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[Cerebrovascular Surgery Center]

Neurovascular Surgery
Brain Aneurysm & AVM Center

Computerized Tomographic Angiography (CTA)
Assists in the Evaluation of Patients with Intracranial Aneurysms

Christopher S. Ogilvy, M.D., Elizabeth S. Lustrin, M.D., James H. Brown, M.D.

Figures

Over the past several years, many new high resolution techniques have been developed to assist with the imaging of intracranial lesions. Standard angiography has traditionally been the best test to delineate the anatomy of intracranial vascular lesions. The relationship of vascular lesions with bony anatomy close to the skull base had to be discerned by studying various projections of the two dimensional angiogram. CT scans could be used to assist in this evaluation, especially if areas of bony erosion were present which could be examined in more detail on the axial sections of the scans. For more anatomically complex skull base lesions, the exact relationship of the bony anatomy with the vascular pathology were often difficult to detail. The use of magnetic resonance (MR) imaging has further assisted in delineating vascular structures to adjacent soft tissue anatomy. Due to the lack of mobile protons, bone has no MR signal and appears as a signal void on MR images. This is often difficult to separate from the signal void produced by blood flow within vessels.Thus CT remains the modality of choice for high resolution bone imaging. By using a high resolution thin cut CT scan combined with intravenous contrast injection, details of vascular anatomy can be rendered in three dimensions and the adjacent bony structures can be visualized as well. This technique has become known as computerized tomographic angiography (CT angiography or CTA). The resulting three-dimensional CT-angiographic picture can simultaneously demonstrate the bony skull base and its related vasculature. The technique involves infusion of 90 ml of Conray 60 via a power injector at 3 ml per second with a 15 second scan delay. Patients are scanned with thin section axial or spiral acquisition mode on a GE Advantage scanner and three- dimensional images are generated. Once the information is gathered, a work- station which is available on the scanner can be used for postprocessing reconstruction. By choosing the appropriate threshold of imaging, the operator can then develop shaded surface displays of the actual vascular anatomy. Although at the MGH Aneurysm/AVM Center we initially made use of this technique for aneurysms in relation to the skull base, it has also proved useful for aneurysms in relation to the circle of Willis, well away from the skull base. One area of particular interest has been in the evaluation of aneurysms related to the carotid artery in the region of the anterior clinoid process. These so-called 'paraclinoid' aneurysms are often in association with the ophthalmic artery. Aneurysms in this location may originate in the cavernous sinus and extend into the subarachnoid space. Once an aneurysm in the 'paraclinoid' location projects to the subarachnoid space, it carries with it the threat of potential hemorrhage. Thus radiographic studies which can help delineate the borders of the aneurysm in relation to the subarachnoid space are extremely helpful in deciding whether or not to treat an aneurysm in this location. Such aneurysms may require careful removal of the anterior clinoid process in order to safely expose the entire neck of the aneurysm for clipping (Fig 1). Alternatively, these aneurysms may originate completely intradurally and not require any removal of the anterior clinoid process (Fig 2). For surgical planning, it is of great benifit to visualize the relationship of the artery and aneurysm to the anterior clinoid process using the CTA. The CTA can be viewed from the predicted angle of surgical approach in order to help plan consicely how much bone needs to be removed. Another use of the CTA has been to confirm the presence of small aneurysms associated with the circle of willis. Figure 3 shows a standard angiogram of a 48 year old man who had suffered a small subarachnoid hemorrhage manifest by headache while lifting weights. Although the region of the anterior communicating artery is suspicious for a small aneurysm, it was difficult to be certain about the exact location of the lesion. A CTA clearly demonstrated the aneurysm to arise from the junction of the anterior cerebral artery on the left and the anterior communicating artery (Fig 3). As experience is gained with the technique of CTA, we anticipate that the applications will expand. Several skull base tumors have already been studied using this technique at our institution. CT angiography can be performed using a minimum of modifications to exsisting CT scan software.

Figures

Figure 1
. A) Lateral angiogram of a 30 year old woman who developed slight visual loss in the left eye. The angiogram demonstrates a "paraclinoid" aneurysm associated with the ophthalmic artery. B) CT angiogram of the same patient demonstrates the dome of the aneurysm (arrow) projecting above the anterior clinoid process. C) Intraoperative photograph. The frontal and temporal lobes are retracted to expose the aneurysm (arrow) adjacent to the optic nerve. The aneurysm was successfully clipped and the patient made an uneventful recovery.


Figure 2. A) Lateral angiogram of a patient who developed a severe headache and was found to have an ophthalmic artery aneurysm. B) The CTA demonstrates the aneurysm (arrow) to be entirely in the intradural compartment and therefore it was not necessary to remove any bone at the skull base to completely dissect and clip the aneurysm. C) Intraoperative photograph demonstrating the aneurysm (arrow) in association with the optic nerve.


Figure 3. A) Anterior posterior angiogram of a left carotid injection of a 45 year old man with subarachnoid hemorrhage which suggests an aneurysm in the region of the anterior communicating artery (arrow). B) A CT angiogram confirms the presence of an aneurysm. Abbreviations: LA1, proximal left anterior cerebral artery; LA2, distal left anterior cerebral artery; RA1, proximal right anterior cerebral artery; RA2, distal right anterior cerebral artery; AComm, anterior communicating artery; aneurysm as noted.

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