Very-low-field magnetic resonance imaging offers significant advantages in portability and cost but faces challenges in maintaining a high signal-to-noise ratio (SNR). In vertical-field configurations, conventional quadrature coils often suffer from inherent structural asymmetry, leading to complex decoupling requirements and significant orthogonal losses. To address this challenge, we propose a symmetrical quadrature coil composed of two sets of series-connected spiral coils based on the working principle of circularly polarized magnetic fields. By employing structurally identical units for both orthogonal channels, the design inherently ensures channel consistency and achieves high interport isolation without additional decoupling circuits. A multiobjective particle swarm optimization algorithm was employed to optimize the geometric parameters, balancing the trade-off between SNR and field homogeneity. Furthermore, an “orthogonal loss” metric was introduced to quantify the deviation from ideal circular polarization. Importantly, comparative analysis reveals that the proposed symmetrical coil exhibits superior $B_1^{-}$ field strength and significantly lower orthogonal loss compared to conventional Helmholtz-Saddle configurations, achieving an average $B_1^{-}$ field strength enhancement of 1.8 times in a large region of interest (ROI). A prototype symmetrical quadrature coil was fabricated and experimentally validated. Simulation and experimental results demonstrate excellent agreement, with an orthogonal loss of only 1.9% observed at the coil isocenter. Measured data confirm that the orthogonal loss remains well controlled within the central ROI, demonstrating that the structural symmetry effectively preserves quadrature purity. This work offers new perspectives on the design of circularly polarized volume coils and receiving coils for vertical-field MRI systems.

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• Fluctuations & noise
• Magnetism
• Medical image quality
• Polarization of light
• Devices
• Medical magnetic resonance imaging
• Microwave techniques
• Radio frequency techniques