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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 permeability of rock to hydrocarbons. It is now a commonly used form of medical imaging.
Nomenclature
Magnetic resonance imaging was developed from knowlege gained in the study of nuclear magnetic resonance. The actual name for the medical technology is nuclear magnetic resonance imaging (NMRI), but the word nuclear is almost universally dropped. This is done to avoid the negative connotations of the word nuclear, and to prevent patients from associating the examination with radiation exposure. Scientists still use NMR when discussing non-medical devices operating on the same principles.
Principles
 First, the spins of the atomic nuclei of the tissue molecules are aligned in a powerful magnetic field. Then, oscillating magnetic field pulses at radio frequency are applied in a plane perpendicular to the magnetic field lines so as to cause some of the hydrogen nuclei to change from their aligned positions. The radio frequency of the pulses used is governed by the Larmor Equation. Magnetic field gradients are applied in the three dimensional planes to allow encoding of the position of the atoms. After this, the nuclei go back to their original configuration, but before doing so, their new alignment can be measured by coils wrapped around the patient. These signals are recorded in the temporary memory termed Kspace; this is the spatial frequency weighting in two or three dimensions of a real space object as sampled by MRI. The information is subsequently inverse Fourier transformed by a computer into real space to obtain the desired image. 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.
T1-weighted MRI scans rely on relaxation in the longitudinal plane, and T2 weighted MRI scans rely on relaxation in the transverse plane. 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. Different scan sequences allow different tissue types and pathologies to be highlighted. A contrast agent is sometimes injected in the sample to augment these differences and improve sensitivity.
Advantages
One of the advantages of an MRI scan is that, according to current medical knowledge, it is harmless to the patient. It only utilizes 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 the presence of a ferromagnetic foreign body (say, shell fragments) in the patient, or a metallic implant (like surgical prostheses, or pacemakers) can present a (relative or absolute) contraindication towards MRI scanning: interaction of the magnetic and radiofrequency fields with such an object can lead to mechanical or thermal injury, or failure of an implanted device.
MRI allows manipulation of spins in many different ways, each yielding a specific type of image contrast and information. With the same machine a variety of scans can be made and a typical MRI examination consists of several such scans.
Another advantage of MRI is that the contrast resolution of soft tissues is much better than in CT, leading to higher-quality images, especially in brain imaging and spinal cord scans. The spatial resolution achieved per second of scanning time, however, is better in CT, giving CT the advantage in assessing, for example, bony abnormalities.
Nobel prize (2003)
Reflecting the fundamental importance and applicability of MRI in the medical field, Paul Lauterbur and Sir Peter Mansfield were awarded the 2003 Nobel Prize in Medicine for their discoveries concerning MRI. Lauterbur discovered that gradients in the magnetic field could be used to generate two-dimensional images. Mansfield analyzed the gradients mathematically. The Nobel Committee ignored Raymond V. Damadian, who demonstrated in 1971 that MRI can detect cancer and filed a patent for the first whole-body scanner, which he successfully defended against infringement by General Electric with an award of $129 million in 1997, and settling out of court for further millions from other MRI scanner manufacturers. It is still not clear if Damadian's method of detecting cancer is working, and it is not used in modern MRI imaging and diagnostics. His description of a whole body scanner does only concern itself with searching the complete body for points with cancer, and does not discuss the use of the data for generating pictures showing different tissues. The procedure as described would take a very long time to perform. There is a big difference between this scanner and the contemporary MRI machines.
Specialised MRI scans
Magnetic resonance spectroscopy
Magnetic resonance spectroscopy (MRS), also known as MRSI (MRS Imaging) and Volume Selective NMR Spectroscopy, 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, increasing the amount of oxygenated hemoglobin ("haemoglobin" in British English) relative to deoxygenated hemoglobin. Because 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. The BOLD effect also allows for the generation of high resolution 3D maps of the venous vasculature within neural tissue.
Diffusion MRI
Diffusion MRI measures the diffusion of water molecules in biological tissues. Following an ischemic stroke, brain cells die, trapping water molecules inside them (cellular pumps are no longer functioning). The resultant areas of restricted diffusion are detectable by diffusion weighted imaging (DWI). This finding is identifiable much earlier after a stroke than findings on CT or on conventional MRI, making DWI one of the most sensitive methods for the detection of early stroke.
Diffusion MRI is also a tool to study connections in the brain. 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 for this application is still at the research stage. Identifying fibers on diffusion MRI is called tractography.
Interventional MRI
Because of the lack of harmful effects on the patient and the operator, MRI is well suited for "interventional radiology", where the images produced by an MRI scanner are used to guide a minimally invasive procedure intraoperatively and/or interactively. However, the non-magnetic environment required by the scanner, and the strong magnetic radiofrequency and quasi-static fields generated by the scanner hardware require the use of specialized instruments. Often required is the use of an "open bore" magnet which permits the operating staff better access to patients during the operation. Such open bore magnets are often lower field magnets, typically in the 0.2 Tesla range, which decreases their sensitivity but also decreases the Radio Frequency power potentially absorbed by the patient during a protracted operation.
Current Density Imaging
Current Density Imaging is a sub branch of MRI that endeavors to use the phase information from the MRI images to reconstruct current densities within a subject. Current Density imaging works because electrical currents generate magnetic fields, which in turn affect the phase of the magnetic dipoles during an imaging sequence. To date no successfull CDI has been performed using biological currents, however several studies have been published which involve applied currents through a pair of electrodes.
See also
External links
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