Vascular diseases of the lung may affect arteries, veins, and/or capillaries. They are primarily evaluated using modalities such as echocardiography, contrast-enhanced CT, or conventional angiography, but in selected cases, HRCT may give an insight not provided by other imaging studies.

HRCT provides an accurate assessment of lung parenchymal abnormalities, but may also allow for a limited assessment of abnormalities directly involving the pulmonary arteries, pulmonary veins, and heart. This chapter focuses upon the pulmonary manifestations of pulmonary vascular disease.

PULMONARY HYPERTENSION

HRCT Findings

Pulmonary hypertension (PH) may be seen with a variety of diseases that affect the lung parenchyma, pulmonary arteries, pulmonary veins, and heart. While pulmonary artery (PA) pressures cannot be directly estimated using HRCT, it may show signs that suggest the presence and relative severity of PH (Table 7.1).

Enlargement of the Main PA

Main PA diameter correlates with the presence or absence of pulmonary arterial hypertension. This correlation is significantly more accurate in patients without parenchymal lung disease than in those with lung disease.

PA diameter should be measured perpendicular to the long axis of the vessel. This may be challenging using HRCT with spaced sections. The threshold above which PH can be said to be present varies in the literature. A threshold diameter of 3.3 cm is relatively specific for this diagnosis. The PA should also be smaller than the adjacent aorta (Fig. 7.1).

Increased Right Ventricular Size

Right ventricular (RV) size may be difficult to assess on HRCT images obtained without contrast injection; however, in many cases the location of the interventricular septum may be estimated even on non-contrast images (Fig. 7.2). The transverse diameter of the RV should be less than the left ventricle. PH is a common cause of RV enlargement.

Increased Size of the Right Atrium, Superior Vena Cava, and Inferior Vena Cava

In the setting of PH, the right atrium (Fig. 7.3), superior vena cava, and inferior vena cava are often enlarged due to tricuspid regurgitation. The size of these structures can usually be determined on non-contrast images. There are no specific size thresholds to use, and enlargement is mainly determined by reader experience. These signs are the least specific for PH as enlargement may be caused by multiple other etiologies.

Figure 7.1

Mosaic Perfusion

Heterogeneous lung attenuation due to regional alterations in blood flow is termed mosaic perfusion (see Chapter 5). Mosaic perfusion may be seen with either airways disease or vascular disease, the former being significantly more likely. Mosaic perfusion may be seen with any cause of PH, but is most common with chronic pulmonary thromboembolic disease.

Figure 7.2

Figure 7.3

The appearance of mosaic perfusion may help to distinguish airways and vascular causes. Vascular disease usually presents with larger, peripheral, non-lobular regions of decreased lung attenuation (Fig. 7.4). Small airways disease typically shows smaller patchy regions of mosaic perfusion, some of which appear lobular.

Figure 7.4

Centrilobular Nodules

The distal pulmonary arteries are centrilobular in location, and diseases that affect the pulmonary arterial system may manifest as centrilobular nodules. Centrilobular nodules from a vascular etiology are often of ground glass opacity (Fig. 7.5). These may be seen in association with pulmonary arterial hypertension of any cause, but they are most common with idiopathic pulmonary arterial hypertension (IPAH), capillary hemangiomatosis, pulmonary veno-occlusive disease, and vasculitis. They often reflect the presence of pulmonary edema or hemorrhage, or cholesterol granulomas, which are an indication of previous hemorrhage.

Centrilobular nodules are not specific for a vascular etiology as they may also be seen with pulmonary edema or hemorrhage (Fig. 7.6) of any cause and airways diseases such as hypersensitivity pneumonitis. In a patient with known PH, however, they are likely due to the vascular disease.

Diseases Associated with PH

The differential diagnosis of PH is very broad (Table 7.2). HRCT may be obtained in the diagnostic evaluation of a patient with PH of unknown cause. One major role of HRCT in this setting is the detection of diffuse lung disease, primarily emphysema and lung fibrosis. Contrast-enhanced CT may also be obtained to evaluate for chronic pulmonary thromboembolism (CPTE).

Figure 7.5

Figure 7.6

There are many causes of PH that are not well evaluated using HRCT. These are unified by a relative lack of lung abnormalities or the presence of nonspecific lung findings. Examples include systolic or diastolic left ventricular dysfunction, valvular heart disease, cardiovascular shunts, hypoventilation disorders, and sleep disorders. The discussion of specific diseases associated with PH will be restricted to those that commonly show lung abnormalities on HRCT (Table 7.3).

Parenchymal Lung Disease

PH may result from parenchymal lung disease because of hypoxia (with vasoconstriction) or obliteration of the pulmonary capillary bed. The most common lung parenchymal processes to result in PH are emphysema (Fig. 7.7A, B) and diffuse lung fibrosis (Fig. 7.8A, B). Cystic lung disease is a rare cause of PH.

Lung fibrosis causing PH is most commonly due to idiopathic pulmonary fibrosis, connective tissue disease, or sarcoidosis. Chronic airways disease, such as cystic fibrosis, and cystic lung disease, such as lymphangioleiomyomatosis or Langerhans histiocytosis, may also be associated with PH. As lung disease associated with PH is typically extensive, it should be evident on HRCT. The lack of significant abnormality on HRCT makes a lung etiology of PH unlikely. In general, the severity of lung disease correlates with the degree of PH.

Sarcoidosis and connective tissue disease– related interstitial lung disease (Fig. 7.9A–C) may show PH disproportionate to the extent of lung disease. In these cases, vasculitis, thrombosis, stenosis, or occlusion of small pulmonary arteries likely contribute. Of note, in the presence of lung disease, there is a poor correlation between main PA diameter and the severity of PH. This is in contrast to IPAH and other diseases without significant lung manifestations in which pulmonary arterial diameter shows better correlation with the presence and severity of PH.

Figure 7.7

Figure 7.8

Chronic Pulmonary Thromboembolism

The diagnosis of CPTE may be challenging. Patients present with nonspecific symptoms and initial objective examinations, such as pulmonary function tests, are nonspecific. Contrast-enhanced CT pulmonary angiography, while effective in the setting of acute pulmonary thromboembolism, has limited sensitivity in the diagnosis of CPTE.

Figure 7.9

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