CT Scan for Kidney Stones: Optimal Imaging Approach

Non-Contrast CT: The Gold Standard

Non-contrast CT (NCCT) is considered the reference standard for kidney stone evaluation with a sensitivity as high as 97% and specificity of 95% 1, 2
Virtually all renal calculi are radiopaque on CT, allowing for accurate detection of even small stones without the need for IV contrast 2
NCCT provides rapid acquisition with high spatial resolution and ability for multiplanar reformations 2
Helical (spiral) NCCT allows for precise measurement of stone size and clear depiction of where in the ureter a stone has become lodged 3
Secondary signs of urolithiasis and complications such as periureteral inflammation, perinephric inflammation, and ureteral dilatation can also be visualized 2

Low-dose CT protocols (<3 mSv) should be used in place of conventional dosing to reduce radiation exposure while maintaining excellent diagnostic performance 3, 4
Meta-analysis shows low-dose CT maintains a pooled sensitivity of 97% and specificity of 95% for detecting urolithiasis 2
Low-dose CT yields equivalent stone measurements compared to standard-dose CT 2

Stone location and size can be accurately depicted with NCCT, which is crucial for determining management as more proximal and larger stones have higher intervention needs 2
CT techniques that improve accuracy of stone measurements include:
- Use of coronal reformations 4
- Viewing on bone window settings 4
- Use of magnified views 4
Thin (1-1.5 mm) axial slice images are preferred over thick (5 mm) coronal maximum intensity projection images for optimal stone detection 1

CT with IV contrast is usually not appropriate as a first-line test for kidney stones as the enhancing renal parenchyma may obscure stones within the collecting system 1, 5
If contrast-enhanced CT has already been performed, it can still detect larger stones (≥6 mm) with approximately 98% accuracy 1
CT abdomen and pelvis without and with IV contrast offers no additional benefit over non-contrast CT alone for kidney stone evaluation 5, 4

For pregnant patients, ultrasonography is the imaging tool of choice due to radiation concerns 3, 6
In patients with recurrent stone disease, limiting the CT scan to the area of interest or using ultra-low-dose protocols can help reduce cumulative radiation exposure 6

Relying on contrast-enhanced CT as first-line imaging for suspected kidney stones may lead to missed small stones 1, 5
The sensitivity for small stone detection decreases with increasing dose reduction, so ultra-low-dose protocols may miss stones <2mm in size 2

The American College of Radiology recommends that in patients with moderate to severe hydronephrosis on ultrasound and moderate or high risk of ureteric calculi, ultrasound may provide sufficient diagnostic certainty without requiring CT, with up to 100% sensitivity and 90% specificity for diagnosing ureteral obstruction 7
The American College of Radiology notes that the absence of hydronephrosis on ultrasound does not rule out stones, with a negative predictive value of only 65% 7
The American College of Radiology suggests considering reserving CT for cases where stone size and location are needed to plan surgical management, using low-dose protocols, in patients with high clinical suspicion for stones and moderate to severe hydronephrosis on ultrasound 7
The American College of Radiology found that in patients with flank pain and negative ultrasound, CT may identify important alternative diagnoses, with 15% of patients with negative urinalysis and no history of urolithiasis having diagnoses best characterized by contrast-enhanced CT 7