Less is more? Performance of loops without distributed capacitors for 7 T MRI applications

We present an exploration of loops without distributed capacitors (i.e. plain loops) for their suitability as RF coils in 7 T MRI applications. Herein, we report on the differences between a circular -i.e. round- plain loop, an elliptic -i.e. elongated- plain loop, a conventional loop - i.e. a loop with distributed capacitors - and a dipole. This comparison highlights the benefits and limitations of plain loops, providing insights into their viability as an alternative to more complex coil designs for the construction of RF coil arrays at 7 T. We characterized the coils in terms of surface current distribution, robustness to loading, transmit efficiency, maxSAR_10g, coupling, and "flexibility" - defined as the ability of a coil to stay tuned (S₁₁≤-10dB) after mechanical deformation at a constant distance from the load. Additionally, we discussed the best practice to fabricate plain loops. A 12 cm round plain loop was more sensitive to the coil load distance than a conventional loop, but it was flexible and can be operated at a roughly constant distance from different patients. The antenna showed similar transmit properties to a 12 cm round conventional loop but with lower coupling (∼44 % less) when the loops were gapped or overlapped less than 20 %. A 62 mmx280mm elongated plain loop was more robust than a dipole to the coil-load distance, providing similar transmit efficiency for imaging of the prostate, with lower worst case maxSAR_10g (∼25 % less), but higher coupling (S₂₁ ∼ -15 dB). This paper provides the groundwork for further optimizing plain loops in UHF-MRI coil arrays.