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MRI (magnetic resonance imaging)
Medical expert of the article
Last reviewed: 04.07.2025
MRI (magnetic resonance imaging) produces images by using a magnetic field to induce changes in the spin of protons within tissue. Normally, the magnetic axes of the many protons in tissue are arranged randomly. When they are surrounded by a strong magnetic field, as in an MRI machine, the magnetic axes align along the field. Applying a high-frequency pulse causes all the proton axes to instantly align along the field in a high-energy state; some protons then snap back to their original state within the magnetic field. The amount and rate of energy release that occurs with the return to the original alignment (T1 relaxation) and with the wobble (precession) of the protons during the process (T2 relaxation) are recorded as signal strengths spatially confined by a coil (antenna). These strengths are used to produce images. The relative signal intensity (brightness) of tissues in an MR image is determined by numerous factors, including the high frequency pulse and gradient waveforms used to acquire the image, the tissue's inherent T1 and T2 characteristics, and the tissue's proton density.
Pulse sequences are computer programs that control high-frequency pulses and gradient waveforms that determine how the image appears and how different tissues appear. Images can be T1-weighted, T2-weighted, or proton density weighted. For example, fat appears bright (high signal intensity) on T1-weighted images and relatively dark (low signal intensity) on T2-weighted images; water and fluids appear as intermediate signal intensity on T1-weighted images and bright on T2-weighted images. T1-weighted images optimally demonstrate normal soft tissue anatomy (fat planes appear well as high signal intensity) and fat (eg, to confirm the presence of a fat-containing mass). T2-weighted images optimally demonstrate fluid and pathology (eg, tumors, inflammation, trauma). In practice, T1- and T2-weighted images provide complementary information, so both are important for characterizing pathology.
[ Indications for MRI (magnetic resonance imaging)
Contrast may be used to highlight vascular structures (magnetic resonance angiography) and to help characterize inflammation and tumors. The most commonly used agents are gadolinium derivatives, which have magnetic properties that affect proton relaxation time. Gadolinium agents may cause headache, nausea, pain and coldness at the injection site, taste distortion, dizziness, vasodilation, and a lowered seizure threshold; serious contrast reactions are rare and much less common than those with iodine-containing contrast agents.
MRI (magnetic resonance imaging) is preferred over CT when soft tissue contrast resolution is important - for example, to evaluate intracranial abnormalities, spinal abnormalities, or spinal cord abnormalities, or to evaluate suspected musculoskeletal tumors, inflammation, trauma, or internal joint disorder (imaging of intra-articular structures may involve injection of a gadolinium agent into the joint). MRI is also helpful in evaluating liver pathologies (eg, tumors) and female reproductive organs.
Contraindications to MRI (magnetic resonance imaging)
The primary relative contraindication to MRI is the presence of implanted material that can be damaged by strong magnetic fields. These materials include ferromagnetic metal (containing iron), magnetically activated or electronically controlled medical devices (eg, pacemakers, implantable cardioverter defibrillators, cochlear implants), and electronically controlled non-ferromagnetic metal wires or materials (eg, pacemaker wires, some pulmonary artery catheters). Ferromagnetic material may be displaced by the strong magnetic field and damage a nearby organ; dislocation is even more likely if the material has been present for less than 6 weeks (before scar tissue has formed). Ferromagnetic material may also cause image distortion. Magnetically activated medical devices may malfunction. In conductive materials, magnetic fields may produce a flux, which in turn may generate high temperatures. MRI device or object compatibility may be specific to a particular device type, component, or manufacturer; prior testing is usually required. Also, MRI mechanisms of different magnetic field strengths have different effects on materials, so safety for one mechanism does not guarantee safety for another.
Thus, a ferromagnetic object (e.g. oxygen tank, some IV poles) may be drawn into the magnetic channel at high speed upon entering the scanning room; the patient may be injured and separation of the object from the magnet may become impossible.
The MRI machine is a tight, confined space that can cause claustrophobia even in patients who are not claustrophobic. Also, some very heavy patients may not be able to fit on the table or in the machine. For the most anxious patients, a pre-sedative (eg, alprazolam or lorazepam 1-2 mg orally) 15-30 minutes before the scan may be helpful.
Several unique MRI techniques are used when specific indications exist.
Gradient echo is a pulse sequence used to produce images quickly (eg, magnetic resonance angiography). The movement of blood and cerebrospinal fluid produces strong signals.
Repeated planar imaging is an ultra-fast technique used for diffusion, perfusion, and functional imaging of the brain.