Capacitive versus Overlap Decoupling of Adjacent Radio Frequency Phased Array Coil Elements: An Imaging Robustness Comparison When Sample Load Varies for 3 Tesla MRI

Abstract

Phased array (PA) receive coils are built such that coil elements approximate independent antenna behavior. One method of achieving this goal is to use an available decoupling method to decouple adjacent coil elements. The purpose of this work was to compare the relative performance of two decoupling methods as a function of variation in sample load. Two PA receive coils with 5 channels (5-ch) each, equal outer dimensions, and formed on 12 cm diameter cylindrical phantoms of conductivities 0.3, 0.6, and 0.9 S/m were evaluated for relative signal-to-noise ratio (SNR) and parallel imaging performance. They were only tuned and matched to the 0.6 S/m phantom. Simulated and measured axial, sagittal, and coronal 5-ch PA coil SNR ratios were compared by dividing the overlap by the capacitive decoupled coil SNR results. Issues related to the selection of capacitor values for the two decoupling methods were evaluated by taking the ratio of the match and tune capacitors for large and small 2 channel (2-ch) PA coils. The SNR ratios showed that the SNR of the two decoupling methods were very similar. The inverse geometry-factor maps showed similar but better overall parallel imaging performance for the capacitive decoupled method. The quotients for the 2-ch PA coils’ maximum and minimum capacitor value ratios are 3.28 and 1.38 for the large and 3.28 and 2.22 for the small PA. The results of this paper demonstrate that as the sample load varies, the capacitive and overlap decoupling methods are very similar in relative SNR and this similarity continues for parallel imaging performance. Although, for the 5-ch coils studied, the capacitive decoupling method has a slight SNR and parallel imaging advantage and it was noted that the capacitive decoupled coil is more likely to encounter unbuildable PA coil configurations.

External waveguide magnetic resonance imaging for lower limbs at 3 T

“…Under normal circumstances, MR images are obtained using a coil array with multiple elements, not just to fully cover the region of interest (ROI), but also to accelerate the image acquisition itself 24 . Such coil arrays offer significant advances, in terms of functionality and image quality; however, their complexity imposes additional design and implementation challenges 20,31–39 …”

“…24 Such coil arrays offer significant advances, in terms of functionality and image quality; however, their complexity imposes additional design and implementation challenges. 20,[31][32][33][34][35][36][37][38][39] F I G U R E 1 Illustrations of LF MR imagers for lower extremity applications (a) and (b), and a whole-body MR imager (c), commonly used in clinical practice. Schematics of RF coil arrays with multiple elements used to acquire images of lower limbs with whole-body MR imagers: (d), 12 (e), 13 (f), 14 (g), 15 and (h) two standard birdcage coils for lower limbs 17 The traveling-wave (tw) MRI approach provides an alternative to coil arrays with a larger number of coil elements.…”