CT Angiography: A Tool for Managing Cerebral Aneurysms?
Matthew T. Walker, MD*
Jill Blackburn, RT
Cameron N. Nourani
Shahram Partovi, MD
Division of Neuroradiology, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona
*Current Address: Division of Neuroradiology, Northwestern Memorial Hospital, Chicago, Illinois
The diagnosis of cerebral aneurysms has relied on conventional digital subtraction angiography (DSA) and magnetic resonance (MR) angiography. However, recent advances in computed tomographic (CT) angiographic technology now provide an alternative modality for the diagnosis of most aneurysms. This imaging modality has several advantages. The procedure can be performed faster and with less risk than that associated with conventional DSA. Reformatted and three-dimensional data provide morphologic information unavailable from DSA such as the relationship of the aneurysm to the clot, the presence of associated calcification, and bony landmarks. This article reviews the different imaging modalities used to diagnose and manage cerebral aneurysms and presents an evolving imaging algorithm that incorporates CT angiography, MR angiography, and DSA.
Key Words: aneurysms, computed tomographic angiography, digital subtraction angiography, magnetic resonance angiography
Intracranial aneurysms represent a significant health risk. They are reported in 1 to 2% of the general population; however, autopsy series identify aneurysms in more than 5% of the population. With each hemorrhagic event, the morbidity and mortality rate associated with ruptured aneurysms is greater than 50%. More than half of all ruptured aneurysms rehemorrhage, the greatest percentage within the first 24 hours. Diagnosis and management can be further complicated by multiple aneurysms in the same patient; between 20 to 30% of affected patients will have at least one other aneurysm. Patients who do survive may have substantial complications related to vasospasm, hydrocephalus, and stroke.[10,25] Given this grim natural history, appropriate and timely treatment of cerebral aneurysms is crucial and relies heavily on diagnostic imaging.
The cornerstone diagnostic examination for intracranial aneurysms has been digital subtraction angiography (DSA). Magnetic resonance (MR) angiography has played a supportive role in some screening algorithms. Advances in computed tomography (CT) technology have reintroduced a third modality, CT angiography, which had remained largely a curiosity. The exact role of CT angiography in the diagnosis of cerebral aneurysms remains to be defined, but our experience suggests that its role in this diagnostic pathway will be important. This article reviews the angiographic modalities used to evaluate cerebral aneurysms, focusing on new developments in CT angiography. A new diagnostic algorithm for the evaluation of patients with acute nontraumatic subarachnoid hemorrhage (SAH) is presented.
Digital Subtraction Angiography
Much of the pioneering work in cerebral angiography was performed by Egas Moniz, a Portuguese physician best known for his Nobel prize-winning work on prefrontal lobectomy in psychiatric patients. At the Society of Neurology in Paris in July 1927, Moniz presented a series of angiographic experiments involving animals and humans. The patients were chronically ill with the diagnoses of general paralysis of the insane, postencephalitic parkinsonism, and brain tumors. Of the original nine patients, useful images were obtained from only two and one patient died from a complication. However, the potential for the technique was clear.
Since then angiography has evolved considerably, and DSA is now available in most neuroradiology departments. Toxic contrast agents such as Thorotrast have been replaced with nonionic iodinated contrast. Filming capabilities also have improved. In the last 20 years enhancements to x-ray detectors for DSA have substantially improved image quality. DSA image resolution has increased by a factor of 4, from a 512x512 to a 1024x1024 pixel matrix, and the increased resolution permits smaller lesions to be identified. In concert with the increase in resolution, the response time of the x-ray detector--the minimum duration required to capture information--has decreased allowing more rapid filming speeds. Rapid filming is paramount for the dynamic evaluation of the cerebral circulation because it permits the construction of a "movie" that depicts how blood flows intracranially. This attribute is unique to DSA, a key feature that distinguishes DSA from all other forms of angiography, including CT angiography. Recently, three-dimensional (3D) reconstruction has become available; this format can be particularly useful when the neck of an aneurysm is difficult to define.
While DSA is considered the gold standard for the evaluation of the cerebral circulation, it still has limitations and associated risks. These risks prevent the use of such an invasive test in screening algorithms and restrict its use to instances where the pretest probability of finding a lesion is high, such as in the presence of SAH. Even in this setting, for example, the procedure may fail to detect the source of acute SAH in as many as 5 to 15% of patients.[4,19] The false-negative rate of DSA is partially a reflection of its limitations, which include factors such as the presence of a thrombosed aneurysm, adjacent vasospasm, poor vessel opacification, and inadequate angiographic views. As an invasive technique, angiography is associated with complications, most importantly a risk of permanent stroke, which should be less than 1% at performing institutions as recommended by the American Society of Neuroradiology and the American Society of Interventional and Therapeutic Neuroradiology. Complications related to the arterial puncture site and to the administration of iodinated contrast also can occur. Lengthy procedures performed in medically unstable patients outside an intensive care setting can further compromise patients' safety. Moreover, angiographic examinations require considerable personnel--a radiologist, technologist, and nurse at minimum. In a busy angiography department, these requirements can contribute to significant time delays in completing an examination. Finally, in some geographic locations, deployment of a costly angiography suite and the required personnel may be financially prohibitive.