magnetic resonance imaging magnetic resonance imaging magnetic resonance imaging (mri) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. mri scanners use strong magnetic fields, magnetic field gradients, and radio waves to generate images of the organs in the body. mri does not involve x-rays or the use of ionizing radiation, which distinguishes it from ct and pet scans. mri is a medical application of nuclear magnetic resonance (nmr) which can also be used for imaging in other nmr applications, such as nmr spectroscopy. this article is about magnetic resonance imaging. for x-ray tomographic imaging, see ct scan. magnetic resonance imagingpara-sagittal mri of the head, with aliasing artifacts (nose and forehead appear at the back of the head)synonymsnuclear magnetic resonance imaging (nmri), magnetic resonance tomography (mrt)icd-9-cm88.91meshd008279medlineplus003335 mri is widely used in hospitals and clinics for medical diagnosis, staging and follow-up of disease. compared to ct, mri provides better contrast in images of soft-tissues, e.g. in the brain or abdomen. however, it may be perceived as less comfortable by patients, due to the usually longer and louder measurements with the subject in a long, confining tube, though "open" mri designs mostly relieve this. additionally, implants and other non-removable metal in the body can pose a risk and may exclude some patients from undergoing an mri examination safely. mri was originally called nmri (nuclear magnetic resonance imaging), but "nuclear" was dropped to avoid negative associations. certain atomic nuclei are able to absorb radio frequency energy when placed in an external magnetic field; the resultant evolving spin polarization can induce a rf signal in a radio frequency coil and thereby be detected. in clinical and research mri, hydrogen atoms are most often used to generate a macroscopic polarization that is detected by antennas close to the subject being examined. hydrogen atoms are naturally abundant in humans and other biological organisms, particularly in water and fat. for this reason, most mri scans essentially map the location of water and fat in the body. pulses of radio waves excite the nuclear spin energy transition, and magnetic field gradients localize the polarization in space. by varying the parameters of the pulse sequence, different contrasts may be generated between tissues based on the relaxation properties of the hydrogen atoms therein. since its development in the 1970s and 1980s, mri has proven to be a versatile imaging technique. while mri is most prominently used in diagnostic medicine and biomedical research, it also may be used to form images of non-living objects, such as mummies. diffusion mri and functional mri extend the utility of mri to capture neuronal tracts and blood flow respectively in the nervous system, in addition to detailed spatial images. the sustained increase in demand for mri within health systems has led to concerns about cost effectiveness and overdiagnosis. mechanism construction and physics main article: physics of magnetic resonance imaging schematic of construction of a cylindrical superconducting mr scanner in most medical applications, hydrogen nuclei, which consist solely of a proton, that are in tissues create a signal that is processed to form an image of the body in terms of the density of those nuclei in a specific region. given that the protons are affected by fields from other atoms to which they are bonded, it is possible to separate responses from hydrogen in specific compounds. to perform a study, the person is positioned within an mri scanner that forms a strong magnetic field around the area to be imaged. first, energy from an oscillating magnetic field is temporarily applied to the patient at the appropriate resonance frequency. scanning with x and y gradient coils causes a selected region of the patient to experience the exact magnetic field required for the energy to be absorbed. the atoms are excited by a radio frequency (rf) pulse and the resultant signal is measured by a receiving coil. the rf signal may be processed to deduce position information by looking at the changes in rf level and phase caused by varying the local magnetic field using gradient coils. as these coils are rapidly switched during the excitation and response to perform a moving line scan, they create the characteristic repetitive noise of an mri scan as the windings move slightly due to magnetostriction. the contrast between different tissues is determined by the rate at which excited atoms return to the equilibrium state. exogenous contrast agents may be given to the person to make the image clearer. the major components of an mri scanner are the main magnet, which polarizes the sample, the shim coils for correcting shifts in the homogeneity of the main magnetic field, the gradient system which is used to localize the region to be scanned and the rf system, which excites the sample and detects the resulting nmr signal. the whole system is controlled by one or more computers. a mobile mri unit visiting glebefields health centre, tipton, england mri requires a magnetic field that is both strong and uniform to a few parts per million across the scan volume. the field strength of the magnet is measured in teslas – and while the majority of systems operate at 1.5 t, commercial systems are available between 0.2 and 7 t. whole-body mri systems for research application operate in e.g. 9.4t, 10.5t, 11.7t. even higher field whole-body mri systems e.g. 14 t and beyond are in conceptual proposal or in engineering design. most clinical magnets are superconducting magnets, which require liquid helium to keep them at low temperatures. lower field strengths can be achieved with permanent magnets, which are often used in "open" mri scanners for claustrophobic patients. lower field strengths are also used in a portable mri scanner approved by the fda in 2020. recently, mri has been demonstrated also at ultra-low fields, i.e., in the microtesla-to-millitesla range, where sufficient signal quality is made possible by prepolarization (on the order of 10–100 mt) and by measuring the larmor precession fields at about 100 microtesla with highly sensitive superconducting quantum interference devices (squids). t1 and t2 effects of tr and te on mr signal examples of t1-weighted, t2-weighted and pd-weighted mri scans each tissue returns to its equilibrium state after excitation by the independent relaxation processes of t1 (spin-lattice; that is, magnetization in the same direction as the static magnetic field) and t2 (spin-spin; transverse to the static magnetic field). to create a t1-weighted image, magnetization is allowed to recover before measuring the mr signal by changing the repetition time (tr). this image weighting is useful for assessing the cerebral cortex, identifying fatty tissue, characterizing focal liver lesions, and in general, obtaining morphological information, as well as for post-contrast imaging. to create a t2-weighted image, magnetization is allowed to decay before measuring the mr signal by changing the echo time (te). this image weighting is useful for detecting edema and inflammation, revealing white matter lesions, and assessing zonal anatomy in the prostate and uterus. the standard display of mri images is to represent fluid characteristics in black and white images, where different tissues turn out as follows: signalt1-weightedt2-weighted high fat subacute hemorrhage melanin protein-rich fluid slowly flowing blood paramagnetic or diamagnetic substances, such as gadolinium, manganese, copper cortical pseudolaminar necrosis anatomy more water content, as in edema, tumor, infarction, inflammation and infection extracellularly located methemoglobin in subacute hemorrhage fat pathology intermediate gray matter darker than white matter white matter darker than grey matter low bone urine csf air more water content, as in edema, tumor, infarction, inflammation, infection, hyperacute or chronic hemorrhage low proton density as in calcification bone air low proton density, as in calcification and fibrosis paramagnetic material, such as deoxyhemoglobin, intracellular methemoglobin, iron, ferritin, hemosiderin, melanin protein-rich fluid diagnostics usage by organ or system patient being positioned for mr study of the head and abdomen mri has a wide range of applications in medical diagnosis and more than 25,000 scanners are estimated to be in use worldwide. mri affects diagnosis and treatment in many specialties although the effect on improved health outcomes is disputed in certain cases. mri is the investigation of choice in the preoperative staging of rectal and prostate cancer and has a role in the diagnosis, staging, and follow-up of other tumors, as well as for determining areas of tissue for sampling in biobanking. neuroimaging main article: magnetic resonance imaging of the brain