Magnetic susceptibility-induced echo-time shifts: Is there a bias in age-related fMRI studies?

PURPOSE: To evaluate the potential for bias in functional magnetic resonance imaging (fMRI) aging studies resulting from age-related differences in magnetic field distributions that can impact echo time and functional contrast. MATERIALS AND METHODS: Magnetic field maps were taken on 31 younger adults (age: 22 ± 2.9 years) and 46 older adults (age: 66 ± 4.5 years) on a 3T scanner. Using the spatial gradients of the magnetic field map for each participant, an echo planar imaging (EPI) trajectory was simulated. The effective echo time, time at which the k-space trajectory is the closest to the center of k-space, was calculated. This was used to examine both within-subject and across-age-group differences in the effective echo time maps. The blood oxygenation level-dependent (BOLD) percent signal change resulting from those echo time shifts was also calculated to determine their impact on fMRI aging studies. RESULTS: For a single subject, the effective echo time varied as much as ±5 msec across the brain. An unpaired t-test between the effective echo time across age groups resulted in significant differences in several regions of the brain (P < 0.01). The difference in echo time was only ∼1 msec, however, which is not expected to have an important impact on BOLD fMRI percent signal change (<4%). CONCLUSION: Susceptibility-induced magnetic field gradients induce local echo-time shifts in gradient echo fMRI images, which can cause variable BOLD sensitivity across the brain. However, the age-related differences in BOLD signal are expected to be small for an fMRI study at 3T. LEVEL OF EVIDENCE: 1 J. Magn. Reson. Imaging 2017;45:207-214.

Simultaneous dynamic and functional MRI scanning (SimulScan) of natural swallows.

In studies of swallowing, dynamic and functional MRI are increasingly used to observe motor oropharyngeal behaviors and identify associated brain regions. However, monitoring of motor performance during a functional examination requires disruptive monitoring sensors, visual or auditory cued tasks, and strict subject compliance to stimuli. In this work, a simultaneous acquisition (SimulScan) was developed to provide dynamic images to monitor oropharyngeal motions during swallowing (1 mid-sagittal slice at 14.5 frames per second) simultaneous with functional MRI (24 oblique-axial slices with a TR of 1.6512 s). Data were acquired while three healthy adult subjects passively viewed a movie during three 15-min scans with the purpose of covertly studying uncued natural swallows. Dynamic MR images were used to determine timing of swallow onsets for subsequent functional analysis. Resulting functional maps show significant areas of activation that agree with previous functional magnetic resonance imaging studies of cued water swallows, except for regions associated with processing the task stimulus. SimulScan may prove a useful tool in developing new techniques for studying swallowing and associated neuromuscular disorders.