Comparative Analysis of Inter-Element Coupling in Sleeve and Dipole Antennas for Ultra-High Field Magnetic Resonance Imaging
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Purpose: This study aims to evaluate sleeve antennas that can reduce electromagnetic coupling compared with conventional dipole antennas while maintaining comparable performance at 10.5 tesla (T). Materials and Methods: We designed and fabricated dipole and sleeve antennas with matching resonant frequencies (447 MHz at 10.5 T) and similar geometric configurations, enabling a fair comparison. The sleeve antenna design consisted of monopole conductors with inductive shortening and 5-cm floating cable traps, allowing a total length of 20 cm. Two-element coupling was measured over a range of 4 to 14 cm on a phantom with tissue- equivalent properties (εr = 49, σ = 0.6 S/m). Additionally, eight-channel dipole and sleeve antenna arrays were assembled on 3D-printed formers with identical elliptical dimensions and an average inter-element spacing of 8.8 ± 1.4 cm. Initially, bench measurements were conducted to evaluate the S-parameters of the dipole and sleeve antennas. Noise covariance matrices and relative B1 + field (transmit magnetic field) maps of each array were obtained through 10.5-T magnetic resonance imaging. Results: The sleeve antenna consistently showed lower coupling than that of the dipole antenna across all distances, with S21 values reaching below -7.2 dB at 4 cm, compared to -5.1 dB for the dipole. In the eight-channel array configuration, both antenna types exhibited similar S11 values, indicating adequate impedance matching. However, the sleeve array demonstrated improved adjacent-element isolation (lower S21) and reduced noise covariance (≤0.07), suggesting greater channel independence. The relative B1 + field maps revealed independent transmit field patterns for both arrays, enabled by sufficient interelement spacing. Conclusion: The sleeve antenna array provides superior decoupling and lower inter-element noise correlation than that of a conventional dipole antenna. Its simplified structure and performance advantages make it a promising solution for scalable, high-density, ultrahigh-field magnetic resonance imaging transmit/receive arrays.
Abstract
Purpose: This study aims to evaluate sleeve antennas that can reduce electromagnetic coupling compared with conventional dipole antennas while maintaining comparable performance at 10.5 tesla (T). Materials and Methods: We designed and fabricated dipole and sleeve antennas with matching resonant frequencies (447 MHz at 10.5 T) and similar geometric configurations, enabling a fair comparison. The sleeve antenna design consisted of monopole conductors with inductive shortening and 5-cm floating cable traps, allowing a total length of 20 cm. Two-element coupling was measured over a range of 4 to 14 cm on a phantom with tissue- equivalent properties (εr = 49, σ = 0.6 S/m). Additionally, eight-channel dipole and sleeve antenna arrays were assembled on 3D-printed formers with identical elliptical dimensions and an average inter-element spacing of 8.8 ± 1.4 cm. Initially, bench measurements were conducted to evaluate the S-parameters of the dipole and sleeve antennas. Noise covariance matrices and relative B1 + field (transmit magnetic field) maps of each array were obtained through 10.5-T magnetic resonance imaging. Results: The sleeve antenna consistently showed lower coupling than that of the dipole antenna across all distances, with S21 values reaching below -7.2 dB at 4 cm, compared to -5.1 dB for the dipole. In the eight-channel array configuration, both antenna types exhibited similar S11 values, indicating adequate impedance matching. However, the sleeve array demonstrated improved adjacent-element isolation (lower S21) and reduced noise covariance (≤0.07), suggesting greater channel independence. The relative B1 + field maps revealed independent transmit field patterns for both arrays, enabled by sufficient interelement spacing. Conclusion: The sleeve antenna array provides superior decoupling and lower inter-element noise correlation than that of a conventional dipole antenna. Its simplified structure and performance advantages make it a promising solution for scalable, high-density, ultrahigh-field magnetic resonance imaging transmit/receive arrays.