Specific absorption rate and temperature in neonate models resulting from exposure to a 7T head coil

• Published in: Magnetic resonance in medicine Vol. 86; no. 3; pp. 1299 - 1313
• Main Authors: Malik, Shaihan J., Hand, Jeffrey W., Satnarine, Ryan, Price, Anthony N., Hajnal, Joseph V.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01-09-2021
• Subjects: 7T MRI; Absorption; Adult; Adults; Computer Simulation; Dielectric properties; Electrical properties; Head - diagnostic imaging; Humans; Infant, Newborn; Magnetic Resonance Imaging; Moisture content; neonatal imaging; Neonates; Phantoms, Imaging; Radio Waves; RF safety; Temperature; Thermal insulation; Thermal simulation; Tissues; Water content; 7T MRI; neonatal imaging; RF safety
• ISSN: 0740-3194; 1522-2594
• DOI: 10.1002/mrm.28784

Purpose
To investigate safe limits for neonatal imaging using a 7T head coil, including both specific absorption rate (SAR) and temperature predictions.
Methods
Head‐centered neonate models were simulated using finite‐difference time domain–based electromagnetic and thermal solvers. The effects of higher water content of neonatal tissues compared with adults, position shifts, and thermal insulation were also considered. An adult model was simulated for comparison.
Results
Maximum and average SAR are both elevated in the neonate when compared with an adult model. When normalized to B1+, the SAR experienced by a neonate is greater than an adult by approximately a factor of 2; when normalized to net forward power (forward‐reflected), this increases to a factor of 2.5‐3.0; and when normalized to absorbed power, approximately a factor of 4. Use of age‐adjusted dielectric properties significantly increases the predicted SAR, compared with using adult tissue properties for the neonates. Thermal simulations predict that change in core temperature/maximum temperature remain compliant with International Electrotechnical Commission limits when a thermally insulated neonate is exposed at the SAR limit for up to an hour.
Conclusion
This study of two neonate models cannot quantify the variability expected within a larger population. Likewise, the use of age‐adjusted dielectric properties have a significant effect, but while their use is well motivated by literature, there is uncertainty in the true dielectric properties of neonatal tissue. Nevertheless, the main finding is that unlike at lower field strengths, operational limits for 7T neonatal MRI using an adult head coil should be more conservative than limits for use on adults.