Detection and Quantification of Large-Vessel Inflammation with C-11-(R)-PK11195 PET/CT

We investigated whether PET/CT angiography using C-11-(R)-PK11195, a selective ligand for the translocator protein (18 kDa) expressed in activated macrophages, could allow imaging and quantification of arterial wall inflammation in patients with large-vessel vasculitis. Methods: Seven patients with systemic inflammatory disorders (3 symptomatic patients with clinical suspicion of active vasculitis and 4 asymptomatic patients) underwent PET with C-11-(R)-PK11195 and CT angiography to colocalize arterial wall uptake of C-11-(R)-PK11195. Tissue regions of interest were defined in bone marrow, lung parenchyma, wall of the ascending aorta, aortic arch, and descending aorta. Blood-derived and image-derived input functions (IFs) were generated. A reversible 1-tissue compartment with 2 kinetic rate constants and a fractional blood volume term were used to fit the time-activity curves to calculate total volume of distribution (V-T). The correlation between V-T and standardized uptake values was assessed. Results: V-T was significantly higher in symptomatic than in asymptomatic patients using both image-derived total plasma IF (0.55 +/- 0.15 vs. 0.27 +/- 0.12, P = 0.009) and image-derived parent plasma IF (1.40 +/- 0.50 vs. 0.58 +/- 0.25, P = 0.018). A good correlation was observed between V-T and standardized uptake value (R = 0.79; P = 0.03). Conclusion: C-11-(R)-PK11195 imaging allows visualization of macrophage infiltration in inflamed arterial walls. Tracer uptake can be quantified with image-derived IF without the need for metabolite corrections and evaluated semiquantitatively with standardized uptake values.