Using forward calculations of the magnetic field perturbation due to a realistic vascular model to explore the BOLD effect

Abstract

This paper assesses the reliability of the infinite cylinder model used previously in the literature to simulate blood oxygenation level dependent (BOLD) signal changes. A three-dimensional finite element method was applied to a realistic model of the cortical vasculature, and the results compared with those generated from a simple model of the vasculature as a set of independent, randomly oriented, infinite cylinders. The realistic model is based on scanning electron microscopy measurements of the terminal vascular bed in the superficial cortex of the rat. Good agreement is found between the two models with regard to the extravascular R 2 Ã and R 2 dependence on the cerebral blood volume and blood oxygenation fraction. Using the realistic model, it is also possible to gain further understanding of the relative importance of intravascular and extravascular BOLD contrast. A simple parameterisation of the dependence of the relaxation rates on relative cerebral blood volume and blood-tissue susceptibility difference was carried out, allowing discussion of the variation in the form of the haemodynamic response with field strength.

Quantitative BOLD: The effect of diffusion

“…Fitting an interpolated lookup table of static dephasing curves produced indistinguishable parameter maps to fitting Eq. [14] directly, validating the method.…”

Development of Magnetic Resonance Imaging Biomarkers for Traumatic Brain Injury

“…Having the ability do to so provide a means to understand better the flow and function of the vasculature system and to model the vascular system for any individual. In return, modeling the system can predict what we expect to see from an activated to resting state (Marques and Bowtell, 2008). This will surely lead to a better understanding of why different individuals respond differently to specific stimuli, and certainly to separating out structural effects from real functional effects.…”

“…However, this very non-local behavior contains everything we need to predict the inherent local susceptibility source Salomir et al, 2003). This has garnered great interest in the last few years to create a quantitative susceptibility map (QSM) of the tissue Marques and Bowtell, 2008;Salomir et al, 2003) for the study of brain iron and oxygen saturation. The latter is possible because the resulting field inside the veins is directly related to the hematocrit and the local oxygen saturation.…”

BOLD background gradient contributions in diffusion‐weighted fMRI—comparison of spin‐echo and twice‐refocused spin‐echo sequences

“…Dx values of 0.36 ppm (baseline level, Y ¼ 0.6) and 0.18 ppm (activated state, Y ¼ 0.8) were used to simulate BOLD susceptibility (these values were calculated using Eq. 5 with HCT ¼ 0.4) (37). As an intermediate value, a cerebral venous blood volume fraction (CBV) of 2% was considered in the simulation.…”