X-ray of the kidneys is called a KUB because it includes the kidneys, ureters and bladder. We don’t actually see the kidneys, ureters and bladder most of the time on these X-rays. Sometimes we can faintly see the outline of the kidneys and bladder.
What are kidney X-rays best for?
X-ray of the kidneys, ureters and bladder is best for detecting stones. Stones can be found in the kidneys, along the ureters and in the bladder. Stones appear as white rounded areas in these locations. We can’t actually see the kidneys, ureters and bladder because they blend in with the other soft tissues next to them.
We know the kidneys are located in the upper abdomen on both sides. With kidney stones, we see white areas in their expected locations. These stones are usually denser then the soft tissues because they are harder and calcified. They can be one and tiny, or multiple and large.
X-rays of the kidneys are best for stones
X-rays of the kidneys can not tell us if there are other abnormalities like masses or blockages. CT is best for that. X-rays also do not show us stones in every case. Some stones are not seen on X-rays. X-rays are good to see if stones are worsening or improving over time.
X-rays for passing stones
We can sometimes see if a stone is being passed from the kidney into the ureter. The ureters are thin tubes that go from the kidneys to the bladder. They carry urine made in the kidney to the bladder. Stones in the kidneys can pass into the ureters and cause a blockage of the kidney. Patients are often in severe pain and present to their doctors or to the emergency room.
Stones that pass into the ureters from the kidneys will sometimes be seen along their expected course next to the spine. We don’t actually see the ureters. If we see a white dense spot along their expected course, then we raise the possibility of a stone passing. White spots along their course can also be seen with other calcifications like vascular ones. The finding can always be investigated with CT which is more accurate.
We can not tell if the kidney is blocked by the passing stone since we can not see the kidney in X-ray. We also can not tell if there are complications like infection or a collection of urine from rupture of the kidney urine collecting system. X-rays are a good quick start in evaluating a suspected stone passing.
X-ray of the kidney is called a KUB. X-rays of the kidneys are best for stones. They are not good for other abnormalities of the kidneys. CT is best for that. We can also see a stone that is passing from the kidney into the ureters. X-ray is a quick, relatively inexpensive start to evaluating or monitoring kidney stones. KUB X-rays can also sometimes tell us if there are any abnormalities of bowel or other findings which may be causing symptoms.