Role of Neuroradiology in Evaluating Cerebral Aneurysms

Abstract

Intracranial aneurysms are common lesions with an adult prevalence rate between one and five percent in autopsy studies 1. Fortunately, most aneurysms are small and an estimated 50 to 80 percent of all aneurysms do not rupture during the course of a person's lifetime 2. Intracranial aneurysms are considered to be sporadically acquired lesions, although a rare familiar form has been described3. Clinically, cerebral aneurysms can be silent or give rise to focal neurological symptoms or rupture leading to the dramatic event of subarachnoid haemorrhage. Nowadays the gold standard technique for the detection of cerebral aneurysms is considered to be digital subtraction angiography (DSA) offering both dynamic and morphological information on the intracranial circulation. However, DSA is relatively expensive and not widely available. DSA also carries a potential risk of transient, reversible or permanent neurological and non neurological complications even though the actual risk is much lower than is perceived 4-13` In view of this, the ideal technique to evaluate cerebral aneurysms should be non invasive, easy and quick to perform, offering high spatial resolution and yielding optimal information on aneurysm location, size of its dome and neck, any parietal abnormality, collateral arteries originating from the dome and the relation between the parent artery and surrounding vascular and/or nervous structures. Nowadays, the diagnostic work-up in patients with cerebral aneurysms or subarachnoid haemorrhage (SAH) includes several imaging techniques such as DSA, Computed Tomography Angiography (CTA) and Magnetic Resonance Angiography (MRA). Worldwide neuroradiological experience during the past 15 years has led to a consensus on the major role of CTA in evaluating patients with intracranial aneurysms. CTA is a minimally invasive, less expensive technique than DSA and MRA, easy and quick to perform and often able to provide correct information on the presence, dimensions and anatomical location of cerebral aneurysms. Volumetric acquisition allows the neuroradiologist to obtain oblique, axial and para-axial views for better display of the aneurysm dome and neck 12-19. The most important limit of CTA, widely accepted and common to most multislice CT scanners, is its dimensional constraints: most aneurysms with a dome diameter smaller than 3 mm (so-called baby aneurysms) can often be missed by CTA examination. MRA, with different acquisition techniques (Time of flight - TOF or Phase Contrast - PC), is increasingly available, but its potential role remains limited by motion and vessels artefacts, sometimes even with contrast media injection. In addition, MRA is still considered a high cost procedure, strongly affected by the already known absolute and relative contraindications and some technical difficulties (particularly when dealing with SAH patients) 20-26. In spite of these drawbacks, MRA has an important role in the follow-up of embolized cerebral aneurysms after endovascular treatment. In these cases, MRA sequences (TOF or PC), even without contrast media injection, are highly reliable in displaying major residual flow within the coil mesh or in the neck region of the aneurysm. Figure 1 A-D CT and CTA (MIP and SSD) of a basilar aneurysm.