## 4 Experimental Results
### LR-DOCT with In Vitro Blood Flow  
In analogy to the Intralipid flow experiment, measurements are performed with the identical capillary model in combination with diluted fresh human blood. The measured Doppler angle \(\vartheta\) amounts to 1.7\({}^{\circ}\). Since the flow rate is set to 8 mL/h, the measurement parameters concerning the tracked transverse velocities are similar to the Intralipid flow phantom in Section 4.1. Exemplary structural images of the detected M-scans, FL-scans, and BL-scans are found in Figure 9a,d for two data sets, each consisting of 400 A-scans. Figure 8: Mean error (standard deviation) \(\sigma_{\Delta\varphi}\) is shown theoretically (solid line [20]) and experimentally (measurement points of the M-scan, FL-scan, and BL-scan) as a function of \(C\!Q\). Figure 9: Grayscale structural images (**a**,**d**) and phase-resolved Doppler images (**b**,**_e_) are shown of the M-scans, FL-scans, and BL-scans each consisting of 400 A-scans for two experiments A and B with identical settings. In addition, the standard deviation of the mean Doppler phase shift of 400 A-scans is overlaid to the structural image for both experiments (**c**,**f**). Obviously, the speckle pattern is different to the experiments with Intralipid flow, first due to the larger size of the highly scattering wheel-shaped erythrocytes with a diameter of \(\mathcal{O}\approx 7.5~{}\upmu\)m and a thickness of \(d\approx 2.0~{}\upmu\)m in comparison to the small spherical oil particles of the Intralipid, with a mean diameter of \(\mathcal{O}\approx 100\) nm relative to the size of the sample beam of \(w_{0}=9.3~{}\upmu\)m at the focal plane. Secondly, this pattern is due to the inhomogeneous distribution of the erythrocytes of the diluted blood sample within the capillary. Nevertheless, the dilution described in Section 3.1 is necessary for the tracking of erythrocytes at the capillary center, otherwise the erythrocytes are already too dense at \(r=0.5R\). Unfortunately, no backscattering signal is achieved at a larger depth, probably due to the orientation of the erythrocytes in this capillary zone and the resulting angular intensity modulation in the glass capillary cross section [28]. However, the velocity of the galvanometer scanner can be matched with the transverse flow velocity of the erythrocytes at the capillary center with the chosen dilution, as seen in the FL-scan of Figure 9a,d. The FL-scans of the measurements A and B differentiate, because the group of erythrocytes in experiment A is tracked over a shorter distance than the group in B. Therewith, the challenge of LR-DOCT for blood flow measurement becomes apparent, which implies the random catch of erythrocytes at the capillary center and the selected depths within the capillary, respectively. Because of the shear flow-dependent movement and orientation of the erythrocytes, the appearance of erythrocytes at the capillary center in blood samples with reduced hematocrit is small. Generally, for blood samples and especially for diluted blood, sedimentation of the erythrocytes occurs, where the flow velocity is small, as within the syringe and the tubing system connecting the glass capillary with the infusion pump [29]. Consequently, the erythrocytes build aggregates in the form of chain-like stacks [30]. With the falling distance of the connecting tube in front of the capillary entry, sedimentation and agglomeration within the capillary are avoided. The aggregates are dispersed, but erythrocytes are still grouped and randomly orientated at the capillary center in the presented FL-scan. Considering the images with an overlaid Doppler phase shift and standard deviation in Figure 9b,c,e,f, it becomes apparent that the Doppler noise is decreased for the resonantly scanned erythrocytes (FL-scan). On closer examination, the phase shift of the resonantly detected erythrocytes still shows an increased noise in dependence on the amount of erythrocytes flowing above, which is considered in the analysis presented below.