A dielectric structure with an extended radiofrequency field coverage area for human brain magnetic resonance imaging

Magnetic resonance imaging (MRI) of the extended region of interest (ROI) included both the brain and the cervical spinal cord, presents a significant technical challenge due to the substantial longitudinal extent of the anatomy and the inherent inhomogeneity of the radiofrequency (RF) field at high magnetic field strengths. Conventional dielectric waveguide does not provide uniform excitation across such an extended ROI. The primary objective of this study was to design and numerically evaluate a novel dielectric structure (DS) capable of providing a balanced and homogeneous RF field В1+ distribution simultaneously in the brain and cervical spinal cord tissues, while maintaining local specific absorption rate (SAR) within safe limits. The study was conducted using a numerical phantom and full-wave electromagnetic simulations (CST Studio Suite). A reference dielectric waveguide (DW) was used for comparative analysis. The proposed DS comprised a base cylindrical segment (external diameter 28 cm, length 35 cm, wall thickness 3 cm, permittivity ε = 21) combined with an additional dielectric insert of variable length. Two configurations were investigated: DS «1» (insert length l = 8 cm) and DS «2» (insert length 2l = 16 cm). For each configuration, a parametric analysis was performed by varying the relative permittivity ε of the additional insert from 15 to 25. The RF field characteristics were evaluated within three regions of interest (ROIs): the brain (16 cm), the cervical spinal cord (10 cm), and a combined ROI (26 cm). The analyzed metrics included the mean В1+ amplitude (<В1+>), field homogeneity quantified by the coefficient of variation (COV), and the 10g-averaged specific absorption rate (SAR10g). The parametric analysis revealed that for the DS «1» configuration, none of the permittivity values provided balanced excitation, as the field enhancement was primarily localized in the brain region, with a significantly lower field strength in the cervical spinal cord. In contrast, the extended DS «2» configuration demonstrated a substantial increase in the longitudinal coverage of the RF field. For DS «2», the optimal performance was achieved at ε = 19. At this permittivity, the mean В1+ values were 0.311 μT for the brain ROI, 0.312 μT for the spinal cord ROI, and 0.314 μT for the combined ROI, indicating an excellent balance. Comparative analysis with the reference DW showed that while the proposed DS «2» yielded a 24% lower mean field in the combined ROI, it provided a remarkable 85% increase in the field strength within the cervical spinal cord. Furthermore, the DS «2» exhibited a 26% reduction in SAR10g and substantially improved field homogeneity, with В1+ COV improvements of 25% for the combined ROI, 16% for the brain, and 5% for the spinal cord, compared to the reference DW. Thus, the proposed optimized dielectric structure (DS «2» with ε = 19) effectively addresses the challenge of simultaneous RF excitation of the brain and cervical spinal cord. By extending the longitudinal field coverage and achieving a balanced В1+ distribution, the designed structure significantly improves field homogeneity in the extended ROI in longitudinal direction.