A method for determining the distribution of an imaging agent in a volume comprising the steps of: acquiring at least one three-dimensional functional image of the volume having an imaging agent distributed therein; segmenting the volume into one or more compartments; assigning at least one three-dimensional imaging agent distribution basis function to one or more compartments within the volume; calculating a projected imaging agent activity from the at least one assigned three dimensional imaging agent distribution basis function for at least one compartment on at least one planar surface; acquiring at least one planar image of the imaging agent activity in the volume; registering the projected imaging agent activity from at least one compartment with at least a portion of the at least one planar image of the imaging agent activity in the volume; comparing the intensity of at least a portion of the acquired planar image of the imaging agent activity in the volume with at least a portion of the calculated projected imaging agent activity on substantially the same planar surface; characterised in that: the three dimensional imaging agent distribution basis function comprises a representation of the three-dimensional imaging agent activity by the product of a scaling factor and a non-affine transformation of a template imaging agent activity wherein the template imaging agent activity is determined from the three-dimensional functional image of the volume; and further characterised in that following the step of comparing the intensity, at least one of the following is modified in order to obtain a better fit between the projected imaging agent activity and the acquired planar image of the imaging agent activity: i) at least one scaling factor and ii) at least one non-affine transformation of the template imaging agent activity. acquiring at least one three-dimensional functional image of the volume having an imaging agent distributed therein; segmenting the volume into one or more compartments; assigning at least one three-dimensional imaging agent distribution basis function to one or more compartments within the volume; calculating a projected imaging agent activity from the at least one assigned three dimensional imaging agent distribution basis function for at least one compartment on at least one planar surface; acquiring at least one planar image of the imaging agent activity in the volume; registering the projected imaging agent activity from at least one compartment with at least a portion of the at least one planar image of the imaging agent activity in the volume; comparing the intensity of at least a portion of the acquired planar image of the imaging agent activity in the volume with at least a portion of the calculated projected imaging agent activity on substantially the same planar surface; characterised in that: the three dimensional imaging agent distribution basis function comprises a representation of the three-dimensional imaging agent activity by the product of a scaling factor and a non-affine transformation of a template imaging agent activity wherein the template imaging agent activity is determined from the three-dimensional functional image of the volume; and further characterised in that following the step of comparing the intensity, at least one of the following is modified in order to obtain a better fit between the projected imaging agent activity and the acquired planar image of the imaging agent activity: i) at least one scaling factor and ii) at least one non-affine transformation of the template imaging agent activity. the three dimensional imaging agent distribution basis function comprises a representation of the three-dimensional imaging agent activity by the product of a scaling factor and a non-affine transformation of a template imaging agent activity wherein the template imaging agent activity is determined from the three-dimensional functional image of the volume; and further characterised in that following the step of comparing the intensity, at least one of the following is modified in order to obtain a better fit between the projected imaging agent activity and the acquired planar image of the imaging agent activity: i) at least one scaling factor and ii) at least one non-affine transformation of the template imaging agent activity. i) at least one scaling factor and ii) at least one non-affine transformation of the template imaging agent activity.