18F-FDG PET/MRI of patients with chronic pain alters management.

Objectives: Pain is the most common reason to seek medical attention worldwide. The chronic pain patient, however, is faced with a lack of objective tools to identify the source of pain. In chronic pain, neural tissues as well as any associated inflamed tissues are hypermetabolic and, therefore, glucose avid, showing increased uptake of radiolabeled glucose analogs, such as [18F]FDG (1,2). The goal of this work is to develop a clinical [18F]FDG PET/MRI method to accurately localize sites of increased inflammation related to sources of pain. The aims are to 1) determine whether imaging findings correlate with location of pain, and to 2) determine whether the imaging results affect management decisions.

Methods: Patients suffering from chronic pain have been referred directly from pain physician specialists. Sixty-five chronic pain patients have thus far been imaged with a GE SIGNA PET/MRI system (time-of-flight PET; 3.0T bore; 4-8 min/bed position) from the head through the feet. All patients underwent imaging one hour after a single 10 mCi injection of [18F]FDG. MRI sequences obtained include a coronal DESS, coronal PSIF (isotropic), axial LAVA FLEX (with water/fat separation) and axial T2W FSE with fat-saturation. Two radiologists evaluated PET/MR images (one blinded and the other unblinded to patient exam and history). Maximum standardized uptake values (SUVmax) and target-to-background were measured using image analysis software (OsiriX v.8.0 64-bit). The radiologist unblinded to the patient exam and history determined if increased [18F]FDG uptake occurred in the site of symptoms and in other areas of the body. Imaging results were discussed with the referring physician, who then determined whether a change in the management plan would follow.

Results: Focal increased uptake of [18F]FDG in affected nerves and muscle (approximately 2-4 times more than background tissue) were identified in 58 of 65 patients at the site of pain and other areas of the body (SUVmax of lesions from 0.9 to 4.2 vs. SUVmax of background from 0.2 to 1.2. A modification in the management plan was needed based on imaging findings: 13/65 no change, 16/65 mild modification (e.g. additional diagnostic test) and 36/65 significant modification (e.g. new invasive procedure suggested or ordered). For example, PET/MR imaging helped to direct treatment of a patient with occipital neuralgia, leading to relief from the pain (Figure 1), as well as release of a fibrotic plantaris muscle causing foot pain, and placement of blood patches to treat cerebrospinal fluid leaks.

Conclusions: Our data suggest that [18F]FDG PET/MRI can identify hypermetabolic or inflammatory abnormalities in patients suffering from neuropathic pain. New management plans have been implemented in 40 out of 65 patients (62%), which had not been anticipated by the referring physician. Further following up with a large cohort of patients should be conducted to establish the effectiveness of [18F]FDG PET/MRI to guide interventions for relieving pain.

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