What types of kidney stones are not visible on a computed tomography (CT) stonogram?

Stones Not Visible on CT Stonogram

All kidney stones are visible on CT stonogram—there are no truly "invisible" stones on CT imaging. However, uric acid stones appear with significantly lower attenuation values (lower Hounsfield units) on CT compared to other stone types, making them appear less dense but still detectable 1.

CT Detection Characteristics by Stone Type

Uric Acid Stones

• Uric acid stones have lower CT attenuation values but remain visible on noncontrast CT 1
• These stones can be distinguished from calcium, cystine, and struvite calculi based on their lower Hounsfield unit measurements 1
• Dual-energy CT can accurately differentiate uric acid stones from non-uric acid stones with 89% sensitivity and 98% specificity 2
• The mean CT attenuation for radiolucent stones (primarily uric acid) is approximately 358 HU, compared to 817 HU for radio-opaque stones 3

Cystine Stones

• Cystine stones may be "barely opaque" on standard fluoroscopy but are visible on CT 1
• CT attenuation coefficients vary based on stone surface characteristics (smooth versus rough external surface) 1
• These stones are detectable on CT stonogram despite their variable opacity on plain radiography 1

Critical Distinction: CT vs. Plain Radiography

The confusion arises because "radiolucent" refers to plain X-ray visibility, not CT visibility:

• Uric acid stones are "radiolucent" on plain KUB radiography but visible on CT 1
• Plain abdominal X-ray has only 48% sensitivity for detecting stones that are clearly visible on CT 4
• A stone with CT attenuation below 498.5 HU will likely be radiolucent on plain X-ray (87.3% specificity), but it remains visible on the CT itself 3

Clinical Algorithm for Stone Detection

When evaluating for kidney stones:

1. Order noncontrast CT abdomen/pelvis as the reference standard—it detects all stone types with 97% sensitivity 1, 5

2. If a low-attenuation stone is identified (suggesting uric acid composition):

   • Look for clinical clues: low urinary pH (typically <5.5) 1
   • Consider dual-energy CT if available for definitive stone composition analysis 6, 2
   • Initiate urinary alkalinization therapy (potassium citrate to achieve pH 6.0-7.0) as these stones may dissolve with medical management 1, 7

3. Do not rely on plain radiography (KUB) for initial stone detection:

   • KUB misses 52% of stones visible on plain abdominal X-ray and has even worse performance on scout CT (83% miss rate) 4
   • KUB has only 53-62% sensitivity for ureteral calculi 8

Common Pitfalls to Avoid

• Do not assume "radiolucent" means invisible on CT—this term refers only to plain X-ray appearance 1
• Do not order contrast-enhanced CT for initial stone evaluation—contrast in the nephrographic phase may obscure stones within the renal collecting system 1
• Do not use ultrasound as first-line imaging in non-pregnant, non-pediatric patients—it has only 24-57% sensitivity for detecting renal calculi compared to CT's 97% 5, 9
• Recognize that stone size affects plain radiography detection but not CT detection—stones <4mm have only 60% sensitivity on KUB but remain visible on CT 8, 3