# Can OCT Angiography Be Made a Quantitative Blood Measurement Tool?
## 4 OCTA Algorithms
### Intensity- or Amplitude-Based OCTA Algorithms  
Intensity-based OCTA algorithms use \(\mathrm{I}(\mathrm{x},\mathrm{z},\mathrm{t})=|\mathrm{S}(\mathrm{x},\mathrm{ z},\mathrm{t})|^{2}\), while amplitude-based OCTA algorithms use \(|\mathrm{S}(\mathrm{x},\mathrm{z},\mathrm{t})|\) in Equation (1). The first class of intensity-based OCTA algorithms is the speckle variance method. Speckle [46] can be described as the random interference of scattering fields (indexed by \(\mathrm{m}\)) that cannot be resolved within a coherence volume:  
\[\mathrm{S}(\mathrm{z})=\sum_{\mathrm{m}}\mathrm{S}_{\mathrm{m}}(\mathrm{z}), \tag{2}\]  
\(\mathrm{S}_{\mathrm{m}}\) represents the fields within a coherence volume, each weighted according to the point spread function at the scatterer location (Figure 3). The intensity (as well as the phase and field) changes over time as the configuration of scatterers changes, causing decorrelation (Figure 4D-F). Decorrelation can occur as RBCs pass through a coherence volume, but may also occur due to rotational motion or diffusion. In 2005, [PERSON] and [PERSON] showed the feasibility of flow speed measurement without phase information by evaluating speckle pattern changes [47]. In 2008, [PERSON] et al. [48] used interframe speckle variance to visualize microcirculation. In [48], speckle variance was defined as:  
\[\mathrm{SV}(\mathrm{x},\mathrm{z})=\frac{1}{\mathrm{N}}\sum_{\mathrm{t=0}}^{( \mathrm{N}-1)\mathrm{T}}[\mathrm{I}(\mathrm{x},\mathrm{z},\mathrm{t})-\overline {\mathrm{I}(\mathrm{x},\mathrm{z})}]^{2}, \tag{3}\]  
where \(\mathrm{t}=0,\mathrm{T},\mathrm{2T},\ldots,(\mathrm{N}-1)\mathrm{T}\) represents the OCT acquisition time; \(\mathrm{T}\) is the time interval; \(\mathrm{N}\) is the total number of acquisitions at the same position; \(\mathrm{x}\) and \(\mathrm{z}\) denote lateral and depth indices, respectively; and \(\overline{\mathrm{I}(\mathrm{x},\mathrm{z})}=\overline{\mathrm{I}(\mathrm{x},\mathrm{z})}^{2}\) is the time-averaged intensity at position \((\mathrm{x},\mathrm{z})\). This is a temporally averaged,  
Figure 6: Comparison of complex field (**A**), intensity (**B**), and phase (**C**) time courses when both static and dynamic scattering are present in a coherence volume (with Doppler shift and decorrelation of the dynamic component). Such coherence volumes are present at the edges of vessels. variance-based algorithm without normalization. By ignoring the phase in Equation (1), the method is not sensitive to pure Doppler shifts. Consequently, speckle variance is not susceptible to phase noise. However, the speckle variance method may be compromised due to interframe bulk tissue motion. While in-plane (xz) motion can be compensated in principle, out-of-plane motion is more challenging to correct. To minimize motion effects, later in 2010, [PERSON] et al. [49] optimized the frame number and frame rate for a given level of bulk tissue motion, through maximizing the speckle variance signal-to-noise ratio (SNR) between a \"dynamic\" and \"static\" pixel. Speckle variance SNR is calculated as:  
\[\mathrm{SV}_{\mathrm{SNR}}(\mathrm{N},\overline{\mathrm{I}(\mathrm{x}, \mathrm{z})})=\frac{\mathrm{SV}_{\mathrm{dynamic}}(\mathrm{N},\overline{ \mathrm{I}(\mathrm{x},\mathrm{z})})-\mathrm{SV}_{\mathrm{static}}(\mathrm{N}, \overline{\mathrm{I}(\mathrm{x},\mathrm{z})})}{\sqrt{\sigma_{\mathrm{dynamic }}^{2}(\mathrm{N},\overline{\mathrm{I}(\mathrm{x},\mathrm{z})})+\sigma_{ \mathrm{static}}^{2}(\mathrm{N},\overline{\mathrm{I}(\mathrm{x},\mathrm{z})} )}}, \tag{4}\]  
where \(\overline{\mathrm{I}(\mathrm{x},\mathrm{z})}\) is the time-averaged intensity for both \"dynamic\" and \"static\" pixels, \(\mathrm{SV}_{\mathrm{dynamic}}\) and \(\mathrm{SV}_{\mathrm{static}}\) are speckle variances calculated from Equation (3), and \(\sigma_{\mathrm{dynamic}}^{2}\) and \(\sigma_{\mathrm{static}}^{2}\) are variances of \(\mathrm{SV}_{\mathrm{dynamic}}\) and \(\mathrm{SV}_{\mathrm{static}}\), respectively.