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Background
18-FDG PET is a nuclear medicine study that utilizes a radiotracer-tagged glucose analog to assess metabolic activity. This is commonly ordered for a variety of pathologies, including oncologic staging, treatment response, and infection of unknown origin. However, there are many incidental PET findings with potential patient consequences, most notably within the nervous system. Although PET is not a primary examination for most nervous system pathologies, many are identifiable and critical for radiologists to appropriately diagnose. For example, up to 1% of PET/CTs will show new brain metastases without associated neurologic symptoms on standard screening which includes the entire brain. Given these studies are often read by radiologists, directed and continued education is essential to recognize the findings that necessitate patient follow-up and intervention.

Educational Goals / Teaching Points
To help educate radiologists on this topic, we will review the normal biodistribution of F18-FDG-PET and the related inherent pitfalls. We then will utilize a case-based approach to demonstrate the imaging appearance of important abnormalities on PET within the nervous system for both common and atypical pathologies. This will lead to the discussion of pertinent differential diagnoses and the next-step recommendations for radiologists to consider and convey to clinicians. Finally, we will provide appropriate CT and MRI correlation for these cases to confirm the diagnosis and compare to the prior PET findings.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Although FDG-PET has intrinsically low spatial resolution, one of the major strengths is that it is high in contrast resolution; however, the contrast and conveyed sensitivity are lost in tissues with high physiologic uptake of tracer, specifically within the brain and spinal cord. Therefore, it is critical to differentiate normal from abnormal to prevent misdiagnosis or false negative examinations. An example case is a 66 year-old man with history of prostate cancer and mantle cell lymphoma (both in remission) who presented with progressive left lower extremity weakness. Patient has no prior neurologic history, and a PET/CT was ordered to evaluate for potential disease progression. Imaging demonstrated a markedly FDG-avid mass within the left cerebellum, concerning for neoplasm. MRI demonstrated homogenous enhancement, restricted diffusion, and no evidence of necrosis, most concerning for CNS lymphoma. Lymphoma is a common CNS neoplasm (approximately 3 - 5% of primary brain neoplasms) which has a unique appearance on FDG-PET as a lesion that has increased FDG-avidity as compared to normal brain parenchyma.

Conclusion
Although MRI remains the predominant modality for the evaluation of the nervous system, numerous pathologies can be incidentally identified on routine 18-FDG PET/CT examinations. Though these may be nonspecific in nature, they can lead to critical changes in treatment and management. Given the ubiquitous nature of PET/CT in radiology, it is imperative for radiologists to understand and confidently identify pathology to guide further imaging and work-up appropriately.