The present disclosure relates to a medical apparatus including a hadron therapy device directing an imaging hadron beam along a beam path. The beam path crosses a subject of interest including a plurality of tissues having upstream and downstream boundaries and a target tissue having a target spot. The apparatus further includes a magnetic resonance imaging device for acquiring magnetic resonance data within an imaging volume including a portion of the subject of interest including the target spot and at least the portion of the beam path between the upstream boundary and the target spot. The apparatus further includes a hadron radiography system (HRS) acquiring a signal generated by the imaging hadron beam and a controller for determining a water equivalent path length of the beam path between upstream and downstream boundaries based on at least the signal.
