Imaging Tests to Diagnose Kidney Stones
Ultrasound This test uses high-frequency sound waves to produce pictures of internal structures (e.g., organs, kidney stones). Ultrasound can detect a dilated (stretched) upper urinary tract and kidney caused by a stone lodged in the ureter, but usually cannot detect small stones, especially those located outside the kidney. It is the preferred imaging method for kidney stone patients who are pregnant.
Intravenous Pyelogram (IVP) This test involves taking a series of x-rays after injecting a contrast agent (dye) into a vein. The contrast agent flows through the veins, is excreted by the kidneys, and improves the x-ray images of the kidneys and ureters. If a kidney stone is blocking a ureter, the contrast agent builds up in the affected kidney and is excreted more slowly. Most kidney stones can be precisely located using this procedure. There is a slight risk for an allergic reaction to the contrast agent during this study and overall kidney function must be normal. IVP can take a very long time if the blockage to the kidney is severe and it requires a radiologist be present.
Retrograde Pyelogram A cystoscopy (i.e., a procedure in which a telescopic instrument is inserted into the bladder) is performed to locate the opening from the ureter to the bladder. The contrast agent is injected directly into this opening and an x-ray is taken to locate the kidney stone.
This procedure eliminates the risk of an allergic reaction to the contrast agent because the dye does not reach the bloodstream, but it may require anesthesia. While a retrograde pyelogram is the most reliable method for visualizing the urinary system and detecting stones, it is generally used only when other imaging methods are inadequate or unsuccessful.
Computerized tomography (CT Scan) This test uses a scanner and a computer to create images of the urinary system. It is performed quickly but may have difficulty detecting small stones located near the bladder. CT scans can also help identify other medical conditions (e.g., ruptured appendix, bowel obstruction, abdominal aortic aneurysm) that cause symptoms similar to kidney stones.
Newer scanners do not require a contrast agent. The non-contrast CT scan is the most common imaging test used to evaluate a possible kidney stone attack. If any stones are found, a plain abdominal x-ray is also taken to determine their size, shape, and orientation. X-rays are used for follow-up studies to monitor the stones' progress.