When is a high resolution computed tomography (HRCT) scan recommended?

When is High-Resolution CT (HRCT) Recommended?

HRCT is recommended when evaluating suspected diffuse lung disease, chronic cough that has failed initial treatment, or when chest radiography is normal but clinical suspicion remains high for interstitial lung disease, bronchiectasis, or hypersensitivity pneumonitis. 1

Primary Indications for HRCT

Diffuse Lung Disease (DLD)

• HRCT is the reference standard for evaluating diffuse lung disease and should be performed using thin-section images (≤1.5 mm) with high-spatial-frequency reconstruction algorithms 1
• CT demonstrates significantly increased sensitivity and specificity over chest radiography for detecting lung parenchymal changes related to DLD 1
• HRCT pattern characterization is especially important for diagnosing usual interstitial pneumonia and guides appropriate biopsy sites when tissue confirmation is needed 1
• Multidisciplinary discussion incorporating HRCT interpretation results in improved diagnostic accuracy and confidence for DLD 1

Hypersensitivity Pneumonitis (HP)

• HRCT findings should be integrated with clinical findings to support HP diagnosis, but CT findings alone should not be used to make a definite diagnosis 1
• Characteristic HRCT features of HP include profuse centrilobular nodules of ground-glass attenuation, inspiratory mosaic attenuation and air-trapping, and the three-density sign 1
• For nonfibrotic HP, typical patterns include diffusely distributed ground-glass opacity, mosaic attenuation, and ill-defined small (<5 mm) centrilobular nodules on inspiratory images with air trapping on expiratory images 1
• For fibrotic HP, coexisting lung fibrosis and signs of bronchiolar obstruction are highly suggestive, with irregular fine or coarse reticulation with architectural lung distortion 1
• Early in nonfibrotic HP, a minority of chest HRCT scans may be normal, so a normal CT does not entirely exclude HP diagnosis 1

Chronic Cough

• HRCT is recommended after appropriate clinical evaluation and empiric treatment have failed to resolve chronic cough lasting more than 8 weeks 1
• Both the American College of Chest Physicians and German Respiratory Society recommend HRCT after failed empiric sequential treatment for common etiologies 1
• HRCT is the reference standard for evaluating bronchiectasis, which has been identified as an etiology for chronic cough in up to 8% of patients 1
• Bronchiectasis and bronchial wall thickening were identified in 27% and 21% of patients, respectively, referred to chest CT for chronic cough evaluation when radiographs were reportedly normal 1
• Chest radiography has shown poor sensitivity for bronchiectasis, failing to detect ectatic airways in up to 34% of patients 1

Hemoptysis

• HRCT without contrast can identify a cause of hemoptysis in 94% of cases and is useful when chest radiography is normal or nondiagnostic 1
• Early studies using HRCT identified a cause in 41% of patients with hemoptysis who had normal chest radiographs 1

Technical Requirements for HRCT

Optimal Scanning Parameters

• Noncontrast examination with volumetric acquisition using submillimetric collimation and shortest rotation time 1
• Tube potential typically 120 kVp with <240 mAs, with lower tube potentials (100 kVp) encouraged for thin patients 1
• Reconstruction of thin-section CT images (<1.5 mm) that are contiguous or overlapping using high-spatial-frequency algorithm 1
• Supine position with both inspiratory (volumetric) and expiratory (sequential or volumetric) acquisitions 1
• Recommended radiation dose for inspiratory volumetric acquisition is 1-3 mSv (reduced dose), with strong recommendation to avoid ultra-low-dose CT (<1 mSv) 1

Additional Sequences

• Expiratory images and prone images are frequently beneficial for evaluating diffuse lung conditions 1
• Volumetric CT data acquisition facilitates multiplanar thin-section reconstructions, which aids evaluation of DLD distribution 1

Important Clinical Caveats

Radiation Considerations

• HRCT delivers approximately 6.5 times less radiation than standard chest CT (0.98 mSv vs 6.5 mSv), though still about 12 times higher than conventional chest radiography (0.085 mSv) 2
• Appropriate use based on established clinical indications is essential, as the clinical benefit will generally be much higher than the radiation risk when CT has a well-established indication 3
• Patients' cross-sectional body size should be taken into account to deliver lower radiation doses to smaller patients 4

When HRCT May Not Be Needed

• In chronic cough, if chest radiography reveals a clear cause and empiric treatment is successful, HRCT may not be necessary 1
• For suspected HP, assessment should consider the prevalence of disease in the particular setting, clinical context, and exposure history alongside HRCT findings 1
• A normal HRCT does not entirely exclude HP, particularly early in the course of nonfibrotic disease 1

Contrast Enhancement

• HRCT protocols typically do not require IV contrast, as the focus is on lung parenchymal evaluation 1
• Contrast may be added when improved visualization of mediastinum, chest wall, soft tissues, or vascular structures is needed 1
• For hemoptysis evaluation, contrast-enhanced CT or CTA may be beneficial for preprocedural planning before bronchial artery embolization 1

Progressive Fibrotic Disease Monitoring

• For patients with confirmed fibrotic interstitial disease on initial HRCT at non-referral centers, referral to an expert center is recommended 1
• Side-by-side comparison of baseline and follow-up HRCT examinations is strongly recommended to reduce variability and increase reproducibility when assessing disease progression 1
• Multiple specialist approach is essential, with radiologist contribution crucial alongside clinical and functional data from pathologist and pulmonologist 1