Functional Magnetic Resonance Imaging Using Photic and Motor Stimulation: Comparison of Results Obtained at 1.0 T and 1.5 T

Abstract

Since the demonstration of blood oxygenation level dependent (BOLD) contrast in magnetic resonance (MR) imaging at 7 and 8.4 T by Ogawa et al. in 1990 [1], the concept of functional magnetic resonance imaging (fMRI) has developed rapidly. Techniques of different complexity have been employed, ranging from conventional gradient echo imaging to advanced echo planar imaging, and a variety of applications have been suggested [2–4]. However, an accurate interpretation of observed BOLD effects in MR images is not always trivial, since the contrast mechanism reflects the combination of several parameters, for example, alterations in blood flow, blood volume, blood oxygenation and oxygen consumption. Furthermore, the possibility to correctly localize an activated region is hampered by the fact that larger vessels or draining veins can generate signal at a distance from the site of true tissue activation, and this large vessel signal contribution is not readily distinguishable from the signal arising from the capillary system within the tissue. In gradient echo imaging, the signal from the capillary system of the activated region is dominated by the T2*-sensitivity, while the signal from large vessels is likely to be a combination of susceptibility effects and inflow effects [3]. Theoretical modelling has suggested that the relative signal enhancement during stimulation (ΔS/S₀) is the sum of a contribution from small vessels, showing a quadratic dependence on field strength, and a large vessel component with a linear dependence on field strength [5]. Consequently, experiments at different field strengths could provide further information on the origin of the fMRI contrast in gradient echo imaging. The aim of this study was to experimentally demonstrate activation of human visual and motor cortex at 1.0 T as well as at 1.5 T, and to investigate different aspects of the field strength dependence in fMRI using identical pulse sequences and identical volunteers at both field strengths.