Remote Detection Coil Setup for Imaging High Velocity Flow Dynamics of Gases with Hyperpolarized 3Helium

Introduction High-frequency oscillatory ventilation (HFOV) is a ventilation technique used in patients with acute respiratory distress syndrome (ARDS). Compared to conventional ventilation, the tidal volume of HFOV is reduced in order to minimize lung injuries. In return respiratory frequency is increased to maintain ventilatory gas transport of O2 and CO2. Using hyperpolarized He or thermally polarized fluorinated gases MRI can contribute to the understanding of the complex gas transport mechanisms in HFOV [1-3]. To optimize this technique a deeper understanding of the gas flow dynamics has to be conceived. One approach is the usage of phase contrast imaging. High flow velocities up to 40 m/sec and the small dimensions of the airways are a big challenge in this context. To achieve high spatial resolution on a trachea phantom a remote detection coil setup using an eight rung birdcage resonator for excitation of rapidly moving spins and two separate receive elements for placement downstream has been designed and tested. Methods Transmit coil: The transmit coil is an eight rung birdcage tuned and matched to 48.51MHz for imaging hyperpolarised He. To compensate asymmetries in the two resonant modes two extra endrings which were connected to the first by a trim capacitor. Receive elements: The receive elements were realized with three turn spiral loops tuned to 48.51MHz. To achieve a sufficient decoupling between the elements the coils were decoupled by active preampdecoupling [4] which is known from phased arrays. Because elements will be separated over several cm this was considered to be sufficient.