Main Page | See live article | Alphabetical index

Magnetic resonance imaging

MRI Image

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, radio frequency pulses are applied in a plane perpendicular to the magnetic field lines so as to cause some of the hydrogen nuclei to change alignement. After this, the radio frequency is turned off and the nuclei go back to their original configuration but, as they do so, they release radio frequency energy which can be picked up by coils wrapped around the patient. 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.

One of the advantages of an MRI scan is that, according to current medical knowledge, it is harmless to the patient. It only utilises 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. It must be noted, however, that patients with metallic foreign bodies (say, shell fragments) or metallic implants (like artificial Titanium bones, or pacemakers) cannot be scanned in MRI machines, due to the very strong magnetic fields involved.

Another advantage of MRI scans is that the quality of the images obtained is usually of much better resolution than a CT scan. This is especially so for scans of the brain and spinal cord though it is noted that CT scans can sometimes be more useful for bony abnormalities.

Reflecting the fundamental importance and applicability of MRI in the medical field, Paul Lauterbur and Sir Peter Mansfield was awarded the 2003 Nobel Prize in Medicine for their discoveries concerning MRI.

Specialised MRI scans

Magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS) is a technique which combines the spatially-addressable nature of MRI with the spectroscopically-rich information obtainable from nuclear magnetic resonance (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

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.

Diffusion MRI

Diffusion MRI measures the diffusion of water molecules in biological tissues. In an isotropic medium (inside a glass of water for example) water molecules naturally move according to Brownian motion. In biological tissues however the diffusion is very often anisotropic. For example a molecule inside the axon of a neuron has a low probability to cross a myelin membrane. Therefore the molecule will move principally along the axis of the neural fiber. Conversely if we know that molecules locally diffuse principally in one direction we can make the assumption that this corresponds to a set of fibers. Diffusion MRI is a tool for scientists (and medical doctors) to study connections in the brain. Diffusion MRI is still at the research stage. The problem of finding a fiber from a Diffusion MRI image is called tractography.