From the Guidelines
CT scans, particularly non-contrast CT, are the preferred imaging modality for diagnosing kidney stones, due to their high accuracy and ability to detect small stones and provide detailed information about stone size, location, and potential complications. While ultrasound can detect some kidney stones and is useful for identifying hydronephrosis, it may miss smaller stones and provides less detail about stone composition and exact location 1, 2. According to a recent study, non-contrast CT has a reported sensitivity as high as 97% for detecting urolithiasis, making it the reference standard for evaluation 2.
The choice between ultrasound and CT scan depends on the patient's specific situation, medical history, and symptom severity. Ultrasound may be recommended first if the patient is pregnant, has had multiple previous scans, or needs to limit radiation exposure 1. However, if symptoms are severe or a definitive diagnosis is needed, a CT scan would likely be recommended. A study of 835 symptomatic patients who underwent point-of-care ultrasound (POCUS) prior to CT of the abdomen and pelvis without IV contrast found that CT identified 54 acutely important alternate findings, highlighting the importance of CT in evaluating patients with suspected kidney stones 1.
Some key points to consider when choosing between ultrasound and CT scan include:
- Ultrasound is radiation-free and less expensive, but may miss smaller stones and provides less detail about stone composition and exact location 1, 2
- CT scans, particularly non-contrast CT, are highly accurate and can detect small stones, but may expose patients to radiation 2
- The presence of hydronephrosis on ultrasound can increase the likelihood of a ureteral stone on CT, but a lack of hydronephrosis does not rule out the diagnosis 1
- CT scans can provide important information about stone size, location, and potential complications, which is essential for planning urologic management 2
Overall, CT scans are the preferred imaging modality for diagnosing kidney stones, due to their high accuracy and ability to provide detailed information about stone size, location, and potential complications. However, ultrasound may still be useful in certain situations, such as in pregnant patients or those who need to limit radiation exposure 1, 2.
From the Research
Imaging Modalities for Kidney Stones
- Abdominal ultrasound (US) can be used to detect kidney stones, but its sensitivity and specificity are limited compared to computed tomography (CT) scans 3, 4, 5.
- The sensitivity of US for detecting renal calculi is around 45-54%, and its specificity is around 88-91% 3, 4.
- US is more accurate for detecting larger stones, but its sensitivity decreases for stones smaller than 3 mm 3.
- CT scans are considered the gold standard for detecting kidney stones due to their high accuracy and ability to provide detailed information about the stone and surrounding tissue 3, 6, 7.
- However, CT scans expose patients to radiation, whereas US is a safer and more cost-effective option 3, 6.
Factors Affecting US Accuracy
- The accuracy of US can be improved by adjusting factors such as gain and depth, and using alternate modes like flash angle imaging, harmonic mode, and S mode 3.
- The use of shadow for size measurement can also help improve the accuracy of stone size measurement 3.
- Automated computerized stone-sizing programs can improve the accuracy of stone size calculation and reduce user variability 3.
Clinical Implications
- US can be used as a first-line imaging modality for nephrolithiasis due to its low cost, absence of radiation, and easy availability 3.
- However, CT scans may be necessary for patients with suspected kidney stones, especially if US is inconclusive or if accurate stone sizing is required for treatment planning 4, 6.
- The combination of US and plain abdominal film of kidney, ureter, and bladder can increase sensitivity, but may still lead to inappropriate counseling in some cases 4.