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. 1 1 1 3 2 1 3 4 1 1 Blanc, J.-F. Meder, C. Oppenheim, X. Leclerc, M. Zins ; Paris/ 2 3 4 FR, SAINT-DENIS/FR, Paris Cedex 14/FR, Lille/FR Keywords: Diagnostic procedure, Technical aspects, MR-Angiography, MR, Neuroradiology brain, CNS, Arteriovenous malformations, Ischaemia / Infarction DOI: 10.1594/ecr2013/C-1347 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. www.myesr.org Page 1 of 21
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. Page 2 of 21
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. 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. Page 3 of 21
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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. Fig. 2: Cerebral blood flow (CBF) is measured using the signal-intensity change between the two data sets: labeled and control Page 5 of 21
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. Page 6 of 21
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. Page 7 of 21
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. 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. Page 8 of 21
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). Page 9 of 21
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 Page 10 of 21
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. Page 11 of 21
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. 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). Page 12 of 21
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". Page 13 of 21
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, green arrows) related to a luxury perfusion. A strong increase in CBF is observed using the ASL sequence (ASL source image, arrow). Page 14 of 21
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). Page 15 of 21
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). Page 16 of 21
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" Page 17 of 21
and is frequently observed in patients with aneurysm, brain vascular malformations or hypervascular tumors. Page 18 of 21
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. Page 19 of 21
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 Page 20 of 21
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