[¹¹C]acetate and PET/CT assessment of muscle activation in rat studies

Purpose: The purpose of the present study is to apply kinetic analysis to investigate exercise-related changes in the metabolism of the skeletal muscle of the rat hindlimb by [¹¹C]acetate positron emission tomography and computed tomography (PET/CT). Methods: Contractions were induced in Wistar rats’ left hindlimb by electrostimulation of the Vastus Lateralis muscle motor point. After 15 min of muscle contractions, [¹¹C]acetate was injected and PET/CT of both hindlimbs was acquired. The resting hindlimb was used as a control reference. The kinetic parameters _K1 and _k2 were calculated for the target muscles (exercised and control) and correlated with the corresponding standardized uptake values (SUVs). The ratio between each kinetic parameter values and the SUV extracted for the exercised muscle and the muscle at rest was computed (^{K1 Ex}/ ^{K1 Re}, ^{k2 Ex}/ ^{k2 Re} and SUV ^Ex/ ^SUVRe, respectively). Results: Kinetic analysis quantitatively confirmed that net tracer uptake (_K1) and washout (_k2) were significantly higher in exercised muscles (_K1: 0.34 ± 0.12 min ^{- 1} for exercised muscles vs. 0.18 ± 0.09 min ^{- 1} for resting muscles, P= 0.01 ; _k2: 0.22 ± 0.05 min ^-1 for exercised muscle vs. 0.14 ± 0.04 min ^{- 1} for resting muscle, P= 0.002). On the other hand, SUV was not significantly different between active and inactive muscles (0.7 ± 0.2 for exercised muscles vs. 0.6 ± 0.1 for resting muscles). Linear regression analysis revealed a good correlation (R²= 0.75 , P= 0.005) between net tracer uptake ratio (^{K1 Ex}/ ^{K1 Re}) and the SUV ratio (SUV ^Ex/ ^SUVRe). A lower correlation was found between the net tracer washout ratio (^{k2 Ex}/ ^{k2 Re}) and the SUV ratio (R²= 0.37 , P= 0.1). Conclusion: The present study showed that kinetic modelling can detect changes between active and inactive skeletal muscles with a higher sensitivity with respect to the SUV, when performed with [¹¹C]acetate PET/CT.