Effect of surrounding vasculature on intravoxel BOLD signal

The nonlocal influence from distant magnetization will affect the magnetic field at a voxel in question. Existing reports on BOLD simulation only consider vasculature inside a single voxel, thus omitting the contribution from the surrounding regions. In this article, the authors study the effect of the surrounding vasculature on the magnetic field and the BOLD signal at a cortical voxel by numerical simulation.

A cortical voxel is generated as a cubic bin filled with randomly networked capillary vessels. First, the authors generate a cortical voxel with a random vessel network and embed it in a greater voxel by filling its surrounding region with vasculatures by different strategies. Next, they calculate the blood-susceptibility-induced magnetic field (BOLD field) at the voxel of interest (VOI) by a Fourier transform technique for different surrounding scenarios and varying surrounding extent. The BOLD field inhomogeneity is described by a radial distribution with a collection of cubic shell masks. The surrounding extent is defined by a collection of concentric cubes, which encase the VOI. Given a BOLD field in the presence of surrounding vasculature, they calculate BOLD signals by intravoxel dephasing.

The influence from the surroundings on the BOLD field at a voxel in question mainly happens at the boundary. The most influence to the BOLD signal is from the inner surroundings. For a 160 x 160 x 160 microm3 voxel embedded in a 480 x 480 x 480 microm3 greater region, the surroundings could disturb the magnetic field by an amount in the range of [-0.002, 0.010] ppmT and could change the BOLD signal ratio in the range of [2.5%, 10%]. (These results were generated from the setting of delta(chi b)B0 = 3 ppmT, capillary = {2.5,6,9} microm, and relaxation time = 60 ms).

The surrounding vasculature will impose a magnetic field disturbance at the voxel in question due to the nonlocal influence of magnetization. Simulation results show that the surrounding vasculature significantly alters the magnetic field (up to 0.01 ppmT) and BOLD signal (typically no more than 10%) at the central voxel and thus should be considered in accurate BOLD modeling.