Resolution and reproducibility of BOLD and perfusion functional MRI at 3.0 Tesla

Abstract

Visual and somatosensory activation studies were performed on normal subjects to compare the spatial discrimination and reproducibility between functional MRI (fMRI) methods based on blood oxygen level-dependent (BOLD) and perfusion contrast. To allow simultaneous measurement of BOLD and perfusion contrast, a dedicated MRI acquisition technique was developed. Repeated experiments of sensory stimulation of single digits of the right hand showed an average variability of activation amplitude of 25% for BOLD data, and a significantly lower variability of 21% for perfusion data. No significant difference in the variability of the locus of activity was observed between the BOLD and perfusion data. In somatotopy experiments, digits II and V were subjected to passive sensory stimulation. Both the BOLD and perfusion data showed substantial overlap in the activation patterns from the two digits. In a retinotopy study, two stimuli were alternated to excite different patches of V1. Again there was substantial overlap between the activation patterns from both stimuli, although the perfusion performed somewhat better than the BOLD method. Particularly for the visual studies, the overlap in activation patterns was more than expected based on the fine-scale retinotopic mapping of cortical activity, suggesting that both BOLD and perfusion contrast mechanisms contribute substantially to the point-spread function ( Changes in tissue perfusion with brain activation can be measured with MRI methods sensitized to perfusion contrast (1) or blood oxygenation level-dependent (BOLD) contrast (2). Since perfusion changes are the source of and potentially precede blood oxygenation changes, it can be argued that perfusion MRI is a more direct method for detecting brain activity. This can lead to distinct differences between perfusion and BOLD measures of activity, with potentially important implications for functional activation studies.One potential difference between perfusion and BOLD functional MRI (fMRI) is in the accuracy of localization (3). Perfusion fMRI is thought to be primarily sensitized to the arterial side of the capillary bed, with a potential contribution from the arterioles and larger arteries. BOLD fMRI, on the other hand, has a bias toward the venous side of the capillary bed and can include venules and the larger veins, as well as their surrounding tissue. This sensitivity to venous or arterial effects can result in localization errors of a few to several millimeters depending on the contribution of signals from outside the capillary bed (i.e., the macrovascular contribution).The difference in contrast mechanisms between BOLD and perfusion MRI can also lead to differences in the reproducibility of the localization and the amplitude of the activation. Since BOLD fMRI relies on an imbalance between changes in vascular oxygen extraction and perfusion, one could argue that perfusion contrast, which does not rely on this imbalance, might be a more robust and reproducible measure.Despite the potential problems related to loc...

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