Brain Aneurysm Imaging

Overview

The word "aneurysm" comes from the Greek word aneurysma (ana, meaning across, and eurys, meaning broad) and denotes an abnormal dilatation of an artery. 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.^{[1, 2]}
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.

Preferred examination

A strong clinical suspicion of aneurysm may be validated by the use of several diagnostic studies, including computed tomography (CT) scanning, lumbar puncture, magnetic resonance imaging (MRI), and cerebral angiography. CT is typically the first diagnostic test ordered when there is a possibility of subarachnoid hemorrhage (SAH). Findings on a nonenhanced CT scan may confirm subarachnoid blood in more than 90% of patients with acute SAH. 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.
In the author's opinion, the use of high-resolution CT angiography combined with the use of digital substraction angiography 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.
For excellent patient education resources, visit eMedicine's Headache Center. Also, see eMedicine's patient education article Aneurysm, Brain.

Computed Tomography

CT is usually the initial diagnostic procedure when SAH is suspected (see the image below). A good-quality nonenhanced CT scan may depict SAH in more than 90% of patients who undergo scanning within 48 hours, depending on the location and extent of the subarachnoid blood and the time elapsed since ictus. The location of the subarachnoid blood identifies the presumed location of the ruptured aneurysm, a finding often supported by the demonstration of an aneurysm in the area of maximum clot localization or the area of the maximum amount of subarachnoid blood.^{[3, 4, 5, 6, 7, 8]}
Nonenhanced CT scan of a middle-aged man with headaches. The patient had a giant aneurysm of the left internal carotid artery in its intracavernous segment. This aneurysm is densely calcified and is easily depicted. In particular, CT is useful in patients with multiple aneurysms. In addition to indicating the location of the vascular lesion, a CT scan may show unsuspected anomalies, such as a related arteriovenous malformation, intraparenchymal hematoma, or hydrocephalus. Finally, by providing a quantitative measure of the amount of blood in the subarachnoid cisterns and ventricles, the initial CT scan provides a reliable predictive index that may be used to identify patients who are likely to have a vasospasm. Most often, the Fisher grading system is used to classify SAH; this system is based on the amount of blood visible on the CT scan. The Fisher grading system is as follows:

• Grade 1 - No subarachnoid blood detected
• Grade 2 - Diffuse vertical layers thicker than 1 mm
• Grade 3 - Localized clot and/or vertical layer thicker than 1 mm
• Grade 4 - Intracerebral or intraventricular clot with diffuse or no subarachnoid blood

The advent of high-resolution CT angiography with the implementation of dynamic 3D reconstructions allows for a superb visualization of the intracranial vessels, as well as the anatomy and characteristics of the aneurysm; images with a similar resolution enable the visualization of intracranial vessels. However, it does not show progression on the pattern of flow. Although some authors have suggested that high-resolution CT angiography may replace cerebral angiography, in the author's opinion, high-resolution CT angiography complements digital substraction angiography but should not replace it.

Degree of confidence

Subarachnoid bleeding is demonstrated in more than 90% of patients, depending on the location and extent of the subarachnoid blood and the time elapsed since ictus.

False positives/negatives

Lumbar puncture is usually reserved for the screening of patients with potential sentinel bleeding or for confirming the presence of bleeding in patients whose clinical history is suggestive but whose CT findings are negative.

Magnetic Resonance Imaging

MRI can provide additional details about the regional anatomy and the size, shape, and content of an aneurysm (see the images 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. T2-weighted MRI of a middle-aged woman with progressive headaches, aphasia, and right-sided hemiparesis. The lesion is a giant internal carotid artery aneurysm. Note the flow void, the blood breakdown products within the layers of mural thrombus, and calcification within the aneurysm that produces a marked hypointense signal. Significant surrounding edema is depicted. Most intracranial aneurysms appear as an area of flow void larger than the healthy vessels in that region. Their interior usually enhances significantly after the intravenous administration of gadolinium–diethylenetriamine penta-acetic acid. Most giant aneurysms have calcifications and an intraluminal clot, but their residual lumen may be depicted as a region of flow void. The thrombosed areas may have variable signal intensity, which represents blood products at different stages. MRIs may also depict small amounts of parenchymal blood surrounding the aneurysms; this finding indicates which of the multiple aneurysms have bled.^{[9, 10, 11, 12, 13, 14]}
Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MR angiography (MRA) scans.
NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see Medscape.
MRA is useful in detecting intracranial aneurysms in both symptomatic patients and asymptomatic patients. MRA is a noninvasive and sensitive method of testing for an aneurysm in symptomatic patients who are at high risk (ie, patients with polycystic renal disease and those with 1 or more first-order family members with documented cerebral aneurysms).
In the symptomatic patient, MRA often reveals the site of aneurysmal dilation. Two main types of MRA—the time-of-flight technique and the in-flow technique—are used more often than the phase-sensitive or the phase-contrast technique. For both time-of-flight and in-flow sequences, a large set of axial source images are acquired; these are then reformatted into images that appear similar to conventional angiograms. The most common method used is the maximum intensity projection (MIP) method.
Advances in MRI technology, including the use of newer 3T units, enable excellent visualization of the intracranial vessels. The use of high-resolution magnetic resonance angiography is appropriate for the follow-up of patients who have undergone treatment with endovascular coiling; it accurately delineates residual aneurysm necks and parent vessel patency (in the absence of a stent) and enables excellent visualization of contrast filling within the coil mass. However, when a stent has been placed in the parent artery, MRI may not be appropriate, owing to the presence of artifacts.^[14]

Degree of confidence

MRI alone is sensitive in the evaluation of subarachnoid and intraparenchymal hemorrhage. Small aneurysms may be missed. MRA is more sensitive to small aneurysms, and it can reliably depict lesions as small as 3-4 mm; however, for optimal sensitivity, MIP images should always be viewed in conjunction with the source images; small aneurysms may be missed if only the MIP images are reviewed. At the present time, angiography should still be considered the criterion standard for the detection of small aneurysms.

Ultrasonography

In patients with subarachnoid hemorrhage (SAH), transcranial Doppler (TCD) ultrasonography is a noninvasive technique that is useful in detecting vasospasm of the intracranial arteries. Most measurements are obtained by using particular cranial windows of relatively thin bone. The most common cranial window is the transtemporal one, which is located above the zygoma. This window is used to measure velocities of the middle cerebral artery, the anterior cerebral artery, the distal internal carotid artery, and the proximal posterior cerebral artery. The transorbital window enables measurement of the ophthalmic artery and the internal carotid artery; the suboccipital window enables measurement of the vertebral arteries and the basilar artery.
In addition, TCD ultrasonography provides repeated serial measurements that may show a pattern of increasing velocities, which lead to clinical deterioration. Clinical decision-making, such as decisions concerning the initiation and duration of hypervolemic, hypertensive therapy, may be aided by TCD.

Nuclear Imaging

Some giant aneurysms in the cavernous segment of the internal carotid artery may be treated with occlusion of the artery. Treatment of these lesions depends heavily on the demonstration of cerebrovascular reserve, which is the ability to tolerate temporary or permanent carotid artery occlusion. This reserve is assessed by means of an endovascular balloon occlusion test, with qualitative or quantitative cerebral blood flow measurements at single-photon emission CT (SPECT) scanning. Patients who can tolerate balloon occlusion and have no significant areas of hypoperfusion on SPECT scans are candidates for carotid occlusion.

Angiography

Cerebral angiography remains the definitive preoperative diagnostic tool in patients with intracranial aneurysms (see the images below). Angiography may also be used to detect and evaluate aneurysmal multiplicity or other associated vascular diseases, assess collateral circulation, identify congenital anomalies, and diagnose cerebral vasospasm and aid in their treatment.
Left oblique cerebral angiogram in a patient with multiple intracranial aneurysms shows an anterior communicating aneurysm and a middle cerebral artery aneurysm. The patient underwent a frontotemporoparietal craniotomy, during which surgical clips were placed in both lesions in one setting. Left oblique cerebral angiogram in a patient with a proximal intracranial internal carotid artery aneurysm. The surgical approach to this aneurysm requires a craniotomy with an orbitotomy and drilling of the anterior clinoid process; however, this aneurysm has a favorable neck-to-fundus ratio for endovascular coil placement. Image obtained after the placement of a Guglielmi detachable coil in the aneurysm. The patency of the internal carotid artery and all its branches is preserved. Contrast material does not fill the aneurysm. A routine angiographic examination consists of a selective 4-vessel study, including both internal carotid and both vertebral arteries. This study enables the evaluation of the cerebral circulation to determine the source of subarachnoid hemorrhage (SAH) and to identify other concomitant lesions that may influence the surgical plan.
Multiple views are often necessary to delineate the origin of vessels overlapping the aneurysms and the configuration of the aneurysm neck. In the anterior circulation, carotid ophthalmic aneurysms are often best seen on the lateral or 45° oblique projection. Posterior communicating and anterior choroidal aneurysms are usually well profiled on the lateral and oblique projections.
Carotid bifurcation aneurysms and some middle cerebral aneurysms may warrant the use of a straight anteroposterior, or Caldwell, projection. The basal, or submental vertex (SMV), projection may help define the anatomy of middle cerebral aneurysms. In the posterior circulation, oblique projections are useful in showing the basilar bifurcation, posterior inferior cerebellar aneurysms, or aneurysms of the vertebrobasilar junction. Occasionally, a straight anteroposterior view may be required to depict an aneurysm of the posterior inferior cerebellar artery.
Possible findings that may affect the patient's treatment include the presence of multiple intracranial aneurysms; areas of intracranial arterial stenoses; associated arteriovenous malformations; and anatomic variations, such as a fetal origin of a posterior cerebral artery or a persistent primitive trigeminal artery. In patients with multiple aneurysms and SAH, angiographic clues to the bleeding source include the size of the aneurysm, vessel displacements from adjacent hematomas, local vasospasm, and an irregular aneurysm contour or nipplelike protrusion on the aneurysm. The escape of contrast agent during the study is an ominous sign that indicates a repeat rupture of the aneurysm.
Hwang et al compared angiographic outcomes at 2 years postembolization for stent- and nonstent-assisted coiled unruptured aneurysms and found that the additional hemodynamic and biologic effects of stents designed for neck remodeling did not appear to enhance progressive occlusion and prevent recanalization.^[15] They concluded that stent placement may not contribute to a positive long-term angiographic outcome for unruptured aneurysms with an unfavorable configuration for coiling.

Possible complications

Aneurysmal rupture during cerebral angiography may occur as a result of traction, the performance of maneuvers near the aneurysmal neck, or catheter misplacement in the neck of an aneurysm that increases dynamic pressures in the aneurysm. One should always be prepared to perform open craniotomy promptly if the aneurysm ruptures. If a thrombus obstructs the vessel, a fibrinolytic such as urokinase or tissue plasminogen should be used immediately.