Transcript Arterial spin labeling

Arterial spin labeling
Edith Liemburg
Ze Wang et al., 2008
Introduction to ASL
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PET-like for direct CBF measurement
Measurement of slow neural changes
Absolute quantification of blood flow
http://www.umich.edu/~fmri/asl.html
Principle of ASL
1. Tag inflowing arterial
blood by magnetic
inversion
3. Repeat experiment without
tag
4. Acquire the control image
2. Acquire the tag image
http://www.umich.edu/~fmri/asl.html
Image subtraction
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Terms:
• CASL: continious arterial spin labeling
• PASL: pulsed arterial spin labeling
http://www.umich.edu/~fmri/asl.html
ASL vs BOLD
ASL
 Less slow drifts
 Changes localized
in cappilaries
 Reduced intersubject variability
 Better functional
localization
BOLD
 Short TR
 Many thin slices
 Higher intrinsic
SNR
http://www.umich.edu/~fmri/asl.html
Introduction: ASL analysis
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Low intrinsic SNR: signal only 1 – 5%
of mean MR signal intensity
CBF calculation  intesity difference
Critical preprocessing steps:
• (Bit resolution)
• Motion correction
• Spatial smoothing & normalization
• Global spike elimination
• Measure of global signal as covariate
ASL-toolbox
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Ze Wang et al.
http://www.cfn.upenn.edu/perfusion
Raw image to CBF conversion
3 methods of calculation
• Simple subtraction
• Zinc subtraction
• Surround subtraction
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Also unscaled (perf) and pseudoBOLD images
Material and Methods
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Visual stimuli and finger tapping task
in blockdesign (n = 10)
CASL: 64 x 64 x 12 voxels (slice 6
mm), TR = 3, labelling 1.6 s, delay
800 ms
Pseudo-CASL sequence
CBF calibration:
f = CBF, ΔM = control – label signal, R1a = longitudinal
relaxation rate of blood (R1a=0.67 s−1), τ = labeling time (1.6 s), ω = delay
time (0.8 s), α = labeling efficiency (0.68), λ = blood/tissue water partition
coefficient (0.9 g/mL) & M0 ~ control image intensity
Motion correction
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Signal intensity
difference  motion
Seperate realignment
vs combined: PCASL or
background suppression
Smoothing & normalization
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Smoothing before CBF calcutation 
increased SNR & outlier reduction
Normalization after CBF calculation &
first level  before minor signal
changes & incorrect
time lag estimation
(reslicing)
Spike elimination
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Combined realignment  average &
substraction of motion paramaters of
adjecent control & label images
Criteria: averaged translation or
rotation > 3 mm/o, subtracted
translation or rotation > 0.8 mm/o,
global CBF + 3 standard deviations
Statistical analysis
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Simple subtraction increases peak tvalue in visual cortex
Including the global signal increases
peak t-value & cluster size
Proposed preprocessing steps
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Realignment: seperate for control & label
Coregistration: labelled to control
Smoothing
Exclude extracranial voxels (brain mask)
CBF calculation: simple subtraction
First level analysis: global CBF as covariate (no
explicit masking, no high-pass filter, no HRF)
• Masking: from mean CBF image
• Global spike elimination: according to criteria
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Coregistration: mean control to anatomy, con
as other
Normalization: anatomy to template
Second level