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Conventional angiography is the standard procedure for the evaluation of cerebral aneurysms. This modality may be used to evaluate the location, size, and morphology of the aneurysms whether the aneurysm is acute or chronic. Angiography is very essential before surgery can take place. The conventional catheter angiography has several limitations even with the resent development of digital angiography and magnetic resonance angiography. The visualization of the neck of the aneurysm is needed and the pattern of arterial branching are required, these methods cannot be relied upon. In recent years, we have seen rapid evolution in the area of three-dimensional reformation of cross sectional imaging by use of 3-D computer tomographic angiography that allows neuroradiologists to evaluate the angioarchitecture of cerebral aneurysms at higher resolution.
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When the ability of three-dimensional angiography is compared with that of DSA and magnetic resonance (MR) angiography in detection of cerebral aneurysms and provision of information regarding the anatomy of aneurysms, the comparison is there. The clinical use of 3D CT angiography to cerebral aneurysms has progressed remarkably in response to development of helical CT scanning technology. Past studies show that three dimensional computer tomographic angiography is able to provide good images for evaluating cerebral aneurysms and also planning therapeutic procedures which are noninvasive.
During surgical intervention, 3D CT angiography is applied for depicting the neck and body of aneurysms relative to the branches of arteries. It is important for providing the most crucial information during surgical planning. 3D computer tomographic angiography provides accurate relationship of cerebral aneurysms with major branches as well as the shapes of aneurysms. It may also reveal unique information regarding cerebral aneurysms thereby making the presurgical assessment easier.
Advances in Diagnosis of Cerebral Aneurysms
Not long ago, cerebral angiography was the sole essential imaging technique present for visualization of intracranial vascular features or structures. Some of the drawbacks of the conventional imaging technique include its invasiveness, it is highly expensive, and the technique is time-consuming. In addition, the technique requires several contrast injections that lead to relatively high rates of complication (Ernest, Forbes, Sandok et al 1983; Hankey, Warlow & Sellar 1990).
Several recent literature describe magnetic resonance imaging/angiography (MR) as a sensitive technique for imaging cerebrovascular diseases. However, its turbulence and the technique’s time consuming attributes complicates the demonstration of cerebral aneurysms. MR is also sensitive to moving artifacts. A typical MR scan takes hours to complete in addition to fasting. It is not only slow but also torturous to the patient. Little wonder there had to be a technology that was faster and reliable that would present little inconvenience to both the neuroradiologist and the patient.
Until recently, neuroradiologists were presented with a problem while using the earlier angiography techniques in differentiating small aneurysms from the larger aneurysms. The time taken to correctly diagnose the aneurysms also presented another challenge. Digital subtraction angiography (DSA) can provide information that is helpful in evaluation of ruptured aneurysms. However, the conventional cerebral angiography techniques cannot easily show the neck of an aneurysm and adjacent arteries. Three dimensional computed tomography angiography has made the evaluation of small and large aneurysms easier as well distinguishing between them in the shortest time. This ideally makes three-dimensional CT angiography ideal for preoperative evaluation of cerebral aneurysms in contrast the conventional angiography techniques.
3D computer tomographic angiography is used to show the relationship of an aneurysm with adjacent structures such as the bones, veins, or brain parenchyma. It can also demonstrate the calcification of aneurysm and major arteries. It confirms the blood vessels, which flow into the aneurysm and the vessels of blood that flow out of the aneurysm, and allows the lumen calcification of the aneurysm. For the large aneurysms, observation is restricted in the presurgical evaluation, which leads to the obscurity of the neck and peripheral blood vessels of the aneurysm. Thus, application of the 3D CT angiography in assessment of aneurismal lumen and blood vessel lumen gives crucial information useful for determining treatment and indications for cerebral aneurysms.
Computed tomography (CT) is now being used for detailed diagnosis and description of facial anomalies, brain tumours, measurement of hepatic volume, vertebra, and determination of fractures of the temporal and pelvic bones. Recently, CT angiography has been applied for the diagnosis of cerebral aneurysms and other cerebrovascular diseases (Schmid, Steiger & Huber 1987). The three dimensional CT (3D-CT) is a useful diagnostic technique for profiling of cerebral aneurysms. CT angiogram is taken to confirm the presence of cerebral aneurysms with contrast medium (Yamamoto et al 1986).
Computed tomographic (CT) angiography is a popular technique for evaluation of cerebral aneurysms and for planning of medical intervention (Teasdale , Statham , Straiton & Macpherson 1990). In spite of various studies in angiography and an improvement of quality of image quality because of application of multisectional CT and more advanced post-processing tools like direct volume rendering, computed topographic angiography is yet to replace digital subtraction angiography (DSA) as the reference standard for evaluation of cerebral aneurysms. Computed tomographic angiography is yet to be a standardized method, especially as regards image post-processing (Gillespie, Adams &Isherwood 1987). A number of methods for 2- and 3-dimensional visualization may be used including multiplanar reformation, direct volume rendering, maximum intensity projection, and shaded surface display (Gholkar & Isherwood 1988).
Limitations of CT Angiography
Limitations of CT angiography are that the small arteries are usually invisible, differentiation of infundibular dilatation at the beginning of an artery from the aneurysm is difficult, “the kissing vessel artifact,” display of vein structures that may simulate aneurysms, the inability to spot thrombosis and calcification on 3-D images, and artifacts that harden the beam by clips of aneurysm (Aoki et al 1988).
In addition patient’s exposure to radiation and the iodinated contrast medium in CT angiography could pose health risks to the patient. It has been shown that long periods of exposure to X-rays can be fatal whether by human error or by mechanical fault (Katada et al 1990). There is always the possibility of radiation overdose that may result from a technical error. This is in contrast to the magnetic resonance angiography whose diagnostic technique differs with that of CT angiography. The same is true for the iodinated contrast medium. We should always remember that the medium is composed of an isotope which if it stays longer in the body could cause harm (Napel et al 1992; Schwartz, Tice, Hooten & Hsu et al 1994).
Recent advancements in imaging technology, especially three-dimensional CT angiography has revolutionized the evaluation and elucidation of cerebral aneurysms. The neuroradiologists and neurosurgeons can now correctly evaluate a patient with cerebral aneurysm in the shortest time possible. This is contrary to the often time-consuming conventional cerebral angiography procedures, which also provide limited accuracy regarding the aneurysms. The new developments have made it possible to reveal extra information, which the traditional procedures could not provide. While there are some limitations to the CT angiography, the benefits of far outweigh the risks given that the clinical results help the neurosurgeon to plan better for surgery and therapeutic treatment.