Three-dimensionally accelerated radial parallel MRI with a 32-channel coil system

Introduction: Established parallel-imaging techniques include the one-dimensional or two-dimensional acceleration of data acquisition with Cartesian or non-Cartesian trajectories [1–4]. However, state-of-the-art receiver coil arrays with 32 and more coil elements that are distributed approximately uniformly in space should also enable a three-dimensional (3D) parallel-imaging acceleration, i.e. simultaneous sparse sampling in all three k-space directions. Unfortunately, a reduced sampling density in the frequency-encoded (readout) direction cannot be efficiently employed for scanning acceleration. Thus, simultaneous parallel-imaging in all three spatial directions requires either acquisition techniques without spatial frequency encoding (e.g., chemical-shift imaging) or techniques with varying frequency-encoding directions. The purpose of this study was to demonstrate 3Daccelerated parallel-imaging with high acceleration factors based on a 3D radial gradient-echo sequence. Methods: A fast gradient-echo pulse sequence with 3D radial trajectories was implemented on a 3-Tesla 32-channel MRI system (Magnetom TimTrio, Siemens Healthcare, Erlangen, Germany) equipped with a 32-channel cardiac array consisting of a flexible anterior part with 16 elements and a posterior part with 16 elements (Rapid Biomedical, Rimpar, Germany). The radial k-space trajectories were distributed approximately uniformly in all spatial directions within a 3D sphere (Fig. 1). The number of radial readouts of the non-accelerated trajectory was chosen such that the Nyquist condition was fulfilled on the surface of the sphere; consequently the center of k-space was substantially oversampled. Accelerated trajectories were obtained by uniformly removing readouts, i.e. by reducing the number of used zenith and azimuth angles.