A simple and accurate method for 13C coil sensitivity estimation

Introduction: Metabolic imaging with Magnetic Resonance (MR) allows a selective identification of molecules and molecular distribution mapping within various organs. However, both weak nuclear polarization levels and low natural abundance reduce the MR experiment sensitivity. Hyperpolarization methods have been proposed to enhance the polarization of nuclear spins such as C and the feasibility of using C hyperpolarization enhanced MRI to monitor pyruvate metabolism in the heart [1]. Efficient imaging of such molecules, however, requires new multifrequency RF coils. Typically, an exam in which hyperpolarized compounds are injected involves at least two coils: one tuned at the H frequency, and one at the C frequency. Alternatively, a single, dual-tuned (H/C) coil can be used. However, when the coil are tuned at lower frequency with respect to H frequency, such as for C experiments, the SNR (Signal-to-Noise Ratio) decreases, thus reducing the data quality. Since the SNR performance increases as the sensitivity of the coils it is important to estimate this parameter for an optimized coil design. Coil sensitivity can be estimated by using magnetic field mapping methods applied at a fixed point in space and many methods have been described in literature, divided into probe and imaging techniques. The purpose of this work is to verify the accuracy of perturbing spheres method, which is able to provide, in a fast and easy way, coil sensitivity estimation. In particular, we describe the application of the method by testing two C quadrature birdcage coils and demonstrating its efficacy for coil sensitivity estimation. Materials and Methods: The coil sensitivity is an important parameter that characterizes the RF coils performance. It is defined as the magnetic field (B1) induced by the RF coil at a given point per unit of supplied power P, as follows: