Secondary ultrasound is also called two-dimensional ultrasound, i.e. general ultrasound with cross-sectional imaging, which has a wide range of applications and can be divided into the following areas according to the purpose of examination: 1. fetal examination: it is used to check the size of the fetus, fetal position, placental position, placental maturity, amniotic fluid volume, fetal heart and other examinations, and also includes screening for malformations in the heart, limbs, head, abdominal organs, spine and other organs; 2. gynecological examination: it can check the uterus 2, gynecological examination: the uterus can be viewed for abnormal masses or cysts, the amount of pelvic fluid can be judged, and the size, shape and internal structure of the uterus and ovaries can be observed; 3, examination of parenchymal organs: such as the liver, gallbladder, spleen, pancreas and kidney, lesions can be detected based on the echo of tissue structure; 4, other: 2D ultrasound can also observe the gallbladder, bile ducts, the direction and function of the bladder, or be used to examine the heart, blood vessels and peripheral blood vessels. structure, function and their hemodynamic status, and also fine needle aspiration and contrast examination under ultrasound guidance.