Functional magnetic resonance imaging using RASER

Although functional imaging of neuronal activity by magnetic resonance imaging (fMRI) has become the primary methodology employed in studying the brain, significant portions of the brain are inaccessible by this methodology due to its sensitivity to macroscopic magnetic field inhomogeneities induced near air-filled cavities in the head. In this paper, we demonstrate that this sensitivity is eliminated by a novel pulse sequence, RASER (rapid acquisition by sequential excitation and refocusing) (Chamberlain et al., 2007), that can generate functional maps. This is accomplished because RASER acquired signals are purely and perfectly T2 weighted, without any T2⁎-effects that are inherent in the other image acquisition schemes employed to date. T2-weighted fMRI sequences are also more specific to the site of neuronal activity at ultrahigh magnetic fields than T2⁎-variations since they are dominated by signal components originating from the tissue in the capillary bed. The RASER based fMRI response is quantified; it is shown to have an inherently less noisy time series and to provide fMRI in brain regions, such as the orbitofrontal cortex, which are challenging to image with conventional techniques.

Research Highlights►In RASER, time-encoding with frequency-swept pulses is used for imaging. ►The T2-weighted RASER sequence is insensitive to static magnetic field variations. ►RASER is utilized to map activation in brain regions which are challenging for EPI. ►RASER provides excellent activation maps of the orbitofrontal cortex at 7 T.