Introduction <p>The aim of this work was to investigate the effect of an air layer of different thicknesses on the design of high-permittivity materials (HPM) helmets for ultrahigh field (UHF) magnetic resonance imaging (MRI).</p> Method <p>We used a recently proposed analytical formulation of scattering from layered spheres to model an MRI experiment with a variable air layer between a homogenous human head and an HPM helmet. Such model expresses the fields as a superposition of progressive and regressive traveling waves by exploiting the theory of inhomogeneous transmission. Analytical results were compared with numerical simulations, in terms of changes in the radiofrequency (RF) magnetic induction field employing a surface and volume coil, to validate the proposed method.</p> Results <p>The presence of an air layer, due to differences in head size, results in a slight variation in the optimal permittivity value required to optimize the performance of the helmet, with a maximum relative variation of no more than 12%. This can be explained by the invariance of the impedance at the outer air–HPM interface, due to the high conductivity typical of biological tissues. In both cases, a clear increase in the magnetic induction field is observed, suggesting that the HPM design is robust to the introduction of a small dielectric insulating layer. Also, good agreement was found between the analytical and numerical results suggesting that the model could be employed to optimize the HPM also in real experiments, particularly when canonical geometries, such as cylindrical or spherical shapes, are employed to design the helmet.</p>