MR Radar: Parallel Radiofrequency Transmission in Principle and Practice

Magnetic resonance imaging (MRI) has been driven towards high magnetic fields in order to benefit from correspondingly high signal-to-noise ratio and spectral resolution. However, technological challenges associated with high magnetic field strength, such as increase in radiofrequency (RF) energy deposition and RF excitation inhomogeneity, limit realization of the full potential of these benefits. Parallel RF transmission enables decreases in RF energy deposition and in the inhomogeneity of RF excitations by using multiple-transmit RF coils driven independently and operating simultaneously. In this work, the behavior of RF excitation and RF energy deposition is explored from an MRI system perspective. New parallel RF excitation techniques are introduced to measure subject-specific electric field interactions between transmit elements. These new techniques are demonstrated in phantom and in vivo studies, and are shown to enable decreases in RF energy deposition while maintaining RF excitation fidelity. Since the capacity of MRI systems for RF power delivery and handling are subject to both technological and regulatory limits, a method was developed to predict the RF power consequence of transmission on each individual channel during parallel RF transmission, and this method was used to design parallel transmission RF pulses obeying strict technical and safety limits. Additionally, MRI system-subject interactions during parallel RF transmission were studied as a function of the distance between the subject and the transmit RF coils. Lastly, inner-volume RF excitations were demonstrated as one of the promising potential applications of parallel RF transmission. In summary, this work represents a step forward in overcoming technical challenges to demonstrate potential applications of high field MRI with parallel RF transmission.