MY9700684 CH2 - CHEST High Resolution CT in Diffuse Lung Disease dependent lung collapse is often seen in both normals and abnormals, and having scans in both positions allows us to differentiate this finding from true W. Richard Webb pathology. Patients with suspected obstructive disease have scans performed at 1 cm intervals from the lung Dept of Radiology, University of California of San apices to bases in the supine position only. Francisco, San Francisco The use of HRCT following expiration, or dynamic ultrafast high-resolution CT (DUHRCT) in which a The development of high-resolution CT (HRCT) in rapid series of ultra, fast HRCT scans are obtained recent years has revolutionized our ability to detect during a forced inspiratory and expiratory maneuvers, and characterize diffuse pulmonary disease. Using can demonstrate dynamic morphologic and lung HRCT, lung morphology can be assessed in detail. attenuation changes associated with airways obstruc- tion." HRCT TECHNIQUE High-resolution CT techniques attempt to optimize NORMAL HRCT FINDINGS the demonstration of lung architecture.14 The use of Structures as small as 0.2 to 0.3 mm can be seen on thin collimation (1-2 mm) and image reconstruction HRCT images; this level of resolution allows the with a high-resolution algorithm are essential in imaging of lung anatomy at the level of the secondary obtaining HRCT.' Additional modifications of pulmonary lobule.2'1213 Within the peripheral lung, technique can improve image quality further, but are interlobular septa measuring 100 |jm or 0.1 mm in not necessary. These include increasing the kVp or thickness are at the lower limit of HRCT resolution,13 mA scan settings in order to reduce image noise, and but nonetheless can sometimes be seen on HRCT targeting image reconstruction to a small field of scans performed in vitro.12 These are better seen view.2-5"* within the peripheral lung than in the central lung, as septa are better developed in this location. On clinical The average skin radiation dose associated with HRCT HRCT in normal patients, a few interlobular septa can scans has been compared to that of conventional CT.9 often be seen, but they tend to be inconspicuous.8 Using a scan technique of 120 kVp, 200 m A, 2 sec, the Numerous clearly defined interlobular septa are mean skin radiation dose was 4.4 mGy for 1.5 mm usually abnormal. HRCT scans at 10 mm intervals, 2.1 mGy for scans at 20 mm intervals, and 36.3 mGy for conventional 10 The central portion of the 'secondary lobule, referred mm scans at 10 mm intervals. Thus, HRCT scanning to as the lobularcore orcentrilobular region,14contains at 10 and 20 mm intervals, as is done in clinical the pulmonary artery and bronch iolar branches which imaging, results in 12% and 6%, respectively, of the supply the lobule. The pulmonary artery supplying a radiation dose associated with conventional CT. secondary lobule measures approximately 1 mm in diameter, while intralobular acinar artery branches Recently, the utility of "low-dose" HRCT (120 kVp, measure 0.5 mm in diameter, vessels of this size are 20 mA, and 2 second scan time) has been evaluated.10 easily seen using HRCT. The visibility of bronchi or Image quality is generally not as good as with standard bronchioles on HRCT is determined by their wall HRCT technique, but low-dose scans may be adequate thickness rather than their diameter. As an appro- ximation, the thickness of the wall of a bronchus or for the follow-up of patients with an established 15 diagnosis. bronchiole measures from 1/6 to l/10th of its diameter. Thus, for a 1 mm bronchiole supplying a secondary lobule, the thickness of its wall measures approxi- Most patients who have HRCT in our institution for mately 0.15 mm; the wall of a terminal bronchiole the diagnosis of suspected restrictive lung disease or measures only 0.1 mm in thickness, and that of an who have diffuse lung disease of unknown type, have acinar bronchiole only 0.05 mm. Bronchioles are scans performed at 2 cm intervals from lung apices to below the resolution of HRCT technique for a tubular bases in both the supine and prone positions. Some 45 ABNORMAL HRCT FINDINGS occurring in relation to small arterial or bronchiolar Numerous pathologic studies have shown that HRCT branches or in the periphery of acini.712 This finding accurately depicts lung anatomy and pathology. is most common can be seen in patients who have Generally speaking, HRCT findings of lung disease fibrosis, and is visible as a fine network of lines.7'12 can be considered in 4 groups or categories, which reflect the histologic abnormalities present. These Honeycombing reflects extensive fibrosis with lung are 1) reticular opacities, 2) nodular opacities, 3) destruction, and results in a cystic, reticular appear- increased lung opacity, and 4) decreased lung opac ity ance on HRCT which is characteristic.12-24 When or cystic opacities. honeycombing is present, normal lung architecture is distorted, and secondary lobules are difficult or RETICULAR OPACITIES impossible to recognize. The cystic spaces of honey- Thickening of the interstitial fiber network of the lung combing can range from several mm to several cm in by fluid, fibrous tissue, or because of interstitial diameter, and are characterized by thick, clearly infiltration by cells, results in an increase in reticular definable, fibrous walls. lung opacities as seen on HRCT. Reticular interstitial abnormalities can often be characterized accordi ng to Centrilobular opacities2327 can reflect their relation to secondary lobular structures. peribronchovascular interstitial thickening such as occurs in patients with fibrosis or interstitial infiltra- Interlobular septal thickening occurs in patients tion, or can reflect bronchiolar abnormalities. For with a variety of interstitial lung diseases, and in the example, centrilobular (peribronchiolar) abnormali- presence of interstitial fluid, fibrosis, or cellular ties have been reported as early HRCT findings in infiltration. In the peripheral lung, thickened septa patients with asbestosis28 andsilicosis,29 and can also measure 1-2 cm in length and are often seen extend- be seen in patients with pulmonary edema, ing to the pleural surface; in the central lung, the sarcoidosis1930 histiocytosis X ,31 and hypersensitivity thickened septa can outline lobules which are 1-2.5 pneumonitis [32]. In some of these conditions, the cm in diameter and appear polygonal in shape. Such presence of centrilobular abnormalities reflects visible lobules commonly contain a central dot-like peribronchiolar inflammation or air-space disease. In or branching artery. patients with small airways diseases such as panbronchiolitis,26cystic fibrosis,27andbronchiolitis, Septal thickening can be smooth, nodular, or irregular centrilobular opacities and bronchiolar wall thicken- in contour in different pathologic processes. Pulmo- ing have been seen. On HRCT, centrilobular nary edema results in smooth septal thickening; abnormalities can appear as an abnormal prominence Iymphangitic spread of tumor characteristically results of the centrilobular artery or bronchiole because of in smooth, nodular, or "beaded" thickening,1"8 while peribronchovascular interstitial thickening or as a "beaded septa" or septal nodules can also be seen in nodular opacity. The appearance of "tree-in-bud" is patients with sarcoidosis and coal workers often indicative of airways disease or inflammation, pneumoconiosis."'22 Septal thickening is not common as in cystic fibrosis or endobronchial spread of TB.33 in patients with interstitial fibrosis, except for those with sarcoidosis and asbestosis;23 when visible, septal NODULES thickening due to fibrosis is often irregular in Nodules as small as 1-2 mm in diameter can be appearance. detected on HRCT in patients with a variety of diseases. Nodules can be "perilymphatic", random, When interlobular septa are visible, and lobules are or centrilobular in distribution in different diseases well outlined, it is important to note whether the and recognizing one of these distributions can be lobules are normal in shape and appearance or whether important in differential diagnosis.34 "Perilymphatic" they are distorted. Thickened septa without nodules affect the peribronchovascular, interlobular architectural distortion is characteristic of edema, septal, subpleural, and centrilobular interstitial com- Iymphangitic spread, or infiltration, while distortion partments, and are usually due to sarcoidosis, which strongly suggests fibrosis. tends to have a peribronchovascular and subpleural predominance,"'2230'35"37 silicosis and coal-worker's Intralobular interstitial thickening represents an penumoconiosis, which predominates in the 2 121 29 38 abnormality of the intralobular interstitium, perhaps subpleural and centrilobular regions, " ' - and 46 lymphangitic spead of tumor, which is typically abnormalities which can be precisely characterized peribronchovascular and septal.'"39 Nodules with a and localized on HRCT.12-24'54 Honeycomb cysts are random distribution are most typical of milary often peripheral in location, and are characterized by tuberculosis" and hematogenous metastases.40 thick, clearly definable walls. Centrilobular nodules, often reflect bronchiolar or peribronchiolar abnormalities,41 and can be seen in Emphysema is accurately diagnosed using HRCT, silicosis and coal-worker's pneumoconiosis,29 and this technique is more sensitive than conven- asbestosis,42 endobronchial spread of tuberculosis1 W3 tional CI or plain radiographs