When should imaging be performed in cellulitis and which imaging modalities are preferred?

Imaging for Cellulitis

Uncomplicated cellulitis is a clinical diagnosis that requires no imaging; however, imaging becomes essential when clinical features suggest complications such as abscess, necrotizing fasciitis, or failure to respond to appropriate antibiotics within 48-72 hours. 1

When Imaging Should NOT Be Performed

• Typical uncomplicated cellulitis presenting with localized erythema, edema, warmth, and tenderness without systemic toxicity should be diagnosed clinically without any imaging studies 1
• Plain radiographs have minimal diagnostic value and should not be used to rule out cellulitis or necrotizing infection 2, 1
• Over-ordering imaging for straightforward cellulitis wastes resources and delays appropriate antibiotic therapy 1

Critical Red Flags Requiring Immediate Imaging

Imaging must be obtained urgently when any of the following are present:

• Pain out of proportion to physical findings – the hallmark of necrotizing fasciitis 2, 3, 4
• Rapid clinical progression despite appropriate antibiotics 3, 4
• Systemic toxicity: fever, hypothermia, tachycardia, hypotension, or shock 2, 4, 1
• Skin changes including violaceous bullae, crepitus, skin sloughing, or dusky discoloration 3, 4, 1
• Failure to improve with appropriate first-line antibiotics after 48-72 hours 3, 1
• Bone palpability through a wound, suggesting underlying osteomyelitis 3, 4
• Persistent fever despite therapy, suggesting occult abscess 3, 4

Preferred Imaging Modalities: A Hierarchical Approach

First-Line: Point-of-Care Ultrasound

For suspected abscess, bedside ultrasound is the initial imaging modality of choice, achieving 98% sensitivity and 88% specificity, which markedly exceeds clinical examination alone and alters management in 56% of cases. 4

• Provides immediate, radiation-free results at the bedside 4
• Can differentiate simple cellulitis from necrotizing fasciitis with 88.2% sensitivity and 93.3% specificity when diffuse subcutaneous thickening with fluid >4 mm along the deep fascia is present 3
• Highly sensitive for detecting joint effusions and superficial abscesses 3
• Can guide aspiration for microbiologic culture and drainage procedures 3
• Limitation: Cannot assess bone marrow for osteomyelitis 3

Second-Line: MRI with IV Contrast (Stable Patients)

When cellulitis fails to improve with antibiotics or deep/complicated infection is suspected, MRI with and without IV contrast is the preferred advanced imaging modality, demonstrating approximately 93% sensitivity for necrotizing fasciitis. 3, 4

• The American College of Radiology rates MRI with and without IV contrast as "usually appropriate" (9/9) for soft-tissue infections with suspected complications 4
• Superior soft-tissue contrast enables visualization of fascial fluid/edema, abscesses, myositis, and bone-marrow involvement 3
• Absence of fascial fluid on MRI effectively excludes necrotizing fasciitis, providing high negative predictive value 3
• IV contrast enhancement is essential: it improves detection of small abscesses, differentiates abscess from phlegmon, and highlights necrotic tissue 3, 4

Key MRI findings that mandate urgent surgical consultation:

• Fascial thickening ≥3 mm on T2 fat-suppressed sequences 3, 4
• Involvement of ≥3 muscular compartments by deep fascial edema 3, 4
• Lack of fascial enhancement after contrast administration (indicates fascial necrosis) 3, 4
• Rim-enhancing fluid collections (characteristic of abscesses requiring drainage) 3, 4

Alternative: CT with IV Contrast (Unstable Patients or MRI Contraindications)

Contrast-enhanced CT is the preferred modality when MRI is contraindicated, unavailable, or the patient is hemodynamically unstable for prolonged scanning. 3, 4

• In case series, CT achieved 100% sensitivity for necrotizing soft-tissue infections (specificity approximately 81%) 2, 3, 4
• Most sensitive modality for detecting soft-tissue gas (approximately 89% sensitivity), a hallmark of necrotizing fasciitis 3, 4
• CT acquisition is faster than MRI, facilitating rapid assessment in critically ill patients 2, 3
• With IV contrast, CT yields 85-95% sensitivity and specificity for abscess detection 4
• Demonstrates fascial thickening and absent fascial enhancement, findings specific for necrotizing fasciitis 2, 3
• The American College of Radiology rates CT with IV contrast as "may be appropriate" (6/9) for soft-tissue infections 4

Critical pitfall: Absence of soft-tissue gas on CT does NOT rule out necrotizing fasciitis, especially early in disease or in diabetic patients 2, 3, 4

Clinical Imaging Algorithm

For Suspected Abscess:

1. Perform point-of-care ultrasound for all patients with suspected abscess 4
2. If ultrasound positive for simple superficial abscess → proceed to incision & drainage; no further imaging needed 4
3. If ultrasound negative but high clinical suspicion persists:
   • Stable patient → obtain MRI with IV contrast 4
   • Unstable patient or systemic toxicity → obtain CT with IV contrast immediately 4

For Suspected Necrotizing Fasciitis or Unresolved Cellulitis:

Stable patients:

• Obtain MRI of the affected extremity with and without IV contrast 3
• If MRI shows fascial involvement, arrange urgent surgical consultation 3
• If MRI is negative but clinical suspicion remains high, repeat MRI in 24-48 hours to assess for progression 3

Unstable patients or those with systemic toxicity:

• Perform immediate contrast-enhanced CT; do not delay for MRI 3
• Consider bedside ultrasound while CT is being arranged 3
• Imaging must never postpone surgical consultation when necrotizing infection is suspected 2, 3, 4

Patients contraindicated for MRI (e.g., pacemaker, severe claustrophobia):

• Use contrast-enhanced CT as the primary advanced imaging modality 3, 4

Special Anatomic Considerations

• Orbital/periorbital cellulitis: CT of the orbits with IV contrast is the preferred initial study to differentiate preseptal from postseptal involvement and to detect abscess formation 2, 4
• Deep neck or sublingual infections: The ACR advises against ultrasound due to limited visualization; CT with IV contrast is recommended 4
• Diabetic foot infections: Plain radiographs and MRI are best for detecting bone involvement, and MRI provides anatomic information about sinus tracts, abscesses, or muscle involvement 1

Critical Pitfalls to Avoid

• Never delay surgical consultation awaiting imaging results in the presence of systemic toxicity, rapid clinical deterioration, or high suspicion for necrotizing fasciitis 2, 3, 4
• Do not assume that absence of soft-tissue gas excludes necrotizing infection; gas may be absent early or in aerobic infections (e.g., Streptococcus pyogenes) 2, 3, 4
• Do not assume normal imaging excludes infection; cellulitis remains a clinical diagnosis even with negative imaging 1
• In unstable patients, ultrasound may be useful to differentiate simple cellulitis from necrotizing fasciitis, but should not delay definitive imaging or surgical consultation 2