Alginate moulding: an empirical method for magnetic resonance imaging/positron emission tomography co-registration in a tumor rat model.

Background And Purpose: In the experimental field of animal models, co-registration between positron emission tomography (PET) and magnetic resonance imaging (MRI) data still relies on non-automated post-processing using sophisticated algorithms and software developments. We assessed the value of an empirical method using alginate moulding for PET-MR co-registration in a tumor rat model.

Methods: Male WAG/RijHsd rats bearing grafted syngenic rhabdomyosarcoma were examined under general anesthesia by MRI using a clinical whole-body 3-T system equipped with a sensitivity-encoding four-channel wrist coil and by a small animal PET system using labelled [(18)F]-fluorocholine as tracer. An alginate mould including a system of external fiducials was manufactured for each animal, allowing strict immobilization and similar positioning for both modalities. Fourteen rats (27 tumors) had only one MR/PET imaging session. Five rats (9 tumors) had a similar MR/PET session before and 3 days after external radiation therapy (13 Gy in one fraction) using the same mould. Co-registration was performed using the Pmod release 2.75 software (PMOD Technologies, Ltd., Adliswil, Switzerland) with mutual information algorithm.

Results: The manufacture of the alginate moulds was easy and innocuous. Imaging sessions were well tolerated. PET-MR co-registration based on mutual information was perfect at visual examination, which was confirmed by the superimposition of external fiducials on fused images. Reuse of the same mould for the post-therapeutic session was feasible 3 days after the pre-therapeutic one in spite of tumor growth.

Conclusion: The empirical method using alginate moulding with external fiducials for PET-MR co-registration in a rodent tumor model was feasible and accurate.