High positive contrast generation of a subvoxel susceptibility deviation in ultrashort TE (UTE) radial center-out imaging at 3T

IntroductionA number of positive contrast techniques for the detection of (super-) paramagnetic particles/agents, such as (U)SPIO or iron-oxide labeled cells have been developed recently. Most of these positive contrast (PC) techniques exploit the frequency shift that is induced by the magnetic susceptibility difference of the contrast agents. Positive contrast techniques may improve detectability (sensitivity) and/or specificity over conventional negative contrast techniques. In this work the artifact of a spherical subvoxel susceptibility deviation in 3D ultrashort TE (UTE) radial center-out imaging is investigated in the context of high positive contrast generation. Contrast to noise ratios (CNR) for UTE images are compared to CNR of conventional 3D images acquired with Cartesian and radial sampling schemes. Simulations are performed to investigate the influence of varying the size (volume) and strength (Δχ,) of the susceptibility inclusion and the value of the read gradient on contrast generation in UTE. Materials & MethodsTheory: The dipolar magnetic field perturbation of a sphere with susceptibility Δχ and radius a can be described by Eq. (1), in which B0 is the field strength and r and θ designate spherical coordinates with respect to the sphere’s center and B0.