Dependence of the MR signal on the magnetic susceptibility of blood studied with models based on real microvascular networks.

PURPOSE

The primary goal of this study was to estimate the value of , the exponent in the power law relating changes of the transverse relaxation rate and intra-extravascular local magnetic susceptibility differences as . The secondary objective was to evaluate any differences that might exist in the value of obtained using a deoxyhemoglobin-weighted distribution versus a constant distribution assumed in earlier computations. The third objective was to estimate the value of β that is relevant for methods based on susceptibility contrast agents with a concentration of higher than that used for BOLD fMRI calculations.

METHODS

Our recently developed model of real microvascular anatomical networks is used to extend the original simplified Monte-Carlo simulations to compute from the first principles.

RESULTS

Our results show that for most BOLD fMRI measurements of real vascular networks, as opposed to earlier predictions of .5 using uniform distributions. For perfusion or fMRI methods based on contrast agents, which generate larger values for , for 9.4 T, whereas at 14 T can drop below 1 and the variation across subjects is large, indicating that a lower concentration of contrast agent with a lower value of is desired for experiments at high B .

CONCLUSION

These results improve our understanding of the relationship between R and the underlying microvascular properties. The findings will help to infer the cerebral metabolic rate of oxygen and cerebral blood volume from BOLD and perfusion MRI, respectively.