Radiofrequency and Gradient Fields

In contrast to the main static magnetic field, radiofrequency (RF) pulses and magnetic gradients are only present during scanning. RF energy (64 MHz-between AM and FM radio) is exchanged with the patient in order to create MR images. A relatively powerful amplifier (25kW) generates this energy and software controls limit the absorption rate in patients. The effects of RF absorption are the heating of the tissue and the patient's ability to dissipate excess heat. This can be expressed in terms of the specific absorption rate (SAR), which is the FDA limit for RF exposure and is primarily set to avoid warming of the patient. The recommended SAR level for imaging in the US is 0.4W/kg (whole body), 3.2W/Kg (head) and 8 W/kg (small volume). The RF field is focused within the bore of the magnet and is negligible external to it.

While software limits RF exposure to safe levels, looped conductors (e.g. wires) within the bore of the magnet can focus these RF fields, producing elevated energy deposition. These concerns are greatest on high field scanners and have been known to cause substantial burns. Accordingly, looped conductors within the bore must be avoided at all cost.

Care should be taken to ensure that the patient's tissue do not directly come into contact with the inner bore of the magnet during the MR imaging process. Pads and other such insulating devices are provided for this purpose. It is also important that the patient's own tissues do not form large conductive loops. Therefore, care should be taken to ensure that the patient's arms and legs not be positioned in such a way as to form a large caliber loop within the bore. For this reason it is preferable to instruct patients not to cross their arms or legs in the MR scanner.