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

3. 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

4. Image subtraction - - - -

5. Terms: CASL: continious arterial spin labeling
PASL: pulsed arterial spin labeling

6. 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

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