A comprehensive electromagnetic evaluation of an MRI anthropomorphic head phantom

Electromagnetic simulations are an important tool for the safety assessment of RF coils. They are a useful resource for MRI RF coil designers, especially when complemented with experimental measurements and testing using physical phantoms. Regular-shaped (spherical/cylindrical) homogeneous phantoms are the MRI standard for RF testing but are somewhat inaccurate when compared with anthropomorphic anatomies, especially at high frequencies. In this work, using a recently developed anthropomorphic heterogeneous human head phantom, studies were performed to analyze the scattering parameters (S-parameters) and the electric and magnetic field distributions using (1) the B₁⁺ field mapping method on a 7 T human MRI scanner and (2) numerical full-wave electromagnetic simulations. All studies used the following: a recently developed six-compartment refillable 3D-printed anthropomorphic head phantom (developed from MRI scans obtained in vivo), where the phantom itself is filled in its entirety with either heterogeneous loading, or homogeneous brain or water loading, in vivo imaging, and a commercial homogeneous spherical water phantom. Our results determined that the calculated S-parameters for all the anthropomorphic head phantom models were comparable to the model that is based on the volunteer (within 17% difference of the reflection coefficient value) but differed for the commercial homogeneous spherical water phantom (within 45% difference). The experimentally measured B₁⁺ field maps of the anthropomorphic heterogeneous and homogeneous brain head phantoms were most comparable to the in vivo measured values. The numerical simulations also show that both the anthropomorphic homogeneous water and brain phantom models were less accurate in terms of electric field intensities/distributions when compared with the segmented in-vivo-based head model and the anthropomorphic heterogeneous head phantom model. The presented data highlights the differences between the physical phantoms/phantom models, and the in vivo measurements/segmented in-vivo-based head model. The results demonstrate the usefulness of 3D-printed anthropomorphic phantoms for RF coil evaluation and testing.