Functional MRI has been performed routinely in the cortex for well over a decade, yielding thousands of publications per year, but fMRI of the brainstem and spinal cord presents additional significant technological challenges. The spinal cord is much smaller than the brain, necessitating improved image resolution while maintaining the temporal resolution needed to resolve changes in the BOLD effect. The adjacent vertebral bodies, which have different magnetic susceptibility to the intervertebral disks and surrounding soft tissues, cause spatially periodic variation in the static magnetic field which is not well-corrected by the standard static shimming solution based on spherical harmonics. T2*-weighted imaging methods are particularly affected by static magnetic field inhomogeneity. The lungs also cause temporally periodic variation in the static magnetic field due to their filling and voiding of paramagnetic O2. Respiration and subject motion also cause non-rigid motion of the spinal cord. Finally, the cerebrospinal fluid (CSF) in the cervical spinal canal pulsates with the cardiac cycle, causing oscillatory variation in its signal intensity. These physiologic noises must be isolated from the BOLD signal.
Our research seeks to exploit the increased BOLD sensitivity and signal-to-noise ratio (SNR) at 7 T to better localize the BOLD signal in human cervical spinal cord (CSC) and brainstem fMRI. The transition to 7 T involves several technical advances:
- Signal-to-noise ratio (SNR) is the currency of MRI, which can be spent to increase either spatial or temporal resolution. The transition from 3 T to 7 T yields a substantial increase in SNR, which can be used to achieve the improved image resolution necessary to precisely localize activity in the spinal cord gray matter. The BOLD effect, based on variation in the susceptibility of venous blood, is enhanced by a stronger static magnetic field.
- A transmit/receive (T/R) radiofrequency (RF) coil specifically tailored to the brainstem and cervical spinal cord also increases SNR, the efficiency and intensity of the transmitted RF field, and facilitates acceleration of image acquisition by parallel imaging.
- Spatial and temporal periodicity of the static magnetic field can be compensated for by dynamic shimming. A solution in which y- and z-gradient current offsets are updated dynamically with respect to imaging slice and respiratory phase can achieve the necessary magnetic field homogeneity despite proximity to the vertebrae and lungs. A more comprehensive solution comprising rapidly-switching aftermarket gradient power amplifiers and accompanying control software can provide more robust magnetic field control.
- Signal from pulsating CSF can also be suppressed by motion-sensitive magnetization preparation, such as delay alternating with precession for tailored excitation (DANTE), into the fMRI sequence.