Prostate ultrasound need to hold urine, the main purpose of holding urine is to be able to measure the amount of residual urine after the bladder is emptied, to see if the bladder outlet obstruction due to prostate disease, caused by the phenomenon of increased residual urine volume. For many older men, the resistance to bladder voiding increases due to narrowing of the posterior urethra caused by prostate hyperplasia. Therefore, there will be a lot of urine left in the bladder that cannot be discharged when the bladder urinates, causing an increase in residual urine. You can tell if the amount of residual urine in the bladder is normal by holding your urine before the ultrasound and looking at the amount of residual urine in the bladder after urination. If the amount of residual urine increases, it is usually a strong indicator of bladder outlet obstruction and prostate enlargement.