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Neuroimage Clin. 2016 Apr 12;11:539-547. doi: 10.1016/j.nicl.2016.03.022. eCollection 2016.

Choosing the polarity of the phase-encoding direction in diffusion MRI: Does it matter for group analysis?

Author information

1
Brain Center Rudolf Magnus, Department of Psychiatry, University Medical Center Utrecht, Utrecht, The Netherlands; Research Center, Military Mental Healthcare, Ministry of Defence, Utrecht, The Netherlands. Electronic address: mitzykennis@gmail.com.
2
Brain Center Rudolf Magnus, Department of Psychiatry, University Medical Center Utrecht, Utrecht, The Netherlands; Research Center, Military Mental Healthcare, Ministry of Defence, Utrecht, The Netherlands; Department of Psychiatry and Behavioral Sciences, Emory University School of Medicine, Atlanta, GA, USA.
3
Brain Center Rudolf Magnus, Department of Psychiatry, University Medical Center Utrecht, Utrecht, The Netherlands.
4
Brain Center Rudolf Magnus, Department of Psychiatry, University Medical Center Utrecht, Utrecht, The Netherlands; Research Center, Military Mental Healthcare, Ministry of Defence, Utrecht, The Netherlands.
5
Image Sciences Institute, University Medical Center Utrecht, Utrecht, The Netherlands.

Abstract

Notorious for degrading diffusion MRI data quality are so-called susceptibility-induced off-resonance fields, which cause non-linear geometric image deformations. While acquiring additional data to correct for these distortions alleviates the adverse effects of this artifact drastically - e.g., by reversing the polarity of the phase-encoding (PE) direction - this strategy is often not an option due to scan time constraints. Especially in a clinical context, where patient comfort and safety are of paramount importance, acquisition specifications are preferred that minimize scan time, typically resulting in data obtained with only one PE direction. In this work, we investigated whether choosing a different polarity of the PE direction would affect the outcome of a specific clinical research study. To address this methodological question, fractional anisotropy (FA) estimates of FreeSurfer brain regions were obtained in civilian and combat controls, remitted posttraumatic stress disorder (PTSD) patients, and persistent PTSD patients before and after trauma-focused therapy and were compared between diffusion MRI data sets acquired with different polarities of the PE direction (posterior-to-anterior, PA and anterior-to-posterior, AP). Our results demonstrate that regional FA estimates differ on average in the order of 5% between AP and PA PE data. In addition, when comparing FA estimates between different subject groups for specific cingulum subdivisions, the conclusions for AP and PA PE data were not in agreement. These findings increase our understanding of how one of the most pronounced data artifacts in diffusion MRI can impact group analyses and should encourage users to be more cautious when interpreting and reporting study outcomes derived from data acquired along a single PE direction.

KEYWORDS:

Data quality; Diffusion tensor imaging; EPI distortions; Fractional anisotropy; Group analysis; Phase-encoding polarity; Susceptibility artifacts

PMID:
27158586
PMCID:
PMC4845159
DOI:
10.1016/j.nicl.2016.03.022
[Indexed for MEDLINE]
Free PMC Article

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