Attenuation Correction for Fixed MR Components in a Simultaneous PET/MR System

Attenuation correction for fixed MR components like MR table and coils in a simultaneous positron emission tomography and magnetic resonance (PET/MR) system is mandatory for accurate quantification of activity distribution. Conventional methods to derive the attenuation maps of MR table and coils include CT scan and transmission scan. Both methods have their advantages and disadvantages. CT scan method is straightforward, however, the images obtained from CT scan have severe metal artifacts due to wires and circuits, and the attenuation coefficients transformation from X-ray energy to Gamma ray energy needs to be remodeled. Transmission scan method avoids the transformation between different energy levels but needs an extra setup of a rotating rod source (usually (68)Gc) and its resolution is poor. In this work, we aim to combine the merits of these two methods. Instead of complex rotating rod source device, a Ge-68 cylinder source was placed at the center of field of view (FOV) as a stationary emission source. By doing so, the full scan reconstruction problem turned to he a limited angle reconstruction problem. Modified maximum likelihood expectation-maximization (MLEM) algorithm incorporated with Split Bregman algorithm was employed to reconstruct the attenuation map directly from the emission data. An additional CT scan provided the structure information of the MR table and coils, which was used as prior knowledge to confine the region of attenuation coefficients. The proposed method was validated by simulations and real data acquired with a simultaneous PET/MR system.