Double Origin of the Posterior Inferior Cerebellar Artery Diagnosed by MR Angiography

Abstract

The posterior inferior cerebellar artery (PICA) frequently arises from the fenestrated segment of the intracranial vertebral artery (VA), and this common variation can be misinterpreted as or confused with a PICA of double origin. Rarely, a PICA of true double origin occurs when two branches of the PICA arise separately from the intracranial VA and fuse to form an arterial ring. We discovered this rare variation incidentally while interpreting images of magnetic resonance (MR) angiography. This is the first report of MR angiographic findings of this rare variation.

Introduction

Normally, the posterior inferior cerebellar artery (PICA) arises from the intracranial segment of the vertebral artery (VA). The fenestration of the intracranial VA (V4 segment) is usually large, and the PICA frequently arises from the fenestrated segment.¹ This common variation may be misdiagnosed as double origin of PICA.^2,3 These two variations should be recognized correctly by the different direction of blood flow in the cranial channel of the double origin of the PICA from that in the distal segment of the VA fenestration from which the PICA arises.

Several cases of double origin of the PICA have been reported using catheter angiography.^4–9 We describe two cases of PICA of true double origin diagnosed by magnetic resonance (MR) angiography. To our knowledge, this is the first report of this variation diagnosed by MR angiography.

Case Reports

Case 1

A 61-year-old woman with an unruptured aneurysm was transferred to our institution for treatment. The patient was examined preoperatively by MR angiography using a 3-Tesla system and standard time-of-flight technique without contrast media. The maximum-intensity-projection (MIP) MR angiographic images showed an unruptured aneurysm at the junction between the right internal carotid artery (ICA) and the posterior communicating artery, the so-called IC-PC junction (Figure 1A). We also found an anomalously originating right PICA, but superimposition of several arterial branches prevented observation of the variation. To confirm the anomalous right PICA, we created partial MIP MR angiographic images of the vertebrobasilar system using the SYNAPSE® VINCENT (Fujifilm Medical Company, Tokyo, Japan) picture archiving and communication system (PACS). These images clearly demonstrated the double origin of the right PICA (Figure 1B). We treated the patient’s aneurysm using Guglielmi detachable coils (GDCs) without complications.

Figure 1.

(Case 1) Magnetic resonance (MR) angiography obtained using a 3-Tesla scanner. A) Right anterior oblique projection of maximum-intensity-projection (MIP) image shows an aneurysm at the junction between the right internal carotid artery and posterior communicating artery. Variation of the right posterior inferior cerebellar artery (PICA) is suspected but not apparent due to superimposition of arterial branches (arrow). B) Partial MIP MR angiographic image of the vertebrobasilar system at the same projection as in (A). The right PICA of double origin is clearly seen (arrows).

Case 2

A 50-year-old woman with a ruptured aneurysm of the right IC-PC junction had been treated by GDC coiling one year previously. Follow-up MR angiography was performed using a 3-Tesla scanner. We found an anomalous left PICA (Figure 2A) and diagnosed that it was PICA of double origin on partial MIP image (Figure 2B).

Figure 2.

(Case 2) MR angiography obtained using a 3-Tesla scanner. A) Slightly left anterior oblique projection of MIP image shows left PICA of double origin (arrow). B) Partial MIP MR angiographic image of the vertebrobasilar system at the same projection as in (A). The left PICA of double origin is clearly seen (arrows).

Discussion

The PICA usually arises from the V4 segment of the VA and supplies the postero-inferior part of the ipsilateral cerebellum. Fenestration of the VA at the V4 segment is not so rare, with a reported prevalence of 0.54% and PICA arising from the fenestrated segment in 10 of 18 VA fenestrations (56%).¹ If blood in the cranial channel flows toward the basilar artery, the variation should be diagnosed as PICA arising from the fenestrated segment of the VA and not PICA of double origin. The distal segment of the PICA of double origin should be supplied by both caudal and cranial channels as shown in Figures 1B and 2B. Thus, blood in the cranial channel must flow toward the distal PICA. Several reports describe catheter angiographic findings of PICA of double origin, but no reports mention this important hemodynamics.^4–9 Figure 3 is a schematic illustration of these two arterial variations. In case of PICA of double origin, the distal segment of the PICA is larger than proximal two channels. In contrast, PICA arising from the VA fenestration has a large caudal channel.

Figure 3.

Schematic illustrations of 2 variations of the arterial ring at the V4 segment of the vertebral artery (VA). A) PICA of double origin. Possible direction of blood flow in the cranial channel is toward the distal PICA (arrow), because the distal segment is larger than proximal two channels. B) PICA arising from the VA fenestration. Blood in the cranial channel flows toward the terminal segment of the VA (arrow), because the caudal channel is larger than the PICA.

We used the time-of-flight technique for MR angiography. If the phase-contrast technique is also used, flow direction in the cranial channel might be confirmed. This additional sequence, however, may not be acceptable in daily clinical practice.

Although the clinical significance of these arterial variations is limited, arterial dissection⁷ or aneurysm⁴ may be seen at the arterial ring. In such cases, one of the channels may play an important role in collateral circulation after coil embolization.

Double or duplicate origin of the cerebral artery has been reported by MR angiography in the middle cerebral artery¹⁰ and in the posterior communicating artery.¹¹ These variations were previously confused with fenestrations. Correct diagnosis should be made during interpretation of cerebral MR angiograms. Partial MIP images are useful for identifying rare cerebral arterial variation.

Conclusions

We describe two cases of double origin of the PICA diagnosed by MR angiography and stress that the direction of blood flow in the cranial channel is important in differentiating the origin of the PICA from the fenestrated segment of the VA.

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

Conflict of interest

The authors declare no conflict of interest.