AJR 2004; 183:1840-1841
© American Roentgen Ray Society
MR Angiography of Subclavian Steal Syndrome: Pitfalls and Solutions
Richard Bitar,
David Gladstone,
Demetrios Sahlas and
Alan Moody
University of Toronto Toronto, ON M2N 7H4, Canada
Sunnybrook & Women's College Health Sciences Toronto, ON M4N 3M5,
Canada
Subclavian steal syndrome may cause brainstem ischemia as a result of
retrograde flow in the vertebral artery secondary to stenosis in the
subclavian artery, proximal to the origin of the vertebral arteries
[1,
2]. Patients may present with
vertigo, ataxia, and visual disturbances (e.g., diplopia). MR angiography
provides noninvasive techniques for the investigation of subclavian steal
syndrome [3,
4]. An understanding of how
these sequences are acquired aids in the interpretation of the images and
helps the radiologist avoid potential pitfalls.
A 63-year-old woman was evaluated for vertebrobasilar ischemia after
experiencing two transient attacks of vertigo accompanied by binocular
horizontal double vision and an episode of acute vertigo, nausea, and vomiting
that occurred while she was washing the kitchen cupboards and that was so
severe that she was forced to lie on the floor for relief. She reported that
her left arm felt "heavy and tired" after repetitive activity;
this feeling was accompanied by vertigo.
Phase-contrast angiography showed the reversal of flow within the left
vertebral artery (Fig. 4A).
Multiple overlapping thin-slab acquisition (MOTSA) time-of-flight images
(Fig. 4B) obtained to depict
the vertebral artery showed loss of flow in the lower left vertebral artery.
Contrast-enhanced MR angiography, however, confirmed that the lower vertebral
artery was patent and revealed that a lesion in the left subclavian artery was
responsible for causing subclavian steal syndrome
(Fig. 4C). Doppler sonography
confirmed severe left subclavian stenosis with a reversal of vertebrobasilar
flow.
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Fig. 4A. —63-year-old woman with subclavian steal syndrome.
Phase-contrast angiogram shows cephalad (dark areas) flow of internal carotid
arteries and right vertebral artery and caudal flow of left vertebral artery
(bright area, arrow).
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Fig. 4C. —63-year-old woman with subclavian steal syndrome.
Contrast-enhanced MR angiogram reveals filling of both vertebral arteries,
showing that left vertebral "occlusion" is false. Lesion
(arrow) responsible for subclavian steal syndrome is seen in left
subclavian artery.
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In phase-contrast angiography, two measurements are obtained (one
insensitive and one sensitive to flow), and the application of subtraction
techniques results in high signal from flowing blood, with minimal signal seen
from stationary tissue [5].
Both magnitude and phase (direction) images of flow are obtained. Phase images
reflect flow along the three gradients (x, y, and z),
providing directional information. MOTSA time-of-flight imaging suppresses the
signal of stationary tissue by applying multiple radiofrequency pulses at
short TRs, causing saturation in the volume that is being acquired. Flowing
blood entering the imaging volume is of higher signal than stationary tissue
because of the refreshment of the blood signal by unsaturated blood
[5]. Saturation bands are
applied at the top of each acquisition to remove high signal from the
inflowing venous system (venous contamination).
The false occlusion seen in the left vertebral artery is a pitfall of the
MOTSA time-of-flight imaging sequence that can be attributed to the saturation
bands applied to prevent venous contamination. In this case, because the flow
in the left vertebral artery was reversed, it was exposed to these saturation
bands, with the resulting signal void seen in the left vertebral artery.
Additional scanning with contrast-enhanced MR angiography provided a solution
to this potential pitfall. In contrast-enhanced MR angiography, administration
of gadolinium (gadopentetate dimeglumine) increases the signal from blood.
Saturation of the blood is overcome by contrast enhancement, and venous
contamination is averted by timing of the delivery of contrast material to the
arterial phase.
This case illustrates how the different techniques available for MR
angiography can complement each other and emphasizes the need to use more than
one technique in cases in which the findings are equivocal. Knowledge of
sequence acquisition can aid in the interpretation of the images by
eliminating potential pitfalls.
References
- [No authors listed]. A new vascular syndrome: the subclavian steal.
N Engl J Med1961; 265:912
–913
- Reivich M, Holling E, Roberts B, et al. Reversal of blood flow
through the vertebral artery and its effects on cerebral circulation.
N Engl J Med1961; 265:878
–885
- Drutman J, Gyorke A, Davis WL, et al. Evaluation of subclavian
steal with two-dimensional phasecontrast and two-dimensional time-of-flight MR
angiography. AJNR1994; 15:1642
–1645[Abstract]
- Van Grimberge F, Dymarkowski S, Budts W, et al. Role of magnetic
resonance in the diagnosis of subclavian steal syndrome. J Magn
Reson Imaging 2000;12:339
–342[Medline]
- Brown MA, Semelka RC. MR imaging abbreviations, definitions and
descriptions: a review. Radiology1999; 213:647
–662[Free Full Text]
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