Ultrasound with stones and CT without stones often suggests that the stones are pure uric acid, which is what we call negative stones. Negative stones in the X-ray examination, the ray can completely through the stone without visualization. CT formally uses X-rays to assist computerized imaging, so it will not be able to detect pure uric acid stones, and will show that no stones exist in the urinary system. And ultrasound is the use of ultrasound to detect stones, when in the presence of stones, the surface of the stones return ultrasound, so as to detect the presence of stones. It is not affected by pure uric acid stones, so it is possible to detect them by doing an ultrasound. There is another situation is that there is a time difference between ultrasound and CT, ultrasound found stones but stones are very small, drink more water, more exercise after the stones have been discharged from the body. At this time to do CT examination, in this time gap within the stone discharge, CT will not find a stone, which results in ultrasound found stones and CT did not find a stone situation.