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 patient's body (time t.sub.0); (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 t.sub.0 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).