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).

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