Magnetic resonance imaging

Magnetic Resonance Imaging (MRI) is a method of creating images of the inside of opaque organs in living organisms as well as detecting the amount of bound water in geological structures. It is primarily used to visualise pathological or other physiological alterations of living tissues as well as to estimate the permability of rock to hydrocarbons.

First, the spins of the atomic nuclei of the tissue molecules are aligned in a powerful magnetic field. Then rapid changes in the field are introduced, which cause the atomic nuclei to release a tiny amount of electromagnetic radiation. These signals are recorded and the resulting data are processed by a computer to generate an image of the tissue. Thus, the examined tissue can be seen with its quite detailed anatomical features. In clinical practice, MRI is used to distinguish pathologic tissue such as a brain tumor from normal tissue.

The technique most frequently relies on the relaxation properties of magnetically-excited hydrogen nuclei in water. The sample is briefly exposed to a burst of radiofrequency energy, which in the presence of a magnetic field, puts the nuclei in an elevated energy state. As the molecules undergo their normal, microscopic tumbling, they shed this energy to their surroundings, in a process referred to as "relaxation." Molecules free to tumble more rapidly relax more rapidly. Differences in relaxation rates are the basis of MRI images--for example, the water molecules in blood are free to tumble more rapidly, and hence, relax at a different rate than water molecules in other tissues.

Though the behavior of atomic nuclei in the sample is central to the technique, the term "nuclear" was dropped from the technique's name to avoid an irrational avoidance of the technique in the face of worries or concerns born from the association of the word "nuclear" with the technologies used in nuclear weapons and the risks of radioactive materials. Unlike nuclear weapon technology, the nuclei relevant to MRI exist and are in place whether the technique is applied or not.

Magnetic resonance spectroscopy (MRS) is a technique which combines the spatially-addressable nature of MRI with the spectroscopically-rich information obtainable from NMR. That is to say, MRI allows one to study a particular region within an organism or sample, but gives relatively little information about the chemical or physical nature of that region--its chief value is in being able to distinguish the properties of that region relative to those of surrounding regions. MR spectroscopy, however, provides a wealth of chemical information about that region, as would an NMR spectrum of that region.

Functional MRI (fMRI) measures signal changes in the brain that are due to changing neural activity. The brain is scanned at low resolution but at a rapid rate (typically once every 2-3 seconds). Increases in neural activity cause changes in the MR signal via a mechanism called the BOLD (blood oxygen level-dependent) effect. Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, greatly increasing the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin. Since deoxygenated hemoglobin reduces MR signal, the vascular response leads to a signal increase that is related to the neural activity. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research.

See also nuclear magnetic resonance