MRI Safe vs. MRI Compatible
"MR safe" means that an object or device, when used in the MR environment, will present no additional risk to the patient but may affect the quality of the diagnostic information.
"MR compatible" means that an object or device, when used in the MR environment, is MR safe and will not significantly affect the quality of the diagnostic information, nor will its operation be significantly affected being in the MR environment.
Before attempting any definition of these terms, it should be noted that they should not be used without reference to the MR environment. This is because the MR environment in which a device was tested can vary greatly. At the most cursory level, the "MR environment" includes anywhere inside the 0.5 mT line (5 Gauss) of the static magnetic field. It also must include consideration of the magnitude of the static magnetic field and associated spatial gradients, rapidly changing magnetic fields (imaging gradients, kHz) and radio frequency magnetic field pulses (MHz). Ideally the test conditions should be specifically stated.
A new system of terms has recently been developed by the American Society for Testing and Materials which offers three categories: MR Safe, MR Conditional, and MR Unsafe. In this system, the "safe" and "unsafe" are two extremes: MR Safe indicates no known hazards in all MRI environments; MR Unsafe indicates that an item poses hazards in all MRI environments. (Notice there are no conditions on MR Safe in this system.) Left in the middle—MR Conditional—are specified conditions (which should be determined as a result of testing) in which the behaviour of the item in the MRI environment is characterised.