Residency · Residency · Diagnostic Radiology
Renal Colic and Urolithiasis Imaging
Introduction
Urolithiasis is one of the most common reasons for emergency department visits, affecting approximately 12% of men and 6% of women during their lifetime. Non-contrast CT of the abdomen and pelvis (NCCT) is the gold standard imaging study for suspected renal colic, offering superior sensitivity and specificity compared to ultrasound and plain radiography.
Clinical Presentation
Patients typically present with acute, severe colicky flank pain radiating to the groin or ipsilateral testicle/labium, associated with nausea, vomiting, and restlessness. Microscopic hematuria is present in 80-90% of cases, but its absence does not exclude stones. Pain location may indicate stone position: flank pain suggests the renal pelvis or UPJ, lower abdominal pain suggests the mid-ureter, and suprapubic or groin pain suggests the distal ureter or UVJ.
Imaging Modalities
Non-Contrast CT (Gold Standard)
Non-contrast CT achieves a sensitivity of 95-98% and a specificity of 96-100% for urolithiasis. It detects all stone compositions except rare pure matrix stones and indinavir stones. Low-dose CT protocols reduce radiation to approximately 1.5-3 mSv while maintaining diagnostic accuracy. In 10-15% of patients without stones, CT provides alternative diagnoses.
Ultrasound
Ultrasound is first-line in pregnancy and pediatric patients. Its sensitivity is approximately 45% for ureteral stones but excellent for hydronephrosis detection. Point-of-care ultrasound in the ED can identify hydronephrosis as a surrogate for obstruction. It involves no ionizing radiation and can be repeated safely.
Plain Radiography (KUB)
Plain radiography has limited sensitivity (approximately 60%) for stone detection. It is useful for follow-up of known radiopaque stones (calcium oxalate, calcium phosphate) but cannot detect radiolucent stones (uric acid, cystine, matrix).
CT Findings in Acute Urolithiasis
Direct Signs
The primary direct sign is a hyperdense calculus within the collecting system, ureter, or bladder. Stone attenuation values help predict composition: calcium oxalate typically measures over 1000 HU, while uric acid measures 200-500 HU. Dual-energy CT can differentiate uric acid from non-uric-acid stones.
Secondary Signs of Obstruction
Hydronephrosis and hydroureter proximal to the stone are the most recognizable secondary findings. Perinephric stranding represents edema surrounding the kidney, and periureteral stranding represents soft tissue edema surrounding the ureter at the level of obstruction. The tissue rim sign describes circumferential soft tissue surrounding the calculus and distinguishes a ureteral stone from a phlebolith. Additional signs include renal enlargement on the affected side and asymmetric perinephric fluid.
Stone Size and Management Implications
| Stone Size | Spontaneous Passage Rate | Management |
|---|---|---|
| <5 mm | 68-80% | Conservative (medical expulsive therapy) |
| 5-10 mm | 20-50% | Trial of passage; may require intervention |
| >10 mm | Low (<10%) | Intervention usually required |
Stones less than 5 mm have a 68-80% spontaneous passage rate and are managed conservatively. Stones 5-10 mm have a 20-50% passage rate and may be given a trial of passage with medical expulsive therapy. Stones greater than 10 mm have a low spontaneous passage rate and usually require intervention. The report should include the maximum diameter, location, and degree of upstream obstruction.
Complications
Forniceal rupture results from high-grade obstruction and manifests as perinephric or periureteral urinoma. Pyonephrosis is an infected obstructed system and represents a clinical emergency requiring urgent decompression. Chronic obstruction leads to cortical thinning and renal atrophy. Xanthogranulomatous pyelonephritis is a chronic inflammatory condition associated with staghorn calculus.
Differential Diagnosis
A phlebolith is round and often has a central lucency (the comet tail sign points away from the bladder). Vascular calcification follows the vessel course and is often curvilinear. An appendicolith is located in the right lower quadrant with surrounding inflammation. Alternative diagnoses identified on CT include appendicitis, ovarian torsion, diverticulitis, and AAA.
Key Clinical Pearls
Use low-dose CT protocols whenever possible; they are diagnostically equivalent for stones larger than 3 mm. The tissue rim sign differentiates a ureteral stone from a pelvic phlebolith with high specificity. Always report stone size, location, degree of obstruction, and signs of complications such as forniceal rupture. In pregnant patients, ultrasound is first-line; MRI without gadolinium is second-line if ultrasound is inconclusive.
References
- Smith-Bindman R, et al. Ultrasonography versus computed tomography for suspected nephrolithiasis. N Engl J Med. 2014;371(12):1100-1110.
- Brisbane W, et al. An overview of kidney stone imaging techniques. Nat Rev Urol. 2016;13(11):654-662.
- ACR Appropriateness Criteria: Acute Onset Flank Pain. American College of Radiology, 2024.
- Ziemba JB, Matlaga BR. Epidemiology and economics of nephrolithiasis. Investig Clin Urol. 2017;58(5):299-306.
For the intern
At 3am, if a patient presents with acute flank pain and hematuria, order a low-dose non-contrast CT abdomen/pelvis immediately—it's the gold standard and will definitively identify stones and complications. Report stone size (critical for management decisions), exact location, and degree of hydronephrosis to the attending. If the patient is pregnant or pediatric, start with ultrasound and call your senior resident before ordering CT. Emergent urology consultation is needed for suspected pyonephrosis (infected obstructed system with fever) or forniceal rupture with large urinoma. Most other cases can wait until morning rounds for formal urology input, especially if the stone is small (<5 mm) with mild hydronephrosis and normal renal function.
Common pitfalls
- Missing alternative diagnoses: In 10-15% of patients with suspected renal colic, CT reveals a different cause (appendicitis, ovarian torsion, abdominal aortic aneurysm, diverticulitis). Always review the entire scan systematically, not just the urinary tract.
- Confusing phleboliths with ureteral stones: Phleboliths are round pelvic vein calcifications with a "comet tail sign" (soft tissue tail pointing away from bladder). True ureteral stones show the "tissue rim sign" (circumferential soft tissue edema around the stone).
- Ordering contrast CT unnecessarily: Non-contrast CT (NCCT) is superior for stone detection—contrast obscures small stones and adds unnecessary radiation and nephrotoxicity risk. Reserve contrast studies for when alternative diagnoses are suspected.
- Underestimating obstruction severity: Report all secondary signs (perinephric stranding, renal enlargement, hydroureter) because high-grade obstruction with infection requires urgent decompression, even if the stone is small.
Make it stick
Imagine you're on call, and a patient presents with classic renal colic symptoms. Your job is to swiftly get the right imaging. Remember, for the vast majority of adult patients, a low-dose non-contrast CT (NCCT) of the abdomen and pelvis is your go-to. It’s like a super-detector for stones, telling you not just if a stone is there, but its size, exact location, and if it's causing backup (hydronephrosis) or other issues. Don't waste time with X-rays or contrast CTs for initial stone detection, as they are less effective or introduce unnecessary risks. Your report should clearly state stone size, location, and the presence of hydronephrosis.
If you remember one thing
Low-dose non-contrast CT abdomen/pelvis is the gold standard for diagnosing urolithiasis; always report stone size, location, and degree of hydronephrosis. For pregnant patients, ultrasound is first-line, followed by MRI if needed.