Arterial spin labeling

1. Arterial spin labeling Edith Liemburg Ze Wang et al., 2008

2. 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

3. 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

4. Image subtraction - - - -

5. Terms: • CASL: continious arterial spin labeling • PASL: pulsed arterial spin labeling http://www.umich.edu/~fmri/asl.html

6. 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

7. 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

8. 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

9. 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

10. Motion correction • Signal intensity difference  motion • Seperate realignment vs combined: PCASL or background suppression

11. 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)

12. 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

13. Statistical analysis • Simple subtraction increases peak t-value in visual cortex • Including the global signal increases peak t-value & cluster size

14. 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