jan 14, 2013 jason su
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Journal Club: Deoni et al. One Component? Two Components? Three? The Effect of Including a Nonexchanging ‘‘Free’’ Water Component in mcDESPOT . Jan 14, 2013 Jason Su. Motivation. mcDESPOT provides fast, whole-brain estimation of myelin water fraction - PowerPoint PPT PresentationTRANSCRIPT
JOURNAL CLUB:Deoni et al. One Component? Two Components? Three? The Effect ofIncluding a Nonexchanging ‘‘Free’’ Water Component in mcDESPOT.
Jan 14, 2013Jason Su
Motivation
• mcDESPOT provides fast, whole-brain estimation of myelin water fraction– However, its accuracy near CSF may be questionable due
to partial voluming– A third non-exchanging component in the tissue model
may allow this to be accounted for
• Lankford has recently criticized the precision of mcDESPOT with an unbiased fitting algorithm– Is this observed in practice with constrained SRC?
Theory• The mcDESPOT signal equations are expanded to include a 3rd
component or free water pool– This is a straightforward extension of the block diagonal matrices
with some key assumptions, 10 total parameters– Adds 3 new parameters: T1, T2, and volume fraction of the free
pool• 1500 < T1free <7500ms; 150ms < T2free < 1000ms; 0 < Ffree < 0.75
– The free pool is nonexchanging, avoids adding 2 possible new exchange rate parameters• Assumes that it models CSF separated by the blood-brain barrier
• The IE pool is at least 5% of the volume of the voxel
Concerns
• Is degeneracy possible? Can myelin and IE pools be confused with each other?– In 2-pool model, this can occur if MWF is allowed to be ≥0.5– What if here free pool is 0.2, the remaining 0.8 could be split
between MWF and IE interchangeably, is this taken care of?
• SRC algorithm– Ignoring some fundamental issues I’ve encountered, i.e. mean
normalization and off-resonance as a cyclic dimension– Not stated whether it usually converges or ends due to hitting
the iteration limit• In my simulations, it’s usually the latter
Methods• Simulation
– Simulated a 3-component model 50,000x each over a range of tissue parameters and fitted with 2- or 3-component models
• In-vivo at 3T – protocol is the same as with 2-component, 8 SPGR and 8 SSFP, and low-res IR-SPGR for DESPOT-HIFI B1 maps– Intra-subject repeatability
• 24yo male, 5 times over 5 weeks– Inter-subject variability
• 10 1yo infants• Why infants? To accentuate variability?
Results – Simulation
Notes
• Seems to be a fair amount of bias in Ffree estimate, almost 8%
• Histograms in general are fairly large with a width of about 0.08-0.11– My own simulations show similar or worse
performance in two-pool models with SNR of 20-30– Not sure how much noise was added in theirs
Product mcDESPOT
Modified mcDESPOT
Notes
• The 2-pool model tends to underestimate the MWF in regions near CSF compared to 3-pool– Makes sense, uses more of longer IE component as
a surrogate
• Would’ve liked to see this in MS brain with variety of lesions– Especially because there is the question of whether
edema would allow exchange with 3rd pool
Results – Curves
Notes
• BIC is a criterion that’s often used to compare fitting models– Penalizes for number of parameters, promotes a
simpler model to avoid overfitting– Closely related to Mallow’s Cp for linear models– Is a heuristic, cross validation is better for evaluating
predictive value of a model but may not work here
• SSFP fits are kind of iffy, particularly phase 0
Results – Reproducibility and Variability
Notes• Shows the characteristic behavior of low MWF CoV in
white matter, high CoV in gray• What registration was used for the inter-subject data?• Hard to interpret the intra-subject vs inter-subject results
– Are 1yo infants supposed to have about the same amount of myelination, how variable is it in development?
– Intra-subject variation is half of inter-subject, either MWF doesn’t vary much between infants or high variability in subject? Is 5 samples enough for CoV?
• Algorithm reproducibility is bad near edges/CSF?– Wonder what this looks like for 2-pool
MWF Variation in Infants
Deoni et al. Investigating white matter development in infancy and early childhood using myelin water faction and relaxation time mapping. Neuroimage. 2012 Nov 15;63(3):1038-53.
Discussion
• 3% deviation in homogeneous WM and GM compared to 2-pool– Is it worth changing model depending on location in brain?
• Is it possible in histology to examine if such a free pool in tissue is real? – The need for a 3rd pool arises out of need to account for
partial voluming, so phantoms should study that not necessarily 3-component mixture
– Examine the effects on 3rd pool as introduce more partial voluming, we could be able to precisely change the Ffree depending on voxel size?
Discussion
• Concerned about even more reduced precision compared to 2-pool– Variability of MWF near edges may indicate 3-pool
model is hard to estimate where it counts.– CoV of Ffree map?– Surprising that there is nothing added to the
acquisition• Can we really get a 3-pool fit for free from what we have
already?• CRLB would say otherwise.