PROJECT SUMMARY Conventional CT imaging is very widely used in modern medicine, with 80 million scans performed per year in the USA. CT has important cardiovascular applications such as calcium scoring and coronary artery angiography. A new form of CT is emerging, which is known as spectral photon counting CT (SPCCT). This type of CT uses photon counting detectors as opposed to the energy integrating detectors used in conventional CT. SPCCT promises improvements over conventional CT such as lower radiation dose, higher spatial resolution and an improved ability to distinguish materials. The current iodine-based contrast agents used in CT imaging have several deficiencies such as very rapid clearance, lack of disease targeting, allergic reactions and potential for kidney damage, and are not ideal for SPCCT in particular since it is difficult to image iodine specifically. There is a need, therefore, to develop new contrast agents designed for SPCCT that can provide more accurate and enhanced diagnoses for cardiovascular disease. We have found that ytterbium is the most potent element for SPCCT contrast generation. We will develop novel, highly biocompatible ytterbium nanoparticle contrast agents for highly precise and low dose cardiovascular imaging with SPCCT.