Background: Tuberculosis (TB) has been around for millennia, and despite initial declines in the incidence during the middle of the 20th century, the disease has been reemerging across the world. The radiologic diagnosis of TB started only about a century ago, after Roentgen's discovery of x-rays. Fluoroscopy was used in the early part of the 20th century to detect cavitary TB, because experienced fluoroscopists could easily detect cavities. Over the years, improvements in technology, coupled with extensive investigation into the radiologic patterns of pulmonary TB, has resulted in diagnostic imaging being an essential adjunct to the clinical and microbiologic diagnosis. With these events, the documentation of cavitary disease and follow-up on film became routine practice.

Pathophysiology: TB results from infection by any of the TB complex mycobacteria: Mycobacterium tuberculosis, Mycobacterium bovis, Mycobacterium africanum, Mycobacterium microti, and Mycobacterium canetti.

TB can be divided into primary, progressive-primary, and postprimary forms on the basis of the natural history of the disease. Postprimary TB usually results from reactivation of a latent primary infection and rarely from the repeat infection of a previously sensitized host. The term postprimary is preferred to reactivation, because firmly distinguishing recurrence from an antecedent infection is impossible in most cases. Approximately 10% of all infected patients are likely to develop reactivation, and the risk is highest within the first 2 years or during periods of immunosuppression.

The major determinants of the type and extent of disease are the patient's age and immune status, the virulence of organism, and the mycobacterial load. Postprimary TB is typically a disease of adolescence and adulthood resulting from reactivation of an initially contained infection by a TB complex mycobacterium. Pulmonary reactivation usually occurs in the apical and posterior segments of the upper lobes or in the superior segments of the lower lobes. This distribution may be related to the higher oxygen tension or the reduced perfusion and lymphatic clearance in these segments.

Frequency:

• In the US: In the United States, approximately 15 million people are infected with TB organisms. Although this number represents less than 10% of the population, it results in a large reservoir of people who are at risk for postprimary TB. The actual annual incidence of disease characterized by positive cavitary smear results is less than 0.1%.

• Internationally: Worldwide, approximately 19-43% of the population is infected with TB. Although most are asymptomatic, the likelihood of reactivation is increasing because of HIV infection. Almost half of all patients co-infected with HIV and TB eventually have a reactivation of the disease. The annual incidence of active TB may approach 4% in certain populations.

Mortality/Morbidity: Untreated postprimary TB has a mortality rate in excess of 50%. Antibiotic therapy can reduce this rate to less than 10%. Residual fibrosis, cavities, and/or bronchiectasis are complications of untreated disease.

Race: Increased susceptibility to TB, such as that found in Native Americans, results in progressive primary disease. Postprimary disease is typically seen in populations that have a high natural resistance to TB that results in initial containment of the infection. Within such groups, TB occurs disproportionately in the disadvantaged, the infirm, the elderly and those with HIV coinfection. True racial or sex-based differences are likely to be minor in postprimary disease.

Sex: When socioeconomic factors are considered, no clear propensity for either sex is observed.

Age: Postprimary TB is primarily a disease of adults, but patients range from adolescents and young adults in endemic areas to middle-aged populations and elderly populations in areas of low prevalence. This distribution may be related to the age at which the initial infection is most likely to occur.

Anatomy: Postprimary TB starts as an expanding granuloma. Unchecked proliferation of the mycobacteria results in rapid necrosis because of preexisting hypersensitivity from the primary infection. Spontaneous arrest can occur, but the disease is usually progressive. This finding is in contrast to that of primary TB in which the development of immunity in a naïve host arrests the spread of disease. Erosion into adjacent anatomic structures results in further spread.

The classic progression involves cavitation into a bronchus with endobronchial spread. Other patterns can cause erosion into the blood vessels; pleural space; or, rarely, the lymphatics.

Clinical Details: The classical clinical presentation includes cough, fever, weight loss, night sweats, hemoptysis, and acid-fast bacilli in the sputum. None of these is universally present, and patients often present with minimal signs or symptoms. Isolated hemoptysis is neither sensitive nor specific for the diagnosis of TB.

Patients with postprimary cavitary TB are more infectious than those with miliary TB. Cavitation into a bronchus results in a high bacterial load in the sputum and, thus, higher infectivity.

Preferred Examination: In nonendemic areas, the initial evaluation of all suspected cases of TB should include tuberculin and sputum testing in conjunction with chest radiography. The value of tuberculin testing is unclear in endemic areas or in countries in which bacille Calmette-Guérin (BCG) vaccination is prevalent.

Limitations of Techniques: No radiologic study shows findings specific for TB. Cavitary process in the upper lobe apical and posterior segments or in the superior segments of lower lobes demonstrated on chest radiographs or CT is likely to be TB; however, differential considerations include other diseases such as histoplasmosis other fungal infections, even bacterial abscesses and necrotic neoplasm, especially lung neoplasm.

In immunocompromised patients, postprimary TB may mimic primary TB, and it can appear with pleural effusion, lymphadenopathy, or miliary spread. The usual pattern of cavitary upper-lobe disease is less common in immunocompromised hosts than in immunocompetent hosts. Findings usually seen in primary TB, such as lymphadenopathy and pleural effusions, may occur in immunocompromised hosts.

DIFFERENTIALS

Blastomycosis, Thoracic
Coccidioidomycosis, Thoracic
Histoplasmosis, Thoracic
Lung, Nontuberculous Mycobacterial Infections
Sarcoidosis, Thoracic
Silicosis and Coal Worker Pneumoconiosis

X-RAY

Findings: Pulmonary TB, especially postprimary disease, nearly always causes abnormalities on the chest radiograph. Typically, the disease is parenchymal without nodal enlargement, and it manifests as cavitary lesions. Upper-lobe involvement with cavitation and the absence of lymphadenopathy are helpful in distinguishing postprimary from primary TB. Anterior or basal segments may be involved in as many as 75% of cases, in addition to the usually involved pulmonary segments, namely, the apical or posterior segments of the upper lobe or the superior segment of a lower lobe (see Images 1-2).

Cavitation is a distinguishing feature of postprimary TB, and it is seen on chest radiographs in about half the cases. Most cases have discernible cavities on chest CT scans (see Image 2). Typical cavities are thick walled and irregular. Air-fluid levels are uncommon and usually indicate superinfection; however, they can be seen in other circumstances in 9% of cases. The persistence of cavitation without healing is unusual and should be investigated, particularly in patients with persisting hemoptysis, to exclude mycetomas (see Image 3). Cavitation can lead to endobronchial spread to the remaining lung (see Image 4) or rupture into the pleural space, where it can cause an empyema or bronchopleural fistula. It can also cause pseudoaneurysms of the pulmonary artery called Rasmussen aneurysms (see Image 7).

Miliary spread is less common in postprimary TB and is caused by erosion of bronchial vessels or pulmonary veins (see Images 5-6).

TB and lung cancer coexist in as many as 5% of cases, but whether TB independently increases likelihood of cancer remains unclear.

Degree of Confidence: TB cannot be confidently diagnosed on the basis of chest radiographic findings alone. Chest radiographic results can often be normal in primary TB. Normal findings are unusual in postprimary lung disease, but they cannot be used to exclude extrapulmonary TB. However, the combined positive predictive value of the typical symptoms and the finding of cavitary upper-lobe infiltrates on chest radiographs is high.

False Positives/Negatives: Sarcoidosis, granulomatous fungal infections, Nocardia infections, and atypical mycobacterioses are the most common mimics of pulmonary TB.