Joelle E. Sarlls1, Francois Lalonde2, Joellyn Stolinski1, Maxim Zaitsev3, and Lalith Talagala1
1NMRF, National Institutes of Health, Bethesda, MD, United States, 2NIMH, National Institutes of Health, Bethesda, MD, United States, 3MR Development and Application Center, University Medical Center Freiburg, Freiburg, Germany
Synopsis
Higher
resolution anatomical images can provide more accurate estimates of
morphometric measures extracted from segmentation algorithms. Acquisition of
submillimeter MPRAGE data at 3T requires long scan times making it prone to
subject motion. Prospective motion correction (PMC) techniques can mitigate
these effects by tracking brain motion and updating scan parameters
accordingly. Here, we compare
morphometric measures from 1.0mm and 0.6mm isotropic resolution MPRAGE data
obtained at 3T while using PMC. Results
show that good quality 0.6mm MPRAGE data can be acquired with PMC in
approximately 30minutes. Increased cortical thickness in some brain regions is
seen with higher resolution data.
Introduction
Morphometry measures are valuable markers in
Neuroscience. It has
been shown that higher resolution (0.75 mm) MPRAGE data at 7T provide more
accurate estimates of morphometric measures than 1mm data1. Submillimeter MPRAGE data at 3T with sufficient
contrast-to-noise ratio (CNR) requires significantly longer scan times than at
7T, making them prone to subject motion.
Subject moton has deleterious effects on high-resolution structural MRI.
The image artifacts from motion not only result in poor image quality, but also
bias the anatomical morphormetric measures extracted from segmentation
algorithms2. Prospective
motion correction (PMC) techniques can mitigate these effects by tracking brain
motion and updating the scan parameters accordingly3. It has been
shown that the reproducibility and accuracy of cortical and subcortical
measures (thickness
and volumes) can be significantly improved when using PMC
techniques in the presence of motion4-8. In this study, we compare morphometric
measures from 1.0 mm and 0.6 mm isotropic resolution MPRAGE data obtained at
3.0 T while using PMC to mitigate effects due to subject motion.Methods
MPRAGE data were acquired
on healthy adults (n=8) at 3T
with a 32-channel head coil using PMC. The
following scan parameters were used for 1mm data: TE/TR=2.2/2530ms, TI=1100ms,
FA=7°, iPAT = 2, FAT
saturation, 1x1x1mm,
scan time = 6 min 2 s. The following
scan parameters were used for 0.6mm data:
TE/TR =2.5/5000ms, TI=1340ms, FA=6°, no iPAT, FAT saturation, 0.6x0.6x0.6, scan time = 33 min 20 s. PMC was accomplished using a Moire phase
tracking marker and optical system (KinetiCor, HI, USA)9. The maker
was affixed to a plastic rod (4 cm) glued to a sports mouth guard (ArmourShield,
Under Armour, MD, USA) customized for each subject by molding to fit the upper
teeth. Motion parameters generated by the camera were captured by the scanner
and used to update the gradient and RF pulses to maintain the scan plane and
FOV before acquisition of each k-space line.
Data was also acquired at 0.6mm without PMC, using the same scan
parameters for 4 subjects. The cortical morphometry
measures were extracted using the
developmental version of FreeSurfer (dated August 7th, 2018) with modification to process submillimeter
data at native resolution10. The
difference in morphometry measures between the 0.6mm and 1mm data were
calculated and normalized to that of 1mm data. All measures were tested for significant
differences using paired T-tests between 1mm and 0.6mm data. After Bonferroni correction for multiple
comparisons, the level for significance was p<0.00074. To assist in comparision with existing literature,
the SNR was calculated as average intensity of whitter matter voxels divided by
the standard deviation, while CNR was the difference between the average white
matter and gray mater voxel intensities divided by the standard deviation of white
matter intensities1. White
and gray matter voxels were defined by FreeSurfer in the aseg.mgz volume.Results
All
subjects completed the scans without any discomfort due to the mouth guard and displayed minimal motion throughout
the scans. The mean SNR was 9.2 and 9.7,
CNR was 3.0 and 2.9 for the 1mm and 0.6mm data, respectively. Representative
images of acquired at 1mm, 0.6mm, and 0.6mm without PMC are shown in Fig. 1. There is greater definition of the gray/white matter boundary and small
structures in the 0.6mm data with PMC compared to other scans. Motion artifacts are present as ringing and
blurring in the 0.6mm data without PMC, indicating the necessity of PMC for
high resolution structural scans that require long scan times. Figure 2 shows magnified regions from the images shown in
Fig. 1 to further illustrate the improved definition of small structures, like
the gray matter of the striatum and white matter in the hippocampus for data
acquired at 0.6mm using PMC. Figures 3 shows cortical thickness measures of
the left hemisphere on an inflated surface, overlaid with boundaries of
cortical parcellation. Five regions (cuneus,
lateral-occipital, lingual, pericalcarine, and post-central) were found to show
significantly different mean thickness between 0.6mm and 1mm data in both the
left and right hemispheres. The cortical thickness measures for the left hemisphere of
these 5 regions are shown in Fig. 4. The
right hemisphere showed similar results.
The cortex is thicker in 0.6mm than 1mm data in all significant regions. The The greatest mean difference is seen in the pericalcarine
region with 24.4% and 26.3% for the left and right hemispheres, respectively.Discussion
This study shows the
feasibility of acquiring high quality MPRAGE data at sub-millimeter resolution
(0.6mm isotropic) at 3T. Because of the long scan times required, prospective
motion correction is necessary to eliminate image blurring due to involuntary
subject motion. Our results indicate
that high resolution data at 0.6mm show significantly increased cortical
thickness in regions that are typically among the thinnest11, similar
to 7T results reported recently1. Increased cortical thickness is
attributed to improved definition of gyral and sulcal surfaces exhibited in
0.6mm data. In this study, high
resolution images did not reveal regions with reduced cortical thickness that
reached the level of significance, although such areas have been identified at
7T1. This maybe due to reduced
CNR in 3T high resolution images compared to 7T (2.9 vs 3.9) or differences in
analysis methods. Acknowledgements
No acknowledgement found.References
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