In addition to the standard MRI examination routinely used to image the brain and central nervous system, a number of advanced imaging techniques are available which are helpful in the diagnosis and management of various neurological conditions, including brain tumors, stroke, and dementia. These techniques include Magnetic Resonance (MR) Angiography, MR Spectroscopy, MR Perfusion, Functional MRI, and Diffusion Tensor Imaging (DTI) and Tractography.
MR Angiography and Venography
MR angiography (MRA) and MR venography (MRV) are MR techniques used to generate 2D and 3D images of blood vessels within the brain, neck, and body. These examinations can be performed with or without the use of intravenous contrast, depending on the area of the body that is being analyzed and the reason the test is being performed.
MR spectroscopy is a specialized MRI sequence that uses a strong magnetic field and radio waves to obtain biochemical information about the area of tissue being analyzed, usually within the brain. In certain instances, this information, when used in conjunction with anatomical imaging, can help in further characterizing an abnormality as a tumor, radiation necrosis, or possibly demyelinating disease.
This MR examination is used to analyze blood volume and flow within a region of the brain. During the injection of intravenous contrast (gadolinium), dynamic imaging is performed which allows for calculation of cerebral blood volume and flow. This technique is helpful in the areas of brain tumor and stroke imaging.
Functional MRI utilizes blood-oxygen-level dependent (BOLD) signal to localize areas of the brain responsible for specific motor, sensory and language functions. During the exam, patients are asked to perform a specific task. This results in increased neural activity within regions of the brain which leads to increased oxygen utilization and metabolism. This change in oxygenation is measured and mapped onto anatomic images of the brain. Functional MRI is most commonly used for presurgical planning although this technique has also been used to study patients with epilepsy, dementia, and neuropsychiatric disorders.
Diffusion Tensor Imaging and Tractography
This MR technique utilizes differences in movement of water molecules within regions of the brain in order to map the direction of white matter tracts responsible for connecting different areas of the brain, so called “fiber-tracking”. This data can be translated into a 2D image (diffusion tensor imaging or DTI) or 3D map (tractography). This technique is most commonly employed in presurgical planning.
Imaging of Acute Stroke
In conjunction with non-contrast CT of the brain, CT angiography and CT perfusion are techniques often utilized in imaging patients suspected of having an acute stroke. These techniques require the administration of intravenous contrast. During injection of the contrast, images are acquired dynamically. The data is then post-processed to calculate mean transit time, cerebral blood flow and cerebral blood volume within regions of the brain. CT perfusion may detect abnormalities before they are visible on a conventional non-contrast CT, therefore helping to guide treatment of patients experiencing an acute stroke.
CT angiography (CTA) is a CT scan performed using rapid infusion of intravenous contrast in order to highlight blood vessels within the region of the body being examined. Within the brain, CTA is used to detect and/or characterize cerebral aneurysms (abnormal localized ballooning or outpouching along an artery), other vascular malformations, and areas of narrowing (stenoses) due to atherosclerosis, vasospasm or vasculitis (inflammation of blood vessels).