# 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).

![Non-contrast axial CT demonstrating a calculus at the left ureterovesical junction with proximal hydroureter](images/uvj-calculus-ct.jpg)

## 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**.

![Coronal CT reformation showing right proximal ureteral calculus with upstream hydronephrosis and perinephric stranding](images/ureteral-calculus-coronal.jpg)

## 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.

![Sagittal CT showing phlebolith with characteristic comet tail sign versus distal ureteral stone with tissue rim sign](images/phlebolith-vs-stone.jpg)

## 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

1. Smith-Bindman R, et al. Ultrasonography versus computed tomography for suspected nephrolithiasis. *N Engl J Med*. 2014;371(12):1100-1110.
2. Brisbane W, et al. An overview of kidney stone imaging techniques. *Nat Rev Urol*. 2016;13(11):654-662.
3. ACR Appropriateness Criteria: Acute Onset Flank Pain. American College of Radiology, 2024.
4. Ziemba JB, Matlaga BR. Epidemiology and economics of nephrolithiasis. *Investig Clin Urol*. 2017;58(5):299-306.