Systems, methods and devices are provided for fitting kinetic models to measurements of dynamic curves where the kinetic models give rise to nonlinear fitting equations in two or more unknowns that can be formulated so that they are linear in one or more of the unknown parameters and nonlinear in one or more of the unknown parameters. Such systems, methods and devices may be utilized to monitor and characterize the attributes of a given tracer such as a radioactive substance within a body, a drug within the body, a concentration of a substance within a particular medium, and the like.