# Can OCT Angiography Be Made a Quantitative Blood Measurement Tool?
## 3 OCTA Signal
Doppler shifts are associated with a change in the phase of the complex field autocorrelation, while decorrelation is associated with a decrease in the magnitude of the complex field autocorrelation, \(|R(\tau)|\), with increasing \(\tau\). Figure 3: The motion of scatterers in a coherence volume gives rise to complex field fluctuations that form the basis for OCTA signals. The contributions to the complex field are shown at two different points in time (t and t + \(\tau\)). (**A,B**) Field fluctuations due to dynamic scatterers in a coherence volume. (**C,D**) Field fluctuations due to a combination of static (blue) and dynamic (red) scatterers in a coherence volume. Both Doppler shifts and decorrelation are present to varying degrees in all vasculature. Note that a Doppler shift due to translational axial motion through the coherence volume implies decorrelation. On the other hand, decorrelation occurs even for transverse motion or rotation, and does not necessarily imply a Doppler shift. To illustrate this, Figure 5 shows a comparison between Doppler OCT and OCTA of mouse brain microvasculature. Doppler OCT detects phase changes caused by translational axial motion [45]. The requirement for axial phase shifts renders Doppler OCT only sensitive to motion parallel to the incident beam. Doppler shifts predominate in larger microvessels which are ascending or descending (Figure 5A); hence when used for angiography, the Doppler effect provides only a  
Figure 4: The major categories of OCT signal fluctuations are Doppler shifts and decorrelation. Comparison of complex field, intensity, and phase time courses, for the case of a pure Doppler shift (**A–C**) and a Doppler shift with decorrelation (**D–F**). For a pure Doppler shift, the field traces out a helical pattern (**A**), whereas decorrelation introduces random deviations from this pattern (**D**). Figure 5: Doppler OCT and OCTA in the mouse brain. (**A**) Doppler OCT can visualize flow based on Doppler shifts, caused by motion in the axial direction, towards or away from the probe beam. On the other hand, OCTA visualizes flow based on decorrelation, usually caused by translational motion through the coherence volume, as well as Doppler shifts. The overlay of both methods (**C**) shows that Doppler OCT is mainly limited to ascending venules or descending arterioles, where Doppler shifts dominate. On the other hand, OCTA, which is sensitive to decorrelation, more comprehensively shows vasculature, including regions with predominantly transverse flow. A standard Kasai algorithm was used on transversally oversampled images for (**A**) and a complex interframe subtraction method was used on rapidly acquired repeated cross-sectional images for (**B**). partial microvascular map. By comparison, decorrelation involves random deviations of the complex field and predominates in vessels with transverse flow. Thus, OCTA, which senses decorrelation via intensity and/or phase, more comprehensively shows the vasculature (Figure 5B,C). Finally, it should be noted that the presence of static scattering can significantly alter time courses. The OCT field, intensity, and phase time courses due to dynamic scattering in the presence of a static scatterer are shown in Figure 6. As suggested by Figure 3D, the presence of static scattering confines the field fluctuations to a portion of the complex plane (Figure 6A). As will be discussed in Section 7.1, the possible presence of static scatterer(s) must be considered in order to recover quantitative information about the Doppler phase shift or the decorrelation rate in OCTA.