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High-Resolution CT of the Lungs

¹ Department of Radiology, 2910 Taubman Center, University of Michigan Medical Center, 1500 E. Medical Center Dr., Ann Arbor, MI 48109-0326.

Received October 12, 2000; accepted after revision March 19, 2001.

 
Address correspondence to E. A. Kazerooni.

Introduction

Top Introduction Background Anatomic and Technical... Pitfalls Clinical Indications for High... Specific Diagnoses Possible with... Conclusion References

 
I first began to hear about high-resolution CT toward the end of my radiology residency in 1991. I was new to radiology, and so was high-resolution CT. In a way, that newness leveled the playing field between residents and faculty; none of us were very sure what we were looking at. I remember seeing new high-resolution CT images and scouring the literature to figure out what we were seeing. When it was my turn to present at our weekly Friday morning CT conference, I spent the entire hour presenting only high-resolution CT cases to our group of thoracic and abdominal CT radiologists and residents. Many squinted their eyes to see what was being described. As a thoracic radiology fellow, I remember trying to figure out what was a normal finding on high-resolution CT and what was not.

Now, a decade later, high-resolution CT continues to be an area of great interest to me, and sometimes a source of considerable frustration. As I have matured as a thoracic radiologist, so has the use of high-resolution CT. It is part of my daily clinical work and research. My collaborations with my pulmonary medicine and thoracic surgery colleagues in a specialized center of research for interstitial lung disease, with our lung transplantation program and lung volume reduction surgery, continue to raise questions about what high-resolution CT can tell us about diffuse lung disease, and what more we need to know to accurately diagnose and predict response to therapy and survival in patients with diffuse lung disease. For many patients, their disease will be the cause of their mortality, and both the disease and the therapy itself, the source of morbidity.

The appearance of most lung diseases on high-resolution CT has already been described, from asbestosis to Hermansky-Pudlak syndrome, and from sarcoidosis to lysinuric protein intolerance [1,2,3,4,5,6,7,8,9,10,11]. The accuracy of high-resolution CT for detecting disease and distinguishing among diseases has been well documented, and the advantages over chest radiography and conventional CT have been elucidated [4, 12,13,14,15,16,17,18,19]. High-resolution CT is now being used extensively to evaluate the response of lung disease to therapy and as a marker of underlying pathophysiology and physiologic processes [20,21,22,23]. New advances in CT technology may allow routine submillimeter scanning, total lung volumetric high-resolution CT, dynamic CT throughout the respiratory cycle (possible now only on ultrafast scanners), reproducible spirometric gating of chest CT acquisitions to specific points in the respiratory cycle, and computer-aided diagnosis [24,25,26].

This article will review the history of high-resolution CT, technical considerations in performing high-resolution CT, test characteristics of the technique, and the clinical indications for high-resolution CT, and will provide an overview of a pattern-based approach to interpretation. For readers interested in more comprehensive resources on this subject, the third edition of the textbook, High-Resolution CT of the Lung, by W. Richard Webb, Nestor L. Müller, and David P. Naidich, is recommended [27]; and the second edition of the textbook, High-Resolution CT of the Chest: Comprehensive Atlas, by Eric J. Stern and Stephen J. Swensen, is a richly illustrated complementary resource [28]. New editions of both textbooks were published in early 2001.

Background

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During the last two decades, high-resolution CT of the lungs has developed into a mature technique for the evaluation of diffuse pulmonary parenchymal abnormality. At its simplest, high-resolution CT is a sampling tool that combines 1- to 2-mm thin-collimation CT images with a high-spatial-frequency reconstruction algorithm to generate images that show exquisite lung detail. The foundation for high-resolution CT began in 1975 with radiologic—pathologic correlative studies of postmortem lungs, resulting in a 1978 publication on small nodules, with special reference to peribronchial nodules, in the American Journal of Roentgenology [29]. The technique of high-resolution CT for diffuse lung disease was initially described by Todo et al. [30] from Kyoto University, in 1982, for 21 patients with either diffuse panbronchiolitis, lymphangitic spread of cancer, sarcoidosis, or tuberculosis. Their report in the Japanese Journal of Clinical Imaging presented careful correlation of the abnormalities seen on high-resolution CT images with the abnormalities seen on corresponding inflation-fixed lung specimens. From this beginning, the foundation of high-resolution CT interpretation has been, and remains, radiologic—pathologic correlation and the relationship of abnormalities to the architecture of the secondary pulmonary lobule.

Great interest in the technique followed the 1985 publication of an article by Zerhouni el al. [31] in the inaugural issue of the Journal of Thoracic Imaging, in which the researchers described their 3-year experience with the technique and presented for the first time an attempt to define patterns of abnormality that could be used to classify diffuse pulmonary diseases. To this day, a pattern-based approach to interpretation, coupled with the distribution of abnormality throughout the pulmonary parenchyma, remains the key to differential diagnosis. In 1990, the radiologist—pathologist team of Nestor L. Müller and Roberta R. Miller published a two-part manuscript [32, 33] on the use of CT in chronic diffuse infiltrative lung diseases in the American Review of Respiratory Disease; and by 1993, sufficient interest existed in high-resolution CT that the Journal of Thoracic Imaging dedicated two consecutive issues to the topic, guest-edited by Müller [34]. The material in those two issues remains some of the best available. For readers interested in becoming more familiar with the terminology and definitions applied to high-resolution interpretations, the "Standardized Terms For High-Resolution Computed Tomography of the Lung: A Proposed Glossary" [35] appeared in the Journal of Thoracic Imaging in 1993, and an illustrated glossary of high-resolution CT terms appears in the textbook High-Resolution CT of the Lung [27].

Anatomic and Technical Considerations

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Anatomy
The interstitium of the lung can be divided into a central (or axial) compartment that surrounds the bronchovascular bundles, and the peripheral (or septal) interstitium that includes the interlobular septa and the subpleural interstitium [36]. The smallest anatomic unit visible on high-resolution CT is the secondary pulmonary lobule (Fig. 1). The walls of the lobules are the interlobular septa; they correspond to the Kerley B lines seen on chest radiographs of patients with left heart failure and interstitial edema. The interlobular septa are not usually visible unless abnormal; at 0.1 mm thick, they are at the lower limit of high-resolution CT resolution [37]. The occasional visible septa may be normal. Structures of 0.2-0.3 mm can be routinely identified on high-resolution CT when they are perpendicular to the plane of imaging. The diameter of the pulmonary artery supplying each lobule is 1 mm, and the diameter of the intralobular acinar arteries is 0.5 mm; both are readily seen on high-resolution CT. Bronchi are visible, depending on the thickness of their walls. The 1.0-mm-diameter bronchiole supplying the lobule has an approximately 0.15-mm wall, just at the limit of high-resolution CT resolution, and barely, if at all, visible on high-resolution CT images [37, 38]. The major types of abnormality involving the secondary pulmonary lobule are illustrated in Figure 2.

View larger version (27K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —Line drawing of a secondary pulmonary lobule. Borders of lobule are interlobular septa. At center of each lobule is a bronchiole and a pulmonary artery (blue). Pulmonary vein (red) run in interlobular septa. Lymphatics (green) are found in interlobular septa and in central or axial interstitium that surrounds bronchovascular bundles.

View larger version (24K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2. —Line drawing of types of abnormalities found on high-resolution CT. (Reprinted with permission from [141])

Thin Collimation and High-Spatial-Frequency Reconstruction Algorithm
The two consistent components of high-resolution CT technique include the use of thin collimation, usually 1-2 mm, coupled with a high-spatial-frequency reconstruction algorithm. These technical adaptations to conventional chest CT are designed to improve spatial resolution and thereby improve the ability to detect small structures and subtle abnormalities such as thick interlobular septa, cyst walls, small nodules, ground-glass opacity, and bronchiectasis [39] (Figs. 3A,3B and 4A,4B). Thin collimation reduces partial volume averaging from adjacent structures and in particular from adjacent aerated lung tissue; and the sharp algorithm reduces the image smoothing that is characteristic of standard or soft-tissue reconstruction algorithms in order to increase spatial resolution, at the expense of increased image noise [39, 40]. In larger patients, the noise can usually, but not always, be countered by increasing the tube current used for scanning (Fig. 5A,5B).

View larger version (114K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —63-year-old man with asbestosis and pleural plaques resulting from exposure to asbestos. Conventional CT scan at 10-mm collimation using standard reconstruction algorithm.

View larger version (122K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —63-year-old man with asbestosis and pleural plaques resulting from exposure to asbestos. 1.5-mm collimation high-resolution CT scan reformatted using high-spatial-frequency reconstruction algorithm obtained at same level shows pleural plaques. However, thickened inter-and intralobular septa of asbestosis (arrowheads) are more clearly seen on B. On A, it is difficult to distinguish partial volume averaging adjacent to pleural plaques from lung abnormality.

View larger version (104K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A. —57-year-old man with obliterative bronchiolitis of chronic lung transplant rejection with normal chest radiograph. Conventional CT scan through lung bases shows subtle areas of ground-glass opacity (arrows), representing partial volume averaging of bronchial walls.

View larger version (109K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B. —57-year-old man with obliterative bronchiolitis of chronic lung transplant rejection with normal chest radiograph. High-resolution CT scan at same anatomic level as A shows diffuse cylindrical bronchiectasis. Signet ring sign of bronchiectasis is illustrated (arrowheads).

View larger version (161K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A. —59-year-old obese woman who underwent high-resolution CT that was nondiagnostic because of patient's size. High-resolution CT scan is degraded by extensive noise and is uninterpretable.

View larger version (121K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B. —59-year-old obese woman who underwent high-resolution CT that was nondiagnostic because of patient's size. Scout topogram from CT examinations reveals patient's body size. Although in most obese patients increasing scanning technique can improve image quality, in very obese patients to do so is not possible.

Image Spacing
High-resolution CT is a sampling examination of the lung, in which thin sections are taken at staggered intervals, revealing both the pattern and the distribution of abnormality, so that a differential diagnosis—or sometimes a single diagnosis—can be rendered [33]. There is a tendency to think that with greater sampling or more images, more diagnostic information is provided. However, there is little consistency in the number of images obtained at different centers. Sampling ranges from one or two images at set anatomic levels, such as the aortic arch, the carina, and just above the diaphragm; to six to eight images evenly spaced throughout the lungs; to images at 1-cm intervals throughout the entire lung [41].

Few studies have actually looked at the appropriate sampling frequency. In a report by Leung et al. [19] comparing the accuracy of high-resolution CT and conventional CT in 75 consecutive patients with chronic diffuse infiltrative lung disease, two observers interpreted three separate sets of CT scans in each patient in random order. These sets included three high-resolution CT scans at the level of the aortic arch, the tracheal carina, and 1 cm above the right hemidiaphragm; three 10-mm collimation CT scans obtained at the same levels as the high-resolution CT scans; and a complete conventional CT scan. The correct diagnosis was made in 71% of the high-resolution CT scans and in 72% of the three-level 10-mm and complete conventional CT scans. A definite confidence level was reached with 49% of high-resolution CT scans, 31% of the three-level 10-mm scans, and 43% of complete conventional CT examinations, with the correct diagnosis made in 92%, 96%, and 94% of these cases, respectively, suggesting that a specific diagnosis can be made with a limited number of high-resolution CT scans in many patients. Similarly, Kazerooni et al. [41] scored the severity and profusion of ground-glass opacity and reticular abnormality in 25 consecutive patients with idiopathic fibrosis on both limited three-level high-resolution CT and high-resolution CT obtained at 1-cm intervals throughout the entire lungs. The scores from both sets correlated equally with the abnormalities shown on open lung biopsy specimens. Other investigators concluded that the gains in high-resolution CT over conventional CT in visualization of small parenchymal structures that allow confident evaluation of diffuse interstitial lung diseases is only possible when the entire lung is studied [13]. Henschke [42] reported a methodologic framework for selecting the appropriate number of high-resolution CT images using simple and stratified random sampling that could reduce the number of high-resolution CT images given prior knowledge of the disease that can be obtained from chest radiographs, pulmonary function tests, radionuclide studies, and clinical parameters.

Patient Position
Most high-resolution CT images are obtained with the patient in the supine position. When the lung abnormality is diffuse in distribution or severe in profusion, inspiratory images alone are usually sufficient. Images with the patient prone may be useful when the only abnormality is in the dependent portion of the lungs; on supine images alone it may be difficult to determine if the findings represent true lung disease or dependent atelectasis [3, 43]. The latter occurs more often in current and former smokers (34-43%) than in nonsmokers (12%), and with increasing age [44]. Because dependent atelectasis occurs in the most dependent portion of the lungs, when a patient is placed prone atelectasis shifts from the anatomically posterior lung to the anatomically anterior aspect of the lung that is now the most dependent lung (Fig. 6A,6B). In contrast, with real lung disease the opacities persist; note the opacity may be less than when the patient is supine, because some, but not all, of the opacity may have been dependent atelectasis (Fig. 7A,7B). If high-resolution CT scans are obtained according to a protocol and not checked routinely before the patient leaves the radiology department, prone images should be included in the routine scanning protocol. A less-recognized site of focal atelectasis that may mimic true interstitial lung disease is the lung immediately anterior to the spine in the azygoesophageal recess, which is particularly common if large osteophytes are present. Similarly, atelectasis may occur adjacent to a large hiatal hernia or bulky callus resulting from a rib fracture.

View larger version (99K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6A. —61-year-old man with dependent opacity mimicking lung disease. High-resolution CT scan through lung bases with patient supine reveals bilateral ill-defined ground-glass and faint reticular opacity confined to dependent portion of lungs.

View larger version (82K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6B. —61-year-old man with dependent opacity mimicking lung disease. High-resolution CT scan at same anatomic level as A and with patient prone reveals that opacity completely clears, indicating opacity shown on A was atelectasis.

View larger version (79K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7A. —29-year-old woman with dependent opacity representing usual interstitial pneumonitis. High-resolution CT scan through lung bases with patient supine reveals bilateral ill-defined ground-glass and reticular opacity confined to dependent portion of lungs.

View larger version (99K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7B. —29-year-old woman with dependent opacity representing usual interstitial pneumonitis. High-resolution CT scan at same anatomic level as A and with patient prone reveals that opacity persists, confirming lung parenchyma is abnormal.

Level of Respiration
High-resolution CT is typically performed at full inspiration. Dynamic or ultrafast high-resolution CT can be performed throughout the respiratory cycle on an electron beam CT scanner [45]. Some of the same information can be obtained by comparing end-inspiration and end-expiration high-resolution CT images [46,47,48]. Expiratory images are usually obtained at maximum expiration. Although inspiratory high-resolution CT may be obtained at 1-cm spacing, usually fewer expiratory high-resolution CT images are obtained, perhaps at 2-cm spacing or less. Normal lung should increase in attenuation at end-expiration, similar to the increased lung opacity seen on end-expiratory chest radiographs. The lungs should also become smaller, and the posterior membranous wall of the trachea appears concave, in contrast to the flat or convex appearance at inspiration. Failure of the lung parenchyma to increase in attenuation on expiration indicates air trapping and suggests small airways disease. In some disease processes, such as bronchiolitis obliterans, a mosaic attenuation pattern of air trapping on expiratory high-resolution CT images may be the only evidence of abnormality, because the lungs may appear entirely normal or near normal on inspiratory images (Fig. 8A,8B). Expiratory images may be particularly helpful when trying to determine if a pattern of mosaic attenuation is primarily caused by airway disease, vascular disease, or infiltrative lung disease (Fig. 9A,9B). Air trapping may also be seen with asthma, hypersensitivity pneumonitis, emphysema, and cystic lung diseases, including Langerhans' cell histiocytosis and lymphangioleiomyomatosis [48,49,50,51].

View larger version (147K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8A. —55-year-old woman with hypersensitivity pneumonitis. Inspiratory high-resolution CT scan shows a few scattered thickened interlobular septa and very faint pattern of mosaic attenuation.

View larger version (145K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8B. —55-year-old woman with hypersensitivity pneumonitis. Expiratory high-resolution CT scan at same anatomic level as A reveals multifocal bilateral air trapping represented by low-attenuation lung parenchyma. High-attenuation areas represent normal lung that has developed atelectasis with expiration. Note internal bowing of posterior wall of bronchus intermedius as evidence that scan was taken at expiration.

View larger version (136K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9A. —54-year-old woman with idiopathic bronchiolitis obliterans. Inspiratory high-resolution CT scan shows diffuse cylindric bronchiectasis, with bronchi larger than adjacent arteries; signet ring sign of bronchiectasis (arrows); and subtle mosaic attenuation. All are findings of small airways disease.

View larger version (142K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9B. —54-year-old woman with idiopathic bronchiolitis obliterans. Expiratory high-resolution CT scan at same anatomic level as A reveals that expected decrease in lung size is absent, and lungs remain low in attenuation, indicating severe diffuse air trapping, with only normal lung parenchyma found as a few individual secondary pulmonary lobules that increased in attenuation (arrowheads).

Reduced-Dose Technique
Low-dose high-resolution CT refers to the use of a reduced tube current, as low as 40 mAs, to obtain high-resolution CT images of the lungs [52]. Although the images are noisier than standard-dose high-resolution CT scans and the image quality is poorer, some researchers have reported that the anatomic detail is equivalent between low-dose and standard-dose high-resolution CT [52, 53]. Other researchers have reported that a minimum of 160 mAs is necessary to reliably identify ground-glass opacity and subpleural lines, and that although ground-glass opacity and emphysema can be identified on the low-dose technique, they may be very subtle [52, 54, 55]. In general, this technique should be avoided in obese patients.

Photography
Although there is no single correct window width and level combination, the appropriate window width is between 100 and 2000 H, and window level is between -500 and -700 H. The exact combination used is generally a matter of personal preference. It is important that consistent photography is used to accustom the observers to the normal appearance of the lung, and to avoid misinterpretations of disease improvement or progression that are created by an artifact of photography. When interpreting hard-copy images, in general fewer images are photographed per sheet of film than for standard chest CT, to make small lines and nodules bigger and easier to see. This may mean six or nine images per film. Retrospective targeted image reconstruction can be used to reduce image pixel size and to further increase spatial resolution. In general, this targeted image reconstruction is not usually done because of the additional time required to reconstruct the images to each lung, and the preference of many radiologists to see both lungs on the same image.

Pitfalls

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Recognizing artifacts and potential interpretive and cognitive pitfalls in the approach to high-resolution CT images is important to avoid confusion of artifacts with real lung disease and other misinterpretation. The pitfalls are summarized in Appendix 1.

Motion
Movement during high-resolution CT image acquisition creates false-positive findings on high-resolution CT images, particularly pseudo—ground-glass opacity, pseudobronchiectasis, and double fissures [56,57,58]. Pseudobronchiectasis occurs because of the motion of a pulmonary blood vessel during the acquisition of an image, creating two parallel vessels on the image that simulate the walls of a bronchus. This effect is most commonly seen adjacent to the left ventricle and the aortic arch. It is particularly problematic because bronchiectasis is commonly a focal disease. Pseudobronchiectasis may skip images, unlike a true dilated bronchus that can usually be followed on consecutive images.

Vascular pulsation artifact most commonly occurs adjacent to the left ventricle and aortic arch. If two parallel opaque lines that are identical in morphology are seen adjacent to the left ventricle, look carefully at the left ventricle border with the lung. If the border of the left ventricle is seen in two places, the distance separating these borders is the distance it moved during the acquisition of the CT image. The distance separating the parallel opaque lines should be the similar to this distance if it was caused by this motion. Care should be taken not to consider that subtle abnormality in these areas is true lung disease when the rest of the lungs are normal.

Respiratory motion creates false opacity throughout the image and may be more difficult to recognize as artificial. A clue to recognizing subtle respiratory motion artifacts that occur throughout the image is that the attenuation of the lung varies diffusely every few images as the lungs move during several respiratory cycles while scanning.

A star-shaped artifact representing movement of a blood vessel perpendicular to the axial plane of imaging is another clue to identifying motion-related artifacts.

Improper Viewing Width and Level
A window width that is too narrow or a level that is too low falsely thickens bronchial walls and creates false ground-glass opacity by making normal parenchymal structures appear too opaque [56]. A window width that is too wide may mask diseases that are characterized by reduced attenuation, including emphysema and cystic lung disease. Serial examinations should be viewed at the same window width and level combinations. A high-resolution CT examination photographed at one window—level combination should not be compared with a high-resolution CT examination photographed at another window—level combination without recognizing this difference, because doing so may lead to the incorrect conclusion that the lung disease has either improved or progressed.

Left Heart Failure
The population of patients with interstitial lung disease, particularly pulmonary fibrosis, is similar in age to patients with ischemic heart disease. In some patients with ischemic heart disease, it may be difficult to determine whether the cardiac disease is the only cause of shortness of breath, particularly in the setting of a low diffusing capacity and mild restrictive pulmonary function tests. The same may also be seen in patients with fluid overload, such as patients with renal failure. Pulmonary edema resulting from left heart failure may mimic interstitial lung disease on high-resolution CT, with smoothly thickened interlobular septa and ground-glass opacity (Fig. 10). Perihilar and dependent ground-glass attenuation, enlarged nondependent blood vessels (cephalization), and smoothly thickened septa that are gravity-dependent are findings that support left heart failure and edema over infiltrative lung disease [59]. When in doubt, the high-resolution CT examination can be repeated in a few days when the patient's fluid status or heart failure has been corrected.

View larger version (112K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10. —68-year-old woman with interstitial edema resulting from left heart failure. High-resolution CT scan through upper lobes shows smooth septal thickening in a gravity-dependent distribution, with no honeycombing or septal nodularity. Mild centrilobular emphysema is shown as small areas of abnormally low attenuation.

Vessels Versus Miliary Nodules
Distinguishing small vessels from miliary nodules may be difficult on noncontiguous high-resolution CT images. Some nodules, particularly nodules of small size and low density, are best seen on high-resolution CT images, whereas obtaining a cluster of contiguous high-resolution CT images or obtaining several thicker 5- to 10-mm contiguous images may be useful if discrete dense nodules are suspected [13].

Pulmonary Vascular Disease
Acute and chronic pulmonary thromboembolic disease may alter the appearance of the lungs and be confused with interstitial lung disease and small airways disease [60,61,62,63,64,65]. The typical high-resolution CT appearance of pulmonary vascular disease is a mosaic pattern of alternating geographic areas of lung attenuation, with reduced attenuation and decrease in size of pulmonary vessels as a result of reduced perfusion in the distribution of the occluded pulmonary vessels, which is referred to as mosaic perfusion or mosaic attenuation (Fig. 11). The remainder of the lung is either normal in attenuation or higher in attenuation than normal because of a relative increase in blood flow away from the areas of pulmonary artery occlusion. Mosaic perfusion also occurs with small airways disease, in which the abnormally low-attenuation lung is created both by trapped air and shunting of blood flow to the more normal lung to optimize matching of ventilation and perfusion [66]. The greatest difficulty occurs in separating mosaic attenuation caused by patchy infiltrative lung disease from mosaic attenuation caused by pulmonary vascular disease [64, 67]. Expiratory images are useful in this setting [63]. With small airway disease, the low-attenuation areas of the lung remain low attenuation with expiration, whereas in pulmonary thromboembolic disease the lungs become increased in attenuation with expiration compared with inspiration images. In infiltrative lung disease with patchy ground-glass opacity, the pulmonary blood vessels are usually the same caliber throughout the lungs.

View larger version (117K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11. —66-year-old man with chronic pulmonary embolism. High-resolution CT scan through upper lobes shows pattern of mosaic attenuation caused by regional alterations in perfusion.

Normal High-Resolution CT and Suspected Interstitial Lung Disease
High-resolution CT may show normal findings in a small percentage of patients with biopsy-proven interstitial lung disease [14, 68, 69]. A normal examination occurs much less frequently with high-resolution CT than with chest radiography. Occasionally, a patient with a good-quality high-resolution CT examination with normal findings has a convincing clinical presentation for interstitial lung disease, including progressive shortness of breath, nonproductive cough, restrictive abnormality on pulmonary function tests, and negative findings for infection or malignancy on bronchoscopy. With a convincing clinical picture and normal high-resolution CT, open or video-assisted thoracoscopic lung biopsy may still be indicated. Some lung disease is occult on high-resolution CT, and normal findings on high-resolution CT should not preclude further workup if the remainder of the clinical picture strongly suggests lung disease.

Clinical Indications for High-Resolution CT

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The clinical indications for performing high-resolution CT are to detect and evaluate bronchiectasis, to evaluate suspected lung disease when chest radiography findings are normal, to clarify the pattern of abnormality from chest radiography in order to narrow the differential diagnosis, to evaluate disease activity, to predict response to therapy and the likelihood of survival, to guide selection of the type and location of biopsy, and to evaluate the effectiveness of medical therapy.

Bronchiectasis
High-resolution CT, having replaced more invasive bronchography, is the technique of choice for identifying and defining the extent of bronchiectasis. Patients with suspected bronchiectasis usually have chronic respiratory symptoms, including cough, recurrent pneumonia, and abundant sputum production. Chest radiographs are notoriously insensitive for detecting bronchiectasis, particularly if it is mild; radiographs show normal findings in as many as 50% of patients with bronchiectasis. Naidich et al. [70] first reported the use of CT to detect bronchiectasis in 1982. Although early investigations showed poor test performance using standard CT compared with bronchography and pathology [71, 72], subsequent reports of high-resolution CT, including the initial work by Grenier et al. [73], have shown consistently high sensitivity of 84-95% and specificity of 93-100% for the detection of bronchiectasis [73,74,75,76,77] (Fig. 4A,4B).

Normal or Equivocal Chest Radiographic Findings, Suspected Lung Disease, and Pattern Clarification
Chest radiographs lack both sensitivity and specificity in the evaluation of diffuse lung disease. To begin with, chest radiographic findings may be normal in patients with suspected interstitial lung disease (Fig. 12A,12B). Three large series of infiltrative lung disease totaling more than 1000 patients showed normal findings on chest radiographs in an average of 13% of patients with open lung biopsy—confirmed disease [14, 78, 79]. Approximately 10% of immunocompromised patients with acute diffuse lung disease in one series [80] and 10% of patients with Pneumocystis carinii pneumonia in another series were reported to have normal chest radiographic findings [81]; and in another series of 112 neutropenic patients with fever of unknown origin and normal chest radiographic findings who had a total of 188 high-resolution CT scans, 60% of the high-resolution CT scans showed abnormal findings and indicated the source of infection [82]. The high-resolution CT findings were abnormal, on average, 5 days before abnormal chest radiographic findings developed, allowing more rapid diagnosis and treatment in this high-risk population, many of whom were bone marrow transplant recipients. In patients with asbestos exposure, high-resolution CT often shows abnormal findings in the setting of normal chest radiographic findings, as reported in 57 (45%) of 169 patients with a score of less than 1/0 using the International Labour Office interpretation scheme [83], and these individuals had significantly lower diffusing capacity (p = 0.024) and vital capacity (p = 0.005) than those with normal high-resolution CT findings [84]. Similarly, in patients with scleroderma or progressive systemic sclerosis, and in patients with rheumatoid arthritis, chest radiographic findings have been reported to be abnormal in 9-59% of published series, whereas corresponding high-resolution CT examinations showed abnormal findings in 71-100%, depending on the population being studied [85,86,87,88]. In as many as 50% of patients with pulmonary lymphangitic carcinomatosis, the chest radiographic findings may be normal; therefore, high-resolution CT should be considered in cancer patients with unexplained pulmonary symptoms and normal chest radiographic findings [89].

View larger version (132K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12A. —56-year-old man with hypersensitivity pneumonitis resulting from bird-fancier's lung. Posteroanterior chest radiograph, originally interpreted as showing normal findings, shows subtle hazy opacity in mid and lower lungs.

View larger version (120K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12B. —56-year-old man with hypersensitivity pneumonitis resulting from bird-fancier's lung. High-resolution CT scan obtained 1 hr after chest radiograph reveals diffuse ground-glass opacity with faint centrilobular nodules in less confluent areas, in addition to air trapping in scattered secondary pulmonary lobules (arrow).

High-resolution CT is also more sensitive than conventional CT for the detection of ground-glass opacity. A series by Remy-Jardin et al. [13] of 150 consecutive patients compared conventional CT and high-resolution CT. The only technique to reveal ground-glass opacity was high-resolution CT, and both fine bronchial and parenchymal lesions were better seen on high-resolution CT than on conventional CT.

When abnormalities are identified on chest radiographs, they are less specific than on high-resolution CT scans. For example, Padley et al. [14] reported a specificity of 82% for identifying normal findings using chest radiography compared with 100% using high-resolution CT in a series of 100 patients, 86 with biopsyproven interstitial lung disease and 14 normal control subjects. Not only is a correct first-choice diagnosis of interstitial lung disease made more often with high-resolution CT than with chest radiography, but the first-choice diagnosis is also made with greater confidence. For example, in a series of 118 consecutive patients with chronic infiltrative lung disease reported by Mathieson et al. [18], three observers were asked to rank their top three diagnoses and the degree of confidence in their first-choice diagnosis. Chest radiography rendered a confident first-choice diagnosis in 23% of cases compared with more than twice as many—49%—with high-resolution CT. High-resolution CT interpretations were correct in 93% of cases compared with 77% for the interpretations of chest radiographs. Similarly, Grenier et al. [90] reported a comparison in 140 consecutive patients with chronic infiltrative lung disease, with three observers also listing their top three diagnoses and confidence. Correct confident diagnoses were made by the three observers on 19-34% of chest radiographs each, compared with 47-57% for the high-resolution CT interpretations (p < 0.001). Nishimura et al. [91] also reported a more frequent correct first-choice diagnosis for high-resolution CT—46%—than for chest radiography—38%—in a series of 134 patients with diffuse infiltrative lung disease (p < 0.01).

In addition to the gains in sensitivity, specificity, and diagnostic confidence over chest radiography, high-resolution CT also reduces interobserver variability in interpretation when compared with chest radiography [92, 93]. For example, in a series of 61 consecutive individuals exposed to asbestos who had chest radiography major category scores of 0 or 1 using the International Labor Organization interpretation scheme [83], mixed with five normal control subjects, Begin et al. [93] used kappa statistics to show significantly better interobserver agreement for CT interpretations than for chest radiography interpretations (p < 0.001) and reduced the frequency of indeterminate interpretations. Collins et al. [92] tested inter- and intraobserver variability of high-resolution CT and chest radiography for determining pattern type and extent of disease in fibrosing alveolitis among two experienced and two inexperienced observers on a total of 126 high-resolution CT examinations and 108 chest radiographs scored on two occasions at least 8 weeks apart. Three of four observers agreed on pattern type in 81% of high-resolution CT examinations ( = 0.48) compared with 54% for chest radiographs ( = 0.16). The greater the observer confidence in identifying pattern type on high-resolution CT, the lower the interobserver variability and the more extensive the disease. Intraobserver variability for high-resolution CT pattern was less for the experienced ( = 0.78 and 0.70) than inexperienced observers ( = 0.50 and 0.37). Interobserver variability for extent of disease was significantly less on CT than on chest radiography (p < 0.001).

Disease Activity and Predicting Response to Therapy and Survival
Honeycombing has been well documented by high-resolution CT—pathologic correlation to represent irreversible end-stage pulmonary fibrosis (Fig. 13), as described in the initial report by Müller et al. [94] of radiologic—pathologic correlation in nine patients with established interstitial fibrosis in 1986. Later, in 1987, Müller et al. [95] reported the first evidence that the high-resolution CT appearance was an indicator of disease activity in a series of 12 patients with idiopathic pulmonary fibrosis who underwent open lung biopsy. Disease activity, measured by a pathologic grading system, identified seven patients with mild disease activity and five with moderate to marked activity. Disease activity was independently scored 0-3 on CT scans on the basis of the presence and density of air-space consolidation. The pathologic score was significantly greater in patients with higher CT scores (p = 0.001), and CT correctly identified five patients with marked disease activity and five of the seven with mild activity.

View larger version (144K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13. —58-year-old man with usual interstitial pneumonitis. High-resolution CT scan through lung bases shows extensive honeycombing, indicating severe irreversible fibrosis. When this pattern is subpleural and lower-lobepredominant, it is characteristic of usual interstitial pneumonitis.

Ground-glass opacity is a less specific finding and may represent active inflammation or fibrosis of alveolar or intralobular septa that is below the resolution of high-resolution CT. In 1993, Remy-Jardin et al. [96] reported a series of 26 patients with extensive ground-glass attenuation as the predominant or exclusive abnormality and no honeycombing on high-resolution CT, with histologic correlation at 37 biopsy sites. The ground-glass opacity corresponded to inflammation in 24 sites (65%) and to fibrosis in 13 sites (35%); in 85% of the cases with fibrosis, associated traction bronchiectasis or bronchiolectasis was seen. Therefore, ground-glass opacity in the absence of bronchiectasis usually represents active inflammation, whereas ground-glass opacity with traction bronchiectasis usually represents fibrosis.

In general, patients with more ground glass respond better to therapy, and patients with greater fibrosis have both poorer response and poorer survival (Fig. 14A,14B). Wells et al. [97] showed significantly greater survival in a retrospective 4-year review of patients with fibrosing alveolitis with predominantly ground-glass opacity (100%; n = 8) compared with patients with a predominantly reticular (15%; n = 50) or mixed pattern (45%; n = 18) (p < 0.001), and this prediction of survival was better than for existing functional measurements and dyspnea duration. Later, using a semiquantitative visual scoring system for the alveolar (ground-glass) and fibrosis (lines and honeycombing) components of interstitial lung disease, Gay et al. [98] showed that high-resolution CT alveolar scores were higher and fibrosis scores lower in patients responding to corticosteroid therapy than in nonrespondents in a prospective study. Using receiver operating curve analysis to identify pretreatment features of longer term survival, these researchers found that only the high-resolution CT fibrosis score (p = 0.009) and the open lung biopsy fibrosis score (p = 0.03) were able to predict mortality, and high-resolution CT did so non-invasively. Pulmonary function and chest radiographic features were not helpful.

View larger version (126K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14A. —53-year-old woman with desquamative interstitial pneumonitis. High-resolution CT scan shows patchy ground-glass opacity.

View larger version (127K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14B. —53-year-old woman with desquamative interstitial pneumonitis. High-resolution CT scan after 6 months of medical therapy with azathioprine reveals that abnormality has almost completely resolved.

Guide Type of Biopsy and Location of Biopsy
Transbronchial and open or thoracoscopic lung biopsy are the two main techniques for the histologic diagnosis of chronic infiltrative lung disease. Transbronchial biopsy is most effective in patients with sarcoidosis and lymphangitic spread of cancer, both disease processes that characteristically involve the central or axial interstitial compartment [99]. Transbronchial biopsy is insufficient to diagnose interstitial pneumonitis and pulmonary fibrosis, but it may be useful to exclude other diagnoses, such as infection, in these patients. The accuracy of transbronchial biopsy has improved for the diagnosis of other less common diseases, such as Langerhans' cell histiocytosis, alveolar proteinosis, and eosinophilic lung disease. Open lung biopsy is more than 90% accurate and has largely been replaced by video-assisted thoracoscopic surgery, a technique associated with less morbidity, faster postoperative recovery, and lower cost [100]. Samples are usually obtained from all lobes on the side undergoing biopsy, three from the right or two from the left. Because the tactile sensation of open lung biopsy is lost with the use of video-assisted thoracoscopic surgery, and the field of view is smaller, high-resolution CT has become more important for directing the surgeon to the areas of ground-glass opacity and nodularity and away from the honeycombing that may yield fibrosis of indeterminate cause at pathology (Figs. 13 and 14A,14B). Because the subtypes of interstitial pneumonitis (desquamative, usual, and nonspecific) are associated with a different prognosis for the patient, it is important to sample the areas of lung parenchyma that maximize the differentiation among the subtypes [101, 102]. CT has been shown to be more accurate than chest radiography for determining whether transbronchial or open lung biopsy is needed for accurate diagnosis [18].

Evaluate the Effectiveness of Medical Therapy
High-resolution CT is used to monitor the response of lung disease to medical therapy, which is particularly important given the morbidity associated with many of the cytotoxic drugs used to treat interstitial lung disease [10, 103,104,105,106,107]. With treatment, ground-glass opacity may regress, as seen in 18 (32%) of 56 patients with fibrosing alveolitis reported by Wells et al. [108]. Ground-glass opacity progressed in five patients, and reticular abnormality progressed in nine patients. In no patient did the reticular abnormality regress. Although ground-glass opacity significantly correlates with improvement in pulmonary function after steroid treatment for pulmonary fibrosis or usual interstitial pneumonitis, areas of ground-glass opacity on high-resolution CT have been shown to precede and predict the development of honeycombing in the same location over time [103, 105, 107]. In sarcoidosis, ground-glass opacity, alveolar or pseudoalveolar consolidation, nodular and irregular linear opacities, and interlobular septal thickening have been shown to represent potentially reversible disease, whereas cystic air spaces, architectural distortion, and septal lines may remain unchanged and may be irreversible [10, 104].

Specific Diagnoses Possible with High-Resolution CT

Top Introduction Background Anatomic and Technical... Pitfalls Clinical Indications for High... Specific Diagnoses Possible with... Conclusion References

 
A full description of the pattern-based approach to high-resolution CT is beyond the confines of this article. The predominant patterns of abnormality—reticular or interstitial abnormality, nodular abnormality, and altered attenuation—are listed in Appendix 2. Some patterns or combinations of patterns together with the anatomic distribution of the abnormality from lung apex to base, and peripheral subpleural versus central bronchovascular, can lead the interpreter to a specific diagnosis. High-resolution CT is particularly accurate in the diagnosis of cystic lung diseases and their distinction from emphysema, in cases of usual interstitial pneumonitis when subpleural honeycombing predominates, in sarcoidosis, and in lymphangitic carcinomatosis [16, 109]. The diseases and patterns that can lead to a single diagnosis include bronchiectasis, emphysema, Langerhans' cell histiocytosis, lymphangioleiomyomatosis, usual interstitial pneumonitis, hypersensitivity pneumonitis, lymphangitic carcinomatosis, pneumoconiosis, and sarcoidosis. The diagnosis in these cases may be sufficiently specific from high-resolution CT to obviate tissue confirmation.

Bronchiectasis
Bronchiectasis has a very specific appearance on high-resolution CT; it appears as dilated bronchi, larger in cross-section than the adjacent pulmonary artery, with or without bronchial wall thickening. The signs of bronchiectasis on CT include the signet ring sign (the artery is the diamond and the bronchus is the ring in cross-section), nontapering of bronchi, dilated bronchi within 1 cm of the visceral pleura, linear and cystic air-filled structures, air—fluid levels in distended bronchi, and bronchial wall thickening caused by peribronchial fibrosis [70, 75] (Figs. 4A,4B and 9A,9B).

Emphysema
Emphysema appears on high-resolution CT as areas of abnormally low attenuation without definable walls, which distinguishes emphysema from the cystic lung diseases [16, 35]. The most common form, centrilobular emphysema, typically occurs as a result of cigarette smoking and is upper-lobe-predominant in distribution. Early centrilobular emphysema is characterized by round black holes that may appear in the central portion of the secondary pulmonary nodule around the centrilobular artery [110]. As emphysema progresses, the low-attenuation areas become confluent and inseparable (Fig. 15A,15B). The pulmonary vessels in areas of severe emphysema are small, with shunting of blood flow to lung parenchyma that can better exchange air to maintain matched ventilation and perfusion. Panlobular emphysema is lower-lobe-predominant, typically occurs as a result of ₁ antiprotease deficiency, and progresses more rapidly if associated with cigarette smoking [111].

View larger version (111K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 15A. —72-year-old woman with severe centrilobular emphysema. High-resolution CT scan at level of aortic arch shows severe emphysema, with normal lung parenchyma almost completely replaced by abnormally low attenuation.

View larger version (123K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 15B. —72-year-old woman with severe centrilobular emphysema. High-resolution CT scan at lung bases shows mild emphysema, appearing as small round areas of low attenuation, often abutting centrilobular artery (arrows).

Langerhans' Cell Histiocytosis
Langerhans' cell histiocytosis, otherwise known as histiocytosis X or eosinophilic granuloma, is considered a smoking-related lung disease that may improve or even resolve with smoking cessation. A history of cigarette smoking is reported in 90-100% of cases. When a combination of cysts and irregular nodules is identified in a cigarette smoker, more severely involving the upper lungs than the lung bases, this diagnosis can be made with confidence [112,113,114] (Fig. 16). The abnormality is more nodular early in the disease course and evolves to be more cystic with time [115] (Figs. 16 and 17). In a series of 48 patients with Langerhans' cell histiocytosis reported by Travis et al. [116], all patients had a history of cigarette smoking, two had pituitary involvement, and four had bone lesions.

View larger version (131K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 16. —65-year-old man with Langerhans' cell histiocytosis and a 3-year history of progressive dyspnea. High-resolution CT scan at level of aortic arch shows mixed pattern of irregular nodules and cysts that was less severe at lung bases.

View larger version (117K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 17. —54-year-old woman with 20-year history of Langerhans' cell histiocytosis. High-resolution CT scan at level of aortic arch shows predominant pattern of cysts that was less severe at lung bases. Irregular nodules are relatively minor component.

Lymphangioleiomyomatosis
Lymphangioleiomyomatosis is a rare cystic lung disease with classic, if not specific, radiologic findings. This disease is characterized by uniformity. Uniformly sized cysts, uniformly distributed from lung apex to base and center to periphery, uniformly occurring in women of childbearing age. Early in the disease the cysts are surrounded by normal lung parenchyma (Fig. 18). Additional findings include chylous pleural effusions, low-attenuation lymph nodes containing dilated spaces filled with chylous material, and spontaneous pneumothorax (Fig. 18). Over time, no normal parenchyma is visualized [16, 117] (Fig. 19). The severity of the cysts measured visually or using attenuation-based quantitative CT directly corresponds to the severity of the associated obstructive pulmonary function abnormalities, the impairment in gas exchange, and the reduction in exercise performance in these patients [118, 119].

View larger version (125K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 18. —39-year-old woman with lymphangioleiomyomatosis. High-resolution CT scan at level of carina displayed at lung window on left and soft-tissue window on right. In addition to large bilateral pleural effusions, note small round low-attenuation areas with faint walls, representing cysts, that were uniformly distributed throughout lung parenchyma.

View larger version (113K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 19. —40-year-old woman lymphangioleiomyomatosis. High-resolution CT scan at level of aortopulmonary window shows severe lung destruction, with almost complete replacement of normal lung parenchyma by cysts that were uniformly distributed throughout lungs.

Usual Interstitial Pneumonitis
Usual interstitial pneumonitis may appear on high-resolution CT as a spectrum of abnormalities ranging from ground-glass opacity early in the disease to the honeycombing of end-stage disease. The appearance of subpleural, lower-lobe-predominant honeycombing on high-resolution CT is characteristic of and highly specific for usual interstitial pneumonitis [97, 109] (Fig. 13). Usual interstitial pneumonitis may be idiopathic or related to collagen vascular disease or drug toxicity [120, 121]. When ground-glass opacity is the predominant pattern on high-resolution CT, the differential diagnosis is long and includes most of the idiopathic interstitial pneumonias, interstitial infections such as cytomegalovirus and P. carinii pneumonia, hypersensitivity pneumonitis, edema, alveolar proteinosis, and diffuse bronchoalveolar cell carcinoma [122]. Most patients with usual interstitial pneumonitis present fairly late in their disease course with honeycombing, compared with patients with either desquamative or nonspecific interstitial pneumonitis who present when their disease is characterized by more active inflammation and ground-glass opacity and is more amenable to therapy [123,124,125]. In 1997, Müller and Colby [101] reviewed the idiopathic interstitial pneumonias, a topic that alone could occupy this entire article.

Hypersensitivity Pneumonitis
Hypersensitivity pneumonitis presents in the subacute stage with diffuse centrilobular nodules, with or without patchy ground-glass opacity and air trapping [126] (Figs. 12A,12B and 20). Clinical history and serologic tests combined with this high-resolution CT appearance are sufficient to make the diagnosis. The CT findings represent a mononuclear cell bronchiolitis and cellular interstitial infiltrate, with poorly defined, scattered nonnecrotizing granulomas [126]. Centrilobular nodules are reported in 40-100% of patients with subacute hypersensitivity pneumonitis, and patchy ground-glass opacity in 52-100% [126,127,128]. The abnormality may be more severe in the mid and lower lungs than in the lung apices. In a population-based study by Lynch et al. [69] of 31 symptomatic recreation center employees referred because of possible hypersensitivity pneumonitis, 11 were diagnosed with hypersensitivity pneumonitis. Chest radiographic findings were abnormal in only one (9%) of 11 patients. High-resolution CT findings were abnormal in five (45%) of 11 patients, and in each of these five patients appeared as poorly defined centrilobular nodules. The appearance of chronic hypersensitivity pneumonitis is less specific, appearing as fibrosis superimposed on patchy ground-glass opacity and centrilobular nodules. Hypersensitivity pneumonitis often spares the lung bases, a distinction that can be used to distinguish chronic hypersensitivity pneumonitis from usual interstitial pneumonitis [129, 130] (Fig. 21). After the antigen that has provoked the lung injury is withdrawn from the environment of the patient with hypersensitivity pneumonitis, the ground-glass opacity and centrilobular nodules may improve or resolve; whereas in patients with persistent antigen exposure, the high-resolution CT findings persist or progress [127].

View larger version (103K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 20. —32-year-old man with hypersensitivity pneumonitis. High-resolution CT scan at level of carina shows diffuse centrilobular nodules.

View larger version (120K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 21. —69-year-old woman with 12-year history of chronic hypersensitivity pneumonitis. High-resolution CT scan through mid lungs shows traction bronchiectasis, reticular abnormality superimposed on patchy ground-glass opacity, and a few centrilobular nodules. Unlike usual interstitial pneumonitis, distribution of abnormality is not predominantly subpleural.

Lymphangitic Carcinomatosis
Lymphangitic carcinomatosis can be recognized by the characteristic appearance of irregular, nodular, or "beaded" interlobular septa forming polygons; irregular and nodular thickening of the bronchovascular core structures in the secondary lobule; and thickening of the central bronchovascular interstitium at the lung hila. This pattern represents the perilymphatic distribution of disease [131, 132] (Fig. 22). There may be associated enlarged lymph nodes, pleural effusions, and thick or nodular fissures. With the advent of chemotherapy, these findings may remain fairly stable or slowly progress on CT over several months [132].

View larger version (123K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 22. —37-year-old man with lymphangitic carcinomatosis resulting from metastatic adenocarcinoma. High-resolution CT scan through right lung base shows irregular and nodular interlobular septa forming polygons, with thickening and irregularity of centrilobular arteries (arrows) and major fissure. Larger nodule in periphery of right lower lobe represents hematogenous metastasis.

Pneumoconioses
Pneumoconioses most commonly encountered radiologically are asbestosis and asbestos-related pleural plaques, silicosis, and coal worker's pneumoconiosis. The high-resolution CT findings, coupled with the appropriate history of exposure, are fairly specific for these diagnoses. As described earlier, high-resolution CT findings are often abnormal in asbestos-exposed patients with normal chest radiographic findings, the characteristic findings being inter- and intralobular septal thickening; subpleural and parenchymal bands; and honeycombing in a subpleural posterior and basilar distribution, with or without pleural plaques [3, 133, 134] (Fig. 23A,23B). Individuals with abnormal high-resolution CT findings have a lower forced vital capacity and poorer gas exchange than asbestos-exposed individuals with normal high-resolution CT findings, and individuals with abnormal chest radiographic findings have a longer duration of exposure than individuals with normal chest radiographic findings and abnormal high-resolution CT findings [5, 84, 135]. Conversely, silicosis and coal worker's pneumoconiosis both appear as 2- to 5-mm centrilobular or subpleural nodules, most severe in the upper lobes, that coalesce to form conglomerate masses leaving peripheral emphysema. Associated lymph node enlargement, calcification of lung nodules, masses and lymph nodes, and a minor component of septal thickening may be present [136,137,138].

View larger version (114K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 23A. —55-year-old man with asbestos exposure. High-resolution CT scans at level of carina (A) and lung bases (B) show parenchymal bands (arrows, A), subpleural bands (arrowheads, B), and thick interlobular septa of asbestosis, in addition to pleural plaques.

View larger version (104K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 23B. —55-year-old man with asbestos exposure. High-resolution CT scans at level of carina (A) and lung bases (B) show parenchymal bands (arrows, A), subpleural bands (arrowheads, B), and thick interlobular septa of asbestosis, in addition to pleural plaques.

Sarcoidosis
Sarcoidosis is one of the more commonly encountered chronic infiltrative lung diseases. Although pulmonary sarcoidosis has many appearances, including nummular or coinlike lesions that resemble metastases, and alveolar consolidation that may make the diagnosis challenging, in many cases the characteristic finding of peribronchovascular and subpleural nodules, in an upper-lobe-predominant distribution, can lead the observer to this diagnosis [9] (Figs. 24 and 25). The pathologic lesion of sarcoidosis is the noncaseating granuloma, and these lesions are typically located along lymphatics in the peribronchovascular sheath and to a lesser extent in the subpleural and interlobular septal lymphatics [139]. This distribution of abnormality is termed perilymphatic. When nodular and upper-lobe-predominant, sarcoidosis is the most likely diagnosis unless the patient has an exposure history indicating silicosis or coal worker's pneumoconiosis. If the disease progresses, irregular opacities, architectural distortion, and honeycombing predominate, and the small nodules coalesce into central masslike opacities [104] (Fig. 26). Patients with predominantly irregular opacities have more severe dyspnea and lower lung volumes than patients with predominantly nodular opacities [140]. Additional findings include bilateral hilar and mediastinal lymph node enlargement seen to involve more anatomic locations on CT than on chest radiography.

View larger version (101K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 24. —38-year-old woman with early pulmonary sarcoidosis. High-resolution CT scan just below level of carina shows miliary nodules predominantly located along central bronchovascular bundles. Her symptoms of arthralgias and erythema nodosum resolved after 4 weeks of daily high-dose oral prednisone.

View larger version (100K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 25. —36-year-old man with sarcoidosis. High-resolution CT scan just below level of carina shows central bronchovascular thickening and nodularity on a background of small nodules, including subpleural nodules.

View larger version (99K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 26. —58-year-old man with end-stage sarcoidosis. High-resolution CT scan through level of inferior pulmonary veins shows central bronchovascular thickening and nodularity with severe architectural distortion and posterior rotation of hila superimposed on background of miliary and subpleural nodules. Note associated peripheral bullous emphysema.

Conclusion

Top Introduction Background Anatomic and Technical... Pitfalls Clinical Indications for High... Specific Diagnoses Possible with... Conclusion References

 
For most practicing radiologists, high-resolution CT is only a small part of the radiology examinations that are interpreted as part of their daily clinical work. The patterns of abnormality and terminology may still seem like a foreign language two decades after the technique was first described and a decade after it has become a well-established tool for the evaluation of diffuse lung disease. High-resolution CT presentations at continuing medical education courses, such as those presented at the annual meetings of the American Roentgen Ray Society and the Radiological Society of North America, continue to draw standing-room-only crowds, a recognition of the level of discomfort that many radiologists continue to have with interpreting high-resolution CT images.

Remember the patterns of abnormality. Remember the anatomic distribution. Remember the clues, such as age, gender, smoking history, and ancillary findings. Use all of these to narrow the differential diagnosis and, in some cases, to make a specific diagnosis.

I have tried to present the circumstances in which high-resolution CT can be used to make a specific diagnosis, including the disease processes that are most common, as well as to convey the limitations of the high-resolution CT technique and the pitfalls in interpretation. I still present high-resolution CT cases to my radiology colleagues, and sometimes they still squint. I tell them those fuzzy centrilobular nodules are really there, and sometimes they still tell me that I am hallucinating.

,

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Top Introduction Background Anatomic and Technical... Pitfalls Clinical Indications for High... Specific Diagnoses Possible with... Conclusion References

 

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