Radiotherapy is a highly effective cancer treatment. The most common type of radiotherapy, external beam radiotherapy (EBRT), delivers radiation to cancer from outside the patient. The key of EBRT is to aim the prescribed dose at cancer while sparing the surrounding normal tissues and organs at risk (OARs) as much as possible. This remains a challenge at the abdominal site that has breathing motion and many OARs. On-board imaging can help determine the target margins and the appropriate respiratory phase for dose delivery. Magnetic resonance (MR)-guided radiotherapy (MRgRT) is an emerging technology that builds on a system combining MR with a therapy linear accelerator, named MR-Linac. MR is advantageous over the conventionally used cone-beam computed tomography (CT) in abdominal radiotherapy. However, MRgRT is limited to real-time 2D imaging on commercial MR-Linac systems, and several 3D imaging approaches by research groups still provide suboptimal image quality. The overarching goal of this project is to develop a next-generation MR imaging strategy at a low field strength, so the potential of MR-Linac as a state-of-the-art radiotherapy platform can be fully unleashed, particularly for abdominal cancers. To achieve this goal, we will develop the technology at 0.55 Tesla through Aim 1 and conduct a preliminary validation in human subjects through Aim 2. Successful completion of this project will deliver a technology that provides high-spatiotemporalresolution real-time 3D images with user-selectable tissue contrast weightings and address both pre-beam adaptive planning and online treatment guidance.