Approaching the Ultimate SNR in Parallel MRI with Finite Coil Arrays

INTRODUCTION: In recent years the radio frequency (RF) electrodynamics (ED) of parallel imaging (PI) have been the subject of several theoretical studies. By calculating the maximally achievable, so-called ultimate SNR, fundamental limitations of PI have been identified and analyzed in detail (1,2). Most importantly it was found that (I) the amount of feasible reduction in PI is inherently limited and (II) increases with the onset of wave behavior at very high B0 and in large objects (3). In theory, the ultimate SNR could be reached with a “complete coil array”, whose coil sensitivities span the entire solution space of the Maxwell equations. Yet it is unclear as yet whether this limit can be approached with real coil arrays with a finite number of elements. In order to investigate this question, in the present work we numerically study the SNR performance of finite RF coil configurations and compare it quantitatively with the corresponding ultimate SNR.