MRI Application
In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain tumor) from normal tissue. One of the advantages of an MRI scan is that, according to current medical knowledge, it is harmless to the patient. It utilizes strong magnetic fields and non-ionizing radiation in the radio frequency range. Compare this to CT scans and traditional X-rays which involve doses of ionizing radiation and may increase the chance of malignancy, especially in children receiving multiple examinations.
While CT provides good spatial resolution (the ability to distinguish two structures an arbitrarily small distance from each other as separate), MRI provides comparable resolution with far better contrast resolution (the ability to distinguish the differences between two arbitrarily similar but not identical tissues). The basis of this ability is the complex library of pulse sequences that the modern medical MRI scanner includes, each of which is optimized to provide image contrast based on the chemical sensitivity of MRI.
For example, with particular values of the echo time (TE) and the repetition time (TR), which are basic parameters of image acquisition, a sequence will take on the property of T2 weighting. On a T2 weighted scan, water and fluid-containing tissues are bright (most modern T2 sequences are actually fast T2 sequences, in which case fat is also bright). Damaged tissue tends to develop edema, which makes a T2 weighted sequence sensitive for pathology, and generally able to distinguish pathologic tissue from normal tissue. With the addition of an additional radio frequency pulse and some more manipulation of the magnetic gradients, a T2 weighted sequence can be converted to a FLAIR (fluid light attenuation inversion recovery) sequence, in which free water now is dark, but edematous tissues remain bright. This sequence, in particular, is currently the most sensitive way to evaluate the brain for changes of multiple sclerosis.
The typical MRI examination typically consists of 5-20 sequences, each of which are chosen to provide a particular type of information about the subject tissues. This information is then synthesized by the interpreting physician.
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