Imaging Brain Regions with Susceptibility-induced Signal Losses using Gradient and Spin Echo Techniques

Abstract

Most fMRI studies use gradient-echo (GE) echo planar imaging (EPI) technique for acquiring blood-oxygenation level-dependent signals. Signal losses occur in the GE-EPI due to macroscopic field gradients occurring at air-tissue boundaries such as the orbitofrontal cortex. The spin-echo (SE) EPI is preferentially sensitized to smaller vessels and is considerably more robust against signal dropouts at air-tissue interfaces. We used double echo EPI pulse sequence with simultaneous acquisition of gradient-echo (GE) and Spin-echo (SE) signal on a 3T Siemens Magnetom Trio Scanner. A custom-built fMRI compatible olfactometer was used to deliver two appetitive and two aversive odours. In an event-related experiment design, the colour of the fixation cue (red or green) prompted participants to breathe-out and breathe-in the odour delivered for 2sec followed by intensity or pleasantness ratings on a visual analog scale. Brain activation was assessed at the onset of breathe-in cue and thresholded maps (p<0.005) were combined for the four subjects and two odours within each valence to depict the number of comparisons (out of a total of 8) that survived the threshold criterion. The results suggest that SE acquisitions might particularly be suitable for studies that focus on frontal, temporal and striatal regions, while the GE acquisitions might be suitable for studies focussing on parietal and occipital regions.


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