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 the control 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 vs BOLD ASL Less slow drifts Changes localized in cappilaries Reduced inter-subject variability Better functional localization BOLD Short TR Many thin slices Higher intrinsic SNR 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