Cerebral aneurysm is a cerebrovascular disorder in which weakness in the wall of an intracranial artery causes a localized dilation or ballooning of the blood vessel. If an aneurysm ruptures, blood leaks into the space around the brain and causes a subarachnoid hemorrhage (SAH). Unruptured intracranial aneurysms (UIAs) are relatively common in the general population, found in and estimated 1-5% of the general population.
They are being discovered incidentally with increasing frequency because of the widespread use of high-resolution magnetic resonance imaging (MRI) scanning. Only a very small percentage (1 in 200 to 400) will ruputure.
Cerebral aneurysms involve both the anterior circulation and the posterior, or vertebrobasilar, circulation. Anterior circulation aneurysms arise from the internal carotid artery or any of its branches, whereas posterior circulation aneurysms arise from the vertebral artery, basilar artery, or any of their branches. (An internal carotid artery aneurysm is shown in the image below.)
T1-weighted magnetic resonance image (MRI) of a middle-aged woman with progressive headaches, aphasia, and right-sided hemiparesis. A large intracerebral mass with a significant amount of surrounding edema is depicted. The lesion is a giant internal carotid artery aneurysm.
Intracranial aneurysms are named according to the artery, the segment of origin, or both; for example, anterior communicating aneurysms arise from the anterior communicating artery, and posterior communicating artery aneurysms arise from the internal carotid artery near the origin of the posterior communicating artery. Intracranial aneurysms are classified into saccular and nonsaccular types on the basis their shape and etiology. Nonsaccular aneurysms include atherosclerotic, fusiform, traumatic, and mycotic types. Saccular, or berry, aneurysms have several anatomic characteristics that distinguish them from other types of intracranial aneurysms. Typically, saccular aneurysms arise at a bifurcation or along a curve of the parent vessel, or they point in the direction in which flow would proceed if the curve were not present.
The methods of imaging of aneurysms have expanded greatly, including advanced magnetic resonance angiography (MRA), CT angiography (CTA), and digital subtraction angiography (DSA). Each modality has advantages and limitations, and they are used variably at various stages in the evaluation of cerebral aneurysms.
After the diagnosis, the specific anatomic details of the aneurysm must be reported to categorize the lesion, select appropriate management, and assess treatment outcomes. For treatment selection, the following are required: accurate measurement of the neck size; a neck-to-dome ratio descriptor; measures of the aneurysm in 3 dimensions; and the relationship of the aneurysm to the surrounding vessels. For treated aneurysms, the presence, measurements, and descriptors of residuals and any parent vessel changes are needed, along with identification of any new aneurysm development.
A strong clinical suspicion of an aneurysm may be validated by the use of several diagnostic studies, including CT scanning, lumbar puncture, MRI, and cerebral angiography. Noncontrast CT remains the initial imaging test of choice to evaluate for suspected SAH because of its high sensitivity for acute hemorrhage, wide availability on a 24-hour basis, lack of absolute contraindications, speed of image acquisition, and ease of patient monitoring. However, its very high initial sensitivity for SAH declines progressively with time, and in these situations, MRI is useful. Noncontrast MRI with susceptibility-weighted sequences is likely to be as sensitive as noncontrast head CT scans in the detection of intracranial hemorrhage.
Diffuse, severe SAH is seldom helpful in identifying the specific site of the aneurysm. Localized SAH, however, may be highly indicative of the site of aneurysm rupture, as in cases in which blood is present in the sylvian fissure as a result of a rupture of a middle cerebral artery (MCA) trifurcation aneurysm or in cases in which interhemispheric blood is present between the anterior part of the frontal lobes as a result of the rupture of an aneurysm of the anterior communicating artery.
Digital substraction angiography (DSA) remains the gold standard imaging test to evaluate cerebral artery aneurysms. However, DSA is an invasive imaging test with potential complications and is therefore usually not a first-line imaging test, except perhaps in patients with acute SAH.
The use of high-resolution CT angiography combined with the use of DSA with dynamic rotational views provides the best possible visualization of the flow pattern and characteristics of any intracranial aneurysm.
Limitations of techniques
In patients with diffuse SAH, CT scans may not depict the site of aneurysm rupture. In severely anemic patients with a small hemorrhage, false-negative CT findings do occur, although rarely. Small amounts of SAH may be cleared from the cerebrospinal fluid (CSF) and may not be visible as areas of increased attenuation on CT scans as soon as 1 or 2 days after the initial severe headache; therefore, a nonenhanced CT scan of the head obtained after this time may show false-negative findings of SAH.
The American Heart Association/American Stroke Association (AHA/ASA) guidelines for the management of unruptured intracranial aneurysms (UIA) includes the following imaging recommendations
Digital subtraction angiography (DSA) can be useful compared with noninvasive imaging for identification and evaluation of cerebral aneurysms if surgical or endovascular treatment is being considered.
DSA is the most sensitive imaging for follow-up of treated aneurysms.
CTA and MRA are useful for detection of UIA.
MRA is a reasonable alternative for follow-up for treated aneurysms, with DSA used as necessary when deciding on therapy.
For patients with UIAs that are managed noninvasively without either surgical or endovascular intervention, radiographic follow-up with MRA or CTA at regular intervals is indicated. An initial follow-up study at 6 to 12 months after discovery, followed by subsequent yearly or every other year follow-up, may be reasonable.
For patients with UIAs that are managed noninvasively and in whom there are no contraindications to MRI, it may be reasonable to consider time-of-flight (TOF) MRA rather than CTA for repeated long-term follow-up.
Coiled aneurysms, especially those with wider neck or dome diameters or those that have residual filling, should have follow-up evaluation, but the timing and duration are uncertain.
Imaging after surgical intervention, to document aneurysm obliteration, is recommended given the differential risk of growth and hemorrhage for completely versus incompletely obliterated aneurysms.
Long-term follow-up imaging may be considered after surgical clipping given the combined risk of aneurysm recurrence and de novo aneurysm formation. Long-term follow-up may be particularly important for those aneurysms that are incompletely obliterated during initial treatment.
Surveillance imaging after endovascular treatment of UIAs lacking high-risk features for recurrence is probably indicated.
Patients with ≥2 family members with IA or SAH should be offered aneurysmal screening by CTA or MRA. Risk factors that predict a particularly high risk of aneurysm occurrence in such families include history of hypertension, smoking, and female sex.
Patients with a history of autosomal dominant polycystic kidney disease, particularly those with a family history of IA, should be offered screening by CTA or MRA.
It is reasonable to offer CTA or MRA screening to patients with coarctation of the aorta and patients with microcephalic osteodysplastic primordial dwarfism.
American College of Radiology (ACR) Appropriateness Criteria for imaging of cerebrovascular disease includes the following key recommendations
DSA is the gold standard for carotid artery evaluation and should be performed if noninvasive imaging is inconclusive or contradictory.
Patients with risk factors for cerebral aneurysms can undergo noninvasive screening with TOF-MRA or CTA. TOF-MRA does not require IV contrast and lacks ionizing radiation exposure to the patient. However, it may not be feasible in claustrophobic or morbidly obese patients and patients with cardiac devices or metal shrapnel. CTA has higher spatial resolution but requires IV contrast and exposes the patient to radiation.
The initial imaging study in patients presenting with suspected nontruamatic SAH should be noncontrast head CT.
Initial evaluation in patients with acute nontraumatic SAH could start with DSA or noninvasive imaging with CTA or MRA. If the initial DSA is negative, then CTA or MRA should subsequently be performed. If CTA or MRA was performed as the initial imaging test and was negative, then DSA should be performed for further evaluation. If both initial DSA and noninvasive studies are negative, DSA should be repeated in 1 to 2 weeks.
Definitive imaging follow-up of treated aneurysms is performed with DSA. However, MRA has shown promise in the follow-up of previously treated aneurysms, whether contrast-enhanced MRA or noncontrast TOF-MRA. CTA is a noninvasive imaging alternative. However, CTA is severely limited in the evaluation of previously coiled aneurysms by metal artifact.
Unruptured aneurysms that are incidentally discovered on noninvasive imaging are best followed up using the same noninvasive imaging modality on which the initial diagnosis was made.