Anatomy and Significance of HRCT
Interstitial lung diseases (ILDs) affect the supportive connective tissue structures: the intralobular and peribronchovascular interstitium, as well as the interlobular septa. The pathogenesis of ILDs comes down to the thickening of these structures due to the accumulation of cells, fluid, or the proliferation of fibrous tissue. Standard chest radiography is poorly suited for early diagnosis due to low sensitivity and high subjectivity of interpretation. The introduction of HRCT fundamentally changed this approach, allowing for the detailed classification of patterns according to the Fleischner Society Glossary.
Reticular Pattern and Fibrosis
The presence of a reticular pattern almost always indicates clinically significant ILD. Changes depend on which specific septa are affected:
- Intralobular septa: their thickening often lies beyond the resolution of the tomograph, thus visually blending into a subtle decrease in transparency referred to as ground-glass opacification.
- Interlobular septa: smooth thickening is typical of pulmonary edema; irregular or beaded thickening is typical of carcinomatous lymphangitis and sarcoidosis.
- Perilobular distribution: creates the illusion of thickened septa, but more often points toward organizing pneumonia.
Severe fibrosis forms coarse reticulation and honeycombing (honeycombing) — cystic spaces with irregular walls. This is a critically important sign indicating a usual interstitial pneumonia (UIP) pattern. The contraction of fibrous tissue also stretches the bronchi, forming traction bronchiectasis.
Nodular and Cystic Patterns
The distribution of nodules on HRCT is the key to diagnosis:
- Centrilobular: involvement of small bronchioles (hypersensitivity pneumonitis, infections, smoking). May have a tree-in-bud appearance.
- Perilymphatic: nodules are located along fissures and interlobular septa (sarcoidosis).
- Random: typical of hematogenous dissemination (tuberculosis, metastases).
The cystic pattern is characterized by rounded, low-density areas with thin walls (less than 2 mm). Cysts, unlike emphysema, have their own walls. This is a classic sign of lymphangioleiomyomatosis (LAM) and pulmonary Langerhans cell histiocytosis (PLCH). It is important not to confuse cysts with bronchiectasis, for which multiplanar reconstructions are used.
Mosaic Attenuation Pattern
This nonspecific pattern reflects regional differences in parenchymal density. It is best evaluated in combination with expiratory CT.
- "Black" lung (decreased density): occurs due to air trapping in small airway disease or due to a reduction in blood flow (e.g., in chronic thromboembolic pulmonary hypertension — CTEPH).
- "Gray" lung (increased density): the ground-glass opacification sign in infiltrative processes (acute respiratory distress syndrome, Pneumocystis pneumonia, subacute hypersensitivity pneumonitis).
Idiopathic Interstitial Pneumonias (IIPs)
A group of chronic diseases of unknown etiology that requires a multidisciplinary team (MDT) meeting (clinician, radiologist, pathologist).
| Pattern | HRCT Findings | Prognosis |
|---|---|---|
| UIP | Reticulation, honeycombing, traction bronchiectasis. | Negative (IPF). |
| NSIP | Ground-glass opacification, traction bronchiectasis, minimal honeycombing. | More favorable. |
| COP | Peripheral consolidation, ground-glass opacification, perilobular pattern. | Responds well to therapy. |
A hallmark of the UIP pattern is temporal heterogeneity (areas of normal lung alternate with dense fibrosis). HRCT reflects this heterogeneity better than a localized biopsy.
Briefly: answers to topical questions
What diseases can manifest with a cystic pattern on HRCT?
A cystic pattern on HRCT can manifest in various interstitial diseases as well as normal lung aging. The main causes include:
- Lymphangioleiomyomatosis (LAM) — a classic disease presenting with a cystic pattern.
- Pulmonary Langerhans cell histiocytosis (LCH) — a classic cause of cysts.
- Pneumocystis pneumonia (PJP) — one of the causes of cyst formation.
- Lymphoid interstitial pneumonia (LIP) — characterized by thin-walled cysts ranging from 1 to 30 mm, often in the setting of Sjögren's syndrome.
- Birt–Hogg–Dubé syndrome (BHD) — a rare cause of a cystic pattern.
- Hypersensitivity pneumonitis (HP) — may be accompanied by cyst formation.
Cysts may also be detected in amyloidosis in combination with solid nodules.
What are the main clinical indications for ordering a chest HRCT?
The main clinical indications for ordering a chest HRCT are the evaluation of interstitial diseases and the detection of comorbid conditions.
- Evaluation of interstitial lung diseases (ILD) — for early detection, specific diagnosis, and prognosis assessment.
- Chronic obstructive pulmonary disease (COPD) — in severe emphysema to determine the appropriateness of lung volume reduction surgery.
- Chronic bronchitis (CB) — to detect associated pathological changes, including bronchiectasis and pulmonary emphysema.
- Characterization of pulmonary nodules (pulmonary nodules) — using thin slices.
Routine use of HRCT in all patients with COPD or chronic bronchitis is not recommended.
How to differentiate true lung cysts from centrilobular emphysema on HRCT?
The differential diagnosis between true cysts and centrilobular emphysema on HRCT is based on the assessment of wall morphology and the presence of central structures.
| Sign | True cyst | Centrilobular emphysema |
|---|---|---|
| Wall | Thin (<2 mm), well-defined border | Without visible walls |
| Contents/Structure | Round low-density area | Small vessels in the center of hypodense areas |
| Association with emphysema | Occur independently of emphysema | Is a subtype of emphysema |
The cystic pattern is best identified by precise assessment of morphology and geometry (round or tubular shape). Centrilobular pulmonary arteries serve as a marker of the lobular center in emphysema.
Mnemonic for nodular patterns: Centrilobular — Center of the lobule (bronchioles); Perilymphatic — Septa and Pleura; Random — Rushed through the blood (hematogenous).
Class and exam questions
What is the difference between «honeycomb lung» and traction bronchiectasis?
«Honeycomb lung» is tissue destruction with cyst formation, a marker of terminal fibrosis. Traction bronchiectasis is the dilation of pre-existing bronchi due to the pulling of their walls by surrounding fibrous tissue.
Why perform expiratory HRCT in mosaic attenuation?
Expiratory CT scanning helps detect «air trapping». This proves that the areas of decreased density are caused by small airway obstruction rather than a blood flow deficit (as in CTEPH).
What does the «ground-glass opacity» sign mean?
This is a reduction in lung transparency where the contours of vessels and bronchial walls remain visible through the altered area. It occurs due to partial alveolar filling or mild interstitial thickening.
What else to study on this topic
- Features of the differential diagnosis between UIP and chronic hypersensitivity pneumonitis patterns
- Smoking-related ILDs (RB-ILD and DIP)
- Diagnostic algorithm for lymphangioleiomyomatosis and Langerhans cell histiocytosis
- Acute interstitial pneumonia and diffuse alveolar damage (DAD)
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