Kidney stones are solid mineral deposits that form inside the kidneys, potentially affecting any part of the urinary tract. These formations vary in size, from a grain of sand to a golf ball. While some stones may pass naturally, larger ones or those that obstruct urine flow can cause severe pain and require medical attention. Imaging techniques play a role in diagnosis.
X-Ray Visibility of Kidney Stones
The ability to detect a kidney stone on a plain X-ray, such as a Kidney, Ureter, and Bladder (KUB) X-ray, depends on its chemical makeup and density. Approximately 70-80% of kidney stones are visible on abdominal X-rays because they contain calcium, which is radiopaque, meaning it absorbs X-rays well and appears bright on the image.
Calcium oxalate and calcium phosphate stones are the most common types, accounting for over 75% of all kidney stones. These stones are readily visible on plain X-rays due to their calcium content. Struvite stones, also known as magnesium ammonium phosphate stones, can also be seen on X-rays, though their visibility can vary. These stones are commonly linked to urinary tract infections.
In contrast, uric acid stones, which make up about 5-10% of kidney stones, are not visible on plain X-rays because uric acid is radiolucent. These stones form in acidic urine and are associated with conditions like gout or a high-protein diet. Cystine stones, a rarer type resulting from a genetic disorder, are poorly visible or not visible on plain X-rays, even though they are radiopaque compared to soft tissue but less dense than calcium stones. Even for visible stones, factors such as small size (less than 3 mm), location within the urinary tract, or obscuring structures like bowel contents, gas, or bones can lead to them being missed on a plain X-ray.
Other Imaging Techniques for Kidney Stone Detection
Given X-ray limitations, other imaging methods are often employed for kidney stone detection. Computed Tomography (CT) scans, particularly non-contrast CT of the abdomen and pelvis, are considered the preferred method for diagnosing kidney stones. CT scans use X-rays to create detailed cross-sectional images, offering high sensitivity (over 95%) in detecting all types of kidney stones, regardless of their composition, and precisely identifying their size and location. This makes CT scans a common choice in emergency settings for quick and definitive diagnoses.
Ultrasound is another imaging option that uses sound waves rather than radiation to create images of the urinary tract. This makes it a preferred choice for specific populations, such as pregnant women and children, who need to avoid radiation exposure. While effective for detecting stones in the kidney larger than about 2-3 mm and identifying conditions like hydronephrosis (swelling of the kidney due to urine backup), ultrasound may be less sensitive for smaller stones or those located in the ureter.
Intravenous Pyelogram (IVP), also known as Intravenous Urogram (IVU), is an older imaging technique that involves injecting a contrast dye into a vein to visualize the urinary tract on X-ray images. While it can show the anatomy of the collecting system and reveal obstructions, IVP has largely been replaced by CT scans due to their superior detail and diagnostic capabilities. MRI is another option, offering 3D imaging without radiation, but it is more costly and less effective at visualizing stones directly compared to CT.
Next Steps When Kidney Stones Are Suspected
If kidney stone symptoms arise, consulting a healthcare professional is advised. Common symptoms include severe pain in the lower back, side (flank pain), or abdomen, which can sometimes radiate to the groin. Other indications may include blood in the urine (hematuria), nausea, vomiting, a persistent urge to urinate, or pain during urination.
A doctor will begin with a physical examination and may order additional tests. These diagnostic steps include urine tests to check for infections, blood in the urine, or stone-forming crystals, and blood tests to assess kidney function and detect elevated levels of substances like calcium or uric acid.
Following initial assessments, imaging studies will be ordered to confirm the presence, size, and location of any stones. This information is important for determining the most effective course of treatment and preventing future occurrences.