What type of CT (Computed Tomography) scan is needed to rule out a kidney stone?

CT Imaging for Kidney Stone Detection

Non-contrast helical (spiral) CT of the abdomen and pelvis is the recommended imaging modality to rule out kidney stones, with low-dose protocols strongly preferred to minimize radiation exposure while maintaining diagnostic accuracy. 1, 2

First-Line Imaging Recommendations

• Non-contrast CT advantages:

  • High sensitivity (up to 97%) and specificity (95%) for kidney stone detection 2
  • Allows precise measurement of stone size and location 1
  • Can detect virtually all renal calculi regardless of composition 2
  • Identifies secondary signs of obstruction (ureteral dilatation, perinephric stranding) 1

• Low-dose protocol requirement:

  • Standard non-contrast CT exposes patients to significant radiation
  • Low-dose protocols should be used instead of conventional dosing 1
  • Ultra-low-dose protocols can achieve radiation doses as low as 0.57 mSv (92% lower than standard low-dose CT) 3

Special Population Considerations

• Pregnant patients:

  • Ultrasonography is the imaging tool of choice 1
  • Avoids radiation exposure but has lower sensitivity (24-57%) for smaller stones 2

• Recurrent stone formers:

  • Low-dose non-contrast CT remains appropriate (rating of 7) 1
  • Consider limiting scan to the area of concern to reduce radiation exposure 1
  • For follow-up of known radiopaque stones, KUB (kidney-ureter-bladder) radiography or ultrasound may be appropriate 2

Clinical Decision Algorithm

1. For initial evaluation in non-pregnant adults:

   • Low-dose non-contrast helical CT of abdomen and pelvis

2. For pregnant patients:

   • Ultrasound as first-line imaging
   • MRI without contrast if ultrasound is inconclusive 2

3. For follow-up of known stones:

   • Low-dose CT for uric acid or small stones
   • KUB or ultrasound for known radiopaque stones 2

Important Clinical Considerations

• Stone size and location are essential for determining the level of intervention required; smaller, more proximal stones are more likely to pass spontaneously 1

• If low-dose CT findings are unclear, secondary signs (ureteral dilatation, perinephric stranding) may help with diagnosis 1

• The American College of Radiology appropriateness rating for non-contrast CT in suspected stone disease is 8 (usually appropriate) 1

• While KUB radiography delivers less radiation than CT, its limited capabilities often lead to repeat imaging, negating the radiation benefit 1

Common Pitfalls to Avoid

• Don't rely solely on KUB radiography for initial diagnosis (sensitivity only 53-62%) 2

• Don't use standard-dose CT when low-dose protocols are sufficient for diagnosis 1

• Don't overlook the need for urgent intervention in cases with complete obstruction 2

• Be aware that automated stone measurement software may report larger stone sizes (by approximately 23%) than manual measurements 3