Magnetic resonance imaging (MRI) machines are noninvasive medical imaging machines that produce detailed pictures of organs, soft tissues, bone, and virtually all other internal body structures. The radiological technique employs magnetism, radio waves, and a computer to produce images of body structures.

MRI machines operate based on nuclear magnetic resonance. A strong magnet is used to produce an electromagnetic field focused on the subject's area of interest aligning the nuclei and protons of water molecules with the direction of the field. Radio frequency (RF) pulses specific to hydrogen are then applied causing unmatched protons to spin in a different direction. As the RF pulses cease the hydrogen ions return to their native states, releasing the energy absorbed from the RF pulses. The receiver coil in the MRI detects these low energy releases and uses Fourier transforms to convert the signal into a picture that can be interpreted by a technician.

MRI exams are used for medical diagnosis of an array of ailments. Board certified radiologists interpret MRI images in order to diagnose problems related to tumors, internal bleeding, blood vessel diseases, infection, or bone and joint injuries. They may also provide more information about a problem initially found on an X-ray, ultrasound scan, or CT scan. Contrast materials, such as dyes, may be ingested or injected to show abnormal tissue more clearly.

Health Risks

MRI scans are a painless radiology technique. There is no radiation exposure as in x-rays although there are two ailments that can cause complications. It is important for a patient to discuss any prior procedures or injuries with a physician prior to having an MRI.

• Metallic implants such as artificial joints, metallic bone plates, prosthetic devices, surgical clips, or other foreign metallic material can significantly distort images produced by an MRI. Patients with certain metallic implants such as heart pacemakers, artificial heart valves, metallic ear implants, bullet fragments, or other metallic elements where risk of movement could jeopardize their well-being cannot have an MRI.
• Claustrophobic sensation during the procedure can be overwhelming. Patients with a history of claustrophobia should discuss this with their physician, practitioner, and radiology staff involved with the procedure.

Standards

SAE R268 -- The use of x-rays, CT scans, and MRI scans to study crash-related injury mechanisms

ASTM F2978 -- Guide describing the recommended protocol for magnetic resonance imaging (MRI) of patients with implanted metal-on-metal (MOM) devices

Resources

Physics Central—MRI

Wikipedia—Magnetic resonance imaging

Image credit:

US Embassy Manila / CC BY-ND 2.0