What are the preferred imaging modalities and characteristic radiologic findings for diagnosing and monitoring allergic bronchopulmonary aspergillosis (ABPA) using high‑resolution computed tomography (HRCT)?

Radiology of Allergic Bronchopulmonary Aspergillosis (ABPA)

Preferred Imaging Modality

High-resolution computed tomography (HRCT) with thin-section technique (1.25–1.5 mm slice thickness) is the gold standard for diagnosing and monitoring ABPA, and should be performed with intravenous contrast at baseline to visualize vessels and assess disease extent. 1, 2

• Plain chest radiography has limited sensitivity and is normal in approximately 50% of ABPA cases, making it inadequate for initial diagnosis 3, 2
• HRCT is normal in only 22–37% of glucocorticoid-naïve ABPA patients, demonstrating far superior sensitivity to chest X-ray 3, 4
• Contrast-enhanced CT (CT angiography) is mandatory at baseline to properly evaluate bronchiectasis extent and vascular anatomy 1, 2

Characteristic HRCT Findings

Pathognomonic Feature

High-attenuation mucus (HAM) is pathognomonic for ABPA with 100% specificity, appearing denser than paraspinal muscle on non-contrast CT, and confirms the diagnosis even when other criteria are incomplete. 1, 5, 2

• HAM is present in 35–36% of ABPA patients and should be assessed on mediastinal windows 1, 4
• When HAM is identified, ABPA diagnosis is confirmed regardless of whether all other diagnostic criteria are fulfilled 1, 5

Bronchial Abnormalities (Most Common)

Central bronchiectasis affecting upper and middle lobes bilaterally is the hallmark finding, present in 63–89% of patients. 1, 2, 4, 6

• Bronchiectasis is typically cylindrical or cystic (varicoid), involving central airways 1, 2, 6
• Important caveat: Peripheral bronchiectasis occurs in 33–43% of cases, so the traditional concept of purely "central" disease is outdated 3
• Bronchial wall thickening is common, seen in approximately 43% of patients 7

Mucus Plugging

Mucoid impaction (mucus plugs) occurs in 59–86% of ABPA patients and constitutes a core diagnostic element. 1, 4, 3

• Mucus plugs appear as branching tubular or nodular opacities filling dilated bronchi 1
• The combination of central bronchiectasis, centrilobular nodules, and mucus impaction strongly favors ABPA diagnosis 4

Parenchymal Findings

Centrilobular nodules with tree-in-bud pattern are present in 86% of cases. 1, 4

• Other common findings include consolidation (17–43%), atelectasis, and mosaic attenuation 1, 7
• Critical distinction: What appears as consolidation on chest X-ray often represents mucus-filled bronchiectatic cavities on HRCT 3
• Parenchymal scarring occurs in 83% of patients, predominantly in upper lobes 7

Advanced Disease Features (ABPA-CPF)

Chronic pleuropulmonary fibrosis (ABPA-CPF) is diagnosed when two or more of the following are present: pulmonary fibrosis, fibro-cavitary lesions, fungal ball, or pleural thickening. 1

• Pleural involvement occurs in 43% of patients, with lesions extending to the pleura 7
• Cavities are seen in 13% of patients 7
• Mandatory consideration: Chronic pulmonary aspergillosis (CPA) must be actively excluded in ABPA-CPF by comparing immunological profiles (dominant IgE + eosinophilia favors ABPA; predominant IgG without marked IgE elevation favors CPA) 1, 5

Radiological Classification System (2024 ISHAM-ABPA Working Group)

The following five categories guide prognosis and treatment decisions: 1, 2

• ABPA-S (Serological): No bronchiectasis on imaging 1
• ABPA-B (Bronchiectasis): Radiological evidence of bronchiectasis 1
• ABPA-MP (Mucus Plugging): Mucus plugs without high-attenuation mucus; patients with both bronchiectasis and mucus plugging are classified as ABPA-MP given greater immunological severity 1
• ABPA-HAM: High-attenuation mucus present (pathognomonic) 1
• ABPA-CPF: Two or more features of chronic pleuropulmonary fibrosis 1

Imaging Algorithm for Diagnosis

Initial Evaluation

1. Obtain thin-section HRCT (1.25–1.5 mm) with intravenous contrast in all patients with suspected ABPA based on clinical and serological criteria 1, 2
2. Assess for central bronchiectasis (upper/middle lobe predominance), mucus plugging, and high-attenuation mucus on both lung and mediastinal windows 1, 2
3. Evaluate for centrilobular nodules, tree-in-bud pattern, consolidation, and atelectasis 1

Pediatric Considerations

• Carefully weigh radiation exposure against diagnostic necessity in children, though HRCT remains the diagnostic standard 2
• Consider lower-dose protocols, though sensitivity for mucus plugs may be reduced 8

Follow-Up Imaging Strategy

Chest radiographs, not CT, should be used for routine treatment monitoring to minimize cumulative radiation exposure. 1, 8, 2

• Repeat HRCT is reserved for: 1, 2

  • New hemoptysis requiring evaluation for vascular complications
  • Suspected treatment failure or significant clinical deterioration
  • Change in clinical status warranting reassessment at 3 or 6 months

• Low-dose CT protocols may be considered for necessary follow-up imaging to reduce radiation burden 1

Imaging Modalities NOT Recommended

PET scanning is not useful for ABPA diagnosis, as aspergillosis causes false-positive uptake mimicking malignancy. 1, 2

• Isometabolic halo and nodule patterns on FDG-PET/CT have been described but lack diagnostic specificity 1

MRI is not recommended for routine ABPA diagnosis despite being radiation-free, as it offers no significant diagnostic advantage over readily available HRCT. 2

• MRI may visualize mucus distribution and bronchiectasis but is not part of standard diagnostic algorithms 8

Common Pitfalls and How to Avoid Them

Pitfall 1: Relying on Chest X-Ray Alone

• Solution: Always obtain HRCT when ABPA is suspected, as chest X-ray is normal in 50% of cases and misses critical findings 3, 2

Pitfall 2: Assuming Only "Central" Bronchiectasis

• Solution: Recognize that peripheral bronchiectasis occurs in one-third of patients; do not exclude ABPA based on peripheral distribution alone 3

Pitfall 3: Misinterpreting Consolidation

• Solution: Correlate chest X-ray consolidation with HRCT, which often reveals mucus-filled bronchiectatic cavities rather than true airspace disease 3

Pitfall 4: Missing High-Attenuation Mucus

• Solution: Always review mediastinal windows to assess mucus attenuation relative to paraspinal muscle; this finding is 100% specific for ABPA 1, 2

Pitfall 5: Confusing ABPA-CPF with CPA

• Solution: In cavitary disease, measure total IgE (≥500 IU/mL favors ABPA), check for eosinophilia (present in ABPA, absent in CPA), and compare Aspergillus-specific IgE (mandatory in ABPA) versus IgG (predominant in CPA) 5

Pitfall 6: Excessive Radiation from Repeated CT

• Solution: Use chest X-ray for routine follow-up; reserve CT for specific clinical indications (hemoptysis, treatment failure, significant deterioration) 1, 8, 2