From light to sound: Seeing and hearing the placenta in health and disease

The placenta is a pregnancy-specific organ, functioning as the maternal-fetal interface and mediating essential processes including exchange, protection, and endocrine regulation. Placental abnormalities contribute to the pathophysiology, onset, progression, and prognosis of major perinatal disorders, making imaging modalities that enable their detection and monitoring crucial. Ultrasound (US) imaging is the principal modality for placental imaging, providing structural assessment with B-mode and blood flow evaluation with Doppler mode. Furthermore, advanced techniques such as 3D power Doppler, quantitative US, and shear-wave elastography expand the capabilities of placental US imaging. Photoacoustic (PA) imaging enables observation of the optical properties of endogenous chromophores (e.g., hemoglobin) with high spatial resolution and measures blood oxygenation, a key factor in placental dysfunction, offering substantial value for placental imaging. In this review, we cover US/PA imaging techniques for placental imaging, including preclinical and clinical studies. In addition, by considering current limitations and potential solutions, we suggest future trajectories for the advancement of US/PA imaging in this field.