Download A New Trend in Vascular Imaging: the Arterial Spin Labeling (ASL

A New Trend in Vascular Imaging: the Arterial Spin Labeling
(ASL) Sequence
Poster No.:
C-1347
Congress:
ECR 2013
Type:
Educational Exhibit
Authors:
J. Hodel , A. GUILLONNET , M. Rodallec , S. GERBER , R.
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Blanc , J.-F. Meder , C. Oppenheim , X. Leclerc , M. Zins ; Paris/
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FR, SAINT-DENIS/FR, Paris Cedex 14/FR, Lille/FR
Keywords:
Diagnostic procedure, Technical aspects, MR-Angiography,
MR, Neuroradiology brain, CNS, Arteriovenous malformations,
Ischemia / Infarction
DOI:
10.1594/ecr2013/C-1347
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Learning objectives
To highlight the clinical value of 3D fast Spin Echo (FSE) Arterial Spin Labeling (ASL)
sequence.
To discuss the technical aspects and the advantages of 3D FSE ASL.
To demonstrate the interest of combining 3D ASL with other standard sequences.
To illustrate imaging findings through case examples.
Background
Technical aspects
Arterial spin-labeling (ASL) is a non-contrast MR perfusion imaging technique that uses
the magnetic labeling of arterial water. ASL is markedly improved at 3T because of higher
signal to noise ratio and longer T1 of blood.
To obtain ASL maps, two data sets are acquired: one without labeling, the control image
and one with labeling, the labeled image (fig. 1). Cerebral blood flow (CBF) is measured
using the signal-intensity change between these two data sets (fig. 2).
Color CBF maps are obtained using the available software (Functool, General Electric).
Quantification of CBF is feasible (fig. 3), however, to the contrary of dynamic susceptibility
contrast imaging, measurement of the cerebral blood volume cannot be achieved.
Post label delay (PLD) is the time duration for spins to travel from the labeling plane to
the imaged slices (fig. 4). It typically ranges between 1,5 and 2,5 secondes.
There is a compromise between PLD values and image quality. Shorter PLDs may lead
to errors in CBF quantification with «pseudo watershed infarcts» (fig. 5). Longer PLDs
improve CBF quantification at the price of a loss in SNR due to the T1-related decay of
the labeled spins.
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Advantages of using 3D Fast Spin echo ASL
Using the 3D Fast Spin Echo (FSE) ASL, all the slices experienced the same PLD, this
is not the case using 2D ASL.
Multiplanar reformations are also feasible (fig. 6) and fusion of 3D ASL with other 3D
sequences (DTI, FLAIR, TOF, Gradient Echo T1 or SWI) can be routinely performed (fig.
7).
FSE imaging improves diagnosis accuracy in patients with aneurysm clips, coils or blood
products because of reduced susceptibility artefacts (fig. 8). This is not the case with EPIbased ASL techniques that are more prone to susceptibility artefacts.
Images for this section:
Fig. 1: To obtain ASL maps, two data sets are acquired: one without labeling, the control
image and one with labeling, the labeled image.
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Fig. 2: Cerebral blood flow (CBF) is measured using the signal-intensity change between
the two data sets: labeled and control
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Fig. 3: Color CBF maps are obtained using the available software (Functool, General
Electric). Quantification of CBF is feasible, however, to the contrary of dynamic
susceptibility contrast imaging, measurement of the cerebral blood volume cannot be
achieved.
Fig. 4: Post label delay (PLD) is the time duration for spins to travel from the labeling
plane to the imaged slices. It typically ranges between 1.5 and 2.5 secondes.
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Fig. 5: ASL CBF color maps in an elderly patient. Shorter PLD value leads to "pseudo
watershed infarcts" (PLD=1.5s, arrows) with most of the ASL signal still located within
brain arteries. In this patient, longer PLD value (PLD=2.5s, arrows) improved CBF
quantification.
Fig. 6: 3D FSE ASL sequence allows for multiplanar reformations in axial, coronal or
sagittal planes.
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Fig. 7: Fusion of 3D ASL with other 3D sequences (DTI, FLAIR, TOF, Gradient Echo T1
or SWI) can be routinely performed. For example, in this patient with an acute stroke, the
diffusion sequence was merged with the ASL CBF color map.
Fig. 8: Thanks to the FSE readout, the susceptibility artefacts are markedly reduced using
the 3D FSE ASL sequence. In this patient with a left frontal hematoma, the CBF increase
beside the hematoma is still visible (ASL source image and CBF color map, arrows).
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Imaging findings OR Procedure details
ASL signal changes involving both brain parenchyma and vessels are important markers
of pathology. Indeed ASL is very sensitive to various vascular pathological states such as
hyperperfusion in brain parenchyma; reduced or increased blood flow in arteries or veins.
ASL hypointensity pattern
Hypoperfused brain parenchyma, surrounding ischemic core, appears hypointense on
ASL images. In patients with acute stroke, fusion of diffusion and ASL images can be
useful to assess the mismatch (fig. 9).
ASL hyperintensity pattern
ASL hyperintensities in brain parenchyma.
Hyperperfused brain parenchyma, as observed in reperfusion syndrome, leads to an
increased ASL signal (fig. 10).
ASL hyperintensities related to reduced blood flow.
In patients with acute stroke, collateral flow may lead to a strong ASL vascular
hyperintensity commonly called "arterial transit artifact" (ATA) (fig. 11).
Intravascular ASL hyperintensity can also be observed in patients with reduced or
stagnant blood flow (fig. 12). These hyperintensities can be explained by the pooling of
ASL signal upstream the arterial occlusion.
ASL hyperintensities related to increased blood flow.
In patients with ischemic stroke, luxury perfusion may lead to a strong ASL hyperintensity
(fig. 13).
ASL hyperintensity is also commonly observed within the venous drainage of
arteriovenous malformation (AVM) (fig. 14) or dural arteriovenous fistula. Such finding
(ASL "bright spot") can be useful for the detection of brain vascular malformations in
patients with brain hematoma (fig. 15).
Stroke mimics
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Increased CBF can be detected using the 3D FSE ASL for two differential diagnoses of
acute stroke: status epilepticus (fig. 16) and migraine (fig. 17).
Steal phenomenon
Pooling of ASL signal within a hypervascular lesion may lead to a strong ASL
hyperintensity associated with a relative extinction of the brain parenchyma: this artefact
is called "steal phenomenon" (fig. 18). Such finding is usually observed in patients
with aneurysm, brain vascular malformations or tumors such as paraganglioma or
meningioma.
Images for this section:
Fig. 9: Patient with internal carotid dissection and acute stroke in the right middle
cerebral artery territory. Hypoperfused brain parenchyma, surrounding the ischemic core,
appeared hypointense on ASL images. Fusion of diffusion (DWI) and ASL images further
improved the detection of mismatch.
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Fig. 10: In this patient with reperfusion syndrome, the ASL sequence revealed a diffuse
hyperintensity related to the hyperperfused brain parenchyma (arrows, ASL source
images).
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Fig. 11: In this patient with acute stroke, the FLAIR sequence revealed a vascular
hyperintensity within the right middle cerebral artery related to reduced blood flow (FLAIR
image, arrow). Note the strong hyperintensity using the ASL sequence (ASL source
image, arrow) contrasting with the hypointensity of the adjacent brain parenchyma, such
finding was previoulsy reported as the "borderzone sign".
Fig. 12: Patient with acute stroke and left middle cerebral artery thrombosis. Intravascular
ASL hyperintensity was observed due to the reduced blood flow (fusion ASL + TOF
image, arrow).
Fig. 13: Diffusion, ASL and TOF MR sequences in a patient with acute stroke in the
left middle cerebral artery territory. Note the dilated lenticulostriate arteries (TOF image,
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green arrows) related to a luxury perfusion. A strong increase in CBF is observed using
the ASL sequence (ASL source image, arrow).
Fig. 14: Post contrast 3D T1 weighted and ASL images in a patient with right parietal
arteriovenous malformation (AVM). A strong hyperintensity was observed in the venous
drainage of the AVM using the ASL sequence (ASL source image, arrow).
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Fig. 15: SWI, ASL and dynamic angiography (TRICKS) in a patient with left frontal
hematoma. ASL images revealed a "bright spot" below the hematoma (ASL source
image, arrow), such finding suggested a brain vascular malformation. TRICKS identified
a small arteriovenous malformation with venous drainage below the hematoma (TRICKS
image, arrow) explaining the ASL hyperintensity.
Fig. 16: Diffusion, FLAIR and ASL sequences in a patient with status epilepticus. Cortical
hyperintensities were observed in DWI and FLAIR images while ASL revealed a strong
increase in CBF (ASL CBF color maps, arrows).
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Fig. 17: ASL source images and color CBF maps in a patient with migraine. Increased
CBF was observed within the left hemisphere (arrows, ASL source images). The ASL
sequence also revealed increased blood flow within the left external carotid branches
(arrows, ASL CBF color maps).
Fig. 18: ASL source images in a patient with a left temporal meningioma. A strong
ASL hyperintensity was observed within the meningioma (arrows) associated with a
relative extinction of the brain parenchyma. Such finding is called "steal phenomenon"
and is frequently observed in patients with aneurysm, brain vascular malformations or
hypervascular tumors.
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Conclusion
Post label delay (PLD) is a critical parameter for ASL image quality.
Fusion of 3D ASL with other 3D sequences may improve diagnosis accuracy.
Fast Spin Echo imaging improves diagnosis accuracy in patients with blood products or
coils.
Artefacts related to flow velocities may impair CBF quantification but are useful markers
of pathology.
A wide range of vascular pathological states may lead to signal increase within the ASL
images.
References
Deibler et al. Arterial spin-labeling in routine cliniqual practice.
AJNR 2008 Aug;29(7):1228-34
Zaharchuk et al. Arterial spin-labeling imaging in patients with normal bolus perfusionweighted MR imaging findings: pilot identification of the borderzone sign.
Radiology 2009 Sep;252(3):797-807
Zaharchuk. Arterial spin label imaging of acute ischemic stroke and transient ischemic
attack.
Neuroimag Clin N Am 2011 May;21(2):285-301
Le et al. Identification of venous signal on arterial spin labeling improves diagnosis of
dural arteriovenous fistulas and small arterioveinous malformations.
AJNR 2012 Jan;33(1):61-8
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