Theoretical basis of in vivo tomographic tracer kinetics

In vivo tracer kinetics, as probed by current tomographic techniques, is revisited from the point of view of fluid kinematics. Proofs of the standard intravascular advective perfusion model from first premises reveal underlying assumptions and demonstrate that all single input models apply at best to undefined tube-like systems, not to the ones defined by tomography, i.e. the voxels. In particular, they do not and cannot account for the circulation across them. More generally, it is simply not possible to define a single non-zero steady volumetric flow rate per voxel. Restarting from the fact that kinematics requires the definition of six volumetric flow rates per voxel, one for each face, minimalist, 4D spatiotemporal analytic models of the advective transport of intravascular tracers in the whole organ of interest are obtained. Their many parameters, plasmatic volumetric flow rates and volumes, can be readily estimated at least in some specific cases. Estimates should be quasi-absolute in homogeneous tissue regions, regardless of the tomographic technique. Potential applications such as dynamic angio-tractography are presented. By contrast, the transport of mixed intra/extravascular tracers cannot be described by conservation of the mass alone and requires further investigation. Should this theory eventually supersede the current one(s), it shall have a deep impact on our understanding of the circulatory system, hemodynamics, perfusion, permeation and metabolic processes and on the clinical applications of tracer tracking tomography to numerous pathologies. PACS numbers: 47.63.Jd, 47.80.Jk, 87.57.-s