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Purpose: The aim of this study was to explore the association of cardiac fibroblast activation with clinical parameters and cardiovascular magnetic resonance (CMR) imaging parameters in patients with chronic thromboembolic pulmonary hypertension (CTEPH)., Methods: Thirteen CTEPH patients were prospectively enrolled. All of the patients underwent cardiac 68 Gallium-labelled fibroblast activation protein inhibitor ( 68  Ga-FAPI-04)-positron emission tomography/computed tomography (PET/CT), right heart catheterisation, and echocardiography, and 11 of them additionally underwent CMR. Thirteen control subjects were selected to establish the normal range of cardiac 68  Ga-FAPI-04 uptake. Cardiac 68  Ga-FAPI-04 uptake higher than that in the blood pool was defined as abnormal. The global and segmental maximum standardised uptake values (SUV max ) of the right ventricle (RV) were measured and further expressed as target-to-background ratio (TBR RV ) with left ventricular lateral wall activity as background. Late gadolinium enhancement (LGE) was visually evaluated, and native-T1 times, enhanced-T1 times, and extracellular volume (ECV) were quantitatively measured., Results: Ten CTEPH patients (77%) had abnormal 68  Ga-FAPI-04 uptake in RV, mainly located in the free wall, which was significantly higher than that in controls (TBR RV : 2.4 ± 0.9 vs 1.0 ± 0.1, P < 0.001). The TBR RV correlated positively with the thickness of RV wall (r = 0.815, P = 0.001) and inversely with RV fraction area change (RVFAC) (r =  - 0.804, P = 0.001) and tricuspid annular plane systolic excursion (TAPSE) (r =  - 0.678, P = 0.011). No correlation was found between 68  Ga-FAPI-04 activity and CMR imaging parameters., Conclusion: Fibroblast activation in CTEPH, measured by 68  Ga-FAPI-04 imaging, is mainly localised in the RV free wall. Enhanced fibroblast activation reflects the thickening of the RV wall and decreased RV contractile function., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)