MRI is a diagnostic procedure that uses a combination of a large magnet, radiofrequencies, and a computer to produce detailed images of organs and structures within the body.
The MRI machine is a large, cylindrical (tube-shaped) machine that creates a strong magnetic field around the patient. The magnetic field, along with a radiofrequency, alters the hydrogen atoms' natural alignment in the body. Computers are then used to form a two-dimensional (2D) image of a body structure or organ based on the activity of the hydrogen atoms. Cross-sectional views can be obtained to reveal further details. MRI does not use radiation, as do X-rays or computed tomography (CT scans).
A magnetic field is created and pulses of radio waves are sent from a scanner. The strong magnetic field causes the hydrogen atoms in your body to align along the same axis. The radio waves knock the nuclei of the atoms in your body out of this aligned position. As the nuclei realign back into proper position, they send out radio signals. These signals are received by a computer that analyzes and converts them into an image of the part of the body being examined. This image appears on a viewing monitor. Some MRI machines look like narrow tunnels, while others are more open.
Magnetic resonance imaging (MRI) may be used instead of computed tomography (CT) in situations where organs or soft tissue are being studied, because bones do not obscure the images of organs and soft tissues, as they do in CT.
Because ionizing radiation is not used, there is no risk of exposure to ionizing radiation during an MRI procedure.
Due to the use of the strong magnet, MRI cannot be performed on patients with implanted pacemakers, older intracranial aneurysm clips, cochlear implants, certain prosthetic devices, implanted drug infusion pumps, neurostimulators, bone-growth stimulators, certain intrauterine contraceptive devices, or any other type of iron-based metal implants. MRI is also contraindicated in the presence of internal metallic objects, such as bullets or shrapnel, as well as surgical clips, pins, plates, screws, metal sutures, or wire mesh. Dyes used in tattoos may contain iron and potentially could heat up during an MRI, but this is a rare occurrence.
Newer uses and indications for MRI have contributed to the development of additional magnetic resonance technology. Magnetic resonance angiography (MRA) is a procedure used to evaluate blood flow through arteries in a noninvasive (the skin is not pierced) manner. MRA can also be used to detect aneurysms within the brain and vascular malformations (abnormalities of blood vessels within the brain, spinal cord, or other parts of the body).
Magnetic resonance spectroscopy (MRS) is another noninvasive procedure used to assess chemical abnormalities in body tissues, such as the brain. MRS may be used to assess disorders such as HIV infection of the brain, stroke, head injury, coma, Alzheimer's disease, tumors, and multiple sclerosis.
Functional magnetic resonance imaging of the brain (fMRI) is used to determine the specific location of the brain where a certain function, such as speech or memory, occurs. The general areas of the brain in which such functions occur are known, but the exact location may vary from person to person. During functional resonance imaging of the brain, you will be asked to perform a specific task, such as recite the Pledge of Allegiance, while the scan is being done. By pinpointing the exact location of the functional center in the brain, doctors can plan surgery or other treatments for a particular disorder of the brain.
Another advance in MRI technology is the "open" MRI. Standard MRI units have a closed cylinder-shaped tunnel into which the patient is placed for the procedure. Open MRI units do not completely surround the patient, and some units may be open on all sides. Open MRI units are particularly useful for procedures involving:
Tell the radiologist if you are claustrophobic and think that you will be unable to lie still while inside the scanning machine; if you have a pacemaker inserted, or have had heart valves replaced; if you have metal plates, pins, metal implants, surgical staples, or aneurysm clips; if you have permanent eye liner or tattoos; if you are pregnant; if you ever had a bullet wound; if you have ever worked with metal (for example, a metal grinder); or if you have kidney disease.
An MRI may be performed on an outpatient basis, or as part of inpatient care. Although each facility may have specific protocols in place, generally, an MRI procedure follows this process:
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