Kidney stones are a common and painful condition, forming when minerals and salts build up in the urine. While computed tomography (CT) scans are used for diagnosing these stones, people wonder if this imaging can miss them. CT scans detect most kidney stones, but no diagnostic method offers 100% certainty.
Understanding CT Scan Accuracy
CT scans, particularly low-dose non-contrast helical CT, also known as CT KUB, are the most accurate imaging method for identifying kidney stones. This technique provides detailed cross-sectional images, allowing radiologists to visualize stones as small as 1 millimeter with high sensitivity and specificity. The scan’s ability to penetrate soft tissues and differentiate stone density makes it effective for most stone compositions.
Despite this high accuracy, certain circumstances can lead to a stone not being identified on a CT scan. Very small stones, called microliths, can be difficult to distinguish from normal calcifications or blood vessels. Stones located in anatomically challenging areas, such as the ureterovesical junction where the ureter meets the bladder, can be obscured by surrounding structures or bowel gas.
Most types of kidney stones, such as calcium oxalate and struvite stones, are visible on CT scans due to their radiodensity. Uric acid stones, which are less dense, are also detectable because CT measures density, differentiating them from surrounding soft tissue. However, artifacts from surgical clips or dense stool can create shadows that obscure a stone.
Other Diagnostic Approaches
While CT scans are effective, other imaging techniques offer alternative ways to diagnose and monitor kidney stones. Ultrasound is a non-ionizing option, making it suitable for pregnant patients or those needing frequent monitoring. It excels at identifying larger stones within the kidney and detecting hydronephrosis, which is swelling of the kidney due to urine blockage. However, ultrasound is less sensitive than CT for smaller stones, especially those located within the ureter, where bowel gas can interfere with visualization.
A KUB X-ray is another method. This technique is used to track the movement of known, radiopaque stones, such as calcium-based stones, over time. However, KUB X-rays have low sensitivity for initial diagnosis due to low density and obscuring factors like bowel gas or bone. Non-calcium stones, such as uric acid stones, are invisible on standard X-rays.
Magnetic Resonance Imaging (MRI) is not a primary diagnostic tool for kidney stones due to its limited ability to visualize the stones themselves. While MRI can show the effects of a stone, such as hydronephrosis, it does not directly image the stone with the clarity of a CT scan. MRI is reserved for specific situations where radiation exposure is a significant concern, such as in pregnant women or young children, where its role is more about assessing obstruction than stone detection.
When to Seek Further Evaluation
Even after a CT scan with no detected kidney stone, seek medical evaluation if symptoms persist or worsen. Ongoing severe flank pain, which often comes in waves and radiates to the groin, is an indicator that a stone could still be present or causing issues. The presence of blood in the urine, either visible or microscopic, is a symptom warranting investigation.
Fevers, chills, or persistent nausea and vomiting could indicate a serious complication like an infection alongside a stone. These symptoms require immediate medical attention. Discuss any unresolved or new symptoms with a healthcare provider, who may recommend repeat imaging using a different modality or a follow-up CT scan. A referral to a urologist, a urinary tract specialist, may also be suggested to explore other causes or consider further diagnostic steps.