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How does MRI work? (1 Viewer)

pkc

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Testpilot said:
How does MRI work?
Wow, a big question. Its probably best answered by reading the plethora of explanations in text books and websites.

If you want a rough nutshell summary then try:

1) A patient is put in a magnetic field.
The magnetic field is used to arrange all water molecules in a patients tissue in the same direction (polarise).

2) Radio waves are passed through the tissue. The emerging radio waves have an intensity pattern that matches the pattern of how the water is distributed in the tissues. (Radio waves are absorbed by polarised water).
This pattern is what is shown on the screen (bones are areas without much water).

Hope it helps.
 

Irskin

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A patient is placed horizontally on a table and placed inside a large donought shaped hollow. This is an extremely strong magnetic field (up to 5 teslas). The nuclei inside the body with a net spin (particularly hydrogen) will allign themselves with this magnetic field. However, allignment will not be perfect and they will precess (the top of an object spinning on it's axis will trace out a cone shape) around the magnetic field lines. When a radio signal is sent into the body at the Lamor frequency all off the nuclei will flip from their lower energy spin up state to the higher energy spin-down state. When the signal is switched off, the nuclei will flip back to their normal spin up state, releasing radio waves as they do. A high frequency of radio waves from a partuclar source in the body means a high water content and thus a larger and faster signal will ve produced by the nuclei in the relaxation process. This can be used to detect cancerous growths as they contain more water than normal tissue. Also, this is why MRI images soft tissue with such great resolution. Each slice of the body must be taken using a different magnetic field strength so that it can be distinguished fom the other slices and put to gether to make voxels. The three shim coils (x, y and z) help with this gradient magnetic field. The MRI procedure generally takes around 45 minutes and it is a problme for claustrophobic people. Also, people with pacemakers or other metal objects on their bodies cannot have an MRI. There are no harmful effects of MRI as it uses no electromagnetic radaiation and is therefore considered the safest imaging technique. Also, it may be hard to perform an MRI on overweight people as their layers of fat may prevent the radio waves penetrating the body to the required areas and will blur internal areas.
 

silent_albert

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if you want further information about how mri works check out the hsc.csu.edu.au website and go to medical physics and there u will find another explanation....
:)
 

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