VAI spesific perfusion maps

The VAI perfusion related maps are generated according to previously published definitions and notations (Emblem et al. Nat Med. 2013; 19:1178-83; Stadlbauer et al. J Cereb Blood Flow Metab. 2017; 37:632-43; Xu et al. Magn Reson Med. 2013; 69:981-91). See Theoretical background - Vessel Architectural Imaging for additional information.

In short, the pixel-wise GRE and SE dynamic signals are converted to change in R2 and R2*, respectively for the SE and GRE signals. The resulting deltaR2 values are then raised to the power of 3/2 and a gamma variate (or Gaussian) function is the fitted to the resulting time curves. The fitted curves are then plotted with delR2^3/2 along the x-axis and delR2* along the y-axis, forming the characteristic hysteresis loops. The hysteresis loops are characterized by the loop direction, long axis, slope of long axis and the area of the loop, properties shown to be related to microvascular properties. In the VAI module, the voxel-wise loop direction is visualized by the arrows making up the loop as well as the arrow color (figure 1).

Figure 1: Characteristic hysteresis loop generated by plotting Gamma fitted SE vs GRE signals. Note that the direction of the loop is indicated by the color and direction of the arrows.

The following metrics are estimated:

. VAI vascular fraction (Vf). Vf is defined as the long axis of the hysteresis loop (figure 1).

A. Vortex area (VA). This is defined as the shaded region in figure 1 b. The cortex area can optionally be scaled by Vf so that VA(scaled) =VA/Vf. VA can also optionally be colored according to loop direction so that loops with clockwise loop directions have warm colors and negative loop directions have cold colors (figure 2).

B. Vortex direction. Binary image where a value of 1 equals clockwise loop direction and a value of -1 equals CCW direction.

C. Peak shift: The shift in the peak (fitted) SE vs GRE signals. Positive shifts for SE leading GRE signal.

D. Vessel calibre. Defined by the slope of the long axis (figure 1 b)

E. Vessel size image (VSI): Defined by  where is defined in figure 1b, ADC is the apparent diffusion coefficient and CBV is normalized cerebral blood volume obtained from the GRE signal.

Examples of different VAI-related perfusion maps are shown below.

Figure 2: Sample VAI related perfusion maps: Vessel size index (a), Vessel calibre (b), peak shift (c), Vascular fraction (d), Vortex area (direction sensitive) (e), Vortex area (f), vortex direction (g) and for reference, normalized CBV (h).

See References.