  Purpose: To simulate the intravoxel incoherent perfusion magnetic resonance
magnitude signal from the motion of blood particles in three realistic vascular
network graphs from a mouse brain. Methods: In three networks generated from
the cortex of a mouse scanned by two-photon laser microscopy, blood flow in
each vessel was simulated using Poiseuille law. The trajectories, flow speeds
and phases acquired by a fixed number of simulated blood particles during a
Stejskal-Tanner monopolar pulse gradient scheme were computed. The resulting
magnitude signal as a function of b-value was obtained by integrating all
phases and the pseudo-diffusion coefficient D* was estimated by fitting an
exponential signal decay. To better understand the anatomical source of the
IVIM perfusion signal, the above was repeated by restricting the simulation to
various types of vessels. Results: The characteristics of the three
microvascular networks were respectively: vessel lengths [mean +/- std. dev.]:
67.2 +/- 53.6 um, 59.8 +/- 46.2 um, and 64.5 +/- 50.9 um; diameters: 6.0 +/-
3.5 um, network 2: 5.7 +/- 3.6 um, and network 3: 6.1 +/- 3.7 um; simulated
blood velocity: 0.9 +/- 1.7 um/ms, 1.4 +/- 2.5 um/ms and 0.7 +/- 2.1 um/ms.
Exponential fitting of the simulated signal decay as a function of b-value
resulted in the following D* [10-3 mm2/s]: 31.7, 40.4 and 33.4. The signal
decay for low b-values was the largest in the larger vessels, but the smaller
vessels and the capillaries accounted more to the total volume of the networks.
Conclusion:This simulation improves the theoretical understanding of the IVIM
perfusion estimation method by directly linking the MR IVIM perfusion signal to
an ultra-high resolution measurement of the microvascular network and a
realistic blood flow simulation.
