The invention provides an active collimator system (100) having a coded collimator aperture (111), the active collimator system (100) comprising scintillator bars (110) and photosensor units (130), wherein the scintillator bars (110) are configured to absorb incoming gamma rays (160) and scintillate, thereby emitting scintillation photons (161), wherein the scintillator bars (110) are further configured to define the coded collimator aperture (111), wherein the photosensor units (130) are configured to detect the scintillation photons (161), and wherein the photosensor units (130) are directed to scintillator bar ends (112) of the scintillator bars (110).