ABSTRACT Changes in the position and orientation of a patient?s anatomical features during radiotherapy, if not properly managed, can lead to underdosage of the target and/or overdosage of neighboring healthy tissues. Deformable image registration (DIR), owing to its ability to geometrically align two images, is becoming increasingly important in radiotherapy for managing these anatomy variations. The accuracy of the DIR directly impacts the success of its clinical applications. Careful assessment of DIR algorithms is therefore a critical necessity before they may be used to inform clinical decision making. Current methods of DIR assessment focus on morphological structures but not on the physiological validity of the entire deformation. Recently, we have demonstrated a novel hyperpolarized 3He tagging MRI technique that is capable of directly, in vivo, and non-invasively measuring physiological lung deformation on a regional basis. This unique imaging technique holds great promise for assessing, validating, and improving the use of DIR algorithms in the lung. Our long term goal is to apply hyperpolarized gas tagging MRI to study lung biomechanics, develop more physiologically sound DIR algorithms for the lungs, and eventually improve radiotherapy of lung cancer. The overall objective of this application is to optimize the hyperpolarized 3He tagging MRI technology and establish its usefulness for DIR assessment. Aim 1 is to develop and optimize a methodology based on 3D hyperpolarized 3He tagging MRI for directly measuring lung deformation. Aim 2 is to develop physiologically sound digital thorax phantoms based on HP 3He tagging MRI and demonstrate their use for DIR assessment in the lung. Successful completion of these aims will yield a novel methodology based on hyperpolarized 3He tagging MRI for DIR assessment in the lung for radiotherapy. It will also yield a number of novel MR imaging techniques and a new series of digital thorax phantoms. The ability to measure true physiological lung deformation makes our technique a promising tool for the assessment, validation, and improvement of lung DIR algorithms. This study will bring important changes to research and the clinic. In the short term, it may lend new insights into the complexities of pulmonary biomechanics, enrich our understanding of DIR, generate gold-standard datasets of lung deformation that may benefit the research community, and provide guidance for clinical implementation of DIR. In the long term, it may lead to development of more sophisticated DIR tools for improving radiotherapy of lung cancer, resulting in more precisely delivered radiation treatment to lung tumors and mitigating radiation- induced injury to surrounding normal tissues.