Question

How are Idiopathic Interstitial Pneumonias (IIPs) classified? Discuss the imaging approach in Idiopathic Interstitial Pneumonias (IIPs) including CT imaging protocol and salient imaging features. [2+(2+6)]

Hint:

IIPs are primarily classified into fibrotic and non-fibrotic types, with IPF being the most common and poor prognosis disease. NSIP and DIP are less aggressive forms.
HRCT is essential for evaluating the pattern and distribution of lung disease in IIPs. The presence of honeycombing, ground-glass opacities, and traction bronchiectasis are key features in distinguishing between different types of IIPs.

Answer 1

Step 1: Classification of Idiopathic Interstitial Pneumonias (IIPs).
Idiopathic Interstitial Pneumonias (IIPs) are a group of interstitial lung diseases (ILDs) characterized by chronic inflammation and fibrosis of the lung interstitium. These diseases primarily involve the lung parenchyma and can lead to progressive pulmonary fibrosis and respiratory failure.
IIPs are classified into the following major categories:
(1) Idiopathic Pulmonary Fibrosis (IPF):
- The most common and well-known form of IIP, characterized by fibrotic changes and honeycombing of the lung tissue. It typically affects the lower lobes and is associated with a poor prognosis.
(2) Non-Specific Interstitial Pneumonia (NSIP):
- A less aggressive form of interstitial lung disease, with a better prognosis than IPF. It is characterized by uniform interstitial fibrosis without the honeycomb appearance.
(3) Desquamative Interstitial Pneumonia (DIP):
- This type of IIP is often associated with smoking and is characterized by alveolar macrophage accumulation and minimal fibrosis.
(4) Respiratory Bronchiolitis-Associated Interstitial Lung Disease (RB-IL:
- Primarily affects smokers, with bronchiolar involvement and peripheral centrilobular nodules on imaging.
(5) Acute Interstitial Pneumonia (AIP):
- A rapidly progressive form of IIP, with a clinical course resembling acute respiratory distress syndrome (ARDS). It has a poor prognosis.
(6) Lymphocytic Interstitial Pneumonia (LIP):
- Often associated with autoimmune diseases or HIV infection, it shows a lymphocytic infiltrate in the interstitium.
(7) Cryptogenic Organizing Pneumonia (COP):
- Characterized by polyps of granulation tissue in the small airways and alveolar ducts. It may present with a patchy distribution and responds well to steroids.
Step 2: Imaging Approach in Idiopathic Interstitial Pneumonias (IIPs).
Imaging plays a critical role in the diagnosis, monitoring, and differentiation of the various forms of IIPs. High-resolution CT (HRCT) is the primary imaging modality used for evaluating IIPs, as it provides detailed information about lung parenchyma, fibrotic changes, and other characteristic features of these diseases.
CT Imaging Protocol:
The CT imaging protocol for evaluating IIPs typically involves the following parameters:
- High-resolution CT (HRCT) scans are acquired with thin slices (1-2 mm) to maximize the ability to detect subtle interstitial changes.
- Inspiratory scans are performed in the supine position to assess the parenchyma in full inspiration. In some cases, expiratory CT may also be done to evaluate air trapping or obstruction.
- Contrast is usually not required unless there is suspicion of an additional vascular or oncologic cause.
- Multi-slice CT (MSCT) technology is often used for better resolution and coverage of the lung fields.
Step 3: Imaging Features in IIPs on CT.
(1) Idiopathic Pulmonary Fibrosis (IPF):
- Honeycombing: Reticular opacities with cystic spaces (honeycombing) predominantly in the lower lobes and peripheral regions. This is the most characteristic feature of IPF.
- Ground-glass opacities: May be present in the early stages and often progress to fibrosis.
- Architectural distortion and traction bronchiectasis due to fibrosis.
- Subpleural and basal predominant involvement.
(2) Non-Specific Interstitial Pneumonia (NSIP):
- Ground-glass opacities: Often bilateral, homogeneous, and lower lobe predominant.
- Reticular opacities without the presence of honeycombing.
- Absence of honeycombing distinguishes NSIP from IPF.
(3) Desquamative Interstitial Pneumonia (DIP):
- Ground-glass opacities with patchy distribution predominantly in the lower lobes.
- Minimal fibrosis and no honeycombing.
- This pattern often improves with smoking cessation.
(4) Respiratory Bronchiolitis-Associated ILD (RB-IL:
- Centrilobular nodules and peripheral ground-glass opacities.
- Hyperinflation and peripheral bronchial wall thickening are common features.
- Smoker’s lung is typically seen in this subtype.
(5) Acute Interstitial Pneumonia (AIP):
- Diffuse bilateral consolidation or ground-glass opacities.
- Rapid progression to fibrosis and fibrotic consolidation may occur in later stages.
- CT findings often resemble acute respiratory distress syndrome (ARDS).
(6) Lymphocytic Interstitial Pneumonia (LIP):
- Mild ground-glass opacities in the lower lobes.
- Nodules and cystic changes in some cases.
- May be associated with autoimmune conditions or HIV.
(7) Cryptogenic Organizing Pneumonia (COP):
- Patchy consolidations with a bronchocentric pattern.
- Air bronchograms within areas of consolidation.
- Peripheral and subpleural distribution.
- Often reversible with steroids.