Detection and quantification of large-vessel inflammation with 11C-(R)-PK11195 PET/CT

We investigated whether PET/CT angiography using 11C-(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.

Seven patients with systemic inflammatory disorders (3 symptomatic patients with clinical suspicion of active vasculitis and 4 asymptomatic patients) underwent PET with 11C-(R)-PK11195 and CT angiography to colocalize arterial wall uptake of 11C-(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 (VT). The correlation between VT and standardized uptake values was assessed.

VT 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 VT and standardized uptake value (R=0.79; P=0.03).

11C-(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.