Ultrasound, especially ultrasound, is the most commonly used adjunctive test because it is convenient and the results are more accurate. Some patients have been instructed by their doctors to hold their urine before having an ultrasound examination, what is the reason for this? Holding urine for ultrasound is usually used for pelvic organs, such as the bladder, prostate and uterus. Holding urine means filling the bladder, which is an organ that stores urine and has a lot of elasticity. When the urine volume is low, the bladder wall is thick and the inner wall forms folds, so small lesions such as tumors and folds are mixed together and are not easily distinguished by ultrasound; as the urine volume increases when holding urine, the bladder wall becomes thinner, the folds are flattened and the inner wall becomes smooth, so it is easier to detect small lesions and the test results are more accurate when doing ultrasound. During ultrasound examination of the bladder, the volume of urine stored in the bladder can also be roughly determined by holding urine and measured again after urination to assess whether urination is complete, whether there is urine left behind, and to determine the amount of residual urine. The requirement to hold urine before performing other organ ultrasounds, such as abdominal ultrasound to examine the pelvic organs such as the prostate and uterus, is related to the nature of ultrasound. Ultrasound can easily pass through parenchymal organs with uniform density, such as muscles, liver and kidneys, but organs containing a lot of gas reflect ultrasound, making it difficult to obtain clear images and obscuring the organs in the rear making examination difficult, therefore, organs containing a lot of gas, such as lung and intestinal diseases, are generally not examined by ultrasound. When ultrasound is done in front of an organ containing gas, some methods are needed to facilitate the examination to obtain accurate results. For example, if the bowel is obscured, the position can be changed. The bowel is highly mobile and the gas-containing bowel may leave the examination area after changing the position. When examining pelvic organs, such as the prostate and uterus, a common method is to hold the urine to fill the bladder. The enlarged bladder will squeeze the gas-containing bowel out of the examination area, while the urine-filled bladder will provide a good ultrasound channel to clearly distinguish organs such as the prostate and uterus.