After its first introduction in the 1990s [1
], optical coherence tomography (OCT) has found a variety of applications, especially in medical research and ophthalmology [2
]. Since 1995 OCT was further developed into Doppler OCT (DOCT) [3
], a technique that is not only capable of providing 2D and 3D micro structural images of the sample but also recognizes moving particles within the object. Various different Doppler OCT systems can be found in literature as for example: phase-resolved Doppler OCT (PR-DOCT) [6
], resonant Doppler flow imaging [9
], joint spectral and time domain imaging [10
], optical micro-angiography (OMAG) [11
] or single-pass volumetric bidirectional blood flow imaging (SPFI) [12
]. Despite the huge variety of DOCT systems, PR-DOCT is still the most widely used technique. PR-DOCT measures the phase difference between adjacent A-scans, which have to be recorded at overlapping positions within the sample. This phase difference Δ
is directly proportional to the velocity of the moving particle:
In this equation vaxial
denotes the speed of the particle in measurement direction, λ is the central wavelength of the measurement beam, τ is the timing interval between two consecutive A-scans and n denotes the refractive index of the medium. Furthermore, the measured velocity depends on the angle α between the incident light and the flow direction of the particle, which is known as the Doppler angle:
From both equations one can see that the maximum detectable flow speed depends on the 2π ambiguity of the phase shifts whereas the minimum measureable velocity depends on the phase noise of the system. The range from the minimum to the maximum detectable velocity can in principle be arbitrarily shifted by changing the time difference τ between both phase measurements. But for in vivo measurements the minimum detectable flow speed is further limited by the measurement time. If one wants to measure the microvasculature within the human retina, where the flow speed is approximately in the range of a few 100 μm/s, the timing between two adjacent A-scans has to be larger than ~1ms. This measurement speed would, on the one hand make the time which is necessary to acquire a 3D capillary flow image in vivo prohibitively long, on the other hand, the phase washout due to bulk sample motions occurring at these long integration times would severely degrade the signal quality. Additionally, when imaging the human retina, the measurement beam is almost perpendicular to the blood vessel and hence the Doppler angle approaches 90° which further degrades the minimum detectable velocity.
Recently important steps towards the visualization of small blood vessels in tissue were introduced. The systems described in [13
] use ultra fast SD-OCT setups to reduce motion artifacts and hence allow contrasting of very small details such as cone photoreceptors or capillary vessels. Other systems are based on Doppler OCT and use different scanning protocols to change the dynamic range of the flow measurement [15
]. Rather than performing the Doppler measurements between adjacent A-scans (fast scanning direction), these systems apply their Doppler analysis between consecutive B-scans (slow scanning direction). Because a single B-scan usually consists of several hundreds of A-scans, the timing between the two required measurements is large enough to contrast the slow flow within a microvasculary network. But nevertheless these methods require strong oversampling between sequential B-scans and hence the measurement time needed for a full 3D scan limits in vivo imaging at human subjects.
Based on this measurement approach and the OMAG technology [11
] Wang et al. [18
] presented a system which is capable of visualizing the microvascular network around the fovea region in vivo. OMAG is based on full range complex OCT [19
] and introduces a constant modulation frequency along the fast scanning direction (x-scan direction) which after applying the algorithm, separates the backscattering signal of a moving object from the surrounding static tissue. In their recent work [18
] they applied this algorithm along the slow scanning axis (y-scan direction) and hence increased the timing between two adjacent measurements. Their system provides good images of the capillary network around the fovea, but it also puts some additional requirements on the measurement apparatus. In order to apply the OMAG algorithm along the slow scanning direction the fast scanning axis time has to be chosen adequately. They used a spectral domain OCT (SD-OCT) system with a CMOS line scan camera and set the integration time to be 7.4 μs. With 256 A-scans per B-scan and a duty cycle of 75% this resulted in a B-scan time of 2.5 ms. This rapid measurement period, which is needed for the OMAG algorithm, places not only high demands on the line scan camera speed but also reduces the system sensitivity (~90 dB in their case). In order to regain sensitivity and hence increase the image quality they had to acquire eight B-scans at the same lateral position [18
In this paper we present a different approach for visualizing the capillary structure within the human retina in vivo. The idea behind our system is related to the work by Makita et al. [23
] and on our dual beam full range complex OCT setup [24
]. The sample beams of two identical SD-OCT setups are scanned over the object at different lateral positions. Therefore two tomograms which are slightly separated in time are recorded. During the post processing we overlay both data sets again and apply an extended PR-DOCT algorithm to extract a 3D capillary network tomogram of the retina. The separation between both sample beams can be adjusted arbitrarily and hence the velocity measurement range can be freely chosen.