Are there instances where renal (kidney) stones are not detectable on a computed tomography (CT) scan without contrast and require a magnetic resonance imaging (MRI) scan for visualization?

From the Guidelines

Renal stones are almost always detectable on a computed tomography (CT) scan without contrast, and magnetic resonance imaging (MRI) is rarely required for visualization, except in very specific cases. The current evidence suggests that non-contrast CT is the reference standard for evaluating urolithiasis, with a reported sensitivity as high as 97% 1. This high sensitivity is due to the fact that virtually all renal calculi are radiopaque on CT, allowing for accurate detection of even small stones.

Some instances where renal stones may not be detectable on a non-contrast CT scan include:

• Certain types of stones, such as indinavir stones or matrix stones, which may be less radiopaque
• Very small stones, below the resolution threshold of CT
• Patients where radiation exposure is a significant concern, such as pregnant women or children requiring multiple imaging studies

In these cases, MRI might be preferred, despite its lower sensitivity for stone detection, as it can provide additional information and help differentiate between a stone and a tumor, or evaluate for complications of stone disease such as obstruction or infection 1. However, it's essential to note that ultrasound is typically the alternative imaging modality chosen when CT is contraindicated, rather than MRI.

The use of low-dose non-contrast CT for stone assessment has also been shown to be effective, with a pooled sensitivity of 97% and a pooled specificity of 95% 1. This approach can help reduce radiation exposure, especially in young patients, while still providing accurate detection of renal stones.

Overall, while there may be rare instances where renal stones are not detectable on a non-contrast CT scan, the current evidence suggests that MRI is not typically required for visualization, and non-contrast CT remains the gold standard for evaluating urolithiasis 1.

From the Research

Renal Stone Detection on CT Scans

• Non-contrast computed tomography (CT) scans are commonly used for diagnosing suspected renal stones, but there are instances where stones may not be detectable on CT scans without contrast 2.
• Certain types of stones, such as those made of n-butyl α-cyanoacrylate, may not be visible on CT scans 2.
• Motion artifact in CT images can cause warping distortion effects, making renal stones appear larger or smaller than they actually are 3.

Limitations of CT Scans in Renal Stone Detection

• CT scans may not always provide accurate information about stone size, composition, or location 3.
• Some stones, such as milk of calcium stones, may be faintly radio-opaque or radiolucent on CT scans, making them difficult to detect 4.
• CT scans may not be able to detect stones that are located in dependent positions within dilated calyces or exhibit fluid level or semilunar patterns 4.

Alternative Imaging Modalities

• Magnetic resonance imaging (MRI) scans may be used to visualize renal stones that are not detectable on CT scans, although they are currently costly and stones can be difficult to visualize 5.
• Ultrasound and plain film radiography are alternative imaging modalities that can be used to detect renal stones, but they have lower sensitivity and specificity than CT scans 6, 5.
• MRI provides the possibility of 3D imaging without exposure to radiation, but further developments are needed to enhance its use in renal stone detection 5.