From the Guidelines
The best outpatient imaging modality for kidney stones is non-contrast computed tomography (CT) scan of the abdomen and pelvis. This recommendation is based on the high sensitivity (up to 97%) and specificity of non-contrast CT for detecting renal calculi, as reported in the most recent study by the Journal of the American College of Radiology 1. The use of non-contrast CT allows for accurate detection of even small stones, visualization of secondary signs of urolithiasis, and assessment of complications such as periureteral and perinephric inflammation and ureteral dilatation.
Some of the key benefits of non-contrast CT include:
- High spatial resolution and ability for multiplanar reformations
- Rapid acquisition time
- Ability to detect stones of all compositions
- Provision of information about stone size, location, and density
- Identification of complications like obstruction or infection
For patients with concerns about radiation exposure, low-dose non-contrast CT protocols can be used, which have been shown to maintain diagnostic accuracy while reducing radiation exposure 1. This is particularly important for young patients or those requiring repeated imaging.
While ultrasound is an alternative option for certain patient populations, such as pregnant patients or children, it is less sensitive (sensitivity of 60-80%) and may miss small stones. Ultrasound has the advantages of being radiation-free and less expensive but provides less detailed information about stone characteristics and surrounding anatomy compared to CT. Therefore, non-contrast CT remains the gold standard for diagnosing kidney stones in outpatient settings.
From the Research
Imaging Modalities for Kidney Stone Detection
- Noncontrast CT of the abdomen and pelvis provides the most accurate diagnosis but exposes patients to ionizing radiation 2
- Ultrasonography has a lower sensitivity and specificity than CT but does not require radiation, and its diagnostic accuracy is equivalent to CT in the emergency department 2
- Kidney, ureter, bladder (KUB) plain film radiography is helpful in evaluating interval stone growth in patients with known stone disease but is less useful in acute stone settings 2
Comparison of Imaging Modalities
- A study comparing CT scans and KUB radiography found that all stones visible on CT scout films were also visible on KUB, but CT scout films detected only 75% of stones visible on KUB 3
- Ultrasonography has a sensitivity of 54% and specificity of 91% for detecting renal calculi, and its accuracy for determining stone size is limited, particularly for stones in the 0-10 mm range 4
- Non-contrast CT has high sensitivity (97-100%) and specificity (96-100%) for detecting pediatric urolithiasis, making it a reliable second-line choice for diagnosis 5
Recommendations for Imaging Modalities
- Ultrasound is recommended as the first imaging modality for initial assessment of pediatric urolithiasis, while non-contrast CT is recommended as the second option 5
- For outpatient imaging, noncontrast CT or ultrasonography may be considered, depending on the clinical context and patient factors, such as radiation exposure concerns 2, 4