Why does color ultrasound hold urine?

Not all color Doppler ultrasound examinations need to hold urine, such as cardiac color Doppler ultrasound, liver color Doppler ultrasound and so on do not need to hold urine, while bladder color Doppler ultrasound, gynecological color Doppler ultrasound and so on need to hold urine examination, holding urine will make bladder become full, can clearly observe the situation in bladder and uterus, conducive to the diagnosis of diseases.