Highly Localized Positive Contrast of Small Paramagnetic Objects Using 3D Center-Out RAdial Sampling with Off-Resonance Reception (RASOR)

Peter Roland Seevinck¹, Hendrik De Leeuw¹, Clemens Bos², Chris JG Bakker¹

We present a 3D imaging technique, applying RAdial Sampling with Off-resonance Reception (RASOR), to accurately depict and localize small paramagnetic objects with high positive contrast. The RASOR imaging technique is a fully frequency encoded 3D ultrashort TE (UTE) center-out acquisition method, which utilizes a large excitation bandwidth and off-resonance reception. By manually introducing an offset, f0, to the central reception frequency (f0), the magnetic field disturbance causing the typical radial signal pile in 3D center-out sampling can be compensated for, resulting in a hyperintense signal at the exact location of the small paramagnetic object. This was demonstrated by 1D simulations and experiments of gel phantoms containing three paramagnetic objects with very different geometry, viz., subvoxel stainless steel spheres, paramagnetic brachytherapy seeds and a puncture needle. In all cases, RASOR is shown to generate high positive contrast exactly at the location of the paramagnetic object, as confirmed by X-ray computed tomography (CT).