Final answer:
Anechoic structures in the breast, such as lactiferous ducts and sinuses containing milk, appear as areas with no echoes in an ultrasound image, indicating that they are fluid-filled.
Step-by-step explanation:
In ultrasound imaging, structures in the breast that are filled with fluid, such as the lactiferous ducts and sinuses when they contain milk, will appear anechoic. This term anechoic refers to the lack of echoes received by the ultrasound probe, indicating that these areas are fluid-filled. The ultrasound technology works by broadcasting a directional beam across the breast and measuring echoes at various depths. A computer then creates an image revealing the shape and density of internal structures. In this context, anechoic structures like the milk-filled lactiferous ducts and sinuses are important for the lactation process, during which the breasts produce and supply milk to an infant. The sinuses serve as reservoirs for the milk before it exits through the nipple pores.