Medical Magnetic Resonance (Mr) Procedures: Protection of Patients

Specific absorption rate in neonates undergoing magnetic resonance procedures at 1.5 T and 3 T

“…A larger potential source of error when scanning at 3 T was found to be the use by the manufacturer of “head” and “body” limited scanning regimes, distinguished by the presence or absence of a head receiver coil. In the body limited case, a scan producing whole body SAR 0.87 W kg −1 and maximum local SAR 10g of 10.1 W kg −1 in an adult model produced whole body SAR 0.55 W kg −1 and maximum SAR 10g of 2.59 W kg −1 in a neonatal model; the local SAR for the neonate is just 26% of the recommended limit for normal operation 2, 3. A neonate positioned identically but scanned in the head limited regime would experience whole body SAR of 1.14 W kg −1 and maximum SAR 10g of 5.4 W kg −1 .…”

“…Indeed, local SAR 10g values have been shown to be only weakly associated with local tissue changes 30. Limits to tissue temperature and temperature change 2, 3 are listed in Table 1. These limits are based on the assumption that adverse health effects are not expected in people with unimpaired thermo‐regulatory and cardiovascular functions if the increase in body temperature does not exceed 1 °C 31.…”

“…These limits are based on the assumption that adverse health effects are not expected in people with unimpaired thermo‐regulatory and cardiovascular functions if the increase in body temperature does not exceed 1 °C 31. Thermal transport mechanisms in neonatal subjects differ from those in adults, however, and the ICNIRP standard 3 states that it is “desirable” to limit core temperature increases in infants to 0.5 °C. Neonates have a 2.5–3.0 times higher surface area to bodyweight ratio compared with adults, increasing the relative potential surface for heat loss 32.…”

“…Since it is not straightforward to directly monitor or accurately predict the spatial and temporal distributions of temperature in a subject, power deposition (as quantified by the specific absorption rate, SAR) is used as a surrogate. National and international regulatory authorities publish maximum limits on exposure in guidelines and standards 2, 3, 4. Table 1 lists temperature and SAR limits cited in the international standard IEC (International Electrotechnical Commission) 60601‐2‐33 2.…”

“…Details can be found in the Methods section. The International Commission on Non‐ionizing Radiation Protection (ICNIRP) statement on protection of patients undergoing MRI 3, which is still current regarding RF exposures 4, does impose limits on local SAR 10g in the head, trunk, and extremities, and it is common practice for research papers on SAR modelling for MRI to consider these limits. This is in part motivated by the fact that there are several examples in the literature of simulations of exposure to body coils that indicate that the SAR 10g limit is exceeded before the whole body SAR limit is reached 5, 6, 7, 8.…”