Described herein are methods and systems for analyzing biomedical images using new models. Example models include a linear reference region model and a reference agent model. In one example aspect, a computer-implemented method is provided. The method may involve determining, based on a set of biomedical images, a first concentration-activity curve and a second concentration activity-curve. Additionally, the method may further include determining a value of at least one pharmacokinetic (PK) parameter based on the first concentration-activity curve and the second concentration-activity curve and a linear model that relates the first concentration-activity curve to the second concentration-activity curve. The value of the at least one PK parameter may be determined based on application of a linear least square fitting algorithm to the linear model. Also, the method may include causing a graphical display to provide a visual indication of the value of the at least one PK parameter.