What Does MRI Mean?

MRI is an acronym for magnetic resonance imaging. MRI scanners make use of the magnetic properties of atoms to construct high-resolution two-dimensional and three-dimensional images of the human body. Two-dimensional images capture "slices" of the body's interior and three-dimensional images generate detailed models of the body's structure. In both cases, MRI provides excellent contrast among different soft tissues, and is useful for oncology, neurology and cardiovascular diagnosis and research.

Theory

MRI relies on the fact that the human body mostly consists of water. The two hydrogen atoms in a water molecule each have a dipole, a measure of the strength and direction of their magnetic field. Under normal circumstances, these dipoles are arranged randomly. However, MRI scanners create an extremely powerful magnetic field that aligns all those dipoles. As the hydrogen dipoles align with the magnetic field, they generate the signal that the scanner can use to construct the image.

Structural MRI

Structural MRI refers to the generation of high resolution 2D and 3D images of the human body's interior. These images are useful for medical diagnosis because they make it possible to examine the interior of the human body without the use of invasive medical techniques. Aberrant tissues like tumors have different magnetic properties than healthy tissue and can also be identified in structural MRI.

Functional MRI

Frequently abbreviated as fMRI, functional MRI refers to the collection of relatively lower resolution images recorded once every two to three seconds. This technique is especially useful for brain research because active neurons are preferentially supplied with a greater ratio of oxygenated to deoxygenated blood. Because oxygenated and deoxygenated blood have different magnetic properties, this Blood-Oxygen-Level Dependent (BOLD) signal can be detected by an MRI scanner and corresponds to the brain's activity.

Misconceptions

fMRI is a valuable research tool for studying human brain activity, but it does not directly measure the activity of neurons alone or in aggregate. Instead, the BOLD signal that underlies fMRI measurements responds to neural activity rather than predicting it. That is, the blood oxygenation that feeds active neurons necessarily lags behind the neural activity that drives it. As a consequence, the temporal resolution of fMRI is very low even though the spatial resolution is relatively high.

Warning

The strong magnetic fields used for MRI can be dangerous. The standing magnetic field can turn ferromagnetic objects like oxygen tanks and even paperclips into high speed projectiles if they are brought too close to the scanner. The same applies to ferromagnetic implants like aneurysm clips and even small impacted foreign bodies like metal shavings. The magnetic field can also generate a current in conductors, and a closed circuit path can result in burns and even fire. Rigorously following established MRI safety protocol will minimize these risks, but caution is always appropriate when interacting with the scanner.

References

• The Magnetic Resonance Imaging Institute for Biomedical Research