and C) show similar details. Cross-sectional angiograms (3-B and D) reveal the retinal vasculature and choroidal vasculature. White arrows in the angiogram B-scans (3-B and D) indicate the outer retinal bands, where angiographic signal is not expected or at least minimal. Spurious signal, presumably due to bulk motion and indicated by the white arrows in the images, is visible in the low-speed (3-B) but noticeably more suppressed in the high-speed (3-D) angiogram. En face projections of the retinal vasculature (3-E and F) demonstrate that the high-speed has similar detail to the low-speed image. White arrowheads in those panels indicate that a small branch which appears fainter in the fast system image. In another scan taken moments later (not shown), the same segment is of similar brightness to nearby vessels. This suggests that a temporary flow reduction occurred during the initial scan. En face projections of the choriocapillaris (3-G and H) demonstrate the improved contrast and detail achieved by the high-speed system. While many superficial similarities are present between the images, the high-speed angiogram reveals a detailed, continuous meshwork of vessels not apparent in most of the low-speed image. In the deeper choroid, the low-speed angiogram (I) is slightly noisier than the high-speed angiogram (J), and the latter reveals better small vessel detail and improved visibility of vessel continuity, though details of the choroid are clearly apparent in both.  ntrast of the ima at regular vesse as a basic m ows the chorio tocol (red) prod w system (blue 8% (+/− 4.3% rast than the lo deviation of th ages. It is evid y exists among at of the other s r s ft r l e s r spectra, ed results ves were ed system protocol, pproaches cyc./mm, age at the els of the metric for ocapillaris duced the e). The 5-%). In all ow-speed he RAPS dent from g subjects, subjects. he utility of th m montages at d e macular regio ively, manually ion of the neur n choriocapill of the macula (c atial frequency veal center, wh tive optics [39 work of well-re 4 (Fig. 7), mu sensitivity ualify this statement in that it is probably only true within a range of flow speeds and signal magnitudes. Our success here was partly serendipitous and partly the result of designing the system for specifically defined goals. It was clear from earlier work by Choi et al. and Braaf et al. that high-quality images of the choriocapillaris and choroid were possible with a high frame speed [14,21,22]. Based on the findings from these studies, we chose the B-scan rate and repetition number of our system to be optimized for imaging this vascular bed. In Choi's work, highquality images of the choriocapillaris were generated with a 400 kHz system. The analogous images taken with our system, whose A-scan speed is about 4 times faster and B-scan speed is about 5 times faster, reveal the choriocapillaris morphology more clearly. The choriocapillaris in our images is even well defined in and around the fovea. There may be many differences between our system and theirs, but the most obvious one is the speed. In general, the parameters that predominantly affect OCTA images are the sensitivity, phase noise, B-scan repetition number, pixel pitch (sampling density), and inter-scan time, and we discuss these below. This study supports our belief that the inter-scan time has the most substantial effect on image quality enhancement.
The fringe amplitude sensitivities of the two systems are predictably different. The fast system was measured to have approximately −95 dB, and the slow system to have −107 dB. The fringe phase noise was measured to be lower in the fast system, but phase noise, by itself, cannot account for the improvement of the images. Even though the characterized phase noise of the slower system's laser is higher than that of the faster laser and higher than the signallimited phase noise (see Table 1), it is our observation that this is not a limiting factor when imaging subjects. Those with exceptional fixation still generate a larger background signal due to movement than a stationary scattering object of similar signal level. It should also be noted that if a lower phase noise was the reason for the improvement, then spectrometerbased OCT systems, which are inherently very phase-stable, would be highly superior to swept-source ones for angiographic imaging. That does not appear to be the case, given the multitude of successful OCTA studies which have not revealed choriocapillaris and choroid images of high contrast. In many of these systems, the algorithms used ignore the phase information altogether.
As for the matter of repeated scans, the fast system captured many more frames within the usual OCT scan duration. It acquired B-scans at a 2 kHz frame rate and captured two mirrored B-scans during the front and back sweeps of the resonant scanner, which are then averaged after the angiogram is generated. Also, the chosen repetition number, 10, is twice as large as that used for the slower system. Both these measures might, at best, render two doublings of SNR. That four-fold improvement, however, does not overcome the theoretical sensitivity advantage of the slower system, which is 16 times ( + 12 dB) that of the fast system. The actual, quantifiable improvement is somewhat unclear and depends on how noise affects the discrete steps of the angiography post-processing method. The method used here is termed the phase variance algorithm, but specifics about how this algorithm theoretically compares to the multitude of others used in the field are not discussed here. We have previously reported an empirical comparison, however [41]. Most relevant is that the slower system should have a signal quality advantage, but it is outperformed by a fast system which scans the same area in much less time.
The sampling densities for both systems are unusually high when compared to those used in other studies. We have sampled at 3 times the density of the point spread function's fullwidth at half-maximum, which many would consider to be unnecessarily dense. Sampling density does make a difference in the appearance of OCTA angiograms, and previous work by our group has briefly explored this idea [17,40]. The angiograms of the slow system are still better than most reported in the literature with systems of similar speeds, possibly improved by the slightly smaller focused spot size as well. Because these parameters are consistent between the two systems, the main finding, that the fast system generates better images, still holds.
The variables in this experiment have been controlled such that the main difference between the two systems is the inter-scan time. The slow system has about 5 ms between subsequent frames, while the fast system has 0.5 milliseconds between them. The effect of scatterer speed and inter-scan time has been explored in controlled flow-phantom experiments [11,12] and in vivo angiography images [14,15,21]. These studies have all demonstrated that the relative flow speed of blood can be inferred from the magnitude of the angiography signal, but the quantitative flow velocity cannot be determined accurately. When flow speeds are high or, equivalently, the inter-scan time is large, the speckle or phase image is maximally decorrelated, and the variance is essentially saturated in magnitude of the OCTA signal. This leads to any local variations of the flow speeds to become indistinguishable and so the vessel image is distorted and somewhat obscured. At the other extreme, when the flow speeds are low, or the interscan time is very short, the magnitude of flow-dependent changes become comparable to and indistinct from the background noise. The processes that contribute to angiography signal and the noise, however, may have different temporal frequency characteristics. The background noise is generally spectrally flat, so it contributes to a wideband noise floor in the temporal spectrum. Patient motion creates some low-frequency variation of the image, but much of that is suppressed by acquiring the frames rapidly, and bulk phase correction. The blood flow, the signal of interest, is probably somewhat bandlimited, assuming we take the samples quickly. Taking many fast samples can help distinguish the signal from the acquisition noise, and motion. This strategy lessens the chance that some of its temporal variance (the sum of all the AC frequency bins) will be aliased into the stationary (the DC) frequency bin of the temporal discrete Fourier transform (DFT), resulting in some minor improvement in OCTA signal which is derived from the AC frequencies of the DFT. While we have demonstrated that increasing acquisition speed improved angiogram quality, we have not concluded that further increases in speed will result in corresponding image quality gains. At some very high speed and correspondingly low sensitivity, flow-dependent variation in the complex signal will fall below the noise floor, and the angiographic signal will be too low to be useable. An ideal frame-rate and dwell time should, we expect, exist for every flow speed range and scatterer signal strength. For imaging the choroid and choriocapillaris, as well as the inner retinal vessels, our chosen frame rate and repetition number worked well.

Conclusions
This study demonstrated that in vivo OCTA imaging of the choriocapillaris and other retinal vasculatures is significantly improved by using an ultrahigh-speed system with a 1.64 MHz A-scan rate. By comparing two similar OCTA systems that had different acquisition speeds, it appears that the critical advantage of the higher-speed system was the short inter-scan time and ability to take a multitude of repeated scans. This result should have a significant impact on how OCT system developers choose to operate their systems for imaging vasculature in the living eye.