# Can OCT Angiography Be Made a Quantitative Blood Measurement Tool?
## 5 OCTA Scanning Protocols  
The efficiency and sensitivity of OCTA measurements are determined by the OCTA scanning protocol. At a fundamental level, scanning protocols can be categorized based on whether the analysis is performed on consecutive A-scans, frames, or volumes. Figure 10 shows the so-called MB-scan,BM-scan and intervolume scanning methods. In Figure 10, the cube represents the imaged object, and t\({}_{1}\), t\({}_{2}\), t\({}_{3}\) are the first, second, and third OCT scanning time scales, respectively, with t\({}_{3}\) > t\({}_{2}\) > t\({}_{1}\). Each protocol can be characterized by the time duration for which a single location is observed.  
To our knowledge, [PERSON] et al. [57] were the first to rigorously compare different OCTA scanning patterns. They compared the MB-scan (Figure 10A) and the BM-scan (Figure 10B), using a phase contrast algorithm. An M-scan is a repeated zero-dimensional scan at a single position, while a B-scan is a one-dimensional scan along a single axis. An MB-scan comprises multiple A-scans taken at one lateral position before switching to the next position (Figure 10A), while a BM-scan comprises repetitive B-scans taken along the same cross-section (Figure 10B). According to [57], the advantages and disadvantages of the two scanning methods are described here.  
The MB-scan is an extension of Doppler OCT protocols. By increasing N, the number of A-scans per M-scan, the dynamic range for the measurement increases. However, the MB-scan is not time-efficient, because the total observation time for a single location is \(\sim\)t\({}_{1}\). Unless the dwell time is very long, \(|\)r(t\({}_{1}\))\(|\)\(\sim\) 1; thus it is challenging to observe decorrelation. However, due to the rapid repeated sampling of the same position, the MB-scan can sample fast Doppler velocities [57] without aliasing.  
On the contrary, a BM-scan compares consecutive frames, thereby more efficiently utilizing the total acquisition time. With a BM-scan, the total observation time for a single location is \(\sim\)t\({}_{2}\). In [57], the BM-scan was able to acquire data 200 times faster than an MB-scan of the same size. Even when using fast systems, the BM-scan may suffer from aliasing of fast Doppler velocities; however, the decorrelation rate can be obtained if the interframe time is short enough, i.e., \(|\)r(t\({}_{1}\))\(|\)\(>\) 0, and provided that t\({}_{2}\) exceeds the intrinsic decorrelation time.  
Figure 10: Volumetric OCTA scanning protocols can operate with respect to A-scan (**A**), frame (**B**), or volume (**C**). A cubic volume is scanned with time scales t\({}_{1}\), t\({}_{2}\), and t\({}_{3}\). Data acquired sequentially along time scale t\({}_{3}\) are shown in red and green. (**A**) MB-scan: multiple A-scans are obtained at one lateral position before switching to the next lateral position; (**B**) BM-scan or interframe scan: multiple B-scans are obtained at one cross-sectional location before switching to the next location; (**C**) intervolume scan: successive scans of the whole volume. Each scan achieves a progressively larger observation time for a single spatial position (t\({}_{3}\) > t\({}_{2}\) > t\({}_{1}\)).  
In a logical extension of the above two scanning methods, in 2016, [PERSON] et al. [65] proposed a volumetric optical microangiography method (Figure 10C) which used intervolume OCT scans to extract dynamic changes. The total observation time for a single location is \(\sim\)t\({}_{3}\). However, in this volumetric protocol, all information about the decorrelation rate is lost as speckles decorrelate between volumes (i.e., \(|\)r(t\({}_{2}\))\(|\sim 0\)) for all but the slowest flows. Nevertheless, the volumetric OCTA is likely to become more prevalent as imaging speeds continue to improve [66].