Final answer:
The imaging modality of choice for placental previa is ultrasound (US) because it is safe, non-invasive, and does not use ionizing radiation.
Step-by-step explanation:
The imaging modality of choice for placental previa is ultrasound (US). Ultrasound is preferred because it is the least invasive imaging technique and uses no ionizing radiation, which makes it safest for monitoring pregnancies. While magnetic resonance imaging (MRI) could also be used, as it provides detailed images without the use of radiation, ultrasound is generally more readily available, faster, and less expensive compared to MRI, making it the first-choice imaging study during pregnancy. Ultrasound works by using high-frequency sound waves to create images of the placenta and the surrounding structures. It can accurately diagnose placental previa and provide valuable information for managing the condition.