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MRI Magnetic Resonance Imaging
Magnetic resonance imaging is a method of taking pictures of the inside of the body. Unlike x-rays, MRI can investigate soft tissue. This non-invasive procedure is particularly useful in detecting cerebral oedema, abnormalities of the brain and spine, and cancer.
Myelin Sheath
How it works
An MRI machine is a large tubular superconducting magnet about 30,000 times stonger than the earth's magnetic field. When a patientlies inside the MRI magnet, the hydrogen atoms in molecules in the body line up along the axis of the magnetic field. Once this is accomplished, a radio wave is pulsed through the body to produce a magnetisation at right angles to the original magnetic field. As the radio frequency chosen is the same as hydrogen, the radio pulse only interacts with the hydrogen atoms, causing them to change the direction of their net magnetisation. Once the radio pulse ends, the hydrogen atoms move back into alignment with the original magnetic field at a rate that is related to the type of body tissue they are contained within. A coil placed around the body then receives a radio signal at a predetermined time which is related to the coherence of the hydrogen atoms. By varying this time we can get different information about tissue and organs within the body such as the brain because of the way hydrogen is bound in various molecules in the body (eg. water, fat, cerebro spinal fluid etc.). For more information about MRI, please check out what Wikipedia has to say.
Diffusion Tensor Imaging
The imaging technique Diffusion Tensor Imaging (DTI) is an extension of diffusion MRI and used to investigate the direction of water movement within the tissues. DTI uses diffusion gradients in 6 directions to calculate for each unit of fluid a "tensor", or 3-dimesional shape of that unit's net diffusion. Unrestricted diffusion will take the shape of a sphere, and is said to has a score "0" asymmetry, or anisotropy. If the direction of diffusion is not a perfect sphere, then there is a net diffusion direction, or a positive tensor. White matter is composed of bundles of axons surrounded by a fatty myelin sheath which insulates the axon from the surrounding tissue. The myelin also serves to reduce the diffusion of fluids out of the axon, confining the possible flow direction to either towards the cell body or away from the cell body along the axon (see above). Bundles of many axons exhibit a detectable flow along the course of the bundle. Knowledge of the net direction of axon fibre bundles helps detect white matter lesions, tumours and infiltrates.TOP
