Glossary of Related Terms MRI Access

Basic MRI Terms

Gradient Magnetic Field Used to select the location and orientation of the image plane or slice through the patients anatomy. Three independent gradient magnetic fields are active during MRI set-up (pre-scan) and image acquisition. When active, the gradient fields are pulsed on and off at a rapid rate, typically hundreds or thousands of times per second.

MR-Conditional Under specific conditions of use, safe in the MR environment.

MR Safe Presents no known hazards resulting from exposure to any MRI environment.

MR Unsafe Presents unacceptable risks in the MR environment.

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 Refers to the relative opaque white appearance which makes a device 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).

B1+RMS The time-averaged RF magnetic field component relevant for creating an MR image that is generated by the scanner during a scan and is measured in units of micro-Tesla (µT).

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

Tesla Unit of measurement (T) to determine the strength of a magnet, equal to 10,000 gauss (G). The earth's magnetic field varies between 0.3 and 0.7 G.

Basic Cardiac Device Terms

Capture Initiation of depolarization of the atria and/or ventricles by an electrical stimulus delivered by an artificial pacemaker or implantable cardioverter defibrillator.

Cardiac Resynchronization Therapy-Defibrillator (CRT-D) An implantable cardioverter defibrillator (ICD) with cardiac resynchronization pacing therapies. The device terminates an erratic, life-threatening cardiac arrhythmia by delivering a high-energy, direct current electrical stimulus to cardiac tissue. In addition, it provides resynchronization pacing therapies to both ventricles of the heart.

Cardiac Resynchronization Therapy-Pacemaker (CRT-P) A device that sends small electrical impulses to both lower chambers of the heart to resynchronize the ventricles.

Implantable Cardioverter Defibrillator (ICD) Device that 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.

Lead (or electrode, catheter) Thin, insulated cable that conducts energy and carries electrical signals to and from the heart.

Pacemaker/Implantable Pulse Generator (IPG) Device that 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 clinicians 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 MRI environment. Programming is performed away from the MRI 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).