The authors develop the 4D extended cardiac-torso (XCAT) phantom for multimodality imaging research. The XCAT phantom is designed to provide highly detailed whole-body anatomies for both adult male and female subjects, using nonuniform rational B-spline (NURBS) and subdivision surfaces based on segmentation of Visible Human datasets from the National Library of Medicine. The phantom includes detailed cardiac and respiratory motion models based on high-resolution CT data. The flexibility of the NURBS surfaces allows for the transformation of the Visible Human anatomies to match body measurements and organ volumes for a 50th percentile male and female. The phantom can simulate realistic imaging data in PET, SPECT, and CT using publicly available simulation packages. The results demonstrate that the 4D XCAT can produce realistic imaging data, making it a valuable tool for evaluating and improving imaging devices and techniques. The phantom's ability to model anatomical variations and motion, combined with its detailed anatomical detail, makes it applicable to a wide range of medical imaging applications, including high-resolution imaging modalities such as x-ray CT and MRI. The authors conclude that the 4D XCAT provides an important foundation for optimizing clinical CT applications, balancing image quality with radiation dose.The authors develop the 4D extended cardiac-torso (XCAT) phantom for multimodality imaging research. The XCAT phantom is designed to provide highly detailed whole-body anatomies for both adult male and female subjects, using nonuniform rational B-spline (NURBS) and subdivision surfaces based on segmentation of Visible Human datasets from the National Library of Medicine. The phantom includes detailed cardiac and respiratory motion models based on high-resolution CT data. The flexibility of the NURBS surfaces allows for the transformation of the Visible Human anatomies to match body measurements and organ volumes for a 50th percentile male and female. The phantom can simulate realistic imaging data in PET, SPECT, and CT using publicly available simulation packages. The results demonstrate that the 4D XCAT can produce realistic imaging data, making it a valuable tool for evaluating and improving imaging devices and techniques. The phantom's ability to model anatomical variations and motion, combined with its detailed anatomical detail, makes it applicable to a wide range of medical imaging applications, including high-resolution imaging modalities such as x-ray CT and MRI. The authors conclude that the 4D XCAT provides an important foundation for optimizing clinical CT applications, balancing image quality with radiation dose.