Engineers and physicists interested to understand a bit more about the safety precautions needed to use a MR system or parts of it safe in case of use of non-approved parts.

- Understand the potential risks of different fields and parts of an MRI system

- Know the important international standards for MR system

- Understand which points need to be covered to use a self-made RF coil safe

- SAR evaluation and safe use of a multi transmit RF coil

MR safety is an important topic for all users performing MR procedures on humans. We want to limit us on the MR system safety in case of imaging a human without any implants. If you use a certified clinical scanner the MR vendor takes care of it and you have “only” to follow the instruction for use. However, as soon as you start to modify any part (software or hardware) the system is no longer approved and you have to take care of the safety.

- Main magnetic field (up to 3T for clinical use, higher fields, always on)

- Gradient field (for special encoding, on during measurements)

- RF field (to excite the spins, on during measurements)

Potential risk:

Static magnetic field:

- Force effects: ferromagnetic materials may strongly be attracted by the magnet

- Torque effects: ferromagnetic materials will align with the main magnetic field

Gradient field:

- Induced currents (peripheral nerve stimulations)

- Acoustic noise

RF field:

- RF heating (tissue burns)

other risks

If you start to use/build hardware which is not approved other risk have to be taken into account:

- SAR management of your RF coil(s)

- Hot surface (if reachable by humans it must be <43°C)

- Flammability: Material should stop fire

- Conductive materials must be isolated and separated from tissue to avoid burns

- No sharp edges

- Gradient coils works with very high voltages and currents which need to be well protected.

- Your device must be correctly recognized by to software to use it appropriate (for example apply the correct limitations)

- Good instruction for use to prevent handling errors

- …

Main standard for MRI is IEC 60601-2-33 (1). This standards covers MR specific safety regulations. The current version is IEC 60601-2-33 edition3 with the 3rd amendment published 2015.

This standard will not cover all safety problems of an MR system. Therefore a medical device must also fulfil the general safety of an electrical medical equipment is in IEC 60601-1 (2). This standard may cite other standards to cover specific problems, like IEC/DIN EN 60695-11-10 (3). Flammability Rating of Plastic Materials for Parts in Devices and Applications.

The paper “Safety testing and operational procedures for self-developed radiofrequency coils” from Jens Hoffmann et. al.(4) is a good overview on testing self-developed RF coils. It gives a nice overview on the different points to be covered. To perform a RF coil safe we do not only have to look, that the new coil will fulfill the local and whole body SAR limits from the IEC 60601-2-33(1) but also the following points need to be avoided.

- RF burns due to currents on conductors in the coil or on cable

- Extended heat on the coil surface reachable from the patient

- high voltage shock

- Hazards due to leakage currents

- Flammability (Burns, asphyxiation, smoke inhalation)

- Pinch Points (patient and/or user)

- Sharp edges (patient and/or user)

- Risks due to splash water (splash water protection)

- Signal from the coil material

- Distortion of the images

The paper goes also into the topic of validation of multi transmit RF coils. However, the safe use of such coils is much more demanding.

The RF transmission is limited by the tissue heating. In the IEC 60601-2-33 currently the maximal temperature increase is used as limit. As an alternative specific absorption rate (SAR, the absorbed power per volume in W/kg) is also valid. Currently, under discussion is the CEM43 methods[5], which takes also the time and the sensitivity of different tissue into account. Temperature mapping during measurement is not practical, but for validation purposes it is very useful in addition to phantom studies.

To predict the SAR, the E-field, the density and the conductivity of the tissue need to be known. Since this cannot be measured in each subject, human models of different size, sex, body shape are available (for example [6]). In simulation software the whole body and local SAR can be estimated. To understand the uncertainty of the results, different models need to be simulated and also the position variation within the RF coil need to be taken into account. Based on this information a worst case SAR estimation can be made and if needed the MR sequence need to be adjusted. Multi transmit RF coils with RF shimming adds a new complexity into the system. However it can also be used to reduce SAR [7].

The predicted RF power must be monitored during the experiment, to avoid too high RF heating due to miss calibration or failure of any part of the transmit chain. In case of the single transmit system, the monitoring was a power monitoring unit. In addition the RF coil need to be checked that the coil is still in a circular mode and not in a linear mode, which would increase the energy deposition by a factor of 1.4. In case of multi transmit coils, the failure of a single coil can have a larger effect [8]. In case of a multi transmit system, the SAR must be calculated out of the sum of the E-field distribution of each coil for the given RF shimming, taking the uncertainty into account given that a model and not the current subject is used and that the position may be shifted. In [9] a safety concept is presented which use pickup coils for each transmit channel to monitor the RF transmission.