Teaching Case

A 50 year-old man was referred to our department for a follow-up brain MR examination of a cavernous angioma on the left side of the brain stem. The lesion had homogeneous low signal intensity on T1W[fig.1], and homogeneous high signal with a hypointense halo on T2W images[fig.2]. It didn’t show any enhancement and remained unchanged compared to the previous MRI one year ago. A subsequent MR angiography revealed the true nature of the lesion.

A 50 year-old man was referred to our department for a brain MR examination for following-up a cavernous angioma on the left side of the brain stem. The lesion had homogeneous signal intensity which was low on T1W [fig.1], and high on T2W images. The lesion presented a hypointense halo on the T2W images [fig.2] suggesting a cavernous angioma. Intravenous contrast was administered but the lesion did not enhance. In comparison with a previous MRI, performed 1 year ago, the lesion was unchanged in terms of size, shape, and signal intensity. The true nature of the lesion was felt to be controversial. A MR angiography (GRE TOF) of the skull base was subsequently performed which proved that the alleged cavernous angioma was in fact a loop of the medullary segment of the left PICA [fig.3,4].

Since the advent of MRI, cavernous angiomas are frequently discovered vascular malformations. The patients are often asymptomatic [1,2]. A hypointense halo surrounding the lesions is shown on T2W images because of deposition of haemosiderin. They are better delineated on T2W GRE images due to the magnetic susceptibility effect of haemosiderin [2,3]. Based on these facts the diagnosis of a cavernous angioma was properly put in the previous examination. However enlargement of the lesion is expected even in asymptomatic subjects and mild to moderate enhancement after intravenous administration of paramagnetic substances [1,2]. In our case the lesion has remained unchanged since the last MRI scan and did not enhance at all. We decided to perform TOF MR angiography [3], in order to demonstrate the vascular anatomy of the region. This sequence proved that the hypointense halo was in fact a loop of the medullary segment of the PICA simulating the rim of haemosiderin. PICA is an artery originating from the last segment of the vertebral artery. A lot of variations have been described [4]. Among the most usual ones is a medullary segment with elongated anomalous course. In our case the medullary segment was situated deep in the cerebellomedullary fissure, forming a loop. In retrospect one can notice that the signal intensity of the “lesion” is in fact following the signal intensity of the subarachnoid space, explaining the homogeneous bright signal on T2W images.