Ethmoidal hemorrhagic mucocele, mimicking a giant internal carotid artery aneurysm

This 67 year old female experienced an episode of severe headache of sudden onset followed by the appearance of diplopia the next morning.
She presented at the hospital three days later. At neurological examination a left 6th nerve palsy was found. As an initial diagnostic imaging modality, MRI examination was elected in order to rule out a parasellar or brain stem lesion.

Ethmoidal mucocele, 0.5 T

Fig.1 Sagittal T1-weighted spin-echo images. Inhomogeneous (hypo-, iso- and hyperintense) rounded structure (arrow) in the left parasellar and posterior ethmoidal region.
Fig.2 Transverse proton density weighted fast spin-echo images. The lesion is in direct contact with the left internal carotid siphon.
Fig.3 Transverse T2-weighted fast spin-echo images. The signal characteristics of the lesion are more prominent with this sequence. The signal void center is surrounded by a hyperintense rim of variable thickness.
Fig.4 Coronal T1-weighted non-enhanced gradient-echo thin section images of the sellar region from a 3D acquisition. There is some isointense material within the ethmoidal sinus, but the lesion, exhibiting high signal intensity on these images, is well delineated, adjacent to the cavernous sinus and the internal carotid artery. The most probable diagnosis is ethmoidal mucocele. However, the possibility of a partially thrombosed internal carotid artery giant aneurysm is also raised.
Fig.5 Transverse source images from a single volume 3D TOF MRA acquisition. The signal intensity from the lesion (yellow arrow) is just slightly lower than in the adjacent internal carotid artery (red arrow). This might be explained either by the T1 contamination phenomenon (short T1 relaxation time tissue, e.g. methemoglobin in a hemorrhagic mucocele) or flow related enhancement (in the patent lumen of a giant aneurysm).
Fig.6 Coronal averaged modulus (left) and corresponding magnitude of complex difference (right) type source images from a 3D PC MRA acquisition. No flow is seen in the high signal intensity area, supporting the diagnostic hypothesis of the mucocele.
Fig.7 Transverse targeted MIP reconstruction from the single volume 3D TOF MRA acquisition data set. The rounded lesion adjacent to the internal carotid artery siphon is clearly visible.
Fig.8 Sagittal (above) and coronal (below) targeted MIP reconstructions from the single volume 3D TOF MRA acquisition data set. Same observations as on Fig.5 and Fig.6, sustaining the differential diagnostic problem.
Fig.9 Sagittal (above) and coronal (below) targeted MIP reconstructions from a 3D PC MRA acquisition data set. There is no evidence for a vascular lesion in conjunction with the left internal carotid artery.
N.B. In this case, through the complex evaluation of parenchymal and vascular MR imaging techniques, the diagnosis of a hemorrhagic mucocele could be established with the necessary degree of confidence. However, this case illustrates the problem of T1 contamination phenomenon, which is an inherent problem with the TOF technique, where short T1 relaxation time tissues (fat, methemoglobin, markedly enhancing lesion) might remain visible (are not saturated, despite the short TR) on both the source and MIP imaging, mimicking flow related enhancement.

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