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Johns Hopkins Hospital Baltimore, MD 21287
Advanced volume-rendering techniques and the development of multidetector arrays have brought CT angiography to the forefront of noninvasive imaging of the cerebral vasculature. Ultrafast acquisition of thin-section data sets is used to generate two- and three-dimensional (3D) reconstructions that offer resolution equivalent or superior to that of high-quality MR angiography [1]. We present a patient in whom CT angiography revealed an accessory middle cerebral artery (AMCA) that had initially been misdiagnosed on MR angiography as a fusiform aneurysm.
A 73-year-old woman presenting with a transient episode of aphasia underwent MR imaging and MR angiography. The images suggested a fusiform aneurysm of the proximal right anterior cerebral artery (A1 segment) (Fig. 3A). She was referred to our tertiary medical center for angiographic evaluation of the lesion. The case was presented at a weekly multidisciplinary neurovascular conference, and CT angiography was recommended. Instead of displaying the expected fusiform aneurysm, CT angiography revealed a type 2 AMCA (Fig. 3B). As a result, further evaluation with conventional diagnostic cerebral angiography was not required.
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CT angiography was performed using volume-rendered 3D reconstructions (Aquilion, Toshiba, Japan), and the parameters were as follows: collimation, 2 mm; table speed (pitch), 3 mm/sec; reconstruction interval, 1 mm; gantry rotation time, 0.75 sec/rotation; contrast rate, 3mL/sec; contrast material volume, 20 mL; and scanning delay, 15 sec. The 3D time-of-flight MR angiography was performed at an outside institution with an open MR imaging scanner (AIRIS II; Hitachi, Tokyo, Japan). The imaging parameters were as follows: 0.35 T; TR/TE, 50/8.0; flip angle, 50°; matrix, 256 x 152; and slab thickness, 1.2 mm. The reconstruction algorithm was generated using maximum intensity projection with 20 reconstructions printed in eight frames of reference.
The anatomy of the AMCA was first described by Crompton in 1962 in an investigation of the pathology of ruptured aneurysms of the middle cerebral artery [2]. Of 347 middle cerebral aneurysms in cadavers, Crompton identified 10 examples of a vessel originating from the internal carotid artery that accompanied the middle cerebral artery into the sylvian fissure and divided to supply the cerebral cortex in the middle cerebral artery distribution. This vessel was termed accessory middle cerebral artery (AMCA) by Crompton, although it occasionally was found to be greater in diameter than the middle cerebral artery itself. Later, Teal et al. proposed the alternate name of a "duplication of the middle cerebral artery" [3].
Abanou et al. [4] reported an AMCA incidence of 0.31% in their study of 6000 angiograms. In addition, they classified three subtypes of AMCA. Type 1 arises from the internal carotid artery proximal to its bifurcation into middle and anterior cerebral arteries, type 2 originates from the proximal portion of the A1 segment of the anterior cerebral artery, and type 3 arises more distally on the A1 segment [4]. Handa and colleagues [5] suggested that the AMCA represents a hypertrophied recurrent artery of Heubner, but other authors disagree because the recurrent artery of Heubner has been repeatedly found in the presence of an AMCA [3]. The clinical relevance of the AMCA lies in its potential to provide collateral blood supply in the event of occlusion of the middle cerebral artery. This variant has not been definitively linked to the occurrence of aneurysms or other cerebrovascular anomalies [4].
In summary, our patient had a rare anatomic variant in which 3D volume-rendered CT angiography proved superior to MR angiography in the evaluation of detailed cerebral vasculature morphology. The discrimination of small overlapping vessels is a well-described limitation of maximum-intensity-projection reconstruction algorithms on both MR angiography and CT angiography because the point at which vessels overlap is not processed as increased signal. Therefore, on MR angiography, the middle cerebral artery and the AMCA appear as one structure. The same problem can occur with maximum-intensity-projection reconstruction algorithms on CT angiography. However, in this patient, CT angiography permitted us to distinguish the two overlapping vessels likely because of a combination of factors such as the multiplanar imaging capabilities of the technique and higher resolution images than those produced with an open magnet. In this patient, CT angiography proved to be an important diagnostic tool, obviating conventional diagnostic cerebral angiography.
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