Construction and physics 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<unk>  scanner that forms a strong<unk>  field around the area to be imaged. First, energy from an oscillating<unk>  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<unk>  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<unk>  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<unk>  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<unk>  scanner are the main magnet, which polarizes the sample, the shim coils for correcting shifts in the homogeneity of the main<unk>  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<unk>  unit visiting Glebefields Health Centre, Tipton, England<unk>  requires a<unk>  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. Most clinical magnets are superconducting magnets, which require liquid helium to keep them very cold. Lower field strengths can be achieved with permanent magnets, which are often used in "open"<unk>  scanners for claustrophobic patients. Lower field strengths are also used in a portable<unk>  scanner approved by the FDA in 2020. Recently,<unk>  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 PD-weighted<unk>  scans Each tissue returns to its equilibrium state after excitation by the independent relaxation processes of T1 (spin-lattice; that is,<unk> ation in the same direction as the static<unk>  field) and T2 (spin-spin; transverse to the static<unk>  field). To create a T1-weighted image,<unk> ation 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<unk> . To create a T2-weighted image,<unk> ation 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<unk>  images is to represent fluid characteristics in black and white images, where different tissues turn out as follows: Signal T1-weighted T2-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 Inter- mediate 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, intracelullar methemoglobin, iron, ferritin, hemosiderin, melanin * Protein-rich fluid
Magnetic resonance imaging
Magnetic field imaging
Magnetic Resonance in Medicine
Electron resonance imaging
Radiography
Autoradiograph
Functional imaging
Myelography