A needle is placed in the femoral or brachial artery; a cannula is then threaded through the needle and along the aorta and the arterial branches to be visualized. In this way, a contrast agent is injected to visualize the arch of the aorta, the origins of the carotid and vertebral systems, and the extensions of these systems through the neck and into the cranial cavity and the vasculature in and surrounding the spinal cord. This allows the visualization of the cerebral and spinal cord vessels to less than 1 mm in lumen diameter. With refinements in technique it is possible to produce images of the major cervical and intracranial arteries with relatively limited amounts of contrast medium introduced through small catheters.
Angiography is not altogether without risk. Overall morbidity from the procedure is approximately 2.5 percent, mainly in the form of worsening of a preexistent vascular lesion or from complications at the site of artery puncture. Occasionally, cerebral or systemic ischemic lesions are produced, the result of either particulate atheromatous material dislodged by the catheter, thrombus formation at or near the catheter tip, vasospasm, or more often by dissection of the artery by the catheter. A cervical myelopathy is a rare but disastrous complication of vertebral artery contrast injection; the problem is heralded by pain in the back of the neck immediately after injection. Progressive cord ischemia from an ill-defined vascular process ensues over the following hours. For these reasons, the procedure should not be undertaken unless it is deemed necessary to obtain a clear diagnosis or in anticipation of surgery that requires a definition of the location of the vessels.
This imaging modality is a contrast-based technique that can be performed with both CT and MRI. Images are rapidly and serially acquired as the contrast transits through the vasculature and parenchyma. A time-intensity curve is produced, from which measurements of cerebral blood flow, cerebral blood volume, and transit time can be derived. Perfusion imaging has provided a means of detecting regions of ischemic tissue, and to monitor the elevated blood volume in certain brain tumors.
The tissue concentrations of a variety of cellular metabolites can be determined with the technique of magnetic resonance spectroscopy (MRS). Among these substances, N-acetyl aspartate (NAA) is a marker of neuronal integrity and is decreased in both destructive lesions and in circumstances in which there is a reduction in the density of neurons (e.g., edema or glioma that increases the distance between neurons). Choline (Cho), a marker of membrane turnover, is elevated in some rapidly dividing tumors. Therefore, compared to normal white matter, the spectrogram of a glioma characteristically shows decreased NAA and increased Cho. It is possible to measure a number of other metabolites such as myoinositol, creatine, and lactate that find occasional clinical utility.
A technique based on DWI, termed diffusion tensor imaging (DTI), integrates measurements of the amount of anisotropy with its directionality to model axonal tracts in the brain. This modality detects damage to, or displacement of white matter tracts because of trauma, vascular injury, or tumor. Tractography is also occasionally used in surgical planning to localize critical white matter tracts in order to avoid their transection during operations.
In the last decades, several techniques of functional imaging have been introduced to study the activation of regions of cerebral cortex during mental and physical actions or experiences. The MRI-based functional imaging technique (functional MRI, or fMRI) shows changes in local cerebral blood oxygenation, a surrogate for local neuronal metabolic activity. These changes are quantified as the blood oxygen level-dependent (BOLD) signal, and evolve over the 10 to 15 s following a change in neuronal activity (Fig. 2-5). In addition to its research application in cognitive neuroscience, this technique also has clinical utility, including presurgical planning in tumor and epilepsy surgery.