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Magnetic resonance imaging (MRI), formerly referred to as ''magnetic resonance tomography (MRT)'' or ''nuclear magnetic resonance (NMR)'', is a method used to visualize the inside of living organisms as well as to detect the amount of bound water in geological structures. It is primarily used to demonstrate for each unit, with several hundred thousand dollars per year upkeep costs. BACKGROUND INFORMATION Nomenclature Magnetic resonance imaging was developed from knowledge gained in the study of Nuclear Magnetic Resonance . The original 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, which is not one of the safety concerns for MRI. Scientists still use '' NMR '' when discussing non-medical devices operating on the same principles. Technique Medical MRI most frequently relies on the relaxation properties of excited to the magnetic field or Antiparallel . Common magnetic field strengths range from 0.3 to 3 teslas, although research instruments range as high as 20 teslas, and commercial suppliers are investing in 7 tesla platforms. An excess of only one in a million nuclei align themselves with the magnetic field, since the thermal energy far exceeds the difference between the parallel and antiparallel states. Yet, the vast quantity of nuclei in a small volume sum to produce a detectable change in field. Most basic explanations of NMR and MRI will say that the nuclei align parallel or anti-parallel with the static magnetic field; however, because of Quantum Mechanical reasons beyond the scope of this article, the individual nuclei are actually set off at an angle from the direction of the static magnetic field, although the bulk collection of nuclei can be partitioned into a set whose sum spin are aligned parallel and a set whose sum spin are anti-parallel. The magnetic dipole moment of the nuclei then Precesses around the axial field. While the proportion is nearly equal, slightly more are oriented at the low energy angle. The frequency with which the dipole moments precess is called the Larmor frequency. The tissue is then briefly exposed to pulses of Electromagnetic energy (RF pulse) in a plane perpendicular to the magnetic field, causing some of the magnetically aligned hydrogen nuclei to assume a temporary non-aligned high-energy state. The frequency of the pulses is governed by the Larmor Equation . In order to selectively image different voxels (picture elements) of the material in question, orthogonal magnetic gradients are applied. Although it is relatively common to apply gradients in the principal axes of a patient (so that the patient is imaged in x, y, and z from head to toe), MRI allows completely flexible orientations for images. All spatial encoding is obtained by applying magnetic field gradients which encode position within the phase of the signal. In 1 dimension, a linear phase with respect to position can be obtained by collecting data in the presence of a magnetic field gradient. In 3 dimensions, a plane can defined by "slice selection", in which an RF pulse of defined bandwidth is applied in the presence of a magnetic field gradient in order to reduce spatial encoding to 2 dimensions. Spatial encoding can then be applied in 2D after slice selection, or in 3D without slice selection. In either case, a 2D or 3D matrix of spatially-encoded phases is acquired, and these data represent the spatial frequencies of the image object. Images can be created from the acquired data using the Discrete Fourier Transform (DFT).
Typical medical resolution is about 1 mm3, while research models can exceed 1 µm3. ''Contrast-enhancement'' Both T1- and T2-weighted images are acquired for most medical examinations. However, these 2 sets of images are not always sufficient to adequately show anatomy or pathology. One option is to use a more sophisticated image acquisition technique - e.g. fat suppression, chemical-shift imaging. The other is to administer a Contrast agent to delineate areas of interest. A contrast agent may be as simple as Water , taken orally, for imaging the stomach and small bowel. Alternatively, substances with specific magnetic properties may be used. Most commonly, a Paramagnetic contrast agent (usually a Gadolinium compound) is given. Gadolinium-enhanced tissues and fluids appear extremely bright on T1-weighted images. This provides high sensitivity for detection of vascular tissues (e.g. tumors) and permits assessment of brain perfusion (e.g. in stroke).
Diamagnetic agents e.g. Barium Sulfate have been studied for potential use in the GI tract, but are less frequently used. APPLICATION In clinical practice, MRI is used to distinguish pathologic tissue (such as a Brain Tumor ) from normal tissue. One of the advantages of an MRI scan is that, according to current medical knowledge, it is harmless to the patient. It 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 and may increase the chance of malignancy, especially in a fetus. While CT provides good Spatial Resolution (the ability to distinguish two structures an arbitrarily small distance from each other as separate), MRI provides comparable resolution with far better Contrast Resolution (the ability to distinguish the differences between two arbitrarily similar but not identical tissues). The basis of this ability is the complex library of ''pulse sequences'' that the modern medical MRI scanner includes, each of which is optimized to provide ''image contrast'' based on the chemical sensitivity of MRI. For example, with particular values of the ''echo time'' (TE) and the ''repetition time'' (TR), which are basic parameters of image acquisition, a sequence will take on the property of T2-weighting. On a T2-weighted scan, fat-, water- and fluid-containing tissues are bright (most modern T2 sequences are actually ''fast T2'' sequences). Damaged tissue tends to develop Edema , which makes a T2-weighted sequence sensitive for pathology, and generally able to distinguish pathologic tissue from normal tissue. With the addition of an additional radio frequency pulse and additional manipulation of the magnetic gradients, a T2-weighted sequence can be converted to a FLAIR (Fluid Light Attenuation Inversion Recovery) sequence, in which free water is now dark, but edematous tissues remain bright. This sequence in particular is currently the most sensitive way to evaluate the brain for demyelinating diseases, such as Multiple Sclerosis . The typical MRI examination consists of 5-20 sequences, each of which are chosen to provide a particular type of information about the subject tissues. This information is then synthesized by the interpreting Physician . Specialized MRI scans 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. The recent development of Diffusion Tensor Imaging (DTI) enables diffusion to be measured in multiple directions (currently up to 99) and the Fractional Anisotropy in each direction to be calculated for each voxel. This enables researchers to make axonal maps to examine the structural connectivity of different regions in the brain (tractography) or to examine areas of neural degeneration and demyelinaton in diseases like Multiple Sclerosis. Another application of diffusion MRI is Diffusion-weighted Imaging (DWI). Following an ischemic Stroke , brain cells die. It is speculated that resultant areas of ''restricted diffusion'' are detectable. This finding appears within 5-10 minutes of the onset of stroke symptoms (as compared with Computed Tomography , which often does not detect changes of acute infarct for up to 4-6 hours) and remains for up to two weeks. As such, DWI sequences are extraordinarily sensitive for acute stroke. Finally, it has been proposed that diffusion MRI may be able to detect minute changes in extracellular water diffusion and therefore could be used as a tool for fMRI. The nerve cell body enlarges when it conducts an action potential, hence restricting extracellular water molecules from diffusing naturally. Although this process works in theory, evidence is only moderately convincing. Like many other specialized applications, this technique is usually used with the use of an Echo Planar Imaging sequence. Magnetic resonance angiography Magnetic resonance angiography (MRA) is used to generate pictures of the arteries in order to evaluate them for stenosis (abnormal narrowing) or aneurysms (vessel wall dilatations, at risk of rupture). MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs (called a "run-off"). A variety of techniques can be used to generate the pictures, such as administration of a paramagnetic contrast agent (gadolinium) or using a technique known as "flow-related enhancement" (e.g. 2D and 3D time-of-flight sequences), where most of the signal on an image is due to blood which has recently moved into that plane. MRV is a similar procedure that is used to image veins. In this method the tissue is now excited inferiorly while signal is gathered in the plane immediately superior to the excitation plane, and thus imaging the venous blood which has recently moved from the excited plane. 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) relative to deoxygenated hemoglobin. Because deoxygenated hemoglobin attenuates the 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. While BOLD signal is the most common method employed for neuroscience studies in human subjects, the flexible nature of MR imaging provides means to sensitize the signal to other aspects of the blood supply. Alternative techniques employ arterial spin labeling (ASL) or weight the MRI signal by cerebral blood flow (CBF) and cerebral blood volume (CBV). The CBV method requires injection of a class of MRI contrast agents that are now in human clinical trials. Because this method has been shown to be far more sensitive than the BOLD technique in pre-clinical studies, it may potentially expand the role of fMRI in clinical applications. The CBF method provides more quantitative information than the BOLD signal, albeit at a significant loss of detection sensitivity. Interventional MRI Because of the lack of harmful effects on the patient and the operator, MR 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. Higher field magnet systems are beginning to be deployed in intraoperative imaging suites, which can combine high-field MRI with a surgical suite and even CT in a series of interconnected rooms. Specialty high-field interventional MR devices, such as the IMRIS system, can actually bring a high-field magnet to the patient within the operating theatre, permitting the use of standard surgical tools while the magnet is in an adjoining space. Radiation therapy simulation Because of MRI's superior imaging of soft tissues, it is now being utilized to specifically locate tumors within the body in preparation for radiation therapy treatments. For therapy simulation, a patient is placed in specific, reproducible, body position and scanned. The MRI system then computes the precise location, shape and orientation of the tumor mass, correcting for any spatial distortion inherent in the system. The patient is then marked or tattooed with points which, when combined with the specific body position, will permit precise triangulation for radiation therapy. Current density imaging Current density imaging is a subbranch 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 successful CDI has been performed using biological currents, however several studies have been published which involve applied currents through a pair of electrodes. Magnetic resonance guided focused ultrasound In MRgFUS therapy, ultrasound beams are focused on a tissue - guided and controlled using MR thermal imaging - and due to the significant energy deposition at the focus, temperature within the tissue rises to more than 65 °C , completely destroying it. This technology can achieve precise " Ablation " of diseased tissue. MR imaging provides a three-dimensional view of the target tissue, allowing for precise focusing of ultrasound energy. The MR imaging provides quantitative, real-time, thermal images of the treated area. This allows the physician to ensure that the temperature generated during each cycle of ultrasound energy is sufficient to cause thermal ablation within the desired tissue and if not, to adapt the parameters to ensure effective treatment. Multinuclear imaging Hydrogen is the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance. However, any nucleus which has a net nuclear spin could potentially be imaged with MRI. Such nuclei include Helium -3, Carbon -13, Oxygen -17, Sodium -23, Phosphorus -31 and Xenon -129. 23Na and 31P are naturally abundant in the body, so can be imaged directly. Gaseous isotopes (3He and 129Xe) must be Hyperpolarized , as their nuclear density is too low to yield a useful signal under normal conditions. 17O and 13C can be administered in sufficient quantities in liquid form (e.g. 17O-water, or 13C- Glucose solutions) that hyperpolarization is not a necessity. Multinuclear imaging is primarily a research technique at present. However, potential applications include functional imaging and imaging of organs poorly seen on 1H MRI (e.g. lungs and bones) or as alternative contrast agents. Inhaled hyperpolarized 3He can be used to image the distribution of air spaces within the lungs. Injectable solutions containing 13C or stabilized bubbles of hyperpolarized 129Xe have been studied as contrast agents for angiography and perfusion imaging. 31P can potentially provide information on bone density and structure, as well as functional imaging of the brain. SAFETY ''Implants and foreign bodies'': Pacemaker s are generally considered an absolute Contraindication towards MRI scanning. Several cases of Arrhythmia s or death have been reported in patients with pacemakers who have undergone MRI scanning. Other electronic implants are, at least, relative contraindications. Ferromagnetic foreign bodies (e.g. Shell fragments), or metallic implants (e.g. Surgical Prostheses , Aneurysm clips) are also potential risks, and safety aspects need to be considered on an individual basis. Interaction of the magnetic and radiofrequency fields with such objects can lead to: trauma due to movement of the object in the magnetic field, thermal injury from radiofrequency induction heating of the object, or failure of an implanted device. In the case of pacemakers, the risk is thought to be primarily RF induction in the pacing electrodes/wires causing inappropriate pacing of the heart, rather than the magnetic field affecting the pacemaker itself. Other significant safety issues include:
Pregnancy No reproducible harmful effects of MRI on the fetus have been demonstrated. In particular, MRI avoids the use of ionizing radiation, to which the fetus is particularly sensitive. However, as a precaution, current guidelines recommend that pregnant women undergo MRI only when essential. This is particularly the case during the first trimester of pregnancy, as Organogenesis takes place during this period. The concerns in pregnancy are the same as for MRI in general, but the fetus may be more sensitive to the effects - particularly to heating and to noise. However, one additional concern is the use of contrast agents; gadolinium compounds are known to cross the placenta and enter the fetal bloodstream, and it is recommended that their use be avoided. Despite these concerns, MRI is rapidly growing in importance as a way of diagnosing and monitoring disease of the fetus because it can provide more diagnostic information than ultrasound without the use of ionizing radiation. Guidance Safety issues, including the potential for biostimulation device interference, movement of ferromagnetic bodies and incidental localized heating have been addressed in the American College of Radiology's 'White Paper on MR Safety' which was originally published in 2002 and expanded in 2004. 2003 NOBEL PRIZE 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. In a controversial decision, the Nobel Committee snubbed MRI pioneer Raymond V. Damadian although Nobel rules allowed for the award to be shared with a third person. Soon after the announcement, Damadian took out expensive, full-page advertisements in major newspapers to protest the decision ( New York Times ad text ). In 1974, Damadian patented the design and use of NMR (US Patent 3,789,832 for detecting cancer. This patent did not describe a method for generating pictures; however, in 1997, he successfully sued General Electric for infringement and received an award of $129 million. He later settled out of court for further millions from other MRI scanner manufacturers. In 1980, he produced the first commercial MRI scanner, though the machine failed to sell and was never used clinically. [http://www.smithsonianmag.si.edu/smithsonian/issues00/jun00/object_jun00.html In recording the history of MRI, Mattson and Simon (1996) credit Damadian with describing the concept of whole-body NMR scanning, as well as discovering the NMR tissue relaxation differences that made this feasible. In 2001 , the Lemelson-MIT program bestowed its Lifetime Achievement Award on Dr. Damadian as "the man who invented the MRI scanner". 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 only concerned itself with searching the body for 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 contemporary MRI machines. SEE ALSO
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