Flexible Dipole Coil Array Design for Efficient Performance of Head and Neck Imaging at 7T MRI
Biomedical Engineering
Sreeram Sreekumar
Abstract
Dipole antennas are being widely used in ultra-high field (UHF) MRI systems due to their deeper penetration depth of the transmit magnetic field (B1). However, traditional rigid dipole designs do not conform well to the highly curved anatomical regions such as neck, wrist, shoulder, etc., resulting in reduced electromagnetic coupling in tissues, increased localized electric-field hotspots, resulting in inefficient operation. To address this limitation, this project aims to compare the traditional rigid dipole antenna with a flexible dipole design to determine whether conformity can improve electromagnetic coupling in tissues, field uniformity, and power efficiency. Both simulations were modeled and simulated in Ansys HFSS using a three-channel configuration at 300 MHz. Both designs were parametrically modeled, allowing for variation of dipole geometry and performance under identical loading conditions. Simulations were carried out with coils placed at the same distance from the phantom, and by placing the coils close to the target region. Bench testing was carried out using a single-channel prototype fabricated using a custom tuning/matching circuit on a 3D-printed phantom model. The simulated and experimental return loss (S11) parameters were used to calculate the simulated and experimental Q-factors of the coils. The simulated and experimental Q-factors were compared using an acceptance criterion of ≤150 to ensure the fabricated prototype and the simulated coil are comparable, while accounting for the variations due to hardware implementation. The results show that the flexible dipole achieved a more uniform magnetic field distribution and eliminated any E-field localizations seen in rigid dipole designs. The flexible coil was able to maintain similar H-field strength to the rigid design while operating at 30% lower input power. Overall, the flexible dipole demonstrates enhanced electromagnetic performance while having lower power requirements, highlighting its potential for a more efficient UHF MRI coil design. Future work will extend this analysis to multi-channel bench tests, SAR optimization, and image tests at 7T using a pre-clinical scanner.