MRI
Magnetic Resonance Imaging. The newest of the 4 imaging modalities.
In an MRI examination, the patient is placed in a strong electromagnetic field. When this happens, the millions of hydrogen atoms in the body align themselves parallel with the magnetic field, either in the same direction or opposite to the direction of the field. All body tissues contain hydrogen atoms, but in different concentrations and compositions depending on the type of tissue. At the level ("slice") where it is desired to 'take a picture', a short, powerful radio signal (yet another form of electromagnetic energy) is sent through the patient's body, perpendicular to the main magnetic field. The hydrogen atoms, which have the same frequency as the radio wave, will become 'excited' (that is, they will be raised to a higher state of energy) and start to resonate with the exciting wave.
When the radio signal is turned off, the hydrogen atoms will, after a period of time, return to their original energy state. The excitation energy, which they had gained, will be released in the form of radio waves, which are detected by the MRI machine. The time it takes for the excited hydrogen atoms to return to their original energy level depends on the number of atoms and characteristic physical properties of the various tissue types. This time is measured and analysed by a computer, which on the basis of these measurements constructs an image of the tissues within the body.
MRI can be done with or without contrast media
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