Magnetic resonance imaging

Magnetic resonance imaging (MRI) uses strong magnetic fields and radiofrequency pulses to generate detailed images of internal body structures, especially soft tissues, without ionizing radiation.

Nuclear magnetic resonance: A physical phenomenon in which nuclei absorb and emit radiofrequency energy in a magnetic field. MRI detects radiofrequency signals produced by nuclear magnetic resonance, chiefly from hydrogen nuclei.
Hydrogen-1: The common hydrogen isotope, whose nucleus is a single proton. Its abundance in water and fat makes hydrogen-1 the main source of clinical MRI signal.
Diffusion-weighted imaging: An MRI method that measures signal changes caused by the microscopic motion of water molecules. It can reveal acute ischemic injury in the brain before conventional images show clear changes.
Computed tomography: An imaging method that uses X-rays and computational reconstruction to create cross-sectional images. CT is faster and often better for bone or acute bleeding, while MRI avoids ionizing radiation.
Felix Bloch: A Swiss-American physicist who co-discovered nuclear magnetic resonance in condensed matter. His NMR research supplied a physical foundation later used to develop MRI.
Proton precession: The rotation of a proton’s magnetic moment around the direction of an applied magnetic field. The precession frequency sets the radiofrequency used to excite hydrogen spins in MRI.
Longitudinal relaxation time: The time constant describing recovery of nuclear magnetization along the main magnetic field. Differences in longitudinal relaxation help distinguish tissues in T1-weighted MRI.
Functional MRI: An MRI method that maps brain activity indirectly through blood-oxygen-level-dependent signal changes. It uses MRI signal changes to study activity-linked changes in cerebral blood oxygenation.
Ultrasound: Medical imaging that uses reflected high-frequency sound waves to depict internal structures and motion. Ultrasound is portable and real-time, unlike MRI's enclosed scanner and longer acquisitions.
Edward Purcell: An American physicist who co-discovered nuclear magnetic resonance in condensed matter. His independent NMR discovery established a method for detecting nuclear magnetic signals.

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