Simulation of microvascular signal changes used on a gadolinium-chelated contrast agent at 3 T MRI in the presence of amyloid-beta plaques

It is important to understand microvascular alterations in the brain of Alzheimer disease (AD) patients which usually has amyloid-beta plaques in an imaging voxel, causing a phase dispersion in an MRI signal. The objective of this study was to simulate the changes of transverse relaxation rates and microvascular indexes with and without the presence of amyloid-beta plaques in an imaging voxel with using a clinically available contrast agent. The Monte Carlo simulations with the finite perturber method were used to calculate microvascular indices of mean vessel diameter (mVD), vessel size index (VSI), mean vessel density (Q), blood volume fraction (BVf), and microvessel-weighted imaging (MvWI) with and without the presence of amyloid-beta plaques in an imaging voxel. The simulation was performed with the conditions of three different magnetic field strengths (1.5 T, 3 T and 7 T), several different echo times for gradient-echo (15, 40, and 60 ms) and spin-echo (20, 80, and 100 ms) images, and two different contrast agents (gadolinium (Gd)-chelated and superparamagnetic iron oxide nanoparticles (SPION)-based contrast agents). The changes of relaxation rates and microvascular indexes were evaluated with increasing the microvascular vessel sizes and with increasing the amyloid plaque loads. In the microvascular structures without amyloid plaques, ΔR2* and ΔR2 increased as the B field became stronger, the susceptibility difference of the contrast agent became larger, and the echo time was increased. Both mVD and VSI increased with increasing microvessel sizes, but Q decreased with increasing microvessel sizes. In the microvascular structures with amyloid plaques, ΔR2* and ΔR2 were not varied with increasing the vessel radius, but higher in the 3.81% concentration of amyloid plaque than 1.83% of that. BVf and MvWI were increased with increasing the plaque load. The BVf and MvWI indices were sensitive to evaluate the integrity of the microvascular in the AD brain since those values were increased with increasing the plaque load. Therefore, it is worthwhile to evaluate microvascular structure changes in the AD human brain using 3 T MRI with a Gd contrast agent.