Design of radiofrequency coils for magnetic resonance imaging applications at high fields: technological and physical feasibility issues

In the field of medical magnetic resonance imaging (MRI), there has been a constant drive towards more powerful magnets and therefore higher operational frequencies. Imaging at higher fields has several benefits, so that images with much greater detail can be obtained using high field MRI systems with high static (B/sub 0/) field strength. Operation at high frequencies, however, adds significant technical challenges to the design of radiofrequency (RF) coils. We address the physics associated with RF coils and their electromagnetic interactions with biological tissue. This is accomplished by examining the B/sub 1/ field distribution and the RF power requirements between 135 MHz and 470 MHz. We also propose methods to optimize the B/sub 1/ field distribution at ultra high field (/spl ges/ 300 MHz). Using the finite difference time domain (FDTD) method, antenna array concepts are utilized to excite a volume head coil (TEM resonator) such that an optimal B/sub 1/ field distribution is achieved in the head. Input excitations with both variable phase and magnitude are evaluated for 2, 3, 4, 6, and 10 excitation ports in a 24-strut TEM resonator loaded with an 18-tissue anatomically detailed human head model. Finally, true phased array is implemented to modify the phase and magnitude of all the 24 possible ports in the 24-strut TEM resonator. By properly exciting each drive port, significantly improved B/sub 1/ field homogeneity was obtained.