Ultrahigh field FMRI - principles and applications

Ultrahigh magnetic fields (UHFs), defined as ≥7T, was developed for human imaging because of anticipated improvements in sensitivity, accuracy, and spatial resolution of functional brain imaging (fMRI) using magnetic resonance techniques and to enable fMRI to reach into the mesoscopic scale organizations in the brain. Since its introduction, incessant developments in UHF techniques and theoretical work have proven that UHF provide linear to supralinear magnetic field depended signal-to-noise (SNR) gains, and the ability to tap into these SNR gains using advanced RF technology. In addition, UHFs provide increased Blood Oxygenation Level Dependent (BOLD) contrast, the most commonly employed contrast mechanism for functional mapping of human brain activity, and elevate the contribution of BOLD signals originating from microvasculature as opposed to larger draining veins. The latter effect is particularly relevant for improved spatial fidelity to the neural activity generating the fMRI signals. Over the past few decades, the expectation that UHFs can expand the spatial scale of fMRI have been amply confirmed by numerous and ever-increasing number of submillimeter resolution human neuroscience applications in the mesoscopic scale. Moreover, even at the more conventional, supramillimeter resolutions, UHFs were shown to increase the detection sensitivity, and model predictive power of fMRI. Most of the UHF efforts to date have taken place at 7T. The gains realized at this magnetic field strength have motivated several initiatives to develop greater than 10T for human imaging; these initiatives are expected to have transformative impact on human neuroscience.