The present invention relates to an apparatus and a method for void size determination of voids within an object into which an aerosol containing magnetic particles has been introduced, in particular for determining the size of a patients pulmonary alveoli, said patient having inhaled an aerosol containing magnetic particles To review information concerning the lung structure, it is proposed to use magnetic particle imaging. First and second detection signals are acquired subsequently at different moments in time after introduction of the aerosol containing the magnetic particles into the object, in particular after inhalation of the aerosol by the patient. These detection signals are exploited, in particular the drop in intensity and/or the signal decay time, to get information about the diffusion of the magnetic particles within the voids, in particular alveoli, and to retrieve information therefrom about the size of the voids, in particular alveoli.
