Study of MR head and neck coils for its use in an integrated MR/PET scanner

Objectives One of the challenges of creating a combined MR/PET scanner is obtaining attenuation maps for PET reconstruction. The presence of MR coils in the field of view has to be accounted for. But, in contrast to PET/CT, there is no information available on the attenuation of these structures.This study describes the development of attenuation models of a pair of head and neck RF coils.

Methods A series of transmission measurements were performed using a Siemens HR+ scanner. These measurements were used to identify potential sources of reconstruction artefacts.After redesigning the coils according to the previous results, a high resolution measurement with an industrial CT scanner was used to create a 3d attenuation model. This was validated by registering it onto the attenuation map of a PET/CT phantom scan and comparing the reconstructions.

Results The transmission scans of the head and neck coils revealed important attenuation in the lock fixing each coil, as well as the signal cables and connectors. However, the worst source of attenuation (25%) proved to be the casing itself, especially at the junction between the coils. Emission scans showed an important artifact at the corresponding axial plane.Using the model, the reconstructed emission of the phantom fitted with the coil was within a 10% of the reference PET/CT reconstruction in the absence of coils. An accurate registration in the axial direction was necessary to avoid reconstruction artefacts.

Conclusions The results show that model-based attenuation correction is possible, given an adequate localization of the coils with respect to the patient.