The application discloses a catheter apparatus comprising a diagnostic assembly having a longitudinal axis, the diagnostic assembly including an optical emitting element (1028) and an optically receiving element (1027) arranged to be spatially offset along the longitudinal axis. The catheter apparatus additionally comprises a processing element to process optical radiation using diffuse reflectance spectroscopy. The application further relates to a tissue monitoring method and an ablation lesion monitoring, measuring, and controlling algorithm including the use of diffuse reflectance spectroscopy (DRS) and/or Arrhenius model thermal denaturation kinetics for determining the characteristics of the lesion of the tissue, especially for identifying the transmurality of the lesion.
