DOI: 10.1148/radiol.2202991498
(Radiology. 2001;220:618-619.)
© RSNA, 2001
The Tau Sign1
Manish Goyal, MD
1 From the Department of Radiology, Aultman Hospital, Canton, Ohio. Received August 13, 1999; revision requested September 24; revision received November 3; accepted November 11. Address correspondence to the author, 430 Nemeiben Rd, Saskatoon, Saskatchewan, Canada S7J 4Z6 (e-mail: mg4763@hotmail.com).
Index terms: Carotid arteries, abnormalities, 1722.14 • Cerebral blood vessels, abnormalities, 1722.14, 1725.14 • Signs in Imaging
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APPEARANCE
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The tau sign is depicted on sagittal magnetic resonance (MR) images of the brain. The configuration of flow voids in the juxtasellar portions of the major intracranial arteries resembles the Greek letter 
(Figure) (1).
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Figure 1a. Persistent trigeminal artery detected incidentally in a patient with frontal glioma. (a) Sagittal T1-weighted MR image (500/15 [repetition time msec/echo time msec]) shows signal void of a trigeminal artery arising from the posterior aspect of the cavernous left internal carotid artery (ICA) (arrow). (b) The tau sign. Magnified image of a. The combination of signal voids (outlined by dotted lines) in the two branches of the presellar ICA (as it turns from a vertical to a horizontal course) and the proximal portion of the trigeminal artery, creates a configuration resembling the Greek letter . (c) Lateral left common carotid angiogram, obtained during the arterial phase in the same patient as in a and b shows the persistent trigeminal artery coursing posteriorly to supply the distal basilar artery. The tau configuration (dotted lines) is also apparent on this image.
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Figure 1b. Persistent trigeminal artery detected incidentally in a patient with frontal glioma. (a) Sagittal T1-weighted MR image (500/15 [repetition time msec/echo time msec]) shows signal void of a trigeminal artery arising from the posterior aspect of the cavernous left internal carotid artery (ICA) (arrow). (b) The tau sign. Magnified image of a. The combination of signal voids (outlined by dotted lines) in the two branches of the presellar ICA (as it turns from a vertical to a horizontal course) and the proximal portion of the trigeminal artery, creates a configuration resembling the Greek letter . (c) Lateral left common carotid angiogram, obtained during the arterial phase in the same patient as in a and b shows the persistent trigeminal artery coursing posteriorly to supply the distal basilar artery. The tau configuration (dotted lines) is also apparent on this image.
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Figure 1c. Persistent trigeminal artery detected incidentally in a patient with frontal glioma. (a) Sagittal T1-weighted MR image (500/15 [repetition time msec/echo time msec]) shows signal void of a trigeminal artery arising from the posterior aspect of the cavernous left internal carotid artery (ICA) (arrow). (b) The tau sign. Magnified image of a. The combination of signal voids (outlined by dotted lines) in the two branches of the presellar ICA (as it turns from a vertical to a horizontal course) and the proximal portion of the trigeminal artery, creates a configuration resembling the Greek letter . (c) Lateral left common carotid angiogram, obtained during the arterial phase in the same patient as in a and b shows the persistent trigeminal artery coursing posteriorly to supply the distal basilar artery. The tau configuration (dotted lines) is also apparent on this image.
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EXPLANATION
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In patients with a persistent trigeminal artery, sagittal MR images may show the anomalous vessel as an abnormal flow void coming off the posterior aspect of the cavernous ICA and running a somewhat horizontal course posteriorly to the basilar artery. The tau sign refers to the unusual configuration of signal voids in the presellar ICA and in a persistent trigeminal artery, as viewed on parasagittal T1-weighted MR images. The combination of the vertical and horizontal segments of the ICA and the proximal portion of the trigeminal artery creates the outline of the Greek letter (1).
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DISCUSSION
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Arterial communications between the carotid and vertebrobasilar systems in the fetus may occasionally persist in the adult. A primitive trigeminal artery is the most cephalad and common of these persistent fetal anastomoses, with an angiographic incidence between 0.1% and 0.6% (1). The persistent trigeminal artery usually arises from the presellar ICA as it exits the carotid canal and enters the cavernous sinus. It extends posteriorly to join the distal third of the basilar artery usually between the origins of the superior and anterior inferior cerebellar arteries. The anomalous vessel may have a parasellar course or may run through the middle of the dorsum sella with nearly equal frequency (2). Flow in the anastomosis is usually from the ICA to the basilar artery, although the reverse has been reported (3).
There frequently are associated small ipsilateral posterior communicating artery, vertebral artery, and hypoplastic basilar artery caudal to the anastomosis. An increased occurrence of other coexisting intracranial vascular abnormalities has been reported (2) in as many as 25% of patients with a persistent trigeminal artery. The commonest of these are intracranial aneurysms seen in 14%–32% of these patients (2,4,5). However, the authors of a recently published report (6) refuted this association and concluded that intracranial aneurysms in patients with persistent trigeminal artery occur no more frequently than in the general population. Other reported (2,7) vascular anomalies include arteriovenous malformations, carotid-cavernous fistulae, moyamoya disease, and other persistent carotid-vertebrobasilar anastomosis.
The radiologic diagnosis of persistent trigeminal artery is generally made at conventional angiography, MR imaging or angiography, or contrast material–enhanced computed tomographic angiography. The tau sign has a typical MR imaging appearance and can frequently be identified noninvasively with standard MR imaging of the brain. An anastomosis that is less than 2 mm in diameter, however, may not be identified with conventional MR imaging (7). The tau sign has been described as a finding indicative of persistent trigeminal artery on sagittal MR images (1,8).
On the basis of angiographic characteristics, Saltzman (9) classified persistent trigeminal arteries into two types, which occur with nearly equal frequency (10). In type I, the trigeminal artery supplies the entire vertebrobasilar system distal to the anastomosis. The basilar artery proximal to the anastomosis is usually hypoplastic, and the posterior communicating artery is often absent. In the Saltzman type II, the anastomosis mainly supplies the superior cerebellar arteries on both sides, and the ipsilateral posterior cerebral artery is supplied through a patent posterior communicating artery. Occasionally, the anastomosis represents a combination of these two variants.
The clinical relevance of a persistent trigeminal artery is debatable. Most cases are discovered incidentally at MR imaging, angiography, or postmortem examination. In the absence of symptoms related to the above-mentioned vascular lesions, the occurrence of neurologic complaints in patients with this vascular anomaly is thought to be mostly unrelated and coincidental (5,11).
There have been a few isolated case reports of persistent trigeminal artery causing tic douloureux or oculomotor paresis (12,13). However, knowledge of the presence of a trigeminal artery is vital before performing intracranial sellar-parasellar or vascular surgery. It is also important to recognize this anomalous vessel while performing neurovascular intervention or the Wada test, to avoid infusion of embolic material or barbiturate into the posterior fossa (2,14). If there is midbasilar hypoplasia or aplasia, it is critical to preserve the anastomosis, which may be the solitary vessel supplying the posterior circulation. The persistent trigeminal artery may also serve as a collateral pathway in vascular occlusive disease (5,15).
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FOOTNOTES
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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REFERENCES
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