Magnetic resonance imaging (MRI) is a medical imaging technique that uses a strong magnetic field, radiofrequency pulses, and computer-based reconstruction to visualize structures inside the body. Its signals arise mainly from hydrogen nuclei in water and fat. MRI provides detailed cross-sectional and three-dimensional images and is used in radiology for diagnosis, treatment planning, and monitoring. Unlike conventional radiography and computed tomography (CT), it does not use ionizing radiation. Different acquisition methods can reveal anatomy, blood flow, tissue microstructure, or changes associated with brain activity. (fda.gov)
Physical principles
MRI applies nuclear magnetic resonance to spatial imaging. A hydrogen-1 nucleus consists of one proton, which possesses intrinsic spin angular momentum and a magnetic moment. Hydrogen is abundant in biological tissues, especially in water and fat. In an external magnetic field, a small excess of nuclear magnetic moments occupies the lower-energy orientation, producing a net magnetization along the field. (fda.gov)
The magnetization undergoes Larmor precession at a frequency proportional to magnetic-field strength. A radiofrequency pulse near this resonance frequency can tilt the net magnetization away from its equilibrium direction. The resulting transverse magnetization induces an electrical signal in receiver coils. MRI therefore detects the collective behavior of many nuclei rather than individual particles. (bcf.technion.ac.il)
After excitation, nuclear spin relaxation changes the magnetization. T1, or longitudinal relaxation, describes recovery along the main field; T2, or transverse relaxation, describes loss of phase coherence among spins. T2* additionally incorporates dephasing caused by magnetic-field inhomogeneity. These properties depend on the molecular environment and help distinguish tissues that would otherwise have similar appearances. (bcf.technion.ac.il)
Spatial encoding and image formation
A scanner combines a main magnet, gradient coils, radiofrequency transmit and receive coils, and control computers. Many clinical systems use superconducting magnets. Gradient coils create controlled variations in field strength across the body, making the frequency and phase of the signal depend on position. Together with radiofrequency pulses, these gradients provide slice selection and spatial encoding. (ncbi.nlm.nih.gov)
Measurements are collected in k-space, a spatial-frequency representation of the object. A Fourier transform converts these measurements into an image. Acquisition and reconstruction determine the field of view, spatial resolution, and imaging time; an individual measurement generally contributes information to the whole image rather than to a single pixel. (ncbi.nlm.nih.gov)
A pulse sequence specifies the timing and arrangement of radiofrequency pulses, gradients, and signal acquisition. Spin-echo sequences use refocusing pulses, whereas gradient-echo sequences form echoes through gradient manipulation. Repetition time, echo time, and excitation angle influence image contrast. Consequently, an MRI examination normally comprises several complementary image series rather than one standardized picture. (bcf.technion.ac.il)
Contrast and specialized methods
T1-weighted images commonly provide anatomical detail, while T2-weighted images often make fluid conspicuous. Fluid-attenuated inversion recovery suppresses cerebrospinal-fluid signal, improving the visibility of some brain abnormalities. These labels describe relative weighting: image brightness also depends on proton density, sequence design, and other tissue properties. (bcf.technion.ac.il)
Diffusion MRI measures sensitivity to the motion of water molecules. Diffusion-weighted imaging is important in detecting early tissue changes associated with ischemic stroke. Direction-sensitive diffusion measurements also provide information about tissue organization. Functional MRI commonly uses blood-oxygenation-level-dependent contrast to study changes associated with activity in the brain. This is a physiological, indirect measure rather than a direct recording of neuronal electrical activity. (ninds.nih.gov)
Other methods evaluate blood vessels, perfusion, and regional chemical composition. Magnetic resonance spectroscopy measures resonances associated with selected metabolites, complementing structural images with biochemical information. (ninds.nih.gov)
Clinical applications and limitations
MRI is particularly useful for examining soft tissues, including the brain, spinal cord, muscles, ligaments, and internal organs. Applications include evaluating neurological disorders, joint injuries, and cancers. Its multiple contrast mechanisms can reveal differences that are difficult to distinguish with other imaging techniques. (nibib.nih.gov)
Limitations include relatively long examinations, sensitivity to movement, and discomfort within the scanner bore. Implanted devices can introduce artifacts as well as safety concerns. CT generally depicts cortical bone better, while the appropriate imaging method depends on the anatomical region and diagnostic question. (fda.gov)
Safety and contrast agents
MRI’s absence of X-rays does not eliminate risk. The static field can attract magnetic objects; changing gradients produce acoustic noise and may stimulate peripheral nerves. Radiofrequency energy can heat tissue or conductive materials. Medical implants have device-specific MR Safe, MR Conditional, or MR Unsafe classifications; conditional devices require specified scanning conditions. (fda.gov)
Some examinations use intravenous gadolinium-based contrast agents to alter tissue relaxation and enhance contrast. Possible adverse effects include allergic reactions. Certain agents are associated with nephrogenic systemic fibrosis, principally in patients with severely impaired kidney function. Gadolinium can remain in tissues for months or years after administration; retention and its clinical consequences are distinct questions. (fda.gov)
Historical development
MRI developed from earlier discoveries in nuclear magnetic resonance. In the 1970s, Paul Lauterbur demonstrated spatial imaging using magnetic-field gradients, while Peter Mansfield advanced mathematical analysis and rapid imaging methods. They jointly received the 2003 Nobel Prize in Physiology or Medicine for discoveries concerning magnetic resonance imaging. (nobelprize.org)