Dynamic or 4D images (in which a section of the body is repeatedly imaged in order to capture physiological motion) are becoming increasingly important in medicine. These images are especially critical to the field of image-guided therapy, because they enable treatment planning that reflects the realistic motion of the therapy target. Although it is possible to acquire static images and deform them based on generalized assumptions of normal motion, such an approach does not account for variability in the individual patient. To enable the most effective treatments, it is necessary to be able to image each patient and characterize their unique respiratory motion, but software specifically designed around the needs of 4D imaging is not widely available. We have constructed an open source application that allows a user to manipulate and analyze 4D image data. This interface can load DICOM images into memory, reorder/rebin them if necessary, and then apply deformable registration methods to derive the respiratory motion. The interface allows output and display of the deformation field, display of images with the deformation field as an overlay, and tables and graphs of motion versus time. The registration is based on the open source Insight Toolkit (ITK) and the interface is constructed using the open source GUI tool FLTK, which will make it easy to distribute and extend this software in the future.