Traveling-wave meets standing-wave: A simulation study using pair-of-transverse-dipole-ring coils for adjustable longitudinal coverage in ultra-high field MRI

At ultrahigh fields (B₀ ≥ 7T), it is challenging to cover a large field of view using single-row conventional RF coils (standing wave resonators) due to the limited physical dimensions. In contrast, traveling wave approaches can excite large fields of view even using a relatively simple hardware setup, but suffer from poor efficiency and high local specific absorption rate in non-imaged regions. In this study, we propose and numerically analyze a new coil which combines the concept of traveling wave and standing wave. The new coil consists of a pair of transverse dipole rings (PTDR) whose separation is adjusted according to the desired imaging coverage. The PTDR coil was validated using electromagnetic simulations in phantoms and human leg models, which showed that coverage can be as long as 60 cm. When the coverage of the PTDR coil was shortened to 20 cm to cover the knees only, it's transmit and specific absorption rate efficiencies were 84% and 37% higher than those of the 50 cm coverage, respectively.