A method to measure arbitrary k-space trajectories was developed to compensate for nonideal gradient performance during rapid magnetic resonance (MR) imaging with actively or nonactively shielded gradients at a magnetic field strength of 4.1 T. Accurate MR image reconstruction requires knowledge of the k-trajectory produced by the gradient waveforms during k-space sampling. Even with shielded gradients, residual eddy currents and imperfections in gradient amplifier performance can cause the true k-space trajectory to deviate from the ideal trajectory. The k-space determination was used for spiral gradient-echo imaging fo the human brain. While individual calibrations are needed for new pulse sequences, the method of k-space determination can be used for any sequence of preparation pulses and readout gradient waveforms and should prove useful for other trajectories, including the rastered lines of echo-planar imaging.

Background/Objectives Resting-state functional magnetic resonance imaging was used to examine for changes in intrinsic functional brain connectivity associated with postoperative changes in cognition; a common complication in seniors undergoing major surgery. Design Objective cognitive testing and functional brain imaging were prospectively performed at preoperative baseline and 6-weeks after surgery, and at the same time intervals in non-surgical controls. Setting Academic medical center. Participants 12 senior patients undergoing cardiac surgery and 12 non-surgical senior controls with a history of coronary artery disease; both groups without cognitive impairment at preoperative baseline (MMSE > 27). Measurements Differences in resting-state functional connectivity (RSFC) and global cognitive change relationships were assessed using a voxel-wise intrinsic connectivity method, controlled for demographic factors and pre- and perioperative cerebral white matter disease volume. Analyses were corrected for multiple comparisons, p-FDR <0.01. Results Global cognitive change after cardiac surgery was significantly associated with intrinsic RSFC changes in regions of the posterior cingulate cortex (PCC) and right superior frontal gyrus (rSFG); anatomical and functional locations of the brain's default mode network (DMN). No statistically significant relationships were found between global cognitive change and RSFC change in the non-surgical controls. Conclusion Clinicians have long known that some older patients develop postoperative cognitive dysfunction (POCD) after anesthesia and surgery, yet the neurobiological correlates of POCD are not well defined. Our results suggest that altered resting state functional connectivity within specific DMN regions is positively correlated with global cognitive change 6-weeks after cardiac surgery, suggesting that DMN activity and connectivity could be important diagnostic markers of POCD and/or intervention targets for potential POCD treatment efforts.

Spatial attention to a visual stimulus that occurs synchronously with a task-irrelevant sound from a different location can lead to increased activity not only in visual cortex, but also auditory cortex, apparently reflecting the object-related spreading of attention across both space and modality (Busse et al., 2005). The processing of stimulus conflict, including multisensory stimulus conflict, is known to activate the anterior cingulate cortex (ACC), but the interactive influence on the sensory cortices remains relatively unexamined. Here we used fMRI to examine whether the multisensory spread of visual attention across the sensory cortices previously observed will be modulated by whether there is conceptual or object-related conflict between the relevant visual and irrelevant auditory inputs. Subjects visually attended to one of two lateralized visual letter streams while synchronously occurring, task-irrelevant, letter sounds were presented centrally, which could be either congruent or incongruent with the visual letters. We observed significant enhancements for incongruent versus congruent letter-sound combinations in the ACC and in the contralateral visual cortex when the visual component was attended, presumably reflecting the conflict detection and the need for boosted attention to the visual stimulus during incongruent trials. In the auditory cortices, activity increased bilaterally if the spatially discordant auditory stimulation was incongruent, but only in the left, language-dominant side when congruent. We conclude that a conflicting incongruent sound, even when task-irrelevant, distracts more strongly than a congruent one, leading to greater capture of attention. This greater capture of attention in turn results in increased activity in the auditory cortex.