High-Resolution CT (HRCT) of the Chest
High-resolution CT refers to a specialized chest CT technique that uses thin-section imaging (1-2 mm slices) with high spatial resolution reconstruction algorithms to provide detailed evaluation of the lung parenchyma, particularly for assessing interstitial lung disease and ground-glass opacities. 1
Technical Specifications of HRCT
The defining technical parameters that distinguish HRCT from standard chest CT include:
- Slice thickness: Sub-millimetric to 1.5 mm thin sections (compared to standard 5-10 mm slices) 1
- Reconstruction algorithm: High-spatial-frequency algorithm that maximizes spatial resolution 1
- Image acquisition: Volumetric acquisition with sub-millimetric collimation, shortest rotation time, and highest pitch 1
- Radiation parameters: Typically 120 kVp with <240 mAs, adjusted for patient size, with tube current modulation to minimize radiation exposure 1
- Reconstruction: Contiguous or overlapping images using iterative reconstruction algorithms when validated 1
Standard HRCT Protocol Components
For comprehensive evaluation of interstitial lung disease with ground-glass opacities, the recommended protocol includes: 1
- Supine inspiratory images: Volumetric acquisition covering entire lung volume 1
- Expiratory images: Can be volumetric or sequential, essential for detecting air-trapping 1
- Prone images: Optional inspiratory scans (sequential or volumetric) to distinguish dependent atelectasis from true pathology 1
Clinical Applications for Ground-Glass Opacities
HRCT is the imaging modality of choice for evaluating ground-glass opacities because it allows detailed characterization of distribution patterns and associated features that guide diagnosis. 1, 2
Key Diagnostic Features HRCT Reveals:
- Distribution patterns: Peripheral vs. peribronchovascular vs. diffuse, upper vs. lower lobe predominance 1, 2
- Associated findings: Reticular opacities, honeycombing, traction bronchiectasis, centrilobular nodules 1, 2
- Extent quantification: Percentage of lung involvement (critical when >30% ground-glass suggests diagnoses other than IPF) 1, 2
- Fibrotic features: Presence of honeycombing or traction bronchiectasis indicates fibrotic disease 1, 2
Specific Relevance to Basilar Ground-Glass Opacities
For a patient with basilar ground-glass opacities and pancytopenia, HRCT provides superior diagnostic information compared to standard chest CT or radiography:
- Earlier disease detection: HRCT detects interstitial abnormalities in patients with normal chest radiographs 1
- Pattern recognition: Distinguishes between UIP pattern (peripheral, basal reticular abnormalities with honeycombing), fibrotic NSIP (peribronchovascular with traction bronchiectasis), and other patterns 1
- BAL targeting: HRCT performed within 6 weeks of bronchoscopy guides optimal sampling sites, with areas of ground-glass opacity providing highest diagnostic yield 1
- Differential diagnosis: Helps distinguish IPF, connective tissue disease-associated ILD, hypersensitivity pneumonitis, drug-induced pneumonitis, and infection 1, 2
Critical Pitfalls to Avoid
Do not confuse standard chest CT with HRCT—the thin-section technique and specialized reconstruction algorithms are essential for accurate assessment of interstitial lung disease. 1 Standard CT protocols using 5 mm slices lack the spatial resolution needed to characterize subtle ground-glass patterns and early fibrotic changes. 1
Ensure images are obtained in both inspiration and expiration phases, as expiratory images are critical for detecting mosaic attenuation and air-trapping that distinguish hypersensitivity pneumonitis from other causes of ground-glass opacity. 1
Visual quantification of disease extent must be performed exclusively on HRCT scans, not on ultrasound or standard CT, as accurate extent assessment has therapeutic implications for antifibrotic treatment eligibility. 1