High-resolution computed tomography (HRCT) is widely used in the evaluation of a variety of diffuse lung diseases. The goal of this introductory chapter is to discuss the basics of HRCT, including indications, technique, and normal lung anatomy, as displayed using this modality.

INDICATIONS FOR HRCT

HRCT has several indications and uses in patients with, or suspected of having, diffuse lung disease (Table 1.1).

Detection of Diffuse Lung Disease

HRCT can be more sensitive and specific in the diagnosis of diffuse lung disease than other diagnostic tests (Fig. 1.1A, B), including plain radiographs and pulmonary function tests. For instance, HRCT may detect abnormalities in asymptomatic patients with connective tissue disease or other conditions, or with various exposures, before pulmonary function tests become abnormal. Detecting abnormalities at an early stage may allow for appropriate treatment, preventing progression of lung disease.

Figure 1.1

HRCT may also be used to exclude certain lung diseases as a cause of symptoms or abnormal pulmonary function test findings. For example, in patients with pulmonary hypertension, HRCT may be used to exclude emphysema and fibrotic lung disease as causative etiologies. As another example, in patients with acquired immune deficiency syndrome and a suspicion of Pneumocystis jiroveci infection, HRCT has a high negative predictive value, and further testing, such as bronchoscopy, is not generally required if the study is normal.

Characterization of Diffuse Lung Disease

The primary role of HRCT is in the identification of specific abnormalities that allow a characterization of diffuse lung disease and formulation of a differential diagnosis. The type and specific location of lung abnormalities may be determined using HRCT, and it may be suggested whether the disease present is primarily inflammatory or fibrotic or whether it is an airways disease (Fig. 1.2), interstitial disease, or alveolar (airspace) disease (Fig. 1.3).

HRCT findings may have important implications for treatment and prognosis. When findings of fibrosis are present on HRCT, patients are less likely to respond to various medications and, in general, have a poorer prognosis. Patients with HRCT findings suggestive of inflammation are generally treated more aggressively in the hope that the lung findings are reversible. HRCT is helpful in making this distinction.

Figure 1.2

Figure 1.3

Differential Diagnosis and Guidance for Further Diagnostic Testing

HRCT is more specific than chest radiography, physical examination, and pulmonary function tests in the diagnosis and characterization of lung abnormalities in patients with diffuse lung disease (Fig. 1.4A, B), and some HRCT findings may be highly suggestive of a specific disease. Nonetheless, most HRCT abnormalities are nonspecific and require a differential diagnosis.

The diagnosis of diffuse lung disease is generally based on a multidisciplinary approach, incorporating clinical information, HRCT findings, and sometimes pathology. Infrequently, one of these is sufficient to make a definitive diagnosis, and a combination of at least two is usually required to maintain a high degree of diagnostic accuracy and confidence. When HRCT is interpreted in conjunction with clinical information, the accuracy and specificity of diagnosis improve significantly (Table 1.2). Examples of clinical information useful in the diagnosis of diffuse lung abnormalities include patient age, duration of symptoms, a history of cigarette smoking, exposures (e.g., inhalational and environmental), immune status, drug treatment, and known systemic disorders such as connective tissue disease.

Figure 1.4

In many cases, a combination of HRCT findings and clinical information may predict a single diagnosis with a high degree of accuracy (Fig. 1.5A, B). In such cases, further diagnostic testing may not be needed. For instance, when centrilobular nodules of ground glass opacity are present on HRCT in a patient with an exposure to birds, the diagnosis is very likely hypersensitivity pneumonitis. In such cases, biopsy is not usually required for diagnosis. Another example is an HRCT showing honeycombing in a subpleural and basilar distribution. This pattern is compatible with usual interstitial pneumonia (UIP) and a biopsy is not generally required for confirmation. In the absence of known diseases or exposures, the patient will be diagnosed as having idiopathic pulmonary fibrosis (IPF).

On the other hand, when the HRCT pattern is nonspecific, the suggested differential diagnosis may help to guide additional tests. For example, when tree-in-bud opacities are present on HRCT, a diagnosis is often obtained using sputum analysis, as this finding usually reflects an infectious cellular bronchiolitis with impaction of small airways. In patients with perilymphatic nodules visible on HRCT, there is a high likelihood of obtaining a histologic diagnosis on transbronchial biopsy, as this abnormality is often due to sarcoidosis or lymphangitic tumor spread (Fig. 1.6). Transbronchial biopsy is useful in the diagnosis of these two diseases, because they involve the airways or predominate in the peribronchovascular interstitium. It is also helpful in diagnosing some infections, when sputum analysis is not sufficient. Histologic samples obtained at transbronchial biopsy, however, have significant potential limitations, primarily because they represent small samples of lung, and the samples obtained may not be representative of the overall process that is present. HRCT, on the other hand, provides a global evaluation of the lung abnormalities present (Fig. 1.7).

Lung biopsy is obtained in cases in which the combination of clinical information and HRCT is not considered sufficient for diagnosis. Video-assisted thoracoscopic surgical (VATS) biopsies are most helpful as they obtain relatively large samples from multiple locations within the lung. In patients with nonspecific fibrosis on HRCT, a VATS biopsy is usually required for diagnosis. HRCT may help guide the surgeon to the most appropriate regions for biopsy. It is important to recognize, however, that even if a VATS biopsy is obtained, it is often important to consider it in association with the HRCT abnormalities.

Figure 1.5

Figure 1.6

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