An online Blood Oxygenation Level-Dependent (BOLD)-magnetic resonance imaging (MRI) method for functional clinical guidance or monitoring a therapeutic modality involving treatment by a sensitizer which, upon excitation by the appropriate sensitizing radiation, initiates local oxygen consumption, comprising: (i) generating a BOLD-weighted MR-image of the target region of interest within the patients body (time to) (ii) administering said sensitizer to the patient (iii) irradiating the target region while the patient is subjected to continuous MR imaging (iv) generating a sole or a plurality of T2* weighted sequential BOLD MR-images during and/or after irradiation (time t) (v) processing the data generated at time to and time t and generating a color-coded difference or ratio map on a pixel by pixel basis and (vi) analyzing the processed data. The method is preferably applied to photodynamic therapy (PDT).
