In MRI all of the atomic nuclei are lined up parallel or anti-parallel to a strong magnetic field. A 180 degree RF pulse or inversion pulse along the Z-axis causes the nuclei to become temporarily non-aligned with the magnetic field. The time that it takes for a nucleus to relax back to equilibrium from this high-energy state is known as T1 (also known as spin-lattice relaxation time or longitudinal relaxation time). One can then repeatedly apply 90 degree RF pulses at time intervals TR. It is possible to fine-tune TR such that one can register differences between the T1 (time to relaxation) in white and gray matter.

T2 (also known as transverse relaxation time or spin-spin relaxation time) measures the time for protons to return into alignment in the XY plane after a transverse RF pulse (echo signal). The duration of T2 (total) is governed by both T2 (molecular interactions) and T2* (imperfections of the magnet). Information on T2 can lead to distinguishing between gray and white matter because protons re-align differently depending on if they are located in white or gray matter. Shorter T2 times come from water molecules hindered by binding to large molecules. Longer T2 come from unattached protons.

Because the differences in T1 and T2 can be captured, the brain can be resolved into white and gray matter. In the T2 scan the water signal is greater than fat signal making the white matter and CSF appear dark and the gray matter appear white. In the T1 scan the fat signal is greater than the water signal making the white matter appear white and the gray matter appear gray. It is possible to achieve image contrast in areas of the magnetically similar areas of the brain by using a paramagnetic contrast agent such gadolinium. This works because shorter T1 times come from water molecules near paramagnetic ions and longer T1 times come from water molecules far from paramagnetic ions.

MRI is especially useful in imaging soft tissue because it can provide good contrast between soft tissue and surrounding muscle, bones and ligaments. An MRI can apply a wide variety of RF pulses by varying the TE and the TR, thereby optimizing the contrast required for the scan of soft tissue. TR is the repetition time or the time between excitations. If one chooses a small TR then the image takes on a T1 weight. TE (echo time) is the time between RF excitation and data acquisition and if it is adjusted to be within the range of T2, the image will have a T2 weight.

The ability of MRI scanning to distinguish soft tissue can be enhanced by improving contrast with a variety of techniques including fat-suppression imaging and gadolinium imaging. Fat suppression decreases the signal from adipose tissue but can decrease variability in signals as well. It can be achieved with fat saturation, inversion-recovery imaging, and opposed-phase imaging. Gadolinium enhanced imaging can achieve contrast in magnetically similar areas of the brain.

Another useful quality of MRI is that it can scan in any plane making multiple angles of soft tissue an option.

References: Toga, A and Mazziotta, J "Brain Mapping: The Methods, 2nd ed." 2002 Academic Press, San Diego, CA.

Related Terms: fMRI

DWI/DTI

Fluid Attenuated Inversion Recovery (FLAIR)^?

Spoiled Gradient Recalled Protocol (SPGR)^?

Echo Planar Imaging (EPI)^?

Proton Density (PD)^?

Fast Spin Echo (FSE)

For more information visit: Introduction to MRI and MRI glossary

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