## 5 Summary and Conclusions  
In the present study, lateral resonant Doppler optical coherence tomography (LR-DOCT) is proposed for velocity measurements of oblique sample motion with very small Doppler angles and high absolute sample velocities in spectral domain OCT (SD-OCT), where the nonlinear relation between the Doppler phase shift \(\Delta\varphi\) and the axial velocity component \(v_{z}\) hampers the accurate velocity calculation by the phase-resolved Doppler OCT (PR-DOCT). The described LR-DOCT overcomes the limitation of the nonlinear \(\Delta\varphi\)-\(v_{z}\)-relationship and allows the calculation of the axial sample velocity \(v_{z}\) by matching the lateral scanner movement velocity with the transverse velocity component \(v_{x}\) of the sample. LR-DOCT is validated by means of two flow phantoms with a 1% Intralipid emulsion and diluted human blood, respectively. The flow velocity at the capillary center is lateral resonantly detected by adapting the scanner velocity to the transverse velocity \(v_{x}\) of the flowing scattering particles. By using an online display of the moving sample and accordingly the capillary flow before data acquisition, the transverse flow velocity \(v_{x}\) can be estimated for the adjustment of the scanner velocity in LR-DOCT. Since the speckle pattern of highly correlated A-scans is clearly identifiable, the adaption of the lateral scanner velocity is simple. Due to the variable scan velocity of the galvanometer scanner, within its specifications, arbitrary flow velocities can be lateral resonantly measured by adapting the scanning protocol. Generally, LR-DOCT works very well for volume scatterers consisting of many small particles (e.g., 1% Intralipid flow) relative to the wavelength \(\lambda_{0}\) and the sample beam width w\({}_{0}\) at the focal plane, whereas the method is limited for particles exceeding one or both of these factors (e.g., highly scattering erythrocytes of blood flow). The validation of the presented study has shown that LR-DOCT performs to a limited extent for diluted blood flow, where small groups of obliquely moving erythrocytes are resonantly detectable. This fact probably makes it difficult for in vivo blood flow measurements. Nevertheless, it is conceivable that LR-DOCT could produce precise blood flow measurements of easily accessible blood capillaries running almost perpendicular to the incident sample beam, containing only a few erythrocytes. Even though LR-DOCT has to be evaluated for in vivo conditions in future research, it is still interesting for many other moving samples in ex  
Figure 12: (**a**) Calculated Doppler phase shift of the FL-scans in Figure 11a,b,d are shown as a function of the radial position \(r\). (**b**) Doppler phase shift of the FL-scan in Figure 11d in comparison to the phase shift of the M-scan in Figure 9b. (**c,d**) Corresponding standard deviation of the mean Doppler phase shift presented in (**a,b**).  
vivo and in vitro applications, or non-medical and non-biological configurations. Under the discussed conditions, LR-DOCT overcomes the nonlinear relation between the Doppler phase shift \(\Delta\varphi\) and axial sample velocity, and with this, the limitation of PR-DOCT in SD-OCT for an oblique sample motion with a strong transverse velocity component. Consequently, LR-DOCT advantageously enables the simple, correct, and unlimited phase-resolved Doppler measurement of sample and flow velocities in SD-OCT.  
We thank [PERSON] for the assistance with the dualband spectral domain OCT system and [PERSON] for the voluntary blood donation.  
All authors contributed to this work. Authors [PERSON] and [PERSON] jointly developed the idea of LR-DOCT, performed the in vitro experiments, processed the acquired data, and analyzed the calculated Doppler information for the validation of LR-DOCT; [PERSON] wrote the paper.  
The authors declare no conflict of interest.