Kim Annink

238 Chapter 11 While studies at 7.0T have now shown to be safe in adults (14,15), literature about safety in children is scarce. In a study of Harris et al. 42 children between five and ten years of age underwent a 7.0T MRI which was well tolerated and safe in all children (16). The Food and Drug Administration (FDA) approved 7.0T MRI in infants of one month and older (17). The limit of the main static field in neonates (infants younger than one month) is 4.0T MRI. SAR limits are the same for adults and neonates (17). We initiated a feasibility pilot study in infants. This study shows the first MR images of infants at 7.0T and provides information about the safety preparations. METHODS Preparation - SAR simulation Using a radiofrequency (RF) simulation, we investigated whether MRI protocols could be translated from the adult to the infant brain without exceeding the SAR limits. Finite difference time domain simulations were performed using Sim4Life (Zurich Med Tech, Zurich, Switzerland) to evaluate transmit efficiency and RF safety limits of the setup, assuming full decoupling to the receiver coils. The geometry and electrical circuits of the Nova head coil (Nova Medical, Inc, Burlington, MA, USA) for 7.0T were implemented in Sim4Life. The simulations were performed on a virtual infant model (Charlie, two months of age, 4.3kg) of the ITIS virtual family (18) in different positions in the coil (Figure 1), using the head coil in quadrature mode. The same simulations were performed on adult heads (male Duke, female Ella) of the virtual family (18) as a reference. Local SAR levels (10g-average) for 1 Watt input power and global SAR levels (average SAR over the whole head) were calculated. Also, the average SAR per B 1 2 was calculated as the average SAR over the whole head divided by the average B 1 2 in a central slice in the brain. Peak local SAR was defined as the highest SAR in the whole infant. SAR simulations were validated by comparing simulations and measurements of B 1 maps of a spherical phantom and power measurements for data scaling (data not shown).