Kim Annink

245 7.0T MRI in neonates DISCUSSION We demonstrated that scanning infants in a 7.0T scanner is feasible and results in good quality images. Whilst optimization of the sequences is ongoing, we already demonstrated that some sequences showed more details compared to 3.0T MRI. Prior to scanning, simulated SAR levels at 7.0T were lower in the virtual infant than in adult models (18). When the infants head was farther in the coil than isocenter, or 5cm in x or y direction, SAR levels did exceed the adult situation. Thus, center position of the infant in the coil is essential. Therefore, the position of the infants head was constrained in the coil, making it mechanically impossible to put the infants head farther in the coil than center position. Differences in SAR due to inter-subject variability cannot be completely ruled out. However, previous simulations at 3.0T showed that different sized infant models and different positions did not result in major differences in simulated SAR levels (21). The SAR simulations had two important limitations. The first limitation is that the Sim4Life model of Charlie uses the dielectric values of adults which might slightly differ from infant dielectric properties. The dielectric values of human infants are unknown and require further research. In the study of Malik et al. conversion of adult dielectric values were based on rat data, however these are not validated (21). Secondly, only one virtual model of a two-month-old infant was available, which cannot be completely translated to a term (equivalent age) infant. Head circumference and body composition differ between term (equivalent) age and two months of age. Malik et al. showed that term neonates with smaller head size or lower body weight had lower SAR depositions (21), suggesting that the two-month- old infant model does not underestimate SAR values. Regarding body composition, Malik et al. simulated the effect of fat percentage on SAR depositions in neonates in two extreme scenarios: one model with only skin and one model with a thick layer of pure fat. The model with only skin had 10% higher peak local SAR depositions (21). The fat percentage of neonates is lower compared to two-month-old infants (22,23). In the worst case scenario, a neonate might have a higher peak local SAR up to 10% compared to a two-month-old infant based on fat composition. This will still not exceed the safety limits of the FDA, since the 7.0T MR scanner of Philips has implemented an additional safety factor larger than 2. Furthermore, the global SAR levels of Charlie in the centered position were 6% lower compared to the adult model 11