With regard to breast ultrasound, the focus is on understanding the tissue structure of the breast parenchyma. One side of the breast consists of 15-20 glandular lobes, with one lobe containing 20-40 terminal ductal lobular units. The lobes can be likened to dendrites that branch at 3-5 levels to connect to the terminal ductal lobule units. The terminal ductal lobule units are weakly echogenic, the ducts are slightly echogenic, the walls are thin, and the lumen is usually not visible, the ducts and terminal ductal lobule units are dendritic and weakly echogenic, and the ductal echoes are slender linear and slightly echogenic in the near central region. The interstitial mammary glands are strongly echogenic. There are fatty echogenicity between the gland and the skin and around the gland, and fat may also extend into the gland. In adolescence, there is mainly ductal development with reduced echogenicity in the central region. In pregnancy, the main focus is on alveolar development, with a nutrient-rich interstitium within the lobules and thinning or even loss of interlobular interstitium. Ductal dilatation is seen in late pregnancy and during lactation. Normal breast ducts are potential ductal lumens with a thin line or two thin lines of less than 1 mm ductal diameter seen on ultrasound images. There are no diagnostic criteria for breast duct dilation, but some literature suggests that a central duct greater than or equal to 1.8 mm and a peripheral duct greater than or equal to 1.1 mm can be used as diagnostic criteria. I personally disagree. How to understand the anatomical book breast duct diameter 3-5mm? Just like the male urethra, it dilates during urination and a catheter can be inserted during catheterization, but what is the diameter of the normal urethra? Can the duct diameter be measured during ultrasonography? I have done transrectal ultrasonography for an elderly male patient, the patient suddenly urinated and the urethra immediately dilated, and the echoless urine showed the lumen and wall of the urethra very clearly, and the diameter of the urethra was about 5 mm. this is my personal experience, and I welcome criticism if it is not correct.