Physical and digital phantoms for 2D and 3D x-ray breast imaging: Review on the state-of-the-art and future prospects

Breast phantoms are a fundamental asset both in routine quality assurance programs and for comparing scanner performance in 2D and 3D x-ray breast imaging. They also play an essential role in the optimization of imaging systems and for testing new technologies before their introduction in the clinical practice. The ideal phantom must reflect and mimic the organ anatomy and pathologies, including details such as simulated tumor masses or microcalcifications. They can also be designed to evaluate a particular technical specification of the detector or of the imaging setup, such as noise or spatial resolution. The introduction of digital breast tomosynthesis (DBT) and computed tomography dedicated to the breast (BCT) in the clinical practice allowed to acquire 3D breast images with relatively high contrast and spatial resolution. The introduction of these innovative technologies encouraged the development of new phantoms for quality assurance and systems comparisons; on the other hand, 3D images of the breasts acquired with DBT and BCT apparatuses along with innovative and low-cost additive manufacturing technologies have been the basis for the development of a new class of digital and physical anthropomorphic breast phantoms. In these cases, the realism has been demonstrated to be fundamental in overtaking the limits of conventional phantoms used in digital mammography (DM) quality assurance programs. This work aims at reviewing the conventional phantoms adopted for testing and optimizing DM, DBT and BCT systems and to furnish a critical insight in emerging physical and digital breast phantoms. The limitations of conventional phantoms will be outlined, also regarding the needs of comparing apparatuses which adopt a compressed breast geometry to modern BCT ones, with pendant uncompressed breast geometry.