## 3 Material and Methods  
### OCT System Setup and In Vitro Flow Phantom  
In the present study, the 1250 nm wavelength band of the dualband spectral domain OCT system of our workgroup is used [23]. This system contains a commercially available supercontinuum laser light source (SuperK Versa Super Continuum Source, Koheras A/S, Birkered, Denmark), where the wavelength band with a center wavelength \(\lambda_{0}\) of 1250 nm has a spectral width \(\Delta\lambda\) of 200 nm and an axial resolution of 5.3 \(\upmu\)m in air. The system comes with a 3D-scanner, whose Gaussian sample beam has a theoretically calculated width of \(w_{0}=9.3\)\(\upmu\)m in the focal plane and a Rayleigh length of about 200 \(\upmu\)m. The scanning unit for 2D-beam deflection at the sample surface is realized by two galvanometer scanners (cp. Figure 3). The system allows 3D-imaging with an A-scan rate of up to 47 kHz. For high backscattering signals with a strong signal-to-noise ratio (SNR) of the sample, the A-scan rate is set to \(f_{\text{A-scan}}=12\) kHz for the flow measurement. The control of the system, the data acquisition and the processing is realized by means of a personal computer and custom software developed with LabVIEW(r) (National Instruments, Inc., Austin, TX, USA).  
LR-DOCT is experimentally validated by a flow phantom consisting of a flow-controlled infusion pump, a glass capillary with a 312 \(\upmu\)m inner diameter, and a tubing system to connect both components. The pumped fluids are a 1% Intralipid emulsion as an established flow phantom model for OCT with well-known flow characteristics, and diluted human blood. Here, the fresh human blood of a young healthy volunteer was extracorporeally stabilized with anticoagulant citrate dextrose solution (ACD-A) and diluted with a solution of 6% dextrose and 0.9% sodium chloride in distillated water with a ratio of  
Figure 2: Schematic of the obliquely moving scattering sample relative to **(a)** a static Gaussian sample beam, **(b)** a forward lateral, and **(c)** a backward lateral scanning beam. **(a)** Due to the limited width of the incident Gaussian sample beam and the transverse motion component, the detected axial displacement \(\Delta z\) is reduced to \(\Delta z_{d}\). **(b)** If the lateral scanning speed is matched with the transverse velocity of the obliquely moving scatterers, the sample movement is detected as purely axial displacement during \(T_{\text{A-scan}}\). **(c)** If the sample beam scanning is opposite to the sample movement, the phase correlation is strongly reduced, for which \(\Delta z\) is measured significantly lower.  
1:2, resulting in a hematocrit of \(\sim\)15%. Despite the enhanced penetration depth into the blood vessels due to the low absorption of hemoglobin and the reduced scattering in the 1250 nm wavelength band, only the dilution enables the lateral resonant flow measurement over the entire capillary lumen of 312 um, which is comparable to the inner blood vessel diameter of the in vivo mouse model used in our cooperative research [24; 25; 26]. To avoid sedimentation of the solid blood components and to guarantee a homogenous dispersion of erythrocytes within the capillary, a drop distance was installed within the tubing system in front of the capillary entry, in accordance with the schematic drawing in Figure 3. For the 1% Intralipid sample, the flow rate is set to 9.5 mL/h, corresponding to a peak center velocity of about 69 mm/s, and for the diluted human blood, the chosen flow rate is 8 mL/h, resulting in a center flow velocity of about 62 mm/s, which conforms well to the maximum flow velocity of the saphenous artery of an anesthetized mouse during the systole prevalent in our research project [24]. For the analysis of the velocity values, a parabolic flow profile was assumed because the capillary had a length of 80 mm, the inlet path was considered, and the Reynolds number was less than ten for both samples. The Doppler angle \(\vartheta\) of 1.7\({}^{\circ}\) was measured by taking a volume scan with the 3D-scanner.