Computed Tomography of Diffuse Lung Diseases and Solitary Pulmonary Nodules

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3 Computed Tomography of Diffuse Lung Diseases and Solitary Pulmonary Nodules

Mario Maffessanti, MD, Giorgia Dalpiaz, MD

Foundations

Living with X-Rays and Working with Computed Tomography

X-Rays and Computed Tomography

Radiology is the science of studying anatomy and pathology using x-rays—electromagnetic waves (like visible light, only with a much shorter wavelength) capable of penetrating the tissues. In computed tomography (CT), the widely recognized imaging standard of reference for assessment of most pulmonary abnormalities, a collimated fan beam of radiation is generated by an x-ray tube inside a gantry (Fig. 3-1).

Figure 3-1 The figure summarizes conceptually the steps of a computed tomography examination, from the acquisition of the images in the gantry (1) to their displaying on a monitor (4). In the computer, the information about a set of volume units of body (voxels) (2) is rendered on the monitor on a scale of grays, according to their average attenuation (3).

The beam is quite homogeneous when entering the body (ingoing radiation) but, point by point, inhomogeneous when exiting (outgoing radiation) because of different attenuations produced by the tissues. The attenuation highly depends on the characteristics of the tissue, calcium being at the highest and air at the lowest end of a scale with soft tissue densities (e.g., organs, muscles, blood vessels, interstitium) and fat in between (see Fig. 3-1).

Always point by point, the outgoing radiation activates a matrix of tiny sensible elements (detectors) during a continuous spiral movement of the tube-detectors system around the body (scan), and the information acquired is stored in a computer. At the end of the process, the system contains a digital three-dimensional map of single unitary elements (voxels) composing the scanned volume (see Fig. 3-1).

Computed Tomography, Spiral Computed Tomography, and High-Resolution Computed Tomography

For viewing, CT is able to return the values of a two-dimensional matrix of voxels on a monitor over a scale of grays (gray-scale), where the brightest (white) spots represent the elements with higher attenuation and the darkest (black) the ones of lower attenuation. Although spiral CT is able to show images of equal quality along planes in any direction, the axial (transverse), frontal (coronal), and sagittal (lateral) views are used more commonly (Figs. 3-2 to 3-4). For each view, a stack of images may be seen in sequence simply by browsing at the workstation through the volumetric data set; at the end of the diagnostic process, the entire volume is investigated from multiple points of view.

Figure 3-2 Computed tomography transverse (axial) view of the lungs at the level of the heart (sun) (case with lung pathology). In the axial images, it is as if the patient were seen from below. Consequently, the right lung is to the left of the viewer. R, right; L, left.

Figure 3-3 Computed tomography frontal (coronal) view of the lungs at the level of the descending aorta (sun) (case with lung pathology). Again, the right of the patient is to the left of the viewer. The patient is always seen vis-à-vis. R, right; L, left.

Figure 3-4 Computed tomography sagittal (lateral) view of the right lung (case with lung pathology). In the sagittal images, the anteroposterior and the craniocaudal directions are explored. A, anterior; P, posterior.

With a diffuse lung disease (DLD), the high-resolution option is used. An actual collimation of 0.5 to 2 mm and a high spatial frequency reconstruction algorithm (edge enhancing) generate the final images.¹ The narrow collimation reduces the voxel size, thereby minimizing the averaging of densities (attenuations) inside them, and this allows the rendering of subtle anatomical details (down to 0.1–0.2 mm in the most favorable conditions¹). A limit is the noise of the image because of the reduced radiation penetrating such small voxels. However, at the pulmonary level, the difference of attenuation (contrast) between lung structures and air is high; thus, the signal is high, and the final signal-to-noise ratio remains adequate for diagnostic purposes.

Terminology

In general, the structures that attenuate more are whiter (thus, more opaque or dense or also hyperdense) than the structures that attenuate less (thus, more transparent or lucent or also hyperlucent). Therefore, the concept of density/opacity/attenuation of an element is a relative one, and for the object of interest it should be expressed in comparison with a reference structure, usually the surrounding background. The mediastinal vessels, for example, are denser than the fat in which they are embedded, but in turn this fat is denser than the tracheal lumen containing air (Fig. 3-5).

Figure 3-5 Effect of different window settings on the same image at the level of the aortic arch (sun). The upper figure has been documented with a mediastinal window that optimizes the contrast on the details at the soft tissue level. The azygos vein (arrow), for example, and a couple of small lymph nodes in the anterior mediastinum (arrowhead) are nicely seen. The lung window (lower image), on the contrary, optimizes the contrast at the lung level and allows recognition of small hyperlucencies (curved arrows) inside faint peripheral lung opacity.

In CT, the observed attenuations also can be described using a quantitative scale measured in Hounsfield units (HUs), where the zero value is given to water. Most common densities are air (−1000 HU), fat (−120 HU), water (0 HU), muscle (+40 HU), radiologic contrast medium (+130 HU), and bone (+400 or more HU).

If a special iodinated substance (contrast medium) is injected intravenously before the examination, the visibility of the tissues is enhanced (contrast enhancement) because iodine is a powerful absorber of radiation (see Fig. 3-5). A contrast medium is used rarely in the studies performed for a suspected DLD but frequently when a mass or a vascular condition is under investigation.

The body structures are best observed on monitors or films where the brightness/contrast is optimized to bring out the details of the image. However, human eye limitations do not allow a real-time appreciation of these details over the entire dynamic range of chest attenuations. Fortunately, all pertinent data are available to the machine, and the operator needs only to press a button to switch, through a dedicated processing referred to as windowing and leveling, from a mediastinal window (where the details in the lung are squeezed down to the absolute blackness) to a lung window (where the soft tissues are leveled out but the lung structures stand out with maximal detail) (see Fig. 3-5).

Lung Anatomy

Arteries, Veins, and Bronchi

When examined using a lung window, the lungs appear as overall grayish structures delineated by the mediastinum and thoracic cage. Their shape depends on how they are cut by the plane of the section (see Figs. 3-2 to 3-4). The homogeneously whitish elements standing out over this background are blood vessels, which appear roundish or linear depending on the plane of section. Their size should be appropriate to their position within the lung (central versus peripheral) (Fig. 3-6). Each artery is joined by a companion airway, characterized longitudinally as a pair of tapering whitish lines separated by air, which branch regularly (“railway track” appearance). The airways appear as white rings when cut transversally (see Fig. 3-6). Actually, the visibility of bronchial structures within an aerated parenchyma is far below the visibility of companion vessels because of the mostly air-containing nature of the former. Consequently, when looking at a normal lung, the general feeling is of a predominance of blood vessels with only sporadic visibility of bronchioles within the outer third of the lungs.

Figure 3-6 Frontal view of a normal right lung, inferiorly delimitated by the diaphragmatic dome (above the sun). White lines and dots within the pulmonary parenchyma are vessels (arrows). Black lines and rings are bronchi (arrowheads). The figure also shows a pleural fissure (curved arrow).

The outer wall of the arteries and both the outer and inner surface of the bronchial walls should present a sharply defined interface with the surrounding parenchyma (Fig. 3-7). As a rule, bronchial walls in corresponding regions of both lungs should be similar in thickness. Moreover, coupled bronchi and arteries should present roughly the same diameter, and this in turn depends on their position (central or peripheral).¹ The arteries tend to divide dichotomously, whereas the veins often present a monopodial branching with several smaller branches flowing into a main collection drain. Arteries and veins also have a different course that becomes almost perpendicular at the level of the vein entrance into the mediastinum and right heart (see Fig. 3-7).

Figure 3-7 Arteries (arrows) and bronchi (arrowheads) run parallel to each other, and they are approximately the same size at every level. The right inferior pulmonary vein (curved arrow) enters the left atrium (sun).

Mediastinal and thoracic pleura are invisible when normal. However, they can appear as subtle tiny linear opacities at the fissural level, where two layers fuse radiologically (see Fig. 3-7). When normal, lymphatics are not visible at any level, their size being inadequate to be perceptible radiologically.

Secondary Lobule

At the periphery of the lung, after 28 generations of arteries and 23 generations of bronchi,² arteries and bronchi become so small that they become invisible. As a consequence, the far peripheral pulmonary parenchyma should have no visible vessels, and the same and even more is true for the bronchi (see Figs. 3-6 and 3-7). Thus, the appreciation of vessels immediately below the pleural surface should point at an abnormality. However, exceptions are possible in the most dependent areas (Fig. 3-8), where the hydrostatic pressure is higher and the vessels larger.

Figure 3-8 Close-up detail of the posterior portion of the right lung in an axial view at the level of the intermediate bronchus (sun). Here we are facing the lobular level, with the centrilobular artery (arrow) and bronchiole (arrowhead) and a perilobular vein (curved arrow).

The centrilobular bronchioles, in particular, should not be visible, and also, when normal, the interstitial framework at the lobular level should not be appreciable per se. Consequently, when an intralobular network of white lines and/or the walls of bronchioles become visible, this means that they are thickened and hence abnormal. In general, under normal circumstances, the lobular architecture is discernible only here and there when fragments of centrilobular arteries and perilobular veins are identified; this is more frequent in the dependent portions of the lung (see Fig. 3-8).

Special Techniques

Increasing Visibility

Multiplanar Reformation

The high-resolution computed tomography (HRCT) technique has existed since the end of the 1970s, but it was the development of the spiral multislice scanning machines in the early years of the 21st century that allowed the generation of consistent high-quality images in every spatial plane in nearly every patient (MPR). The first and most popular way to render the data is called averaged because, pixel by pixel, the images show the average attenuation of the tissues across the plane of section. The natural high contrast of lung tissue and thin collimation of the x-ray beam, coupled with a high frequency algorithm of reconstruction, guarantee sharp details of anatomical and pathologic elements down to well less than 1 mm in size (see Fig. 3-8).

Pathology that occurs in the central or peripheral regions of the lung is best studied in axial images (see Fig. 3-2). Diseases that show upper or lower lung prevalence benefit from visualization in coronal view (see Fig. 3-3). Finally, disorders that prevail in the parahilar regions or in the costophrenic angles are best depicted in the sagittal view (see Fig. 3-4). With the volumetric approach, the planes may be varied according to the needs of the operator and targeted on the suspected disease (Box 3-1).

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 images (curved MPR). This allows a structure to be traced and displayed as if it lay 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. 3-9). The curved reformatted images are not real, but they are effective and easy to generate and therefore suitable for practical purposes (e.g., “virtual” bronchoscopy).

Figure 3-9 Axial view (left) and curved reformatted image (right) of the right paramediastinal region in a patient with emphysema and a cavitary lesion of the lung. The axial image is in a plane. The right is artificially reconstructed along the drainage bronchus (arrows); however, it is effective in showing the bronchial ramifications from the hilum to the periphery. MPR, multiplanar reformation.

Increasing Ambience

However, in supine patients, some whiter atelectatic lung is frequently seen in the most dependent posterior areas (see Fig. 3-8), where it may simulate pathology or, alternatively, hide it. These normal densities disappear with prone positioning (Fig. 3-13), and indeed some experts suggest the routine use of prone scans when diseases under consideration characteristically involve posterior lung (e.g., asbestosis).⁴

Figure 3-13 Supine (top) and prone (bottom) axial views of the right lung approximately at the same level. In the supine scan, there is an area of faint increased attenuation in the subpleural region (arrow). The opacity disappears in the prone position, so it should be functional and not due to lung pathology.

In normal subjects, an expiratory scan shows a uniform reduction in size of the lungs together with a homogeneous increase of their density, due to the reduced amount of air within the alveoli. When an arterial obstructive or a bronchial stenotic disease is present, variable portions of lung become darker than normal because of the reduced blood supply caused directly by hampered vascular filling or indirectly caused by hypoxemic vasoconstriction. However, in the expiratory CT, the hyperlucent areas due to vascular obstruction physiologically increase their density, whereas in the case of bronchial stenosis they do not because the air does not exit from the alveoli (air trapping) (Fig. 3-14). When arterial obstructive or bronchial stenotic diseases are suspected, supplementary expiratory scans should then be added to complete the investigative process.

Figure 3-14 Frontal view of the right lung of a patient with constrictive bronchiolitis. The existence of patchy areas of different density due to air trapping is better demonstrated by the expiratory scan (arrowheads). Insp., inspiratory scan; Exp., expiratory scan.

Diffuse Lung Diseases

Elementary Lesions

The radiologic appearance of each DLD depends on the elementary lesions and their distribution throughout the lung. The beginning of the radiologic process should involve verifying the existence of abnormalities, in particular of elements causing an increased absorption of the x-rays (opacities) or a reduced attenuation of them (hyperlucencies). If abnormalities exist, the next step should entail identifying their prevalent aspect, which in turn depends on the underlying pathology. Another step should involve localizing the abnormalities, both in relation to the lobular architecture (when possible) and their topographic distribution throughout the lung. The lobular approach presents valuable information about the modalities of arrival/onset of the lesions and their spreading routes, and the topographic approach helps discriminate among diseases with similar presentation.

The lobular approach should be quite obvious for the pathologist, who will find it extraordinarily easy to recognize many radiologic aspects of diseases with which he or she is acquainted from gross organ inspection (a radiologic image is essentially a black-and-white representation of gross lung examined with a magnifying glass). The possibility of confirming and specifying a disease using its distribution throughout the lung may be less intuitive but will become excitingly new and beneficial with practice. After all, the two modalities are not mutually exclusive; on the contrary, they strengthen each other through the concept of pattern.

Patterns

Patterns in practice of medicine are the ensemble of characteristic elements that give proof and name to a disease or to a family of diseases. In the DLD universe, basic radiologic patterns play an important role at the beginning of the diagnostic assessment. According to the literature and on the basis of the personal experience, the distinction of six main radiologic patterns is suggested:

• Septal pattern (linear pattern, preserved architecture)

• Fibrotic pattern (linear pattern, distorted architecture)

• Nodular pattern

• Alveolar pattern

• Cystic pattern (focal lucencies pattern)

• Dark lung pattern (diffuse lucencies pattern)

There are some limitations when thinking in terms of patterns. First, the same disease may present with different radiologic patterns. This may result from its variable pathologic expression in a given patient (e.g., pulmonary manifestations of progressive systemic sclerosis may show histologic usual interstitial pneumonia [UIP], nonspecific interstitial pneumonia [NSIP], organizing pneumonia [OP], and even diffuse alveolar damage patterns), from its temporal phase (e.g., a hypersensitivity pneumonitis may present in the acute, subacute, or chronic stage) or from its natural progression (e.g., an NSIP may proceed from a minimal changes pattern to end-stage lung disease). Second, the same pattern may be present in several diseases (a classical model being the systemic collagen vascular diseases [CVDs]), and also this is not unexpected because the lung has a limited number of reactions to different insults. These caveats are not an absolute limit to the diagnostic approach using patterns, but they underscore the necessity of a tight integration of imaging with clinical presentation and pathology in arriving at a meaningful diagnosis for the patient, as widely recognized in the literature.⁵

Septal Pattern

Definition

A septal pattern is present when a thickening of the perilobular interstitium is appreciable, making the lobular boundaries evident. The bronchovascular bundle is also usually thickened, producing changes both at the central parahilar level and in the centrilobular core (Fig. 3-15). The final effect is of that of a regular network of white lines with increased evidence of the perilobular interstitial architecture but without retraction or remodeling of pulmonary structures. For this reason, the septal pattern is also called regular linear pattern or linear pattern with preserved architecture.

Figure 3-15 Radiology (A) and pathology (B) of septal diseases. Thickening of septal and fissural interstitium associated with peribronchovascular cuffing is the key element of this pattern.

High-Resolution Computed Tomography Signs

A network of white lines due to thickened septal, peribronchovascular, and subpleural interstitium is the trademark of this pattern. The thickened interlobular septa appear as white lines 1 to 2 cm in length outlining the polygonal boundaries of secondary lobules (interlobular or perilobular reticulation) (Fig. 3-16). Normally, these septa are not recognizable, so their presence points to an abnormality. A few lines inside the lobule may be also visible.^2,⁶

Figure 3-16 Septal thickening. This sagittal view shows thickened septa in the upper lobe (curved arrows). The thickening of the interlobular septa outlines secondary pulmonary lobules of various sizes.

Centrilobular peribronchovascular thickening becomes manifest as a cuffing of the core structures of the lobule. The bronchiole, usually invisible under normal conditions, becomes evident as a white ring adjacent to a white dot of similar size (the centrilobular arteriole). It is enlarged compared with those identifiable in adjacent portions of pulmonary parenchyma (Fig. 3-17).

Figure 3-17 The centrilobular peribronchovascular thickening manifests itself as increased visibility of the centrilobular structures (arrows).

The thickened peribronchovascular bundle at a more central level is also perceived as arteries of increased size, compared with similar portions of pulmonary parenchyma, and as thickening of bronchial walls (Fig. 3-18). As a rule, vessel size and bronchial wall thickness in corresponding regions of one or both lungs should be similar, and a comparative evaluation of different lung regions is helpful and makes the recognition of the abnormalities easier.²

Figure 3-18 Central peribronchovascular thickening. In this coronal image, there is a thickening of the central peribronchovascular interstitium that manifests as peribronchial cuffing of segmental bronchi (arrow) and increased size of the companion arteries (arrowhead).

The subpleural interstitial thickening should be evaluated at the edges of the lung as a white enveloping line simulating thickened pleura. This sign is often easier to identify at the fissural level, where two layers of subpleural interstitium coexist ⁷ (Fig. 3-19).

Figure 3-19 Subpleural interstitial thickening. The sagittal view shows thickening of subpleural interstitium, which is easily recognizable in relation to the fissures (arrowheads).

Pleural effusion may be an additional finding in some septal disorders. It can be small and may be seen along the costovertebral angles or fissures (Fig. 3-20), where large, significant compressive effects on the adjacent parenchyma may be present.

Figure 3-20 A pleural effusion is visible in this image along the costovertebral angle as a meniscus (arrows) and along the fissure (arrowhead).

Subsets

Morphologic characteristics of the septal thickening pattern allow the distinction of two possible subsets of the septal pattern: smooth and nodular.

Subset Smooth

The anatomy of the three interstitial compartments (perilobular, peribronchovascular, and subpleural) is thickened more-or-less regularly with smooth profiles. The polygonal outlines of the lobules are visible without focal abnormalities. Their shape is variable, depending on the CT plane (Fig. 3-21).

Figure 3-21 Four images of the right lung in a disease presenting with Septal Pattern, subset Smooth. A patchy distribution of thickened septa creates polygonal networks with smooth profiles.

Inside the lobules, enlarged arteries and bronchioles with thickened walls are often visible. Smoothly thickened fissures are recognizable (Fig. 3-22), in particular with multiplanar reconstructions.⁸ Coexisting patches of faint opacities are possible, due to partial alveolar filling (ground glass opacity [GGO]). However, these patches should not overshadow the septal aspects; otherwise, an alveolar pattern should be considered.

Figure 3-22 This coronal view shows smooth thickening of a fissure (arrow), of the perilobular interstitium (curved arrow), and of the peribronchovascular interstitium (arrowhead).

Diseases in the Septal Pattern, subset Smooth, are listed in Box 3-2.

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, due to minimal alveolar edema ⁹ (Fig. 3-23). There is a tendency for the hydrostatic edema to show a symmetrical basal and posterior distribution (in supine patients) (Fig. 3-24), but patchy nongravitational distributions are not impossible.¹⁰

Figure 3-23 This coronal view shows bilateral smooth perilobular, peribronchovascular, and subpleural thickening in a patient with hydrostatic pulmonary edema (Fig. 3-22 is a close-up of this image). Areas of faint GGO are also present (arrows).

Figure 3-24 Supine patient, axial scan at the lung bases. The image reveals the posterior (gravitational) prevalence of bronchial cuffing (curved arrows) and bilateral pleural effusion (arrows) in a patient with congestive heart failure. Note also the enlarged heart (sun).

Heart enlargement and bilateral pleural effusion are common findings in cardiogenic pulmonary edema (see Fig. 3-24), and in a number of patients also a pericardial effusion may coexist. When the flow of the lymph to the systemic veins decreases, an enlargement of mediastinal lymph nodes due to fluid stagnation may occur¹¹ (Fig. 3-25).

Figure 3-25 Axial scan (mediastinal window) in a patient with congestive heart failure. Subcarinal enlarged lymph nodes are visible in the mediastinum (curved arrows). A small bilateral pleural effusion coexists (arrows).

Lymphangitic Carcinomatosis

Lymphangitic carcinomatosis may present with a fully smooth subset ¹² (Fig. 3-26), but not infrequently nodular irregularities (beaded appearance of septa and fissures) and random micronodules in areas of thickening occur. Nodules result from focal growth of cells within the lymphatics and local extensions into the parenchyma.^6,¹³ Subpleural thickening, when present, may also be smooth or nodular. Pleural effusion is unilateral in 50% of cases.

Figure 3-26 Sagittal view in a patient with lymphangitic carcinomatosis. The image shows smooth thickening of interlobular septa in the right upper lobe (curved arrows) and a thickening of the peribronchovascular interstitium, resulting in an increased thickness of bronchial walls and increased size of companion arteries (arrows). A pleural effusion is also present, with basal (sun) and intrafissural distribution.

The lesions of lymphangitic carcinomatosis are typically patchy, often unilateral, and not gravity-dependent ¹² (Fig. 3-27). Hilar lymphadenopathy is visible in 50% of patients. Enlarged mediastinal lymph nodes can be also seen in a number of cases (25% to 50%).¹³ Dedicated CT window settings may demonstrate metastatic lesions elsewhere (Fig. 3-28).

Figure 3-27 This coronal view shows a unilateral lymphangitic carcinomatosis. Note the non–gravity-dependent smooth septal thickening in the right upper lobe (arrowhead), the thickening of the peribronchovascular interstitium (curved arrow), and a pleural effusion (sun), together with lower lobe atelectasis.

Figure 3-28 Sagittal view at the level of the midline in a patient with multiple skeletal metastases. The image has been documented with bone window settings and shows multifocal spotty white areas in the sternum (arrow) and thoracic vertebrae (arrowheads).

Veno-occlusive Disease

The presentation of veno-occlusive 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 ^14,¹⁵ (Fig. 3-29).

Figure 3-29 Pulmonary veno-occlusive disease in a 25-year-old man. The axial CT image shows widespread smoothly thickened interlobular septa (curved arrow), bronchial cuffing in the centrilobular area (arrowhead), and a right pleural effusion (arrow). The sun indicates the heart.

The lesions present a geographical appearance with variable localization. They are always bilateral and may have a gravitational preference ¹⁵ (Fig. 3-30).

Figure 3-30 An axial scan of the same patient as in Figure 3-29 confirms the gravitational distribution of the lesions.

A key radiologic sign is a coexisting enlargement of the central pulmonary arteries compatible with arterial pulmonary hypertension ¹⁶ (Fig. 3-31). The right side of the heart also may 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.¹⁴

Figure 3-31 A contrast-enhanced axial scan (mediastinal window) of the same patient as in Figure 3-29 reveals a dilated central pulmonary artery (arrowhead) and a right pleural effusion (curved arrow). Compare the size of the main pulmonary artery with the diameter of the ascending aorta (arrow): they should be the same in a normal individual.

Erdheim-Chester Disease

Erdheim-Chester disease is a non–Langerhans cell systemic histiocytosis that produces smooth septal and subpleural interstitial thickening, with more-or-less regular contours (Fig. 3-32). Multifocal areas of ground glass attenuation, small centrilobular nodular opacities, and pleural effusion may be also present.^17,¹⁸

Figure 3-32 Computed tomography scan of a patient with Erdheim-Chester disease. The image shows a caricatural bilateral smooth thickening of interlobular septa (curved arrow) and subpleural interstitium along the costal margins (arrowheads) and the fissures.

The septal lesions of Erdheim-Chester disease involve both lungs diffusely (Fig. 3-33), but in some cases they may predominate in the upper or lower lobes.¹⁸ In addition, the pleura and the mediastinal structures may be involved (Fig. 3-34). The superior vena cava, along with the pulmonary trunk and main arteries, may be “coated” by anomalous tissue, and 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 HRCT) or pericardial, the latter being the most frequent and best visible on images, thanks to the contrast provided by the adjacent pericardial fat¹⁹ (see Fig. 3-34).

Figure 3-33 Widespread basal septal thickening in the same patient as in Figure 3-32.

Figure 3-34 Axial scan (mediastinal window) of the same patient as in Figure 3-32. An abnormal dense tissue thickens the subpleural spaces (curved arrows) and infiltrates the mediastinal fat (arrowheads).

Subset Nodular

The interstitial compartments are thickened in nodular form, testifying to the existence of locally growing cells or extracellular deposits within the interstitial boundaries ²⁰ (Fig. 3-35). Being interstitial, these nodules are dense with well-defined margins² (Fig. 3-36), embedded as they are inside thickened interlobular septa and interstitial lines with an overall beaded appearance.²¹

Figure 3-35 Septal Pattern, subset Nodular, showing both smooth and beaded septal thickening of the peripheral interstitium (interlobular septa) in which small nodules are visible (curved arrows).

Figure 3-36 Septal Pattern, subset Nodular. The nodules inside the thickened septa and subpleural interstitium have high-density and well-defined margins (arrows).

The Septal Pattern, subset Nodular, differs from the Nodular Pattern, subset Lymphatic, where the nodules are more-or-less individually seen along appropriate routes that are not thickened. This important distinction helps the radiologist distinguish the two diseases (e.g., lymphangitic carcinomatosis [Septal Pattern, subset Nodular] from sarcoidosis [Nodular Pattern, subset Lymphatic]).

Lymphoid interstitial pneumonia is a multifaceted disease that may present with different patterns, including septal. However, it tends to show more often nodules along lymphatic routes, so it has been placed in the Nodular Pattern, subset Lymphatic. Diseases in the Septal Pattern, subset Nodular, are listed in Box 3-3.

Box 3-3 Diseases Presenting with Septal Pattern, Subset Nodular

Frequent

Lymphangitic carcinomatosis (see Septal Pattern, subset Smooth)

Rare

Diffuse interstitial amyloidosis

Lymphoid interstitial pneumonia (see Nodular Pattern, subset Lymphatic)

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. 3-37).^22,²³ The lesions of diffuse pulmonary amyloidosis show a basal and peripheral prevalence²⁴ (Fig. 3-38).

Figure 3-37 Diffuse interstitial amyloidosis. This axial scan at the level of the heart (sun) shows both smooth and nodular septal thickening (arrowheads) associated with well-defined subpleural nodules (curved arrow).

Figure 3-38 This is a more basal scan from the same patient as in Figure 3-37. Bull’s-eye, liver; sun, heart.

The key to the diagnosis is the existence of subpleural, confluent, calcified consolidations (Fig. 3-39). Associated findings are lymph node enlargement and unilateral or bilateral pleural effusions.²⁵ Tracheobronchial involvement may be also present, with thickening of the tracheal and bronchial walls due to deposition of amyloid.

Figure 3-39 Mediastinal window at the level of the great vessels (bull’s-eyes) in the same patient as Figure 3-37. The image shows patchy peripheral calcified consolidations in both lungs (arrowheads).

Fibrotic Pattern

Definition

A fibrotic pattern is present when signs of retraction and remodeling of thoracic structures are recognizable at the lobular level and/or in correspondence of larger portions of lung (Fig. 3-40).

Figure 3-40 Radiology (A) and pathology (B) of fibrotic diseases. The signs of retraction and remodeling on the pulmonary structures are the key elements to identify this pattern.

High-Resolution Computed Tomography Signs

The signs of fibrotic disease are associated with the direct visualization of fibrotic elements or to the effects of retraction and remodeling on pulmonary structures. Direct signs of fibrosis are irregular linear opacities (irregular reticulation), vessel enlargement, traction bronchiectasis, and bronchiolectasis with bronchial wall thickening, parenchymal bands, and honeycombing. The effects of lung retraction and remodeling are identifiable as irregular displacement of fissures, crowding of vessels, and retraction of the pleural/mediastinal surfaces with interface signs. These features are all due to shrinking of the pulmonary parenchyma with volume loss.

Irregular linear opacities (irregular reticulation) are criss-crossing, not uniform, wavering, white lines that appear as though they were traced by an unsteady hand on the lung background (Fig. 3-41). Sporadically, one could imagine that one or more of these lines represent remnants of interlobular septa (interlobular reticulation), but the visibility of interlobular septa is not the rule. On the contrary, the distortion due to fibrosis tends to reduce the recognition of the lobular architecture.² Most of these lines criss-cross spaces of lobular size, so they are also called intralobular reticulation (see Fig. 3-41).

Figure 3-41 Irregular linear opacities (irregular reticulation) in the periphery of the lung (arrowheads) with remodeling of the lobular architecture, that indeed is no more recognizable. Signs of retraction are also evident on the bronchial structures with bronchiectasis and bronchiolectasis (curved arrows).

The vessels may appear enlarged with shaggy margins (interface sign), and the bronchi are irregularly ectatic with thickened walls and a winding or corkscrew appearance (Fig. 3-42). In the periphery of the lung, the bronchioles may be also ectatic (so visible) with the same appearance (traction bronchiectasis and bronchiolectasis) (see Fig. 3-41). Finally, parenchymal bands are long lines representing thickened connected septa marginating several lobules but also focal scarring or linear atelectasis.²

Figure 3-42 Tiny irregular lines in the anterior portion of the right lung. A long, ectatic winding bronchus with thickened walls is well appreciable (arrows). The ground glass component of the image is reasonably a fibrotic GGO, due to concomitant irregular reticulation and bronchiolectasis (arrowhead).

As a whole, the described lesions may vary in size and aspect, from a more-or-less coarse obvious pattern to a subtle hazy opacification of the lung referred to as fibrotic GGO that is only minimally inhomogeneous. In the latter case, ectatic bronchioles inside the GGO and superimposing irregular reticulation (indicating fibrosis) are the discriminant features ²⁶ (see Fig. 3-42).

A peculiar feature of destructive fibrosis is honeycombing. In honeycombing, small hyperlucent areas of variable size (from 2 mm to 1 cm) and shape separated by well-defined thick walls are crowded in an area where the lung architecture is lost ²⁷ (Fig. 3-43). They should be distinguished (not easy and not always possible, especially in the early cases) from also roundish or elongated, windingly linear, transparencies corresponding to ectatic bronchioles (called microscopic honeycombing by Nishimura²⁸).

Figure 3-43 This is an example of what is called honeycombing radiologically (arrows)—multiple air-containing hyperlucent spaces (black holes) with well-defined thick walls grouped in several layers in an area of complete loss of the lobular architecture.

Signs of retraction and remodeling give further, at times striking, evidence of the existence of a fibrotic disorder. At the pulmonary interface, a pleural line with shaggy margins and connections with parenchymal irregular lines (interface signs) may be evident (Fig. 3-44). A thickening of the subpleural interstitium may also be obvious, as well as an increased thickness of extrapleural/mediastinal fat, the latter compensating for the shrinking lung (see Fig. 3-44). A shaggy thickening may be also observed at the interface of the visceral pleura, which becomes angulated and displaced.

Figure 3-44 Shaggy margins of pleura (arrows) and vessels (arrowheads) (interface sign) and retraction of extrapulmonary structures toward the shrunk lung (curved arrow) are also indications of an underlying pulmonary fibrosing disorder.

When fibrosis advances, one or more lobes and even the entire lung may become reduced in size. The signs associated with this are angulation and displacement of fissures, crowding of vessels and bronchi, and mediastinal and diaphragmatic attraction toward the affected lung (Fig. 3-45).

Figure 3-45 In this advanced fibrosing disease, the volume of the lower lung is markedly reduced, as shown by high diaphragmatic domes (arrowheads) approaching lowered fissures (arrows).

Subsets

Depending on the underlying disease, the morphologic elements and their topographic distribution may be arranged in subsets that are helpful in focusing the observer on definite categories of fibrotic disorders and at times even on specific diseases. The main subsets of the fibrotic pattern are UIP, NSIP, tug-of-war fibrosis, and bronchocentric fibrosis.

Subset Usual Interstitial Pneumonia

The UIP subset is defined by the presence of patchy areas of irregular reticulation and gross honeycombing with prominent signs of architectural distortion (Fig. 3-46). Traction bronchiectasis and microscopic honeycombing in connection with the pathologic areas are also characteristic.²⁸ Some GGO is possible but less extensive than the reticulation.²⁹ Signs of retraction and remodeling of vessels, fissures, lobes, and pulmonary boundaries are common, especially in advanced cases (Fig. 3-47).

Figure 3-46 Honeycombing in the posterior costophrenic lung in a sagittal view. Note the sharp demarcation of the diseased lung with the normal pulmonary parenchyma (arrows).

Figure 3-47 This sagittal view shows several signs of an important fibrosing pulmonary disorder: honeycombing in the anterior portions of the lung (sun), retraction of the mediastinal fat (arrow), and displacement of several ectatic bronchi (arrowheads).

The UIP subset may be seen both in idiopathic pulmonary fibrosis (IPF) and, with identical aspects, in several CVDs or, more rarely, chronic drug toxicity. Aspects of UIP are present also in individuals who develop an acute clinical course (acute exacerbation or acceleration of IPF), where the histologic findings show superimposed features of acute lung injury; in these cases, the radiologic presentation is usually dominated by the alveolar densities of acute lung injury and it is consequently discussed under the Alveolar Pattern, subset Acute. Diseases in the Fibrotic Pattern, subset UIP, are listed in Box 3-4.

Asbestosis

The early lesions of this disease are a combination of centrilobular dotlike and branching ³⁰ opacities, which are often arranged in clusters or connected in form of subpleural curvilinear lines³¹ (Fig. 3-48). These nodular opacities correspond to peribronchiolar nodular fibrosis responsible also for hyperlucent areas (mosaic perfusion) of lobular size from air trapping.³¹ The subsequent evolution may lead to an irregular interlobular and intralobular reticulation with bronchiectasis, architectural distortion, and honeycombing.³²

Figure 3-48 Axial scan at the level of the heart (sun) in a patient with asbestosis. Areas of advanced fibrosis with honeycombing (arrowhead), but also more initial subpleural lines with beaded appearance (arrow) are evident. The pattern is completed in this case by patchy areas of mosaic oligemia (curved arrows).

The lesions are predominantly or exclusively located in the subpleural lobules of the posterior regions of the lower lobes in the early phases of disease,³⁰ but they may become more extensive as the disease progresses (Fig. 3-49).

Figure 3-49 Axial scan at the carinal level in the same patient as in Figure 3-48. At this transversal level, the fibrotic involvement of the lung is only initial, and the honeycomb changes are confined in restricted areas (arrow). Bilaterally, there are pleural plaques of typical aspect (arrowheads).

Diffuse parietal pleural thickening and pleural plaques with or without calcifications (Fig.3-50; see also Figs. 3-48 and 3-49) are considered typical of the asbestos-related disease, but not all patients with asbestosis show pleural abnormalities.² Parenchymal bands are also characteristic of this disease (see Fig. 3-50) 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.^32,³³

Figure 3-50 Calcified pleural plaques (curved arrows), subpleural lines (arrowhead), and parenchymal bands (arrows) are variably distributed in this patient with initial parenchymal asbestosis.

Chronic Hypersensitivity Pneumonitis

Fibrotic GGO and irregular reticulation with traction bronchiectasis and bronchiolectasis are the most common features of hypersensitivity pneumonitis, but honeycombing is also a frequent finding. Characteristically, the fibrotic lesions may be associated to a mixture of lobular areas with decreased attenuation, centrilobular nodules, and cysts inside the GGO ³⁴ (Fig. 3-51).

Figure 3-51 Mixed densities pattern in a patient with chronic hypersensitivity pneumonitis. Coarse irregular linear opacities (arrowhead) coexist with patchy honeycombing (arrow) and areas of hyperlucent lung with reduced vascularity (curved arrow).

Both reticulation and honeycombing may prevail in the periphery of the lung and may show upper lung predominance (Fig. 3-52). However, a random distribution is also common. Lower lobe predominance is uncommon.³⁴

Figure 3-52 The lesions of chronic HP tend to prevail in the upper regions of the lung (arrows), and this is true also for honeycombing. In this sagittal view of the left lung, the base of the lung is relatively free of lesions, and this is an important element in the differential diagnosis with idiopathic UIP.

Accompanying signs of retraction on the pleural surfaces and on the mediastinal profiles are generic consequences of the underlying fibrosis. Some volume loss may occur, particularly in the upper lungs ³⁵ (Fig. 3-53).

Figure 3-53 In chronic HP, interface signs, and evidence of architectural derangement are prevalently located in shrunken upper lobes, indicated here by the upward bowing of the major fissures (arrows).

Idiopathic Usual Interstitial Pneumonia (Clinical Idiopathic Pulmonary Fibrosis)

Patchy areas of dense irregular reticulation and macro,³¹ as well as micro²⁸ honeycombing alternating with normal lung (morphologic heterogeneity), are the most specific feature. Some focal areas of only slightly increased attenuation (due to uneven fibrosis) interspersed with relatively normal alveoli may coexist.²⁸ Rugged pleural surfaces (due to the tendency for fibrosis to occur in the periphery of the secondary lobule) are very frequent.²⁸ Characteristically, the patches of fibrosis are intermingled and sharply marginated with areas of normal parenchyma³⁶ (Fig. 3-54).

Figure 3-54 Axial scan at the level of the right liver dome (bull’s-eye) and of the heart base (sun) in a patient with idiopathic UIP. Areas of patchy honeycombing alternating with normal lung are present (arrowheads) and are typical of this disease.

The disease is typically subpleural,^29,³⁷ with some extension to the inner lung in connection with thickened vessels and ectatic bronchi.²⁸ The longitudinal distribution of the lesions is interesting. Although the more complex lesions with traction bronchiectasis and honeycombing show middle and lower predominance,³⁴ a contemporary irregular reticulation is frequently seen in the upper peripheral lung³⁸ (Fig. 3-55). Especially in advanced fibrosis, the lung becomes smaller and the indirect signs of retraction and remodeling striking.

Figure 3-55 The peripheral regions of both lungs in this frontal view of a patient with idiopathic UIP show an irregular reticulation superiorly (arrows), and typical honeycombing is present at the basal level (arrowheads).

Focal emphysematous hyperlucencies in the upper zones of the lung ²⁸ but also inside the basal lesions are possible,³⁹ and these may create diagnostic problems of differential diagnosis with honeycombing.⁴⁰ A mild enlargement of mediastinal lymph nodes is present in approximately 70% of cases.⁴¹ Occasionally, small nodular foci of calcification²⁵ or a disseminated dendriform pulmonary ossification⁴² may be found. Associated solitary pulmonary opacities from lung cancer are possible,⁴³ as much as in all fibrotic disorders.

Some CVDs ⁴⁴ and, more rarely, drug reactions⁴⁵ may present with aspects indistinguishable from the idiopathic UIP. Consequently, the suspicion of the underlying disorder may be formulated only on clinical grounds, but occasionally specific signs of the original disease are also seen radiologically⁴⁶^–⁴⁸ (Fig. 3-56).

Figure 3-56 Patchy honeycombing alternating with normal lung (UIP subset) in a patient with systemic sclerosis. In this axial scan at the subcarinal level, an enlarged esophagus with air-fluid level is visible (arrowhead) between the intermediate bronchus to the right and the junction of the upper and lower lobe bronchus to the left (arrows).

Subset Fibrotic Nonspecific Interstitial Pneumonia

The NSIP pattern is defined by the presence of homogeneous areas of GGO associated with irregular reticulation ⁴⁹ (Fig. 3-57). The percentage of each likely depends on the proportions of inflammation and fibrosis within the lung, and some authors have even attempted to identify definite subgroups based on the extent of reticulation and traction bronchiectasis.⁵⁰ Traction bronchiectasis is characteristic (Fig. 3-58). Honeycombing, on the contrary, should be absent or minimal.⁵¹

Figure 3-57 The typical fibrotic NSIP subset is characterized by areas of GGO and irregular reticulation associated with more-or-less evident bronchiectasis (curved arrows) but without significant honeycombing.

Figure 3-58 The minIP technique is valuable in showing size and extension of the bronchiectatic abnormalities inside the GGO (arrowheads). Note the absence of honeycomb cysts inside the pathologic area.

The NSIP subset may be seen both in idiopathic NSIP and in several CVD and drug reactions, but NSIP aspects also may be present in patients with acute exacerbation of NSIP (accelerated NSIP) where the histologic findings show superimposed features of acute lung injury. In latter cases, the radiologic presentation is dominated by the alveolar densities of acute lung injury, and this is consequently discussed in the Alveolar Pattern, subset Acute. Diseases in the Fibrotic Pattern, subset Fibrotic NSIP, are listed in Box 3-5.

Idiopathic Fibrotic Nonspecific Interstitial Pneumonia

Characteristic features of disease include reticular/GGO opacities with homogeneous aspect in affected areas. Inside the lesions, traction bronchiectasis and bronchiolectasis are common, and their extent has been shown to be a reliable indicator of fibrosis ⁵⁰ (Fig. 3-59). Dense consolidations, on the contrary, are uncommon, and their presence should raise the suspicion of another disease (such as OP, chronic eosinophilic pneumonia, or bronchioloalveolar carcinoma) or, in the appropriate clinical setting, of an acute exacerbation (see Alveolar Pattern, subsets Acute and Chronic).⁴⁹ Honeycombing, if present, is mild and otherwise should raise the suspicion of UIP⁵¹; however, it has been reported that, over time, a number of NSIP originally presenting with NSIP pattern progress to UIP pattern.⁵²

Figure 3-59 This is a patient with a typical fibrotic NSIP subset, possibly from systemic sclerosis because the esophagus is enlarged. Mainly the periphery of both lungs is involved by a subtle reticular GGO (arrows) without significant honeycombing. A shrunk right lower lobe (arrowhead) is more extensively involved, and it contains some bronchiectasis.

The disease is bilateral and symmetric,⁴⁹ involving mainly the lower lungs in more than 90% of the cases⁵¹ (Fig. 3-60), otherwise equally distributed. On the contrary, primarily upper lobe lesions are very rare.⁴⁹ Axially, the pattern is diffuse in more than 50% of the cases or predominantly peripheral subpleural, but in a number of cases (20% to 43% according to some authors^27,⁵¹) the immediate subpleural regions are relatively spared.

Figure 3-60 Frontal view of both lungs at the level of the descending aorta (sun). A basal fibrotic GGO with reticulation and bronchiectasis and bronchiolectasis is indicated by the arrows. In this patient, several areas of mosaic oligemia are also present (bull’s-eyes).

Volume loss, mostly of the lower lobes, is fairly common,⁵¹ usually in conjunction with other indirect signs of fibrosis. Lymphadenopathy is possible at the mediastinal level,⁴⁹ usually mild and involving not more than two nodal stations.⁴¹

Several CVDs ⁴¹ and adverse reactions to therapeutic drugs^53,⁵⁴ may present with aspects indistinguishable from the idiopathic NSIP. Consequently, the suspicion of the underlying disorder should be formulated on clinical grounds. Occasionally, specific signs of the original disease are visible radiologically⁴⁶^–⁴⁹ (Fig. 3-61).

Figure 3-61 Supine and prone scans at a same axial level (costophrenic angles) in a patient with systemic sclerosis. In the supine image, there is some faint increased attenuation with irregular reticulation in the subpleural lung (arrows). In the prone scan, the GGO is gone (hence reversible) but the reticulation persists (arrows). In both images, the esophagus is enlarged (arrowheads).

Subset Tug-of-War

Tug-of-war subset is defined by the presence of irregular linear opacities stretching between the mediastinum and the thoracic boundaries, bridging over variably involved bronchi, fissures, and more generally anatomical structures and even pathologic elements found on their way (Fig. 3-62). The mediastinal profiles are variably stretched outward and the thoracic pleural profiles inward, hence the proposal for the name of this fibrotic subset (Fig. 3-63). Diseases in the Fibrotic Pattern, subset Tug-of-War, are listed in Box 3-6.

Figure 3-62 Tug-of-war fibrosis, axial scan. Several irregular white lines extend from the hilum, which is stretched outward (arrow) to the pulmonary periphery, which in turn is irregular for the presence of several spicules directed inward (arrowheads).

Figure 3-63 Tug-of-war fibrosis, sagittal image. Straight interstitial connection lines bridge the bronchovascular bundle, entirely stretched anteriorly and superiorly (arrow), and several peripheral irregularities point inward (arrowheads).

Box 3-6 Diseases Presenting with Fibrotic Pattern, Subset Tug-of-War

Frequent

Sarcoidosis

Rare

Berylliosis (see Sarcoidosis)

Sarcoidosis

Fibrosis may present early in the history of the disease, when nodular elements are fairly well visible. Irregularities of the margin of the nodules, distortion of fissures, bronchial irregularities, traction bronchiectasis, and more-or-less coarse linear opacities corresponding to the fibrotic component of the disease.⁵⁵ The elements of the bronchovascular bundle become crowded and show a zigzagging course with angulations^55,⁵⁶ (Fig. 3-64). Progressive fibrosis leads to central conglomeration of parahilar bronchi embedded in a dense agglomerate of tissue radiating from the center to the periphery.⁵⁶ Honeycombing and cystic abnormalities may be also seen, but rarely the honeycombing involves mainly the lower lung zones, mimicking UIP/IPF.⁵⁷

Figure 3-64 Axial scan of a patient with mild fibrosing sarcoidosis. There are some white irregular lines outstretched between the hilum and the periphery (arrows). Slightly ectatic bronchi with thickened wall (curved arrows) contribute to the feeling of a tug-of-war fibrosis. Calcified lymph nodes can be seen inside the mediastinum.

The disease shows parahilar predominance between the central and the peripheral middle and upper lung, with patchy accentuation of parenchymal distortion and severity of the lesions ^55,⁵⁸ (Fig. 3-65).

Figure 3-65 Frontal scan of a patient with sarcoidosis. The tug-of-war aspect of the fibrosing component of the disease is well appreciable in the upper lung fields. Several micronodules are also identifiable, in particular in the right middle lung field (arrow).

Mediastinal lymphadenopathy frequently coexists, often calcified. CT findings suggestive of pulmonary hypertension are possible in the late disease.² Cavitation of conglomerated masses may be seen in patients with necrotizing sarcoid granulomatosis, the entity first described by Liebow characterized by sarcoid-like granulomas and vasculitis associated with variable degrees of necrosis⁵⁶ (Fig. 3-66).

Figure 3-66 Necrotizing sarcoid granulomatosis. Here the lesions extending between the hila and the periphery are dense opacities containing air hyperlucencies from cavitation (arrows). Large bronchi stand out; they are embedded and irregularly stretched inside the opacities (arrowhead).

Subset Bronchocentric Fibrosis

The bronchocentric subset is defined by the presence of a disease in which signs of traction and remodeling prevail at the level of the bronchial elements. The fibrosis may be focal or diffuse.

Focal fibrosis from constrictive bronchiolitis is concentrated on the bronchioli and too subtle to be appreciated radiologically. However, indirect signs of bronchial narrowing are visible, namely a patchy dark lung (Fig. 3-67). This condition is subsequently discussed in the Dark Lung Pattern.

Figure 3-67 Patient with constrictive bronchiolitis post-transplantation. In this sagittal scan, there are vast areas of hyperlucent lung (dark lung) anteriorly (arrows), where vessel size and number is reduced.

Pulmonary Langerhans cell histiocytosis, also a prominently centrilobular fibrotic process, on the contrary is well visible in form of thick walls around enlarged airways (Fig. 3-68) that assume early a cystic aspect.⁵⁹ Consequently, its insertion in the cystic pattern has been considered more suitable. Diseases in the Fibrotic Pattern, subset Bronchocentric Fibrosis, are listed in Box 3-7.

Figure 3-68 Patient with early PLCH. Several ringlike opacities visible in this image represent enlarged bronchi with thickened walls, as indicated by the tiny white dot (the companion artery) nearby (arrowheads).

Box 3-7 Diseases Presenting with Fibrotic Pattern, Subset Bronchocentric Fibrosis

Frequent

Constrictive bronchiolitis (see Dark Lung Pattern)

Langerhans cell histiocytosis (see Cystic Pattern)

Rare

Airway-centered interstitial fibrosis

Airway-Centered Interstitial Fibrosis

The main findings of airway-centered interstitial fibrosis are peribronchovascular interstitial thickening with traction bronchiectasis, thickened airway walls, and surrounding dense tissue with irregular margins (Fig. 3-69). Bronchiolectasis and honeycombing may also occur in a limited number of cases. GGO, poorly defined centrilobular micronodules, and lobular air trapping with the mosaic attenuation of the dark lung pattern are lacking.⁵⁹

Figure 3-69 At a first glance, the lesions in this patient with ACIF mimic a nodular disease. Actually, the faint opacities scattered throughout the lungs are due to thickening of bronchial walls (better seen in the inset, where an enlarged view of the area between the curved arrows is shown).

(Courtesy of Fabrizio Luppi, MD, Modena, Italy.)

The lesions show a central rather than a peripheral distribution. They consistently show scarring around the airways ⁶⁰ (Fig. 3-70).

Figure 3-70 Another image of the same patient as in Figure 3-69. In the posterior left lung (curved arrows), the insistent thickening of the peripheral airways is well evident (inset).

(Courtesy of Fabrizio Luppi, MD, Modena, Italy.)

Nodular Pattern

Definition

A nodular pattern is defined by the presence of multiple roundish opacities ranging in diameter from 2 to 10 mm (Fig. 3-71).

Figure 3-71 Radiology (A) and pathology (B) of a patient with nodular disease. The presence of multiple small roundish opacities scattered throughout the lung is the key element to identify this pattern.

High-Resolution Computed Tomography Signs

On HRCT, lung nodules appear as white, roundish lesions with variable morphology and lobular distribution, depending on the route of arrival and on the modality of spread.^2,^7,⁶¹

Nodules that have low-density and ill-defined margins (nodular GGO) have a characteristic soft aspect, like snowflakes (Fig. 3-72). Sometimes they are very tiny and difficult to recognize.²⁰ They are commonly seen in patients with disease that primarily affects centrilobular bronchioles and the immediate area around them. The low-density CT aspect is due to minimal thickening of the peribronchiolar interstitium or partial filling of the peribronchiolar alveoli.² Both conditions are below the CT spatial resolution, and thus the common final effect is a focal low-density lesion. The ill-defined margins are due to progressive reduction of interstitial or alveolar involvement extending away from the centrilobular area to the periphery. These types of nodules may coalesce, resulting in the appearance of extensive GGO.

Figure 3-72 Nodules with low-density and ill-defined margins (nodular GGO). Innumerable white soft roundish lesions are visible, with an aspect similar to snowflakes.

Nodules with high-density and well-defined margins are commonly seen in patients with diseases primarily affecting the interstitium and growing spherically in it surrounded by aerated parenchyma.^2,⁶¹ They present a solid aspect like opaque beads and obscure the edges of vessels or other structures that they touch (Fig. 3-73). They may have regular or lobulated contours, the latter aspect secondary to asymmetrical growth. The nodules may coalesce with development of larger opacities or pseudoplaques along the costal or fissural margins.⁶²

Figure 3-73 Nodules with high-density and well-defined margins. Several white dense roundish lesions are visible with an aspect similar to opaque beads.

On occasion, the nodules may have shaggy profiles, especially in diseases with a fibrotic component (Fig. 3-74). The existence of small black areas inside these high-density nodules may be due to necrosis (see Fig. 3-74) or traction bronchiolectasis. The presence of faint increased lung attenuation around the nodules (halo sign) is most often an expression of hemorrhage (see Fig. 3-74) or of inflammatory infiltrates from any origin.^62,⁶⁵ Regarding the lobular distribution, this is the result of the route of arrival and of their modality of spread, both underlying their distinction in subsets.