Musings of a Compulsive Thinker

Ever wondered how MRI works?

Magnetic resonance imaging (MRI) is a powerful tool in medical diagnostics, yielding a true three-dimensional image of large tissue volumes. It works by producing a strong graded magnetic field along a specified spatial axis. Most biological tissues contain large concentrations of hydrogen atoms, usually in the form of water. The atomic nuclei of some of these hydrogen atoms align with the field. Then the tissue is bombarded with radio waves of a specific frequency, exciting these nuclei. This causes the nuclei to "flip" their polarity back and forth between their two aligned states; each "flip" is actually a specific excitation event and subsequent "relaxation," which causes an energy emission. These emissions are measured and used to produce an image of the hydrogen content of the tissue. Because the magnetic field gradient can be generated along various spatial axes, a series of images can be taken and compiled into a three-dimensional image.

Contrast-enhanced MRI involves introducing a contrast agent (usually a gadolinium compound) into the tissue's blood supply. These contrast agents appear very bright on MRI images. The contrast agent can be thought of as a probe of sorts, spreading throughout the cardiovascular system and diffusing into the extracellular space as permitted. This technique is especially useful in screening for or imaging cancers, many of which are highly vascular tissues, because the contrast agent will tend to accumulate in highly vascular areas.

Functional MRI (fMRI), which involves the rapid acquisition of a sequence of time-dependent images, can help differentiate cancers from benign lesions. This is because benign lesions generally have a more normal vascularity than malignant lesions, and therefore are usually seen to enhance more slowly on fMRI sequences.

For more in-depth reading:
How MRI Works
MRI - Wikipedia
The Basics of MRI (on-line textbook)

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Labels: engineering, science

posted by the alygator @ 4:02 PM