Current Recycled and Self-Phase Shift to Expand the Length of Radio-Frequency Coils, With Application to Brain and Spine Coil Array at 7T MRI

The use of the 7-Tesla (T) magnetic resonance imaging (MRI) promises improved imaging quality and higher resolution compared with lower-field MRI systems. The design of the loop coil considers the tradeoff between coil size and performance. A larger coil enables deeper field penetration, but it may result in poorer field uniformity and localization. On the other hand, a smaller coil offers improved localization capabilities, however, field penetration reduces, and multiple coil elements are required to cover the same space as large coils. Additionally, safety concerns regarding the high-energy absorption of electromagnetic waves in healthy tissues principally limit the use of the 7T MRI, which is measured with the specific absorption rate (SAR). A coil that can generate a uniform magnetic field while maintaining a low SAR is necessary to comply with the SAR limits. We propose a coil design that recirculates the current and provides a phase shift in the same structure to provide a magnetic field over a broad area, thus reducing the number of channels required to cover the same area. We present electromagnetic (EM) simulations of the proposed coil with a magnetic field and SAR computed for the brain and human spine model. We built the coil and acquired images with a phantom using a 7T MRI system. The proposed coil improves the SAR by 43% compared with the reference coil in the spinal area in the case of EM simulations, indicating the imaging quality improvement potential of our proposed coil.