Constructive Near-Field Interference Effect in a Birdcage MRI Coil with an Artificial Magnetic Shield

Abstract

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.

Potential Reduction of Peripheral Local SAR for a Birdcage Body Coil at 3 Tesla Using a Magnetic Shield

“…In situations with weaker loading than described in this work (e.g., children in a 3 T scanner whole body birdcage, a head in a 400 mm birdcage at 7 T or small animal scanners [28]), this loading problem will be less severe. The energy can propagate in the azimuthal direction without too much losses, which results in a sinusoidal current pattern over the rungs.…”

“…Other studies [22][23][24][25][26] have demonstrated improved performance for local transmit antennas by using a magnetically conducting shield, but for the birdcage coil at 3 T, this has not been investigated. Lezhennikova et al [27,28] have investigated potential improvements of birdcage coils with a magnetic shield under weaker loading conditions, where no adaptation to the driving scheme is required. However, they have not investigated potential SAR hotspots in any scenario where tissue is present close to one of the conductors, which was the focus of this work.…”

“…However, a smaller conventional birdcage with 300 mm diameter still performed better, and the authors were unable to apply their slot resonator to this smaller coil. In other work, Lezhennikova et al [28] described potential improvement of birdcage coils with RF shields of arbitrary impedance, and they designed an artificial magnetic shield for use in a small animal coil (Ø 70 mm) for 7 T. However, their resonant structure has a significant thickness, increasing the diameter of the total coil + shield to 140 mm, thus resulting in a solution that would not be practical to implement at 3 T, where bore space is precious.…”

Extending a birdcage coil for magnetic resonance imaging of a human head with an artificial magnetic shield

“…It was shown that the structure in [22] provides more homogeneous B 1 field and lower electric field. In [23] it was shown that the artificial mag-125 netic shield realized as a miniaturized corrugated structure when used as a shied increases the efficiency of the preclinical birdcage coil and this improvement depends on the size of a conducting sample. The efficiency gain of 13% was numerically and experimentally demonstrated for MRI at 130 7 T for the artificial magnetic shield over a conventional copper shield.…”

“…As a result, by using a corrugated shield one can improve the signal-to-noise ra-135 tio when using the same coil for the reception, but with the same homogeneity. However, since a small preclinical birdcage was considered, no analysis of SAR was made in [23].…”