Computed tomography of diffuse lung diseases and solitary pulmonary nodules

Multiplanar reformation.

Spiral CT allows the generation of high-quality images in every spatial plane in nearly every patient (multiplanar reformation [MPR]).

Pathology that occurs in the central or peripheral regions of the lung is best studied in axial images (see Fig. 4.2 ). Diseases that show upper or lower lung prevalence benefit from visualization in the coronal view (see Fig. 4.3 ). Finally, disorders that prevail in the parahilar regions or the costophrenic angles are best depicted in the sagittal view (see Fig. 4.4 ). With the volumetric approach, the planes may be varied according to the needs of the operator and targeted on the suspected disease ( Box 4.1 ).

Curved multiplanar reformation.

Having the entire volume available and working digitally makes it possible to reconstruct objects traveling in and out of a two-dimensional plane along curved reformatted image (curved MPR). This allows a structure to be traced and displayed as if it were lying along a single plane. For example, an intuitive visualization of the entire course of a bronchus from a cavitated lesion to its origin can be achieved by displaying it along a manually or automatically generated centerline of the bronchial lumen ( Fig. 4.9 ). The curved reformatted images are not real, but they are effective and easy to generate and therefore useful for practical purposes (e.g., virtual bronchoscopy).

Maximum intensity projection.

The maximum intensity projection (MIP) technique renders only the voxels with higher attenuation in a thick slice (0.5–2 cm). The technique is suitable for rendering the vascular tree three dimensionally against a black background; moreover, with MIP, it is possible to obtain a comprehensive representation of the position of various lesions inside the lobular framework ( Fig. 4.10 ). In selected cases, MIP images are useful for distinguishing between vessels and nodules and, when nodules are present, to assess their profusion.

Minimum intensity projection.

The minimum intensity projection (minIP) technique renders only the voxels with lower attenuation in a thick slice (0.5–2 cm). This technique is useful for improving the visualization of hyperlucent elements (bronchi, emphysema, bullae, honeycombing) and some opacities, allowing for a more precise study of their attributes and distribution. The minIP technique is also ideal for investigating bronchial caliper and course, particularly within areas of increased density ( Fig. 4.11 ).

Volume rendering.

Volume rendering (VR) techniques may also be used in assessing DLD. When this is implemented, the machine renders only the structures within a specific range of attenuations and contained within a chosen volume. VR may be useful for studying a volume of lung in three dimensions from within or for inspecting its surface from outside (external VR) ( Fig. 4.12 ). This is particularly useful for concisely looking at the pulmonary surface and its abnormalities and for helping to make medical decisions (e.g., determining the site of a surgical biopsy).

Interstitial hydrostatic pulmonary edema.

The septal lines of pulmonary edema are usually associated with smooth subpleural and peribronchovascular interstitial thickening (peribronchial cuffing). Patchy lobular GGO often coexists because of minimal alveolar edema ( Fig. 4.23 ). There is a tendency for the hydrostatic edema to show a symmetrical basal and posterior distribution (in supine patients) ( Fig. 4.24 ), although patchy nongravitational distributions are not impossible.

Heart enlargement and bilateral pleural effusion are common findings in cardiogenic pulmonary edema (see Fig. 4.24 ); a pericardial effusion may coexist in a number of patients. When lymph flow to the systemic veins decreases, an enlargement of mediastinal lymph nodes due to fluid stagnation may occur ( Fig. 4.25 ).

Lymphangitic carcinomatosis.

Lymphangitic carcinomatosis (LC) may present with a fully smooth subset ( Fig. 4.26 ), but not infrequently nodular irregularities (beaded appearance of septa and fissures) and random micronodules in areas of thickening do occur. Nodules result from the focal growth of cells within the lymphatics and local extensions into the parenchyma. ^, Subpleural thickening, when present, may also be smooth or nodular. Pleural effusion is unilateral in 50% of cases.

The lesions of LC are typically patchy, often unilateral, and not gravity-dependent ( Fig. 4.27 ). Hilar lymphadenopathy is visible in 50% of patients. Enlarged mediastinal lymph nodes can also be seen in a number of cases (25%–50%). Dedicated CT window settings may demonstrate metastatic lesions elsewhere ( Fig. 4.28 ).

Venoocclusive disease.

The presentation of venoocclusive disease is similar to that of hydrostatic pulmonary edema. Smooth septal lines, bronchial cuffing, and patches of GGO related to alveolar wall thickening and pulmonary edema are apparent ( Fig. 4.29 ). ^,

The lesions present a geographical appearance with variable localization. They are always bilateral and may have a gravitational preference ( Fig. 4.30 ).

A key radiologic sign is coexisting enlargement of the central pulmonary arteries compatible with arterial pulmonary hypertension ( Fig. 4.31 ). The right side of the heart may also be dilated without evidence of left atrial or ventricular enlargement. In addition, pericardial or pleural effusion and enlargement of the mediastinal lymph nodes may be visible.

Erdheim-chester disease.

Erdheim-Chester disease (ECD) is a nonLangerhans cell systemic histiocytosis that produces smooth septal and subpleural interstitial thickening with more or less regular contours ( Fig. 4.32 ). Multifocal areas of ground-glass attenuation, small centrilobular nodular opacities, and pleural effusion may be also present. ^,

The septal lesions of ECD involve both lungs diffusely ( Fig. 4.33 ), but in some cases they may predominate in the upper or lower lobes. In addition, the pleura and mediastinal structures may be involved ( Fig. 4.34 ). The superior vena cava, along with the pulmonary trunk and main arteries, may be coated by anomalous tissue; in case of severe involvement, a reduction of the vascular lumen is also possible. The cardiac involvement may be endocardial or myocardial (not visible with high-resolution computed tomography [HRCT]) or pericardial, the latter being the most frequent and most clearly visible on images, thanks to the contrast provided by the adjacent pericardial fat (see Fig. 4.34 ).

In addition, easily appreciable and frequently associated features can contribute to the diagnosis, consisting in a sclerotic lesion of long bones, mainly tibia and femur and along their anterior aspect.

Diffuse interstitial amyloidosis.

The diffuse interstitial form of amyloidosis is characterized by smooth or nodular septal, peribronchovascular, and subpleural thickening, frequently (50%) associated with well-defined subpleural nodules that are often calcified ( Fig. 4.37 ). ^, The lesions of diffuse pulmonary amyloidosis show a basal and peripheral prevalence ( Fig. 4.38 ).

The key to the diagnosis is the existence of subpleural, confluent, calcified consolidations ( Fig. 4.39 ). Associated findings are lymph node enlargement and unilateral or bilateral pleural effusions. Tracheobronchial involvement may also be present, with thickening of the tracheal and bronchial walls due to the deposition of amyloid. In cardiac magnetic resonance (MR) imaging, the left ventricular wall is typically thickened, with associated diastolic dysfunction. Delayed contrast material–enhanced cardiac MR imaging typically shows global transmural or subendocardial enhancement.

Asbestosis.

The early lesions of this disease are a combination of centrilobular dotlike and branching opacities, which are often arranged in clusters or connected by subpleural curvilinear lines ( Fig. 4.48 ). These nodular opacities correspond to peribronchiolar nodular fibrosis, which is also responsible for hyperlucent areas (mosaic perfusion) of lobular size from air trapping. The subsequent evolution may lead to an irregular interlobular and intralobular reticulation with bronchiectasis, architectural distortion, and honeycombing.

The lesions are predominantly or exclusively located in the subpleural lobules of the posterior regions of the lower lobes and anteriorly in upper lobes (helical course) in the early phases of disease ( Fig. 4.49 ), but they may become more extensive as the disease progresses.

Diffuse parietal pleural thickening and pleural plaques with or without calcifications ( Fig. 4.50 ; see also Figs. 4.48 and 4.49 ) are considered typical of the asbestos-related disease, but not all patients with asbestosis show pleural abnormalities. Parenchymal bands are also characteristic of this disease (see Fig. 4.50 ) and may reflect thickening of interlobular septa, fibrosis along bronchovascular sheaths, coarse scars, or areas of atelectasis adjacent to pleural plaques or visceral pleural thickening. ^,

Fibrotic hypersensitivity pneumonitis.

According to the new guidelines of diagnosis of fibrotic HP, on CT the “typical HP” pattern suggestive of a diagnosis of HP requires an HRCT pattern of lung fibrosis and at least one abnormality that is indicative of small airway disease ( Fig. 4.51 ). The distribution of fibrosis may be: random both axially and craniocaudally or mid lung zone–predominant or relatively spared in the lower lung zones ( Fig. 4.52 ).

HRCT abnormalities indicative of small airway disease include the so called “three-density pattern” and/or air trapping (often in a lobular distribution); ill-defined, centrilobular nodules and/or GGOs may be also present.

The term “three-density pattern” describes the presence of three different lung densities, which some radiologists have referred to as the “headcheese sign.” This pattern emphasizes the diagnostic value of lobules with decreased attenuation and vascularity on inspiratory HRCT images, especially when concomitant with air trapping at expiration, both suggesting the presence of severe bronchiolar obstruction (see Fig. 4.51 ). ^,

Sometimes fibrotic HP may present with not-typical pattern appearing similar to UIP pattern ( Fig. 4.53 ).

Idiopathic usual interstitial pneumonia–clinical idiopathic pulmonary fibrosis.

UIP is the hallmark radiologic pattern of IPF. Honeycombing is a distinguishing feature of UIP and must be present for a definite HRCT diagnosis of UIP to be made ( Fig. 4.54 ). ^{, ,}

It is associated with peripheral traction bronchiectasis or bronchiolectasis. The typical distribution of UIP is subpleural with basal predominance, although some upper lobe involvement is common ( Fig. 4.55 ); in some cases, the craniocaudal distribution of UIP may be relatively uniform. Some focal areas of only slightly increased attenuation (due to uneven fibrosis) interspersed with relatively normal alveoli may coexist. Rugged pleural surfaces (owing to the tendency for fibrosis to occur in the periphery of the secondary lobule) are very frequent. Characteristically the patches of fibrosis are sharply bordered and intermingled with areas of normal parenchyma. Asymmetric disease may occur in up to 25% of cases. Several studies have demonstrated that the positive predictive value of a radiologic diagnosis of UIP on HRCT for a pathologic diagnosis of UIP is between 90% and 100%.

The disease is typically subpleural with some extension into the inner lung in connection with thickened vessels and ectatic bronchi. The longitudinal distribution of the lesions is noteworthy. Although the more complex lesions with traction bronchiectasis and honeycombing show middle and lower predominance, a coexisting irregular reticulation is frequently seen in the upper anterior and peripheral lung. Particularly in advanced fibrosis, the lung becomes smaller and the indirect signs of retraction and remodeling are striking.

Diagnostic problems of differential diagnosis between such areas of emphysema and honeycombing may ensue. ^, A mild enlargement of mediastinal lymph nodes is present in approximately 70% of cases. Occasionally calcification and even ossification can be found, both as small, nodular foci and in disseminated, dendritic form. Associated solitary pulmonary opacities from lung cancer are possible, as in all fibrotic disorders.

Some CVDs and, more rarely, drug reactions may present with features indistinguishable from idiopathic UIP. Accordingly, suspicion for the underlying disorder is mainly formulated on clinical grounds, although occasionally specific signs of the original disease are also seen radiologically ( Fig. 4.56 ).