Background/Purpose: To validate high-sensitivity PET/CT as a diagnostic test we have explored the association of total-body and extremity PET/CT measures with the standardized psoriatic arthritis (PsA) outcome measures (DAPSA, Leeds Enthesitis Index, etc). The objective of our study is to determine the association of in vivo PET/CT measures, and use these data to create a catalog of systemic PsA musculoskeletal pathologies and compare with that of RA and OA.

Methods: Patients with RA, PsA or OA were enrolled in IRB-approved studies. All subjects underwent a single-timepoint PET/CT scan on either the total body uEXPLORER scanner or the UC Davis extremity PET/CT scanner for 20 minutes starting at either 40 or 70 minutes after IV injection of ¹⁸F-FDG (~1/4^th of the conventional dose). Qualitative findings and different patterns for the three conditions were evaluated. We quantified the degree of inflammation in the extremities and whole-body, respectively and determine pathologic predilection for anatomical domains of bones/ligaments of hands in PsA, RA and OA.

Results: Here we will share data from recruited 20 patients, all males, with RA (n=7; median age: 67 years), PsA (n=10; median age: 68 years), OA (n=3; median age: 60 years). The RA patients had symmetric joint involvement, most commonly in the hands/knees. The following sites were involved: radio-and/or ulno-carpal compartments (n=5), MCP (n=4), PIP (n=5) joints. Joints of the feet appeared to be less frequently affected. All PsA patients demonstrated multiple sites of enthesitis (n=10/10), affected the tendons of the hands/fingers and seemed more active on the extensor side (n=5). Increased nail matrix/fingernail FDG uptake was observed to be a distinct feature (n=8/10). Several large joints showed positive findings in all patients. Less frequent features included sausage finger (n=2), plantar fasciitis (n=3); sacroiliac joint (n=2). Involvement of the interspinous ligament (n=2) and facet joints (n=2) was also noted. Patients with OA showed unilateral enhanced FDG uptake at least one big joint (shoulder, n=3; elbow, n=1; knee, n=2), and small joints of the hand/feet (1^st CMC/1^st MTP).

Conclusion:

1. Asymmetric synovitis of small joints of hands/feet, enthesitis, nail matrix inflammation, dactylitis and spondylitis stands out to be pathologic features for PsA.
2. There appeared to be an overall concurrence between degree of inflammation by imaging and clinical outcome measures of PsA such as DAPSA and Leeds Enthesitis Index.
3. We expect at the completion of this study these clinico-radiologic domains will provide for unique quantitative diagnostic imaging biomarkers for PsA and a single PET imaging scan will be able to differentiate from other inflammatory/non-inflammatory conditions like RA and OA.

Fig 1: Enthesitis on PET/CT in PsA: Extensor digitorum tendon uptake at the central slip, showing enthesitis; (A) Maximum-intensity projection through the PET scan of the right hand of a participant with PsA showing entheseal uptake at the third PIP joint; axial (B) and sagittal (D) sections through the CT scan of the third PIP joint (grayscale); and axial (C) and sagittal (E) section from the overlay of PET (color) on CT (grayscale).

Fig 2: Nail matrix involvement in PsA: (A) Maximum-intensity projection through the PET scan of the left hand of a participant with PsA showing nail involvement; sagittal (B) CT and (C) PET/CT (PET in color, CT in grayscale) of the second digit showing nail bed inflammation. (D) shows nail bed involvement in a participant with PsA from our previous study for comparison.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/high-sensitivity-18f-fdg-pet-ct-a-diagnostic-tool-for-psoriatic-arthritis/