Abstract
Purpose.To revisit the “loopole,” an unusual coil topology whose unbalanced current distribution captures both loop and electric dipole properties, which can be advantageous in ultra-high-field MRI.Methods.Loopole coils were built by deliberately breaking the capacitor symmetry of traditional loop coils. The corresponding current distribution, transmit efficiency, and signal-to-noise ratio (SNR) were evaluated in simulation and experiments in comparison to those of loops and electric dipoles at 7 T (297 MHz).Results.The loopole coil exhibited a hybrid current pattern, comprising features of both loops and electric dipole current patterns. Depending on the orientation relative to B0, the loopole demonstrated significant performance boost in either the transmit efficiency or SNR at the center of a dielectric sample when compared to a traditional loop. Modest improvements were observed when compared to an electric dipole.Conclusion.The loopole can achieve high performance by supporting both divergence-free and curl-free current patterns, which are both significant contributors to the ultimate intrinsic performance at ultra-high field. While electric dipoles exhibit similar hybrid properties, loopoles maintain the engineering advantages of loops, such as geometric decoupling and reduced resonance frequency dependence on sample loading.
Highlights
Compared to conventional magnetic resonance imaging (MRI) systems, ultra-high-field (UHF) MRI systems promise an enhanced signal-to-noise ratio (SNR) [1,2,3]
Using the far field antenna theory, it was shown that a radiative dipole can be an effective UHF transmit/receive element under certain conditions related to the operating frequency and sample properties [7, 8]. e radiative dipole represented a drastic change from conventional closed loop coils and stripline
It was shown that the resulting configuration can be represented as the sum of the uniform closed-path current pattern of a typical loop coil and an openpath current pattern that could be achieved with an electric dipole [25] (Figure 1)
Summary
Compared to conventional magnetic resonance imaging (MRI) systems, ultra-high-field (UHF) MRI systems (defined as those with B0 field strength of 7 T and above) promise an enhanced signal-to-noise ratio (SNR) [1,2,3]. UHF MRI presents a variety of challenges, some of which are related to accentuated interactions between the applied radiofrequency (RF) magnetic field (B+1 ) and the dielectric biological tissues. Using the far field antenna theory, it was shown that a radiative dipole can be an effective UHF transmit/receive element under certain conditions related to the operating frequency and sample properties [7, 8]. E radiative dipole represented a drastic change from conventional closed loop coils and stripline Using the far field antenna theory, it was shown that a radiative dipole can be an effective UHF transmit/receive element under certain conditions related to the operating frequency and sample properties [7, 8]. e radiative dipole represented a drastic change from conventional closed loop coils and stripline
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