CEM43°C thermal dose thresholds: a potential guide for magnetic resonance radiofrequency exposure levels?

Abstract

Objective: To define thresholds of safe local temperature increases for MR equipment that exposes patients to radiofrequency fields of high intensities for long duration. These MR systems induce heterogeneous energy absorption patterns inside the body and can create localised hotspots with a risk of overheating.

Methods: The MRI + EUREKA research consortium organised a "Thermal Workshop on RF Hotspots". The available literature on thresholds for thermal damage and the validity of the thermal dose (TD) model were discussed.

Results/conclusions: The following global TD threshold guidelines for safe use of MR are proposed: 1. All persons: maximum local temperature of any tissue limited to 39 °C 2. Persons with compromised thermoregulation AND (a) Uncontrolled conditions: maximum local temperature limited to 39 °C (b) Controlled conditions: TD < 2 CEM43°C 3. Persons with uncompromised thermoregulation AND (a) Uncontrolled conditions: TD < 2 CEM43°C (b) Controlled conditions: TD < 9 CEM43°C The following definitions are applied: Controlled conditions A medical doctor or a dedicated trained person can respond instantly to heat-induced physiological stress Compromised thermoregulation All persons with impaired systemic or reduced local thermoregulation

Key points: • Standard MRI can cause local heating by radiofrequency absorption. • Monitoring thermal dose (in units of CEM43°C) can control risk during MRI. • 9 CEM43°C seems an acceptable thermal dose threshold for most patients. • For skin, muscle, fat and bone,16 CEM43°C is likely acceptable.

Fig. 1

Graphs showing various levels of normal tissue damage for a rectum and b small intestine as a function of temperature of original exposure and the CEM43°C (equivalent exposure time in minutes at 43 °C). The CEM43°C model illustrates increasing tissue damage with increasing CEM43°C dose and constant tissue damage for the same CEM43°C dose (mouse 50 % crypt stem cell survival: higher temperature is compensated by shorter exposure). Small symbols data from [14]; large symbols in b only, from [15]. Rat (whole body hyperthermia) green circle 4.6 CEM43°C no effect; red circle 7 CEM43°C significant change

Fig. 2

Graphs showing various levels of tissue damage for a muscle and b fat with respect to temperature during the thermal exposure and the CEM43°C (equivalent exposure time expressed in minutes at 43 °C). Small symbols data from [14]; large symbols in a only, from [15]. Human orange circle 26 CEM43°C threshold to induce thermal damage in the muscle

Fig. 3

Thermal threshold as CEM43°C for normal tissue damage of multiple parts of the eye. Data from [14]; inserts are new data from [15]

Fig. 4

Thermal threshold of brain tissue damage after local heat exposure including a range for the thermal threshold in CEM43°C for an effect of the blood brain barrier (BBB) response after whole body hyperthermia (data from [14, 15])

Fig. 5

Highest CEM43°C for all tissues and all species for which “no damage” or “no effect” was reported

Fig. 6

Lowest CEM43°C for tissue damage for all tissues reported in larger animals and humans only

Fig. 7

Lowest CEM43°C reported for tissue damage for all tissues and all species