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Variable slice thickness (VAST) EPI for the reduction of susceptibility artifacts in whole-brain GE-EPI at 7 Tesla

  • Sascha Brunheim
  • Sören Johst
  • Viktor Pfaffenrot
  • Stefan Maderwald
  • Harald H. Quick
  • Benedikt A. Poser
Research Article

Abstract

Objective

A new technique for 2D gradient-recalled echo echo-planar imaging (GE-EPI) termed ‘variable slice thickness’ (VAST) is proposed, which reduces signal losses caused by through-slice susceptibility artifacts, while keeping the volume repetition time (TR) manageable. The slice thickness is varied across the brain, with thinner slices being used in the inferior brain regions where signal voids are most severe.

Materials and methods

Various axial slice thickness schemes with identical whole-brain coverage were compared to regular EPI, which may either suffer from unfeasibly long TR if appropriately thin slices are used throughout, or signal loss if no counter-measures are taken. Evaluation is based on time-course signal-to-noise (tSNR) maps from resting state data and a statistical group-level region of interest (ROI) analysis on breath-hold fMRI measurements.

Results

The inferior brain region signal voids with static B0 inhomogeneities could be markedly reduced with VAST GE-EPI in contrast to regular GE-EPI. ROI-averaged event-related signal changes showed 48% increase in VAST compared to GE-EPI with regular “thick” slices. tSNR measurements proved the comparable signal robustness of VAST in comparison to regular GE-EPI with thin slices.

Conclusion

A novel acquisition strategy for functional 2D GE-EPI at ultrahigh magnetic field is presented to reduce susceptibility-induced signal voids and keep TR sufficiently short for whole-brain coverage.

Keywords

fMRI Echo-planar imaging Susceptibility artifact Repetition time Slice thickness 7 Tesla ultrahigh field MRI 

Notes

Authors’ contribution

S Brunheim: project development, programming, data collection, data analysis, manuscript writing. S Johst: project development, programming, data collection, manuscript editing. V Pfaffenrot: data collection, data analysis, manuscript editing. S Maderwald: data collection, manuscript editing. HH Quick: project supervision, manuscript editing. BA Poser: project supervision, project development, programming, data analysis, manuscript editing.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflicts of interest.

Ethical approval

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

Informed consent

Informed consent was obtained from all individual participants included in the study.

Supplementary material

10334_2017_641_MOESM1_ESM.tiff (13 mb)
Supplementary Figure 1 Mean signal intensity distribution of a spherical polydimethylsiloxane (PDMS) oil phantom (diameter =  165 mm, T1 = 1150 ms, T2 = 750 ms) for all three VAST techniques and the 3-mm control on a central coronal slice (left) and all from top to bottom head-feet sorted axial slices (right). The common measurement parameters were the same as for the in-vivo tSNR maps (Table 1, dagger), except that no fat saturation was applied. Thus, with a GRAPPA acceleration factor of R = 3 (48 ACS lines), a total of 51 slices could be acquired within TR = 2 s, resulting in different volume coverage. For the different slice thicknesses of VAST GE-EPI, only the slice repositioning in the sequence was conducted, but not the application of the inverse proportional intensity re-scaling factor. Moreover, here, the slices with different thicknesses were not re-gridded and interpolated onto the common voxel size of (1.5 x 1.5 x 3) mm3. Hence, especially the transition between one slice thickness to another can be noticed (TIFF 13343 kb)
10334_2017_641_MOESM2_ESM.tiff (10.6 mb)
Supplementary Figure 2 Mean signal intensity distribution with inverse proportional intensity re-scaling and an interpolation onto the common voxel resolution of 6.75 mm3. In contrast to the data shown in Supplementary Figure 1, the total slice count after re-gridding and interpolation amounts to 26. Accordingly, the corresponding slices of the directly measured 3-mm control were chosen for qualitative comparison. The apparent signal intensity gradient in the head-foot direction is due to the receive channel distribution of the Nova coil (TIFF 10894 kb)
10334_2017_641_MOESM3_ESM.tiff (14.5 mb)
Supplementary Figure 3 tSNR map comparison for the PDMS oil phantom of the three different VAST variants and the 3-mm control GE-EPI, based on the re-scaled and interpolated measurements presented in Supplementary Figure 2. The interpolated smaller Δz blocks of the VAST variants are marked with white boxes and show distinct block-wise tSNR variations in combination with the mean signal intensity reduction from cranial to caudal. Nevertheless, the cranial parts of the different methods, equally measured with Δz = 3 mm, prove a good overall agreement of the slice-by-slice median tSNR between all techniques (TIFF 14883 kb)

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Copyright information

© ESMRMB 2017

Authors and Affiliations

  1. 1.Erwin L. Hahn Institute for Magnetic Resonance Imaging, University of Duisburg-EssenEssenGermany
  2. 2.High Field and Hybrid MR Imaging, University Hospital EssenEssenGermany
  3. 3.Department of Cognitive Neuroscience, Faculty of Psychology and NeuroscienceMaastricht Brain Imaging Centre, Maastricht UniversityMaastrichtThe Netherlands

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