Radiological patterns of tuberculosis
Computed tomography allows a detailed assessment of the nature of changes in the lung tissue in various forms of the tuberculous process.
Post-primary tuberculosis (endobronchial spread):
- Lung tissue consolidation forms, while signs of destruction (cavitation) may be absent.
- A typical pattern of centrilobular nodules is identified, known as the «tree-in-bud» sign (tree-in-bud).
- The presence of bilateral nodular lesions of varying sizes is characteristic.
Miliary tuberculosis: The clinical presentation is usually accompanied by fever and dyspnea. CT reveals a specific pattern: multiple isolated small nodules with a diameter of 1 to 2 mm. Their distribution in the lung tissue is completely random in character.
Non-tuberculous mycobacterial (NTM) infections
Infections associated with non-tuberculous mycobacteria (NTMB) often develop in patients with a burdened medical history. Risk factors include debilitating diseases, immunodeficiency states, previous pulmonary tuberculosis, chronic airway obstruction, silicosis, and post-lung-transplant status. The severity of the course depends directly on the patient's immune status and the presence of underlying pathology.
Clinical features and forms:
- Infection with Mycobacterium avium complex (MAC) can be indolent, clinically manifesting as a cough that may be productive or non-productive.
- The most common form of NTM involvement occurs in elderly men with underlying background lung diseases.
- A special form is Lady Windermere syndrome (Lady Windermere syndrome). It is characteristic of elderly Caucasian women who have a complete absence of any underlying lung diseases.
- Upon infection with Mycobacterium kansasii, the radiological picture is practically indistinguishable from post-primary tuberculosis, whereas changes characteristic of primary tuberculosis are extremely rare.
CT findings in MAC infection: High-resolution computed tomography (HRCT) reveals cylindrical bronchiectasis (mild to moderate severity) and multiple centrilobular «tree-in-bud» opacities 1–3 mm in diameter. Typical localization of changes is the middle lobe and lingula. In some cases, partial collapse (atelectasis) of the affected lobe is observed.
Fungal lung infections
Fungi causing pulmonary infections are classified into two main groups:
- Pathogenic fungi: capable of infecting any host, regardless of their immune status. Diseases in this group include coccidioidomycosis, blastomycosis, and histoplasmosis.
- Saprophytic fungi: affect exclusively individuals with immunodeficiency phenomena. They are the cause of Pneumocystis pneumonia (PCP), candidiasis, mucormycosis, and aspergillosis.
Aspergillus infection
The disease is caused by fungi of the genus Aspergillus (in the vast majority of cases — A. fumigatus). The form of infection development is entirely determined by the host's immune response. Three classic forms of pulmonary aspergillosis are distinguished: invasive, allergic, and saprophytic.
Saprophytic aspergillosis (Aspergilloma / Mycetoma):
- Pathogenesis and localization: Saprophytic proliferation of fungal colonies occurs inside a pre-existing cavity in the lung, which usually remains after previous sarcoidosis or tuberculosis. The upper lobes or upper segments of the lower lobes are predominantly affected.
- Clinical manifestations: The disease is asymptomatic in the majority of cases. The most important and formidable complication is haemoptysis (haemoptysis). Bronchial artery embolization can be used to stop it.
- CT findings: A space-occupying lesion with soft-tissue attenuation is visualized within the pulmonary cavity. This lesion is characteristically separated from the cavity wall by a layer of air — forming the «air crescent» sign ('air crescent' sign). Sometimes the lesion is bilateral and accompanied by the formation of multiple nodules.
Allergic bronchopulmonary aspergillosis (ABPA): This form is based on a pronounced hypersensitivity reaction localized in the lumen of large airways. The pathology is closely associated with respiratory symptoms, elevated serum IgE levels, and the detection of positive serum precipitins.
Briefly: answers to topical questions
What CT signs are characteristic of invasive pulmonary aspergillosis?
Invasive pulmonary aspergillosis on CT is characterized by the appearance of nodules with signs of perifocal necrosis and hemorrhage. The following specific patterns are distinguished:
- Halo sign (halo sign) — multiple nodules surrounded by a zone of decreased attenuation resembling ground-glass opacity.
- Air crescent sign (air crescent sign) — cavitation of the lesion with the formation of an air crescent.
- Subpleural consolidation areas — wedge-shaped zones diffusely distributed across multiple lobes.
- «Amputated bronchus» sign — cutoff of the bronchial tree.
What radiological changes on CT are characteristic of allergic bronchopulmonary aspergillosis (ABPA)?
Allergic bronchopulmonary aspergillosis on CT is characterized by signs of mucoid impaction and consolidation areas. The following changes are distinguished:
- Mucus plugs (mucous plugs) — usually hypodense, but can be hyperdense in 20% of cases.
- Bronchocele (bronchoceles) — opacities of various shapes (linear, branched, V- and Y-shaped) pointing toward the lung hilum.
- «Glove-finger» sign (glove finger opacities) — branching thick tubular opacities due to bronchial dilatation.
- Consolidation (consolidation) — varies from massive to subsegmental, often recurrent.
With which diseases is the differential diagnosis of miliary tuberculosis performed on CT?
Differential diagnosis of miliary tuberculosis on CT is performed with miliary metastases.
- Miliary metastases (miliary metastases) — manifest as multiple small nodules in both lungs that visually resemble miliary tuberculosis. A distinctive feature of metastases is the absence of large nodules and signs of lymphatic obstruction.
What CT signs are characteristic of primary pulmonary tuberculosis?
Primary pulmonary tuberculosis on CT is characterized by the presence of extensive intrathoracic lymphadenopathy. The following signs are distinguished:
- Lymphadenopathy (lymphadenopathy) — involvement of the right paratracheal, bifurcation (subcarinal), and bilateral intrathoracic (hilar) lymph nodes.
- Tracheal compression — may occur as a complication of marked lymph node enlargement.
Morphologically, the process is represented by the primary tuberculous complex, but the changes in the lymph nodes are always more pronounced compared to the primary focus.
For quick memorization of the CT picture of an aspergilloma, imagine a night sky: the old lung cavity forms a dark crater, inside which lies the fungal ball, and above it glows the «air crescent» sign.
Class and exam questions
What does miliary tuberculosis look like on CT?
It manifests as a random distribution of multiple small isolated nodules ranging from 1 to 2 mm in diameter.
What is Lady Windermere syndrome?
This is a specific form of non-tuberculous mycobacterial infection that develops in elderly Caucasian women in the absence of underlying lung diseases.
Which complication of saprophytic aspergillosis is the most dangerous?
The main complication of aspergilloma is hemoptysis, for the treatment of which bronchial artery embolization may be used in clinical practice.
What else to study on this topic
- Differential diagnosis of the "bud-in-tree" sign in various infections
- Pathogenesis and clinical presentation of allergic bronchopulmonary aspergillosis
- Features of the clinical course of infections caused by Mycobacterium kansasii
- Radiological manifestations of pathogenic fungal infections (coccidioidomycosis, histoplasmosis)
- Principles of bronchial artery embolization for hemoptysis
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