Embodiments provide a modular phantom that enables quantitative assessment of imaging performance (e.g., spatial resolution, image uniformity, image noise, contrast to noise ratio, cone-beam artifact) and dosimetry in cone-beam computed tomography (CT). The modular phantom includes one or more modules for various imaging performance tests that may be rearranged in the phantom to accommodate the design of various cone-beam CT imaging systems. The modular phantom includes one or more of a cone-beam module, an angled edge module, or a line spread module. The phantom may be inserted into a larger sleeve and be used to assess imaging performance and dosimetry in whole body CT imaging systems.