Operating Modes

What is meant by the term "operating mode" of an MR scanner?  

In the 1990s the International Electrotechnical Commission (IEC) and US Food and Drug Administration (FDA) jointly worked to develop the first international standards for MR equipment safety, codified as IEC 60601-2-33.  This document recognized three modes of scanner operation based on perceived risk to patients and volunteers: 

1. Normal operating mode. This is the "routine" level at which most clinical MRI today is performed, being considered safe for all patients, regardless of their condition.
2. First level controlled operating mode. This level is defined as one where certain imaging parameters may cause physiologic stress (such as peripheral nerve stimulation or tissue heating). Active medical supervision is required to use this mode to insure that a careful assessment of benefits vs risks have been assessed. The FDA requires equipment operating in this mode to display and record dB/dt and SAR levels, and display a warning that the first level mode is about to be reached. The MR scan operator must acknowledge this at the console before being allowed to proceed with the scan.
3. Second level controlled operating mode.  Also called "research" or experimental" mode, this level is reached when such operation may produce significant risks to patients. Second level mode may only be used under an appropriate ethics/human studies protocol. Special security measures (such as software password lock) must be used to prevent unauthorized operation in this mode.

The original IEC definitions continue to be refined, and several other national and international organizations, including the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the UK Health Protection Agency (HPA), have adopted similar operating mode categories. The basic parameters defining each mode include static field (Bo), time-varying gradients (dB/dt), specific absorption rate (SAR) by body part, and temperature rise. Cutoff values for each can be found in the references below.

IEC SAR Limits. For convenience these have been repeated from the prior Q&A.

IEC SAR Limits (W/kg).All values are based on the environmental temperature being ≤ 25​ºC. For higher temperatures, the First Level whole body limit (4 W/kg) only is derated by 0.25 W/kg for each 1 ºC rise until the Normal operating level limit (2 W/kg) is reached. Partial body limits are scaled according to Mass Ratio (R) = RF-exposed patient mass ÷ total patient mass, giving ranges: a = [10 − 8×R] W/kg and b = [10 − 6×R] W/kg. Transmit/receive knee and wrist coils are covered under the "partial body" limits.

IEC Tissue Temperature Limits.  Maximum allowable core and local temperatures are 39 ºC and 40 ºC for both Normal and First Level operating modes. There are also restrictions on the rise of core temperature, limited to 0.5 ºC for Normal mode and 1 ºC for First Level mode. As before, the local IRB is responsible for setting Second Level temperature limits.

Main Magnetic Field Strength (Bo). The IEC has established the following operating mode limits based on field strength: Normal mode (≤ 3.0T), First Level controlled mode (≤ 8.0T), and Second Level Mode (> 8.0T but below IRB limit).

Gradient Output Limits (dB/dt). The IEC has set limits for the effects of gradient switching based upon observed or theoretical levels of peripheral nerve stimulation (PNS). Several rather technical methods are available that define acceptable boundaries for Normal and First Level Controlled Mode. These are detailed in the Advanced Discussion below.

Advanced Discussion (show/hide)»

The International Electrotechnical Commission Standard (IEC 60601-2-23:2015) has set the prevailing limits for gradient exposure, also adopted by many other organization such as the US Food and Drug Administration (FDA). The IEC provides several methods to satisfy their requirements:

1. Default dB/dt limits for whole body systems.. Use the threshold equation [dB/dt]_TH = rheobase x (1 + 0.36/t_samp) with rheobase of 20 T/s where t_samp is the stimulus duration time in msec. Gradient output limits for Normal Operating Mode and First Level Controlled Mode are defined as 80% and 100% of these values respectively.

2. Default electric field (E) limits for all gradient systems.. Use the threshold equation [E]_TH = rheobase x (1 + 0.36/t_samp) with rheobase of 2.2 V/m where t_samp is the stimulus duration time in msec. Gradient output limits for Normal Operating Mode and First Level Controlled Mode are defined as 80% and 100% of these values respectively.

3. Direct determination in volunteers. (This can apply to both whole body and special purpose gradients.) Using different pulse durations and axes, PNS stimulation thresholds are established in at least 11 volunteers. Gradient output limits for Normal Operating Mode and First Level Controlled Mode are defined as 80% and 100% of the lowest PNS perception level recorded.

IEC Cardiac Stimulation Limits. The IEC uses an exponential model for cardiac limits. (A hyperbolic model is used for PNS). The relevant equation is E < 2 ÷ [1 − exp(−t_stim/3)]

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References
     International Electrotechnical Commission. IEC 60601-2-33:2010: Medical Electrical Equipment - Part 2-33: Particular Requirements for the Basic Safety and Essential Performance of Magnetic Resonance Equipment for Medical Diagnosis. 3rd ed. with amendments. International Electrotechnical Commission; 2015. (accessed July 2020)
     Health Protection Agency. Protection of patients and volunteers undergoing MRI procedures. Documents of the Health Protection Agency Radiation, Chemical and Environmental Hazards, August 2008. [link]
    Medicines and Healthcare Products Regulatory Agency (MHRA). Safety guidelines for magnetic resonance imaging equipment in clinical use. 2015. (See Appendix 2 for comparison of exposure limits suggested by HPA, IEC, and ICNIRP) [link]

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Related Questions
     Who regulates/sets standards for MRI equipment and safety? 

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