A Flexible Quadrature Coil for Very-Low-Field (50.4 mT) Knee Magnetic Resonance Imaging

Very-low-field (VLF) magnetic resonance imaging (MRI) has been developed unprecedentedly because of its low cost and mobility. They are not only becoming inexpensive substitutes for high-field MRI scanners but also making the hospital no longer the only place for diagnosis. However, the low signal-to-noise ratio (SNR) of VLF MRI is the main limitation of product promotion, and the receive coil is an important component in determining imaging quality. Most existing receive coils are rigid, sacrificing patient comfort. Now, several groups have developed groundbreaking wearable knee coils for high-field MRI scanners but are unsuitable for VLF MRI due to the different impedance characteristics caused by different operating frequencies of the coils. Therefore, our work proposed a prototype of a flexible quadrature coil made of litz wire and neoprene. The coil can be adapted to different imaging object sizes, while allowing imaging of knee joints at multiple bending angles, increasing the imaging capacity of joint characteristics. Specifically, we first used uniform design (UD) and particle swarm optimization (PSO) to optimize the geometric parameters of the flexible quadrature coil consisting of saddle and butterfly coils. The optimization goal of the algorithm is to maximize coil SNR and magnetic field homogeneity at 50.4 mT. Then, we fabricated the optimal flexible coil, tested its frequency shift due to stretching and bending, and proposed an adaptive matching and tuning circuit to retune the frequency. Meanwhile, we used capacity to decouple the two coils. Finally, we evaluated the performance of the flexible quadrature coil using the CuSO
$_{4}\cdot 5\text{H}_{2}\text{O}$
phantom and in vivo human knee imaging. The SNR in the CuSO
$_{4}\cdot 5\text{H}_{2}\text{O}$
phantom image is about 40. Importantly, our work has achieved the first high-quality imaging of different degrees of knee flexion at VLF MRI.