document: Multiecho segmented EPI with z-shimmed background gradient compensation -LRB- MESBAC -RRB- pulse sequence for fMRI A MultiEcho Segmented EPI with z-shimmed BAckground gradient Compensation -LRB- MESBAC -RRB- pulse sequence is proposed and validated for functional MRI -LRB- fMRI -RRB- study in regions suffering from severe susceptibility artifacts . This sequence provides an effective tradeoff between spatial and temporal resolution and reduces image distortion and signal dropout . The blood oxygenation level-dependent -LRB- BOLD -RRB- - weighted fMRI signal can be reliably obtained in the region of the orbitofrontal cortex -LRB- OFC -RRB- . To overcome physiological motion artifacts during prolonged multisegment EPI acquisition , two sets of navigator echoes were acquired in both the readout and phase-encoding directions . Ghost artifacts generally produced by single-shot EPI acquisition were eliminated by separately placing the even and odd echoes in different k-space trajectories . Unlike most z-shim methods that focus on increasing temporal resolution for event-related functional brain mapping , the MESBAC sequence simultaneously addresses problems of image distortion and signal dropout while maintaining sufficient temporal resolution . The MESBAC sequence will be particularly useful for pharmacological and affective fMRI studies in brain regions such as the OFC , nucleus accumbens , amygdala , para-hippocampus , etc
doc_bio_tags: B I I O O O O O O O O O O O B O B I I O O O O O O O O O O O O O O O O O O B O O O O O O B I I O O O O O O O O O O B I O O B I O B I O O O O O O O O O O B O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O B I O B I O O O B I O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O
extractive_keyphrases: fmri multiecho segmented epi severe susceptibility artifacts orbitofrontal cortex navigator echoes ghost artifacts signal dropout temporal resolution image distortion
abstractive_keyphrases: z-shimmed background gradient compensation gradient compensation pulse sequence bold-weighted signal event-related functional brain mapping spatial resolution