Artifacts and diagnostic pitfalls in positron emission tomography-magnetic resonance imaging.

Introduction Positron emission tomography-magnetic resonance (PETMR) is a new hybrid imaging modality that has recently been introduced in clinical practice for oncologic imaging and is increasingly being used in various clinical indications. PET-MR unifies the complementary capabilities of magnetic resonance imaging (MRI) and PET in a single imaging modality. Excellent anatomical and morphologic information with high soft tissue resolution and contrast from MRI and the best possible molecular, functional, and physiological information fromPET are complementary andhave the potential to provide maximum diagnostic information in a single procedure. However, the incremental diagnostic value of PET-MRover the current standard imaging procedures in various clinical scenarios is still under investigation. Initial results indicate that PET-MR is particularly promising in oncologic diseases and is at least equivalent to PET-computed tomography (PETCT) in lesion detection. Besides the hope for improved clinical and diagnostic performance, there are further expected benefits of PET-MR over current conventional imaging techniques: PET-MR harbors the potential to improve the patients’ workflow and save the patient and the administrative organization the implications of numerous appointments for multiple imaging procedures. As such “one-stop-shop” imaging modality, PET-MRmaybemore time efficient than conventional imaging techniques, such as CT or even PET-CT. Furthermore, there is the potential to decrease the overall radiation exposure to the patient from diagnostic imaging; an issue that gains particular relevance in the world of pediatric patients and young adult patients with a need for repetitive imaging follow-up. Although, both PET and MRI have been well-established individual standalone imaging modalities for decades, the marriage of the 2 components in 1 device came along with significant hardware and software adjustments. Inherent to such adjustments is the risk and likelihood for new imaging effects and artifacts that need to be addressed to guarantee appropriate, reliable, and reproducible diagnostic interpretation. The fact that today’s commercially available models of PET-MR devices operate with significant vendor-dependent technical differences adds complexity to these systems. Differences in the technical approach to PET-MRI increases the necessity to identify, improve, or if not possible at least, describe technology-related imaging artifacts and effects and bring these effects to the attention of the ultimate user of this new technology. The present review first aims to provide an overview over the most common artifacts and pitfalls encountered with PET-MR. It describes their imaging characteristics and discusses the technical background and potential ways of mitigating these issues. It focuses on the potential clinical implications of these artifacts to increase the awareness of these challenges and helps avoid interpretation errors in the clinical use. Secondly, this review addresses the challenges in workflow and in setting up appropriate functional and practical imaging protocols for the use of this complex combined hybrid imaging modality. It will provide suggestions and examples of practical approaches.