Article

An evaluation of the use of magnetic field maps to undistort echo-planar images.

MRC Cognition and Brain Sciences Unit, Cambridge, United Kingdom.
NeuroImage (Impact Factor: 6.25). 02/2003; 18(1):127-42. DOI: 10.1006/nimg.2002.1281
Source: PubMed

ABSTRACT When a head is placed in an MRI scanner, differences between the magnetic susceptibility of tissue, bone, and air distort the magnetic field. While some of the resulting inhomogeneity can be corrected by the shimming process, much of it cannot, and this causes distortion (sometimes referred to as geometric distortion) of echo-planar images (EPIs). One strategy for the correction of distortion is to acquire a map of the magnetic field achieved in each subject and then to use this to undistort their EPIs after reconstruction (). Here, we present five experiments to evaluate the application of such a strategy on data from a 3-T scanner. We show that after undistortion, the shape of EPIs is more similar to the true shape of the brain, and we investigate the effect of head movement on the efficacy of undistortion. If undistortion was applied first, it was found that less nonlinear warping was required to transform EPIs into a standard space, particularly in the phase-encode direction. We show that if SPM 99 normalization is used to perform a nonlinear warp to standard space, the prior application of undistortion increases the statistical power of group studies with motor and auditory tasks. We show that this increase in power is due to an increase in the overlap of activation of different subjects. Finally, we evaluate where in the brain undistorting EPIs might be expected to have the greatesteffect, in terms both of mislocalization of activationand of a reduction in power. Overall, undistorting EPIs using field maps has proved extremely successful, improving the anatomical localization of activation and increasing statistical power.

0 Bookmarks
 · 
155 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Resting state fMRI may help identify markers of risk for affective disorder. Given the comorbidity of anxiety and depressive disorders and the heterogeneity of these disorders as defined by DSM, an important challenge is to identify alterations in resting state brain connectivity uniquely associated with distinct profiles of negative affect. The current study aimed to address this by identifying differences in brain connectivity specifically linked to cognitive and physiological profiles of anxiety, controlling for depressed affect. We adopted a two-stage multivariate approach. Hierarchical clustering was used to independently identify dimensions of negative affective style and resting state brain networks. Combining the clustering results, we examined individual differences in resting state connectivity uniquely associated with subdimensions of anxious affect, controlling for depressed affect. Physiological and cognitive subdimensions of anxious affect were identified. Physiological anxiety was associated with widespread alterations in insula connectivity, including decreased connectivity between insula subregions and between the insula and other medial frontal and subcortical networks. This is consistent with the insula facilitating communication between medial frontal and subcortical regions to enable control of physiological affective states. Meanwhile, increased connectivity within a frontoparietal-posterior cingulate cortex-precunous network was specifically associated with cognitive anxiety, potentially reflecting increased spontaneous negative cognition (e.g., worry). These findings suggest that physiological and cognitive anxiety comprise subdimensions of anxiety-related affect and reveal associated alterations in brain connectivity.
    Journal of Cognitive Neuroscience 10/2013; · 4.49 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Echo-planar imaging (EPI) is a standard procedure in functional magnetic resonance imaging (fMRI) for measuring changes in the blood oxygen level-dependent (BOLD) signal associated with neuronal activity. The images obtained from fMRI with EPI, however, exhibit signal dropouts and geometric distortions. Parallel imaging (PI), due to its short readout, accelerates image acquisition and might reduce dephasing in phase-encoding distortions. The concomitant loss of signal-to-noise ratio (SNR) might be compensated through single-shot multi-echo EPI (mEPI). We systematically compared the temporal SNR and BOLD sensitivity of single echoes (TE=15, 45, and 75ms) and contrast-optimized mEPI with and without PI and mEPI-based denoising. Audio-visual stimulation under natural viewing conditions activated distributed neural networks. Heterogeneous SNR, noise gain, and sensitivity maps emerged. In single echoes, SNR and BOLD sensitivity followed the predicted dependency on echo time (TE) and were reduced under PI. However, the combination of echoes with mEPI recovered the quality parameters and increased BOLD signal changes at circumscribed fronto-polar and deep brain structures. We suggest applying PI only in combination with mEPI to reduce imaging artifacts and conserve BOLD sensitivity.
    NeuroImage 08/2013; · 6.25 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Respiration affects the bulk magnetic susceptibility, causing frequency shifts as shown in the brain at 7T. Due to the close proximity of the spine to the lungs, this effect is expected to be even larger in the spinal cord, resulting in detrimental B0 offset. The goal of this study was to quantify the effect of respiration on B0 variation in the spinal cord. Seven healthy subjects were scanned at 3T. Field maps were acquired during inspired and expired conditions. Frequency shift was quantified in the brain, brainstem, and cervico-thoracic spinal cord. A skewed Gaussian function with linear term was fitted to the frequency shift as a function of z-location along the spine. Large frequency shifts were measured along the cord, with a maximum of 74 Hz at C7 (P < 0.05), corresponding to 0.58 ppm. The proposed model was adequately fitted to the respiratory-induced frequency-shifts (adjusted R(2) = 0.9954). The morphology of subjects (weight and height) seemed to have an impact on the amplitude of frequency shift, although correlations were not significant. This study provides a deeper understanding of the contribution of respiration to B0 shift in the spinal cord. The proposed model can be useful for designing future hardware or software strategies to compensate for these B0 variations dynamically. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 01/2014; · 3.27 Impact Factor

Full-text (2 Sources)

View
222 Downloads
Available from
May 21, 2014