Secondary ultrasound is also called two-dimensional ultrasound, i.e., general ultrasound with cut surface imaging, which has a wide range of applications and can be classified into the following aspects according to the different purposes of the examination: 1. Fetal examination: it is used to check the size of the fetus, fetal position, location of the placenta, placental maturity, amount of amniotic fluid, fetal heart and other examinations, and also includes the heart, limbs, head, abdominal organs, spine, and other organs for the screening of malformations; 2. Gynecological examination: it can view the uterus Gynecological examination: it can check whether there are any abnormal masses or cysts in the adnexa, determine the amount of pelvic fluid, and also be used to observe the size, shape and internal structure of the uterus and ovaries; 3. Parenchymal organ examination: such as the liver, gallbladder, spleen, pancreas and kidneys, and it can be used to discover lesions according to the echoes of the tissues and structures; 4. Other: 2D color ultrasound can also be used to observe the gallbladder, the bile ducts, the direction of the bladder and its function, etc., or used to check the structure, function, and hemodynamic status of the heart, blood vessels, and peripheral vascular system. structure, function and its hemodynamic status, and also can be used for fine needle puncture and imaging under ultrasound guidance.