Experimental determination of the BOLD field strength dependence in vessels and tissue

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<jats:title>Abstract</jats:title><jats:p>High resolution functional MRI (fMRI) experiments were performed in human visual cortex at 0.5, 1.5, and 4 T to determine the blood oxygenation level dependent (BOLD) field strength response within regions of obvious venous vessels and cortical gray matter (“tissue”). <jats:italic>T</jats:italic><jats:sub>2</jats:sub>*‐weighted FLASH images were collected in single‐ and multi‐echo mode and used to determine the intrinsic BOLD parameters, namely, signal‐to‐noise ratio (Ψ), the apparent transverse relaxation rate (R<jats:sub>2</jats:sub>*) and the change in R<jats:sub>2</jats:sub>* (ΔR<jats:sub>2</jats:sub>*) between the activated and baseline states. The authors find the average percentage signal change (ΔS/S, measured at <jats:italic>TE</jats:italic> = <jats:italic>T</jats:italic><jats:sub>2</jats:sub>*) to be large in vessels (13.3 ± 2.3%, 18.4 ± 4.0%, and 15.1 ± 1.2%) compared with that in tissue (1.4 ± 0.7%, 1.9 ± 0.7%, and 3.3 ± 0.2%) at 0.5, 1.5, and 4 T, respectively. The signal‐to‐noise ratio in optimized, fully relaxed proton density weighted gradient echo images was found to increase linearly with respect to the static magnetic field strength (B<jats:sub>0</jats:sub>). The predicted upper bound on BOLD contrast‐to‐noise ratio (ΔS/R)<jats:sub>max</jats:sub> as a function of field strength was calculated and found to behave less than linearly in voxels containing vessels larger than the voxel itself and greater than linearly in voxels containing a mixture of capillaries and veins/venules with a diameter less than that of the voxel.</jats:p>

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