HOW I DO CMR SCANNING SAFELY Elisabeth Burman Research Sister Royal Brompton Hospital, London UK This presentation is posted for members of scmr as an educational guide – it represents the views and practices of the author, and not necessarily those of SCMR.
Magnetic Resonance One of the safest imaging modalities but there are still safety issues
No ionizing radiation
Repeat studies present no problem
Fewer ethical problems imaging volunteers
Why are there safety issues?
Potential for accidents and even fatalities
Damage to equipment / cost
Safety issues centre around the 3 electromagnetic fields and issues indirectly related Static magnetic field biological effects mechanical effects Gradient Field induced currents (PNS) auditory damage RF Field RF power deposition Burns Other issues contrast agents critically ill patients/resuscitation monitoring quench claustrophobia
Static magnetic field 1.5 T is 30,000 x the strength of earth’s magnetic field Measured in Gauss or Tesla (10,000G equivalent to 1T)
Static Magnetic Fields 1. Biological effects (potential risk) - exposure to static magnetic fields of up to 4T are not thought to be harmful Biological effects relevant to clinical imaging - distorted ECG (magnetohydrodynamic effect) - consider prudency with pregnancy 2. Mechanical effects (very real risk) - translational or attractive forces on metallic objects when brought into the field A superconducting magnet is always switched on!
Static Magnetic Field Biological effects Magnetohydrodynamic effect – augmented T wave Outside field 0.5 Tesla 1.5T
Magnetohydrodynamic effect seen as augmentation of T-wave
Caused by the effect of the static magnetic field on moving blood (systole) as a conducting fluid.
The gradient and RF fields also affect the configuration of the ECG
Morphological ECG changes are therefore difficult to detect and diagnose, but rhythm is usually recognised
Any concern regarding rhythm, remove patient from scanner and perform 12 lead ECG
“ Contraindications” to CMR* Implants & metal Electromechanical implants
Any device electrically or mechanically activated * In some circumstances MR has been performed despite “contraindications”, especially in specialist centres. Work is in progress to make some devices safe. For example, some cerebral aneurysm clips may now be scanned.
Device when used in the MR environment, has been demonstrated to present no additional risk to the patient BUT may affect the quality of diagnostic information
Device has been demonstrated to neither significantly affect the quality of the diagnostic information or to be operationally affected by the scanner in use
Equipment must be MR Safe or MR Compatible
Quench Cryogens maintain the magnetic field - helium QUENCH = the liquid helium ‘boils off’ and becomes a gas Causes - physical - human error (accidental) - intervention (elective) Effects - ratio of gaseous to liquid helium 760:1 - should vent to the outside, but this can fail - pressure build up - asphyxiation/frostbite