Open birdcage coil for head imaging at 7T

Abstract

Purpose: To theoretically describe, design, and test the new geometry of the birdcage coil for 7 Tesla anatomical brain imaging, which includes a large window on top, without deliberately jeopardizing its homogeneity and efficiency. This opencage will not only improve patient comfort but also enable the volunteer to follow functional MRI stimuli. This design could also facilitate the tracking of patient compliance and enable better correction of the movement.

Methods: Via the transfer matrix approach, a birdcage-like coil with a nonperiodic distribution of rungs is constructed with optimized currents in the coil rungs. Subsequently, the coil is adjusted in full-wave simulations. Then, the coil is assembled, fine-tuned, and matched on the bench. Finally, these results are confirmed experimentally on a phantom and in vivo.

Results: Indeed, the computed isolation of -14.9 dB between the feeding ports of the coil and the symmetry of the circular polarized mode pattern transmit RF magnetic field ( $B_1^{+}$ ) showed that the coil was properly optimized. An experimental assessment of the developed coil showed competitive transmit efficiency and coverage compared with the conventional birdcage coil of similar size.

Conclusion: The proposed opencage coil can be designed and work without a dramatic drop of performance in terms of the $B_1^{+}$ field homogeneity, transmit efficiency ( $\left(B_1^{+}\right)$ / $\sqrt{P_{\mathrm{ref}}}$ ), peak local specific absorption rate ( $SA{R}_{10g}$ ) and SAR efficiency ( $\left(B_1^{+}\right)$ / $\sqrt{SA{R}_{10g}}$ ).

Keywords: birdcage coil; brain imaging; head coil; open coil; ultra-high field MRI.