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ASL Papers
Recommended implementation of arterial spin-labeled
perfusion MRI for clinical applications: A consensus of the
ISMRM perfusion study group and the European consortium
for ASL in dementia.
Alsop DC1, Detre JA, Golay X, Günther M, Hendrikse J,
Hernandez-Garcia L, Lu H, Macintosh BJ, Parkes LM, Smits M,
van Osch MJ, Wang DJ, Wong EC, Zaharchuk G.
Magn Reson Med. 2014 Apr 8
Arterial Spin Labeling (ASL) for
Brain Imaging
Consensus and Recommendations by
the Experts
An introduction to ASL labeling techniques.
Wong EC.
J Magn Reson Imaging. 2014 Jul;40(1)
MRI Journal Club 9/18/2014
Schematic Diagram of ASL
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Recommendations
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Hardware considerations
Hardware considerations;
Labeling approaches;
Time delay between labeling and imaging;
Background suppression;
Readout approaches;
Post-processing methods;
ASL in the clinical setting.
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Labeling approaches
• Continuous labeling
• 3T
Over a long period of 1 – 3 second as blood flow
through a labeling plane. (Flow driven adiabatic
inversion)
– CASL: a single long labeling pulse
– PCASL; a series (>1000) short (~1us) pulses
– Higher SNR
– Longer T1 (longer life time of tracer)
• Multichannel head coil
– Higher SNR
– Parallel imaging to reduce echo time
• Pulsed labeling
A single or a few short pulses over 10-20 ms
• Velocity selective labeling
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CASL/PCASL vs PASL
CASL/PCASL vs PASL
~20cm/s
10-20cm
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MRI Journal Club 9/18/2014
Flow Driven Adiabatic Inversion
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Labeling vs Control
Labeling (B1ave ≠ 0, Gave ≠ 0)
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MRI Journal Club 9/18/2014
Control (B1ave = 0, Gave = 0)
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Advantages of PCASL over CASL
Labeling RF train and duration (t)
• Reduced subtraction error due to MT effect
• Higher labeling efficiency with longer duration
• Diminishing returns for label durations much longer
than the T1 of blood, 1-e(-t/T1)
• Longer durations increase TR, and thereby decrease
the number of averages obtained per unit time.
• Just below the inferior border of the cerebellum to
ensure labeling of the posterior cerebral circulation.
• Avoid labeling in regions of strong susceptibility
artifacts (inefficient labeling if off-resonance)
– Larger gradient
– Increased frequency offset to brain tissue
– Less saturation from magnetization transfer
– Reduced subtraction error between control and
labeling
• Compatible with existing RF amplifier
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Magn Reson Med. 2008 December ; 60(6): 1488–1497
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Post Labeling Delay (PLD)
CASL/PCASL vs PASL
• For CBF quantification using PCASL, ideally,
PLD should be just longer than the longest
ATT.
• Areas of low ASL signal may reflect some
combination of low CBF and unusually long
ATT.
• Multi-TI/PLD method is need for the
estimation of ATT or the most precise
quantitation of CBF.
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MRI Journal Club 9/18/2014
PCASL: average labeling gradient 1 mT/m
PCASL: slice-selective labeling gradient 10 mT/m
PCASL: average B1 1.5 mT
PCASL labeling duration 1800 ms
PCASL PLD: neonates 2000 ms
PCASL PLD: children 1500 ms
PCASL PLD: healthy subjects <70 y 1800 ms
PCASL PLD: healthy subjects >70 y 2000 ms
PCASL PLD: adult clinical patients 2000 ms
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PASL TI1 800 ms
PASL TI Use PCASL PLD
(from above)
PASL labeling slab thickness 15–20 cm
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• In gray matter, perfusion replaces 1% of the brain
water with in-flowing blood water every second
• The difference between label and control images is
typically <1% of the relaxed brain signal.
• Subject motion produces signal fluctuations (noise
and/or artifacts) that are proportional to the signal
intensity in the un-subtracted images.
• There is a trade-off in the number of inversion pulses
used for BS.
• BS only nulls the magnetization of static tissue at one
point in time. For a single excitation per TR, such as 3D
readout, BS can be highly effective,
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BS Technique
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Saturation + Multiple Inversion
MAGMA. 2012 April ; 25(2): 127–133
Unconstrained
scheme
Constrained
scheme
Unconstrained
scheme
interleaved
with CASL
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Background Suppression (BS)
Recommended Labeling Parameters
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Chen Lin
Magn Reson Med. 2013 August ; 70(2): 527–536
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Readout Approaches
2D vs 3D readout
• 2D single-shot (past)
– Less sensitive to motion between excitations.
• 3D segmented (present)
– High efficiency
– FSE (T2* insensitivity) + EPI (acquisition efficiency)
– 3D RARE stack of spirals (blur) or 3D GRASE
(distortion)
• 3D single-shot (future)
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2D vs 3D readout
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Vascular Crushing Gradients
• Bipolar gradients as in PC-MRA
• H/F direction with VENC=4cm/s
• Prolongs TE -> reduction in SNR + T2 (or T2*)
weighting
• User selective option (depending on the
application)
– Bright vessel indicates long ATT, an useful info.
– Use PLD > ATT or multiple PLD
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CBF Quantification Assumptions
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CBF Formula
• The entire labeled bolus is delivered to the
target tissue (PLD > ATT)
• Rapid exchange and no outflow of labeled
blood/water
• T1 of labelled spin = T1 of blood (not
influenced by tissue)
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l (blood–brain partition coefficient or tissue water content) = 0.9 mL/g (74)
T1,blood at 3.0T = 1650 ms (10), T1,blood at 1.5T = 1350 ms (75)
 (labeling efficiency) for PCASL = 0.85 (17)
The factor of 6000 converts the units from mL/g/s to mL/(100 g)/min
SIPD (Proton Density weighted signal) acquired with
• Long TR (>5s) or corrected with tissue T1
• No labeling or BS
• Motion corrected and smoothed
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Recommended Visualization
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PLD too short
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ASL Pitfalls
• For PCASL scans, look for areas of low labeling
efficiency.
• Note the overall gray matter CBF value
(Should be 40-100 mL/min/100 mL)
• Check for motion artifacts (ASL signal outside
of brain)
• Look for intravascular artifacts.
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THANK YOU
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