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Military Health System

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) at the National Intrepid Center of Excellence (NICoE) uses a General Electric MR750 with a 3-Tesla static field magnet. Upgrades to the head coil and software make it possible to replicate advanced imaging techniques. The baseline MRI exam consists of the following components:

  1. Clinical examination:

    This exam is done as the first part of a research scan and is designed to include key components of a routine clinical scan

    • T1 weighted images. These images accurately depict the brain and are sensitive to the size and shape of the brain and its structures.
    • T2 weighted images. These are also images that map the anatomy but are sensitive to different characteristics of the brain. They are also used in the processing of diffusion images.
    • T2-FLAIR images. These images portray the anatomy but are sensitive to variations in the brain that are not easily visualized on a T2 weighted image. It uses a technique that blanks out the signal from the cerebrospinal fluid, which makes features at the gray/white boundary more easily detected.
    • Susceptibility weighted images. These images are sensitive to the presence of iron in the brain. This often results from blood that has been incompletely cleared from the brain tissue after an injury. This scan is often positive in moderate and severe TBI.
    • Diffusion images. These images are also acquired as part of the research exam described below.
  2. Research acquisition. (This can vary depending on the study being performed)
    • Resting state functional MRI (fMRI). This scan collects an image every 2 seconds for 11 minutes. Each image is sensitive to the change in the magnetic properties of blood in the capillaries. When at rest, these images reflect the activity of the brain at rest. These images can be used to identify and characterize connections between gray matter regions.
    • Diffusion. This measurement is sensitive to the microscopic movement of water as it diffuses through the brain. It can be used in two ways: to map out white matter tracts as in resting state fMRI (tractography); or it can be used to examine diffusion parameters along specified tracts. This technique is used to identify and stage injuries to the white matter.
    • Perfusion. This modality measures the amount of blood passing through each point in the brain.

MRI is accomplished by placing the participant into a powerful and uniform magnetic field. A system of three much weaker magnets can then be used to precisely adjust the magnetic field at each point in the region to be imaged. Adjusting the strength and duration of these magnets in specific ways can illuminate various characteristics of the region being imaged. The following image types are obtained during studies performed at NICoE:

  • Structural images, also known as T1 and T2 weighted images, faithfully portray the structure of the brain. Lesions can be detected by observing small bright areas (hyperintensities) in the images.
  • Perfusion Images are obtained by "tagging" blood with an RF pulse as it travels up the carotid arteries and then measuring the amount of tag at each point in the brain. This yields an image of how much blood travels through each small area of the brain.
  • Diffusion Images are formed by applying strong magnetic pulses before acquiring each image. The image reconstruction algorithms assume that each molecule is motionless such that the images become darker in regions where the water molecules have moved. This darkening can be used to compute quantitative measures of how much the molecules have moved. This is useful in the context of traumatic brain injury (TBI) because the injuries damage the cell walls of the nerve fibers, so the water diffuses more freely, resulting in the darkening of the images.
  • Susceptibility Imaging relies on the fact that the signal tends to disappear in areas where there is iron. Regions high in iron (or any paramagnetic material) yield darker images in those regions. In moderate to severe TBIs, traces of blood often remain in the tissue after the brain has healed. These "microhemorrhages" can be detected using this technique.
  • Functional MRI (fMRI) uses the blood-flow response to a stimulus to generate an activation image. If, for example, a flashing light is presented to a participant, the neurons in the visual cortex will fire, which triggers a temporary increase in local blood flow. This causes a change in image intensity. These changes provide a map showing which parts of the brain responded to the stimulus.
    • Task-based fMRI uses external stimuli such as mental tasks to study brain function.
    • Resting state fMRI relies on the fact that neurons in the brain are constantly firing and each time one fires it triggers a blood flow response. Calculating the correlations between different brain regions can be used to determine which pathways are active and their degree of correlation.

The MRI scanner at NICoE is a General Electric MR750. This scanner has a 3T static magnet and a NOVA 32 channel coil.

Last Updated: August 26, 2022
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