Magnetic resonance imaging

Principle

MRI uses a strong magnet, radio waves and computers to create detailed images of the body.
It does not use ionizing radiation, unlike conventional radiography and computed tomographic (CT) imaging.
MRI imaging is based on the magnetic properties of atoms.

Patient lies inside a massive hollow magnet, and is exposed to short bursts of powerful non-ionizing radio wave energy, directed at protons of the nuclei of hydrogen or water atoms in the body.
Radio signals that are generated by first "exciting" and then "relaxing" those protons are computer-processed to form digital images, reflecting different types of tissue.
Scanner must be located within a specially shielded room to avoid outside interference.
The patient will be asked to lie on a narrow table which slides into a large tunnel-like tube within the scanner.
Special body coils are placed around the areas to be studied, e.g. knee, shoulders, etc. These coils can add to the feeling of claustrophobia.
These special body coils send and receive the radio wave pulses, and improve the quality of the images.
Hydrogen proton imaging.
Observe behavior of protons in magnet after application of RF signal.

An MRI provides superior tissue contrast resolution.
Because of its ability to show soft tissues in exquisite detail, MRI can detect soft tissue disease and evaluate vasculature.
An MRA, or magnetic resonance angiogram, is a special type of MR that creates three-dimensional reconstructions of vessels containing flowing blood. It is often utilized when conventional angiography cannot be performed due to renal failure or other contraindications. No IV contrast is needed for this study. Flowing blood appears dark in the arteries.
MRI can delineate a cyst from a solid mass.
It can identify the spread of cancer into the vessels and soft tissue (staging).
When contrast CT cannot be done due to an allergy, MRI is useful to evaluate vascular lesions.
No radiation.
Many MRI’s are done with IV gadolinium contrast.

Spinal cord compression: MRI is the procedure of choice. It has replaced myelogram.
Brain lesions: Post traumatic (follow-up of subdural, no IV contrast), acute CVA (CT first to r/o blood, functional MRI for acute CVA), tumors, mets, etc.
Joint disease.
Soft tissue tumors.

Procedure is expensive.
It has limited availability.
A complete scan, depending on the sequences performed, may take up to one hour or longer.

Patients with certain types of metals (heart valves, pacemaker. Etc.) cannot undergo MR studies since the clips can be dislodged by the magnetic field.
If the patient is claustrophobic, the study cannot be done without sedation.
Warn the patient that the scan involves loud noises so they can anticipate this.
Gadolinium contrast in a patient with compromised renal function can cause nephrogenic systemic fibrosis.
Weight limit.