Download - Summer Work
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Brain Atlas
Dan Diner
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Introduction
• PET is a nuclear imaging technique• A radiotracer is injected into subject, and the
signal that it sends off is captured and measured to find out how much activity occurs and where
• The information that is collected is used for brain study, and disorder analysis and diagnosis
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ROI/VOI Mapping
• ROIs(Regions of Interest) are small regions mapped onto brains
• Most common way of mapping is manual• Manual mapping is very time-consuming and
requires highly-paid personnel (raters)• Raters have inter- and intra- subject variation• As PET scan quality increases, so does the
time it takes for raters to do their job
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• PET has very poor spatial resolution
• MRI has very high spatial resolution
• ROIs are therefore mapped onto the MRI of a subject, onto which the PET scan is co-registered
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MRI PET Co-Registered
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Brain Atlases• An “average brain” (co-registered PET/MRI)• Has all VOIs labeled manually onto it• These automated methods have been developed to label
VOIs (Volumes of Interest)• Never have been as accurate as manual methods• Very precise - have no inter- or intra- subject variation• Take drastically less time to label a brain than manual
methods
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Brain Atlases
• Many imaging labs have their own, but we could not use another labs’ because each lab has different MRI acquisition and labeling techniques
• Can be single- or multiple- subject template• Could be made for only one specific region or
many• Probabilistic atlases tell you the probability of
a certain voxel belonging to a certain ROI
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Methods and M aterials: Subject and Image Acquisition
• MRIs and PETs from 176 subjects of past studies
• 123 from a 1.5 Tesla MRI
• 53 from a 3.0 Tesla
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Image Analysis Setup
• Worked on a PowerMac G5 using Mac OS X 10.5.4 Leopard
Major Software used:• Matlab (R2007a)• FSLVIEW and FLIRT• ART• iView
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Image Pre-Processing
• Raw MRI files were
copied to a common
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• Previously made Gray Matter,
White Matter, and Cerebral Spinal Fluid Masks were multiplied(matrix multiplication) against the raw
files to get rid of all extraneous
material (neck, skull, dura matter)
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QuickTime™ and a decompressor
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QuickTime™ and a decompressor
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• Used iView to compare new images against raw ones
• We did this to ensure that all extraneous material was gone and that no brain was accidentally segmented
• I used a scale of 0-3 to rate the new images (0= perfect, 1=some dura, 2=much dura, 3=segmented brain)
• Most images were 0s and 1s.
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Normalization Procedure
• MNI template to which were were normalizing was reoriented towards raw MRI native space
• New MRI images also normalized to this template
• We examined the normalized images, and found that some were very well aligned, but others were misplaced or improperly warped
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• ART uses a 2-step process to warp MRIs: linear registration, then warp
• Linear registration is rotations• Warping is dilations• Because the rotation was bad, we thought
that the linear registration had gone wrong• We tried linear registration(first 9 Degrees of
Freedom) with FLIRT, and then warping(up to 12 dof) with ART
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• The rotations came out looking good, but close inspection revealed that some leftover dura got warped into the new brain
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• An MRI with a particularly large amount of dura was manually cleaned, and then had FLIRT and ART run on it
• The resulting image was perfect
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Conclusion
• ART is very sensitive to extraneous material
• all dura needs to be properly removed from all images before normalization
• A better automatic method for cleaning up dura needs to be developed
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