Arterial spin-labeling in routine clinical practice, part 2: hypoperfusion patterns

Abstract

Arterial spin-labeling (ASL) is a powerful perfusion imaging technique capable of quickly demonstrating both hypo- and hyperperfusion on a global or localized scale in a wide range of disease states. Knowledge of pathophysiologic changes in blood flow and common artifacts inherent to the sequence allows accurate interpretation of ASL when performed as part of a routine clinical imaging protocol. Patterns of hypoperfusion encountered during routine application of ASL perfusion imaging in a large clinical population have not been described. The objective of this review article is to illustrate our experience with a heterogeneous collection of ASL perfusion cases and describe patterns of hypoperfusion. During a period of 1 year, more than 3000 pulsed ASL procedures were performed as a component of routine clinical brain MR imaging evaluation at both 1.5 and 3T. These images were reviewed with respect to image quality and patterns of hypoperfusion in various normal and disease states.

Fig 1.

Ischemic penumbra. DWI reveals multiple borderzone infarcts in the left cerebral hemisphere (arrow). MR angiogram (center) demonstrates a left internal carotid artery occlusion. ASL map shows the left ischemic penumbra, corresponding to the perfusion > DWI mismatch.

Fig 2.

Carotid occlusion and tissue at risk. Axial contrast-enhanced spoiled gradient-recalled-echo image (left) shows chronic occlusion of the left internal carotid artery (arrow). FLAIR (center) shows no evidence of ischemia. Flow asymmetry on the ASL CBF map (ellipse) represents tissue at risk in the left cerebral hemisphere.

Fig 3.

Encephalomalacia. Axial T2-weighted image shows high signal intensity in the frontal lobes representing the remote bilateral anterior cerebral artery (ACA) territory infarcts (yellow arrows). ASL CBF map reveals corresponding focal decrease in flow in the ACA territories (white arrows).

Fig 4.

Localized hypoperfusion in temporal lobe epilepsy. Interictal ASL CBF map in a 37-year-old woman demonstrates low signal intensity in the right lateral temporal lobe (arrow) and right hippocampus. Electroencephalography confirmed a right temporal seizure focus.

Fig 5.

Hypoperfused tumor on ASL. Axial (left) and coronal (center) gadolinium-enhanced T1-weighted images demonstrate a heterogeneously enhancing mass invading the third and lateral ventricles (yellow arrows). ASL (right) shows regional hypoperfusion within the more cystic portion of the tumor (white arrow). The solid portions show hyperperfusion on ASL (arrowhead). The tumor proved to be a pituitary macroadenoma at surgery.

Fig 6.

Paradoxically low signal intensity on ASL. T2*-weighted gradient-recall image reveals a calcified mass arising from the left sphenoid wing. The mass showed avid contrast enhancement (not shown) consistent with meningioma. Despite the enhancement, ASL CBF map shows no increased flow within the tumor (arrow). Underlying hyperperfusion may be masked by competing susceptibility effects.

Fig 7.

Hypoperfused glioma on ASL. Coronal FLAIR (upper left) and axial T2 (upper right) images demonstrate a high-signal-intensity mass in the left thalamus (white arrows). Minimal enhancement is seen after contrast (lower left), with corresponding low perfusion on the CBF map (white arrow, bottom right). The rim of normal perfusion (arrowheads) corresponds to the residual normal tissue in the adjacent thalamus.

Fig 8.

Blood products on ASL. Axial unenhanced CT scan reveals acute parenchymal hemorrhage in the right frontal lobe (yellow arrow). Focal loss of signal intensity on the ASL CBF map (white arrow) corresponds to the area of hemorrhage. Note flow asymmetry with decreased signal intensity throughout the right hemisphere.

Fig 9.

A, Toxoplasmosis on ASL. Decreased perfusion to a toxoplasmosis abscess. Ring-enhancing lesion is demonstrated on corresponding postgadolinium T1-weighted image (arrow). The edema surrounding the abscess decreases the perfusion of the white matter, accentuating the normal perfusion of the adjacent gyri. The area of high signal intensity on ASL surrounding the abscess (arrowhead) represents the normal adjacent gray matter. B, Toxoplasmosis on ASL. Axial T2-weighted image (left) shows a mass with vasogenic edema in the left frontal lobe (arrow). Corresponding ASL image (right) shows diffusely decreased perfusion in the lesion and adjacent white matter (arrow).

Fig 10.

Steal phenomenon associated with AVM. ASL CBF map shows characteristic hyperperfusion corresponding to the AVM (arrow). Also seen is a regional zone of hypoperfusion representing the associated steal phenomenon (arrowhead).

Fig 11.

Cerebral atrophy as a cause of globally decreased ASL signal intensity. T2-weighted image (left) reveals advanced cerebral volume loss, ex vacuo ventricular enlargement, and white matter hyperintensities. ASL map demonstrates poor perfusion signal intensity throughout.

Fig 12.

Bilateral hypoperfusion in CNS vasculitis. Multiple white matter lesions are shown on the T2-weighted image (yellow arrow). ASL CBF map demonstrates symmetrically decreased signal intensity in the bilateral cerebral hemispheres (white arrows) in a 38-year-old woman with CNS vasculitis.