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Radio-frequency (rf) coils are used in all clinical and research magnetic-resonance-imaging (MRI) systems to excite nuclear spins and to receive signals from them. The quality of imaging depends strongly on the signal-to-noise ratio (SNR) and the transmit efficiency of the coils. The birdcage volume coils used in most MRI scanners for homogeneous imaging of a sample are typically shielded from the external systems of the MRI scanner, i.e., the gradient coils, to confine the rf field within the region of interest. However, the near magnetic field of a conventional copper rf shield surrounding a birdcage coil interferes destructively with the primary field of the coil in the sample, which significantly limits the SNR and the transmit efficiency. In the work presented here, we theoretically study and experimentally demonstrate the possibility of creating an artificial magnetic rf shield for a birdcage coil with constructive interference in a sample. This effect is similar to the in-phase reflection of antenna far fields from a magnetic shield but affects the near field in MRI. We build an analytical model of a birdcage coil shielded with a cylindrical impedance boundary and analyze the conditions for increasing the efficiency of the coil by means of the shield. We conclude that by replacing a copper shield with an artificial magnetic one, it is possible to reduce the dissipative intrinsic losses of the coil and increase the power absorbed by the sample, which improves the efficiency. To demonstrate the effect, we perform a detailed numerical simulation and an experiment with a small birdcage in a 7-T 19 F MRI system with a magnetic shield implemented as a periodic cylindrical metal structure with corrugations filled with a ceramic.

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We assessed electromagnetic behavior of metallic implant used for distal femur fracture in typical MRI situation. |B1+| field, SAR and temperature variations were computed at 3T using a surface coil on human model for different positions of the implant relative to the coil. To validate the simulation, we measured the E-field and compared it to simulated E-field. |B1+| field maps showed an interesting augmentation near the implant. Both global SAR and local SAR levels proved that it is possible to safely image bone repair. However, temperature elevation near the tip of the implant was important and is to be considerate.

Parallel transmission for 7T MRI of the spinal cord is a promising area of research, as high specific absorption rate and coil inefficiency in some levels may limit certain applications. In the absence of virtual observation points (VOP) provided by the coil manufacturer, preliminary testing and first in vivo applications on a healthy volunteer were done in a conservative SAR-restricted, showing potential in regions that suffered from signal drops with the default shim parameters. The workflow to ensure radiofrequency safety with virtual observation points, based on simulations of the specific absorption rate and their validation, is also described.