I'm often asked why I routinely perform my CT KUB with the patient prone when the clinical query is renal stones.
Both images below are of the same patient, the one on the left is supine, the right prone. When the patient is supine, gravity causes any loose stones in the urinary bladder to rest posteriorly, which happens to be the exact location as the ureterovesical junction (UVJ). In the supine position, there is no way for the radiologist to tell if the stone is moving freely in the bladder or if it is stuck in the UVJ. Turning the patient prone will move any loose stones anteriorly, while any stones that are stuck in the VUJ will remain there.
This is crucial information because an obstructed UVJ can cause hydronephrosis and may need to be removed urgently.
This patient does not have any stones in the UVJ, however after scanning prone we can clearly differentiate urinary bladder stones (red arrow) from prostate calcification (green arrow)
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