Silke Kreitz1,2, Angelika Mennecke1, Laura Cristina Konerth 2, Armin Nagel3, Frederic Laun3, Michael Uder3, Arnd Doerfler1, and Andreas Hess2
1Department of Neuroradiology, University Hospital Erlangen, Friedrich-Alexander-University Erlangen-Nürnberg, Erlangen, Germany, 2Institute of Experimental and Clinical Pharmacology and Toxicology, Friedrich-Alexander University Erlangen-Nurnberg, Erlangen, Germany, 3Department of Radiology, University Hospital of the Friedrich-Alexander University Erlangen-Nürnberg, Erlangen, Germany
Synopsis
7T MRI is hoped to improve diagnostics, therapy
and research of neurological diseases. Here, we characterize the influence of
field strength on fMRI approaches including task based and RS-fMRI. Quality
metrics revealed a basic separation of 3T and 7T fMRI data, mainly by tSNR,
MDI, CNR and EFC. 7T fMRI showed higher BOLD response amplitudes, more
functional connections and higher connectivity strength especially in inferior
brain regions. Though higher variability between subjects at 7T likely requires
enhanced statistical power in group comparisons, intra individual fMRI
measurements might detect subtle connectivity changes at 7T useful for
diagnosis and therapy.
Introduction
Ultra-high field 7T MRI nourishes the hope for
a better diagnostic accuracy, therapy control and research of neurological
diseases due to improved spatial and temporal SNR. This was recently exemplified
in better detection of MS cortical lesions1 and resting state (RS)
differences in mood-related circuit disturbances in depression2
using 7T compared to 3 T. However, higher B0 inhomogeneity and more
susceptibility artifacts resulting in geometric distortion and signal
disturbances might hamper the advantages of higher field strength especially in
functional MRI (fMRI). Here, we comprehensively characterize the influence of field
strength on the outcome especially of fMRI approaches including task based and
RS-fMRI.Methods
A total of 18 healthy subjects (aged 19-55, 8
females) with no history of neurological diseases or psychiatric disorders were
scanned at 3T (Magnetom Skyra, Siemens, Germany) and 7T (Magnetom Terra,
Siemens, Germany).
Each session consisted of a standard MPRAGE anatomy
followed by three functional EPI scans (3T: TR=2000 ms, TE= 27.8 ms, isotropic
voxel resolution = 2 mm, matrix 126x126, FOV 25.2x25.2 cm, 72 slices, 7T:
TR=2000 ms, TE=21 ms, isotropic voxel resolution= 1.5 mm, matrix 168x168,
FOV=25.2x25.2 cm, 84 slices). The
different voxel resolutions for 3T and 7T were chosen to compensate for field
strength related differences in SNR. The first functional scan measured RS with
300 volumes (10 min), the second included a finger tapping motor task (7
repetitions of 7 sec right hand ft, 105 volumes, 3.5 min) followed by an
additional RS scan. To assess reliability of repeated measures, 9 subjects
performed no motor task in the second session.
Standard preprocessing was performed including
inter-slice time, motion correction, and spatial Gaussian smoothing (FWHM 4.5
mm). RS data were bandpass filtered between 0.009 Hz and 0.08 Hz and after
white matter and ventricle time course regression analyzed using a new approach
combining classical seed correlation analysis and graph theory3,4.
Initially, the following quality metrics were calculated using the first
inter-slice time and motion corrected RS measurement: tSNR, standardized DVARS5,
median distance index (MDI)6, global correlation (GCOR)7,
SNR8, CNR8, foreground to background energy ratio (FBER),
and entropy focus criterion9.
Task driven data were temporally Gaussian
smoothed (FWHM 4s) and analyzed using GLM to detect significantly activated
voxels. Each subject’s brain was individually parcellated into 158 brain
regions using the Havard-Oxford cortical and subcortical and the SUIT
cerebellar probabilistic atlas. For group comparison brain image were
registered to the MNI space using a
symmetric diffeomorphic warping (ANTS).
Reproducibility of repeated RS measurements
were assessed by cross correlation of the individual connectivity matrices
resulting from the RS analysis.Results
PCA of quality measures clearly revealed a
separation of measurements acquired with 3T and 7T along the first PC (Fig. 1A).
The spatial quality metrics CNR and EFC showed the strongest positive and the
temporal metrics tSNR and MDI the strongest negative loadings on that PC (Fig. 1B).
Finger tapping motor task induced activated voxels in
several brain regions including left and right motor and sensory cortex, middle
and superior frontal gyrus and cerebellum. The total activated volume did not
significantly differ according to field strength, though locally the cerebellum
was more activated with 7T (Fig. 2A). However, the regions specific maximum
response amplitude was significantly enhanced in 7T data. Moreover, the shape
of the 7T response amplitude indicated a biphasic response which could not be
observed in the 3T data (Fig. 2B).
RS connectivity was enhanced at 7T, which was
reflected in both enhanced number of significant connections and average
connectivity strength (Fig. 3A). Network based statistics (NBS)10
revealed that especially inferior regions (subcortical, temporal and
cerebellar) showed enhanced connectivity strength (Fig. 3B). Additionally, only
with 7T we could detect significant motor task induces alterations in RS
connectivity (second vs. first RS measurement) especially in the contralateral
motor cortex and middle frontal gyrus (Fig. 3C).
Reproducibility of RS connectivity between
subjects were significantly higher using 3T compared to 7T, but reproducibility
within subjects showed no difference according to the field strength (Fig. 4).Discussion
Since we compensated for spatial SNR by adapting
voxel resolution, the dominant effect of SNR on data quality observed by others11
was reduced in this study. Here, data quality was dominantly impacted by EFC
(related to susceptibility and motion artifacts), MDI, CNS and tSNR. Presumably
due to the higher BOLD
[MO1] amplitude, the field strength mainly
affects the brain’s functional connectivity rather than task related statistical
parametric maps. Though the higher variability between subjects at 7T likely
requires enhanced statistical power in group comparisons, intra individual ultra-high
field functional MRI measurements might detect connectivity changes that are
overlooked using 3T MRI. These small changes can be useful hints for diagnosis
and therapy success. In particular, the improved detectability of the inferior
brain region circuitry with 7T MRI greatly enhances future investigations of
diseases involving such subcortical regions e.g. in depression (as shown by Morris
et al.2) or Parkinson’s disease.Acknowledgements
No acknowledgement found.References
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