Andrew L Janke1, Kieran O'Brian2, Steffen Bollmann1, Tobias Kober3, and Markus Barth1
1Centre for Advanced Imaging, University of Queensland, Brisbane, Australia, 2Siemens Healthcare Pty Ltd, Brisbane, Australia, 3Advanced Clinical Imaging Technology, Siemens Healthcare AG, Lausanne, Switzerland
Synopsis
7T
provides a method to see detailed image contrast in the human brain; the
MP2RAGE sequence allows 500um acquisition resolution.Purpose
Digital
MRI atlases serve to integrate data from differing modalities, stereotactic
localisation, automated region identification, automated segmentation and
direct comparisons between individuals [1]. While paper atlases can provide
exquisite detail of delineated structures, they are typically based upon an
individual subject's histology and as such make it difficult to identify
structures in novel subjects in an automated fashion. Here we generate a
minimum deformation average (MDA) from a population of subjects based upon high
resolution 7T MR imaging.
Method
32 (7 female, average age
33.4±9.4) individuals were imaged using a 7T whole-body Magnetom research
scanner (Siemens Healthcare, Erlangen, Germany) with a gradient strength of 70
mT/m, slew rate of 200 T/m/s and a 32-channel head coil (Nova Medical,
Wilmington, USA). T1w images were acquired using the prototype MP2RAGE sequence
with a range of resolutions: 0.5mm (2 indiv.), 0.75mm (21), 1.0mm (8) and 1.3mm
(1) isotropic. Common image parameters were TR= 4330ms, TI1/TI2=750/2370ms,
TE=2.8ms, flip angle=5,6, and GRAPPA = 3. The image matrix was typically
256x300x320 but was dependent upon coverage and FOV.
The MP2RAGE denoised images [6,
7] were first bias-field corrected using the N3 technique and
intensity-normalised using a histogram clamping technique. A probabilistic model
was then created using the method in Janke et al [2] and Grabner et al [4]. In the present case, the fitting strategy consisted of 2
linear fits to the evolving internal model followed by a hierarchical series of
non-linear grid transforms. These transforms started with a step size of 32mm
followed by 16mm, 12mm, 8mm, 6mm, 4mm and finished with 2mm. These fitting
steps use progressively de-blurred data with a 3D kernel FWHM of half the
current step size. Twenty iterations at each fitting stage were performed using
the ANIMAL algorithm [5]. As the step size decreased the resolution of the
evolving model to which data was being fit was increased, starting with a step
size of 1.0mm and finishing with a resolution of 0.3mm. Given the multiple
overlapping samples it is possible to increase the resolution to this point
without suffering from a lack of information at any point. Our technique
differs from Fonov et al's [3] during the intermediate model generation
in that a robust averaging process is used to reduce the effect of artefacts
and small handling tears in the brain. The averaging technique is a “winner
takes all” approach and as such places a lower weight on data at each voxel
that is greater than two standard deviations from the current model. This increases
the likelihood that a single minimum is achieved for the entire model. The
fitting process took approximately two weeks on a 250 core commodity Debian
GNU/Linux cluster.
Results
Representative
views are show in Figure 1
demonstrating the contrast that can be achieved in a 7T MDA model. Note in
particular the substructures emerging in the Hippocampus, deep brain stem/pons,
and the thalamus, which commonly shows very little internal structures on T1
weighted MRI.
Conclusion
The
increase in resolution and signal from the modelling process means that we can
now readily identify multiple thalamic and neocortical nuclei that are not
visible in individual subjects. In the future, we plan to release a complete
multi-modal model including segmentations and tissue density maps. Code is
available as part of MINC in the volgenmodel package and the model will be
available for download.
Acknowledgements
The authors acknowledge the facilities, and the scientific and technical assistance of the National Imaging Facility at the CAI, UQ.
MB acknowledges funding from ARC Future Fellowship grant FT140100865.
References
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Evans AC, Janke AL, Collins DL, Baillet S. Brain templates and atlases.
Neuroimage 2012
2.
Janke AL, Ullmann JFP, Robust methods to create ex-vivo minimum deformation
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3.
Fonov, V. et al. Unbiased average age-appropriate atlases for pediatric
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(2011).
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(2006).
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Kieran R., et al. "Robust T1-Weighted Structural Brain Imaging and
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Kieran R., et al. "Dielectric pads and low-B1+ adiabatic pulses:
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tesla." JMRI 40.4 (2014): 804-812.