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Arterial spin labeling. Edith Liemburg Ze Wang et al., 2008. Introduction to ASL. 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.
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Arterial spin labeling Edith Liemburg Ze Wang et al., 2008
Introduction to ASL • 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 2. Acquire the tag image 3. Repeat experiment without tag 4. Acquire thecontrol image http://www.umich.edu/~fmri/asl.html
Image subtraction - - - -
Terms: • CASL: continious arterial spin labeling • PASL: pulsed arterial spin labeling http://www.umich.edu/~fmri/asl.html
ASL Less slow drifts Changes localized in cappilaries Reduced inter-subject variability Better functional localization BOLD Short TR Many thin slices Higher intrinsic SNR ASL vs BOLD http://www.umich.edu/~fmri/asl.html
Introduction: ASL analysis • 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 • 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 • Also unscaled (perf) and pseudo-BOLD images
Material and Methods • 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 • Signal intensity difference motion • Seperate realignment vs combined: PCASL or background suppression
Smoothing & normalization • 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 • 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 • Simple subtraction increases peak t-value in visual cortex • Including the global signal increases peak t-value & cluster size
Proposed preprocessing steps • 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 • Coregistration: mean control to anatomy, con as other • Normalization: anatomy to template • Second level