How are arteriovenous malformations and vascular lesions detected?

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Besides arteriovenous malformations (AVMs), three other main types of vascular lesion can develop in the brain or spinal cord: cavernous malformations, capillary telangiectases and venous malformations.

These lesions may form virtually anywhere within the central nervous system. Unlike AVMs, however, they are not caused by high-velocity blood flow from arteries into veins. In contrast, these three other types of vascular lesions are all low-flow lesions. Instead of a combination of arteries and veins, each one involves only one type of blood vessel.
Physicians use different traditional and new imaging technologies to detect AVMs and other vascular lesions.

Two of the most frequently used noninvasive imaging procedures are computed axial tomography (CT) and magnetic resonance imaging (MRI) scans. CT scans use X-rays to create a series of cross-sectional images of the head, brain or spinal cord and are especially useful in revealing the presence of hemorrhage. MRI imaging offers superior diagnostic information by using magnetic fields to detect subtle changes in neurological tissues. 
Physicians use an array of traditional and new imaging technologies to uncover the presence of arteriovenous malformations (AVMs). Angiography provides the most accurate pictures of blood vessel structure in AVMs. The technique requires injecting a special water-soluble dye, called a contrast agent, into an artery. The dye highlights and records the structure of blood vessels on conventional X-rays. Although angiography can record fine details of vascular lesions, it is an invasive procedure and carries a slight risk. Its safety, however, has been improved by the development of more precise techniques for delivering dye to the site of an AVM. Superselective angiography involves inserting a thin, flexible tube called a catheter into an artery; a physician guides the tip of the catheter to the site of the lesion and then releases a small amount of contrast agent directly into the lesion.
Two of the most frequently employed noninvasive imaging technologies used to detect AVMs are computed axial tomography (CT) and magnetic resonance imaging (MRI) scans. CT scans use X-rays to create a series of cross-sectional images of the head, brain, or spinal cord and are especially useful in revealing the presence of hemorrhage. MRI imaging, offers superior diagnostic information by using magnetic fields to detect subtle changes in neurological tissues. A recently developed application of MRI technology-magnetic resonance angiography (MRA)-can record the pattern and velocity of blood flow through vascular lesions, as well as the flow of cerebrospinal fluid throughout the brain and spinal cord. CT, MRI, and MRA can provide three-dimensional representations of AVMs by taking images from multiple angles.
This answer from the National Institute of Neurological Disorders and Stroke has been reviewed and/or edited by Dr. William D. Knopf.