Radiotherapy (RT) treatment planning (TP) based on probability distribution function (PDF) is an evolving approach for tumor motion management. In PDF-TP, the dose distribution is weighted by the probability of the tumor being in that location during the treatment, thus the determination of reliable tumor motion PDF from time resolved dynamic imaging, named 4D-imaging, is essential. Ideally, a 4D-MRI should be as long as or even longer than the real abdominal RT treatment to represent the real motion pattern and account for any motion irregularity in the treatment, but it is actually impractical. Thus, another unanswered question is how long a 4D-MRI scan is really needed in order to obtain a tumor motion PDF as reliable as possible but keep acquisition as short as possible. In this study, we aim to determine the optimal 4D-MRI duration for PDF-TP by analyzing the time dependency of different abdominal organs’ PDF using an ultrafast volumetric 4D-MRI.