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 and in patients with traumatic brain injury.
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).