Clinical diagnosis
Case 178
【Progress】
He was introduced to the major hospital where neurosurgeon can manage aneurysm using intervention or neck clipping. Regrettably, we had no information about his progress thereafter.
【Discussion】
Basilar artery lies in pontine fossa. It situates from 6th nerve (abducent nerve) roots and end to 3rd nerve (oculomotor nerve) roots, or it exists from between pons and medulla oblongata to between midbrain and pons. Basilar artery is formed by fusion of right and left vertebral arteries and ended branching into right and left posterior cerebral arteries. Basilar artery branches anterior inferior cerebellar arteries, several pontine arteries and superior cerebellar arteries.
Blood vessels compose of endothelium, internal elastic lamina, smooth muscle layer, external elastic lamina and adventitia. Brain aneurysm is prone to arise from bifurcation of brain artery. Blood flow pressure pushes the bifurcation site. First, internal elastic lamina at the bifurcation degenerates, weakens and causes vessel lumen outpouching, leading to formation of aneurysm.
Ruptured brain aneurysm is known to occur anterior communicating artery most, followed by posterior communicating artery, internal carotid artery and middle cerebral artery (1-4). The greater size of aneurysm causes the more risk of the rupture. However, the size of the ruptured aneurysm is mostly within 12mm (1-4). Of vertebral-basilar aneurysms which are not so common, basilar tip aneurysm is most. However, in the healthy screening using MRI, carotid aneurysm is most depicted (4).
In order to show brain artery without contrast medium on MRI, time of flight (TOF) and magnetic resonance imaging (MRA) are used. The reason why brain artery is selectively shown on TOF MRI is as follows; the background tissue of the interest slab is first magnetized (saturated) by excite pulse (repeated RF pulse). The signal intensity of the background tissue becomes lower after repeated pulse than the signal intensity after single pulse like 90 degree RF pulse in spin echo method: in this situation, new blood flow in the background tissue. Flow-in blood is greatly magnetized because it is unsaturated (5,6). Flow-in blood contain artery and vein. Pre RF pulse is poured below and above the interest slab simultaneously. Because vein flow is slow compared with artery flow, pre RF pulse repress the signal originated from vein. Then, TOFMRI acquires selective signal intensity from artery flow (5,6).
MRA is an image of magnitude intensity projection (MIP) for TOF. In short, the magnitude magnetic signal is taken out from TOFMRI second dimensional and/or three dimensional, which create an image of selective brain artery without background tissue (5,6).
In our case, TOFMRI depict basilar tip aneurysm but not MRA. The reason is that intensity of blood flow signal in aneurysm is not so high as that in brain artery. Namely, the big aneurysm with slow blood flow is demonstrated better in TOF rather than MRA.
【Summary】
We present a forty nine-year-old male suffering from pain at bilateral temple and at the inside of eyes. Time of flight MRI and T2* weighted image showed high signal intensity and low signal intensity of the outpouching inflated lesion at basilar tip, respectively, indicating the presence of basilar tip aneurysm. It is in borne in mind that basilar artery lies in pontine fossa from 6th nerve (abducent nerve) and end to 3rd nerve (oculomotor nerve). Ruptured brain aneurysm is known to occur anterior communicating artery most, followed by posterior communicating artery, internal carotid artery and middle cerebral artery. Its size is most within 12mm. Of the vertebral-basilar aneurysms, basilar tip aneurysm is most. However, in the healthy screening using MRI, carotid aneurysm is most depicted. TOF and MRA are crucial to detect brain aneurysm.
【References】
1.Hunt WE, Hess RM. Surgical Risk as Related to Time of Intervention in the Repair of Intracranial Aneurysms. Journal of Neurosurgery 1968; 28:14-20.
2.Brisman JL, Song JK, Newell DW. Cerebral Aneurysms. NEJM 2006; 355:928-939.
3.Bederson JB, et al. Recommendations for the management of patients with unruptured intracranial aneurysms. Stroke 2000;31:2742-2750.
4.Imaizumi Y, et al. Detection rates and sites of unruptured intracranial aneurysms according to sex and age: an analysis of MR angiography-based brain examinations of 4070 healthy Japanese adults. J Neurosurg. 2018 Apr 6;130(2):573-578.
5.Saloner D. An introduction to MR angiography. Radiographics 1995;15:453-465. (Older review; good discussion of TOF and PC MRA)
6.Wehrli FW. Time-of-flight effects in MR imaging of flow. Magn Reson Med 1990; 14:187-193.
2020.1.22