EXPLORing Arthritis with Total-body Positron Emission Tomography

Total-body positron emission tomography: a tool for systemic, quantitative evaluation of the inflammatory burden of psoriatic arthritis

OBJECTIVES

To test the hypothesis that recently-developed total body-positron emission tomography (TB-PET) imaging with integrated computed tomography (CT) will enable low-dose, quantitative, domain-specific evaluation of the total inflammatory burden of psoriatic arthritis (PsA) and associate with established outcome measures of the clinical domains of PsA.

METHODS

Seventy-one adult participants (40 with PsA, 16 with rheumatoid arthritis (RA), and 15 with osteoarthritis (OA)) underwent 20-min TB-PET/CT scans using [18F]FDG, a glucose analogue radiotracer. [18F]FDG uptake was assessed qualitatively and quantitatively. Rheumatological examinations were performed prior to the scan. For both evaluations, domain-specific assessments included 68 joints, 6 entheses, 20 nails, axial disease and dactylitis.

RESULTS

[18F]FDG PET uptake consistent with joint involvement and enthesitis was noted in 100% of participants with PsA. Other features included nail matrix pathology (53%), spinal involvement (60%), active sacroiliitis (13%) and dactylitis (10%). Patterns of [18F]FDG uptake in PsA differed from those in participants with RA or OA. There was a high concordance between TB-PET measures and the domain-specific assessments of the joint (75%), entheseal (79%) and nail (65%) pathology. TB-PET was positive for an additional 15% of joints, 20% of entheses and 13% of nails that were negative on clinical assessments.

CONCLUSION

TB-PET/CT identified inflammatory pathologies characteristic to all clinical domains of PsA and thus provided an in vivo evaluation of systemic PsA inflammatory burden. This promising tool may further contribute to identifying pathologies that may be occult, provide biomarkers to diagnose and differentiate PsA at an early stage, and to monitor early treatment response.

Total Body PET/CT: A Role in Drug Development?

Nowadays, total body PET has already entered the medical centers and enabled various clinical applications due to its superior imaging capabilities, especially the high sensitivity. However, the potential of the total body PET in the clinical evaluation of radiopharmaceuticals remains underexplored. The development and regulatory processes for radiopharmaceuticals present unique challenges that total body PET could address. In the safety evaluation of radiopharmaceuticals, the internal radiation dosimetry demands images with high quality and quantitative accuracy, which can be achieved using the total body PET. The current clinical pharmacokinetic study for radiopharmaceuticals still relies on invasively sampling of blood and other body fluid, causing discomfort of participant and difficulty in implementation. With the total body PET, the radioactive concentration of the drug in various blood vessels can be assessed noninvasively, facilitating the pharmacokinetic study. The parametric analysis over the total body based on compartment models also sheds light on the pharmacokinetics of the radiopharmaceutical. A special requirement for multi-center clinical research involving PET and SPECT is the harmonization of the quantitative performance among different imaging equipment, and the discrepancy between the total body PET and short axial field of view PET scanners may add to the complexity. To date, there are several successful examples of clinical trials of innovative radiopharmaceuticals using the total body PET, involving different types of tracers ranging from small molecules, peptides, nanobodies, minibodies, and aptamers. In conclusion, total body PET has the potential to revolutionize the clinical evaluation of radiopharmaceuticals and will play a crucial role in future drug development.

Role of Total Body PET/CT in Inflammatory Disorders

Inflammatory disorders historically have been difficult to monitor with conventional PET imaging due to limitations including radiation exposure, lack of validated imaging biomarkers, low spatial resolution, and long acquisition durations. However, the recent development of long-axial field-of-view (LAFOV) PET/CT scanners may allow utilization of novel noninvasive biomarkers to diagnose, predict outcomes, and monitor therapeutic response of inflammatory conditions. LAFOV PET scanners can image most of the human body (if not the entire body) simultaneously in one bed position, with improved signal collection efficiency compared to conventional PET scanners. This allows for imaging with shorter acquisition durations, decreased injected radiotracer dose, prolonged uptake times, or a combination of any of these. In addition, LAFOV PET scanners enable whole-body dynamic imaging. Altogether, these intrinsically superior capabilities in assessing both local and systemic diseases, have allowed these scanners to make increasingly significant contributions to the assessment of inflammatory conditions. This review aims to further explore the role and benefits of LAFOV scanners for imaging various inflammatory conditions while addressing future developments and challenges faced by this technology.

Focusing on ligamentous soft tissue inflammation for the future understanding of early axial psoriatic arthritis

Imaging has transformed the understanding of inflammatory and degenerative arthritis in both peripheral and axial disease. In axial inflammation, fat suppression magnetic resonance imaging (MRI) has unravelled the role of sub-fibrocartilaginous osteitis in axial spondyloarthritis and the role of peri-entheseal vertebral body osteitis and subsequent spinal new bone formation. Established or late-stage axial psoriatic arthritis (PsA) cases often exhibit impressive para-marginal or chunky syndesmophytosis on conventional X-ray that pathologically represents entheseal soft tissue ossification. However, the spinal entheseal soft tissue and contiguous ligamentous tissues are poorly visualized on MRI in subjects with early inflammatory back pain including those with axial PsA. In this article, we highlight the need for imaging modalities to discern the crucial soft tissue "ligamentous" component of axial PsA towards diagnosis, prognosis and therapy validation. We issue a clarion call to focus advanced imaging methodology on spinal ligamentous soft tissue that represents the last hidden backwater of PsA immunopathology that needs visualization to fully decipher axial PsA pathogenesis. This in combination with the existing ability to visualize ligamentous bony anchorage site osteitis is needed to define a gold standard test for axial PsA.

Multimodal positron emission tomography (PET) imaging in non-oncologic musculoskeletal radiology

Musculoskeletal (MSK) disorders are associated with large impacts on patient's pain and quality of life. Conventional morphological imaging of tissue structure is limited in its ability to detect pain generators, early MSK disease, and rapidly assess treatment efficacy. Positron emission tomography (PET), which offers unique capabilities to evaluate molecular and metabolic processes, can provide novel information about early pathophysiologic changes that occur before structural or even microstructural changes can be detected. This sensitivity not only makes it a powerful tool for detection and characterization of disease, but also a tool able to rapidly assess the efficacy of therapies. These benefits have garnered more attention to PET imaging of MSK disorders in recent years. In this narrative review, we discuss several applications of multimodal PET imaging in non-oncologic MSK diseases including arthritis, osteoporosis, and sources of pain and inflammation. We also describe technical considerations and recent advancements in technology and radiotracers as well as areas of emerging interest for future applications of multimodal PET imaging of MSK conditions. Overall, we present evidence that the incorporation of PET through multimodal imaging offers an exciting addition to the field of MSK radiology and will likely prove valuable in the transition to an era of precision medicine.