What is the significance of a figure 3 sign in medical diagnosis?

The Figure 3 Sign in Medical Diagnosis

The figure 3 sign (also known as the "three-density sign" or previously as the "head-cheese sign") is a highly specific radiographic finding on high-resolution CT imaging of the lungs that strongly indicates fibrotic hypersensitivity pneumonitis (HP) rather than idiopathic pulmonary fibrosis (IPF). 1

Definition and Appearance

The figure 3 sign is characterized by:

• A sharp demarcation of three distinct density patterns within the same lobe of the lung:
  1. Areas of decreased attenuation (air trapping)
  2. Areas of normal lung density
  3. Areas of increased ground-glass opacification

This creates a mosaic attenuation pattern that resembles the number "3" or a "head-cheese" appearance on CT imaging. 1

Diagnostic Significance

The figure 3 sign has important diagnostic value:

• When present in three or more lobes, it has a specificity of 93% and sensitivity of 49% for diagnosing fibrotic hypersensitivity pneumonitis 1
• When present in five or more lobules and in three or more lobes bilaterally, specificity increases to 96% (sensitivity 42%) 1
• It is a critical radiographic finding that helps differentiate fibrotic HP from idiopathic pulmonary fibrosis (IPF), which can be challenging diagnoses to distinguish

Clinical Context

The figure 3 sign is particularly useful in patients with:

• Pulmonary fibrosis of unclear etiology
• History of environmental or occupational exposures
• Diffuse interstitial lung disease
• Mosaic attenuation patterns on CT

Radiographic Identification

To properly identify the figure 3 sign:

1. High-resolution CT (HRCT) imaging is required
2. Both inspiratory and expiratory images should be obtained
3. The sign is best appreciated when viewing multiple lobes
4. The areas of decreased attenuation become more pronounced on expiratory images

Differential Considerations

While highly specific for HP, other conditions to consider when seeing mosaic attenuation include:

• Small airways disease
• Pulmonary vascular disease
• Other interstitial lung diseases with air trapping

Clinical Implications

Identification of the figure 3 sign has significant treatment implications:

• Suggests an exposure-related etiology requiring antigen identification
• May indicate need for environmental/occupational assessment
• Often requires different treatment approach than IPF
• Prognosis may differ from other interstitial lung diseases

Pitfalls to Avoid

• Do not confuse with other mosaic attenuation patterns that lack all three density components
• Interpretation requires expertise in thoracic radiology
• The sign may be absent in early or non-fibrotic HP
• Absence of the sign does not rule out HP (given moderate sensitivity)

The figure 3 sign represents an important radiographic pattern that can significantly impact diagnostic accuracy and treatment decisions in patients with interstitial lung disease, particularly when differentiating between fibrotic hypersensitivity pneumonitis and idiopathic pulmonary fibrosis.