Echo-free in ultrasound means that there is no reflection that appears black on the image, while posterior echogenic enhancement effect is visible, no reflection, and seen in fluidic substances. The absence of echo in normal human tissues is commonly seen in liquid substances such as urine in the bladder, bile in the gallbladder, blood in the blood vessels, vitreous humor in the eye, etc. The absence of echo in abnormal conditions is seen in pleural fluid, ascites, cysts, effusions, abscesses in the liquefaction phase, etc. If the cyst is accompanied by infection, bleeding, etc., the anechoic intra-transmission is poor and dotted and flocculent echogenicity is seen, as well as early pregnancy examination, the intrauterine gestational sac is anechoic and no structures such as yolk sac, fetal bud and fetal heart are seen within it, which cannot be reported as early intrauterine pregnancy and can only describe the anechoic cystic structures visible in the uterine cavity, at which time it is recommended to recheck after one week.