Glossary of Related Terms

Basic MRI Glossary

Gradient Magnetic Field

A variation in the magnetic field with respect to distance. This field makes the energy status of the protons position dependent.

MR Safe

Use of the device in an MR environment presents no additional risk to the patient but quality of the image may be affected.

MR Conditional

No known hazards under specific conditions. Considered safe only when certain specific conditions are met.

MR Unsafe

Hazards posed in all MR environments. Any device not labeled MR safe or MR conditional.

Pulsed Radio Frequency (RF) Field

Changes the energy state of protons and elicits magnetic resonance signals from tissue. Frequencies used in the millions of cycles per second (MHz).

Radiopaque

Visible using standard X-ray techniques.

Specific Absorption Rate (SAR)

Measure of the absorption of electromagnetic energy in the body, measured in units of watts per kilogram (W/kg).

Static Magnetic Field

Magnetic field created by the large magnet found in MRI, used to align protons. Always present, even when scanner is not imaging. The magnetic force of attraction for a 1.5 Tesla MRI system is approximately 30,000 times stronger than the earth's magnetic field.

Tesla

Unit of measurement to determine the strength of a magnet, equal to 10,000 Gauss. The earth's magnetic field varies between 0.3–0.7 Gauss.

Basic Cardiac Device Glossary

Capture

Initiation of depolarization of the atria and/or ventricles by an electrical stimulus delivered by an artificial pacemaker or ICD.

Lead (or electrode, catheter)

Thin, insulated cables which conduct energy and carry electrical signals to and from the heart.

Implantable Cardioverter Defibrillator (ICD)

Device which terminates an erratic, life-threatening cardiac arrhythmia by delivering a high energy, direct current, electrical stimulus to cardiac tissue. An ICD is usually implanted in the upper chest or abdominal area.

Pacemaker/Implantable Pulse Generator (IPG)

Device which provides timed electrical stimuli to the heart to restore the heartbeat to a more normal rate, thereby relieving symptoms of bradycardia. A pacemaker system includes the pulse generator and the lead(s). Pacemakers are usually implanted in the pectoral region.

Programmer

A small computer used in a clinician's office to make changes in the operations of an implanted device. The programmer magnet is often placed over the pacemaker to collect information stored in the pacemaker. Note: Programmers are not labeled for use in the MR environment. Programming is performed away from the MR scanner.

Sensing

The ability of a pacemaker or ICD to recognize and respond to the electrical activity of the heart.

Slew Rate

The rate of change over time of the magnetic fields that make up the gradient fields.

Threshold (or stimulation threshold)

The minimum electrical stimulus needed to consistently elicit a cardiac depolarization. It is expressed in terms of amplitude (volts, milliamps) and pulse width (milliseconds), or energy (microjoules).