Section 1 -

Radiographic Features of Diffuse Pediatric Lung Diseases Alan S. Brody Cincinnati Children's Hospital Cincinnati , Ohio

Overview of Chest Radiography and CT Scanning The imaging modalities primarily used to evaluate diffuse lung disease are chest radiographs and computed tomography (CT). Chest radiographs are universally available in North America , inexpensive, and require a very low radiation dose. Chest radiographs are less sensitive to the presence of pulmonary abnormalities and can overestimate as well as underestimate the severity of lung disease. As would be expected, comparison with CT scanning has shown that both the number of correct diagnoses and diagnostic confidence are higher with CT scanning than with plain radiographs [1]. In this study normal subjects were correctly identified on 12/12 CT examinations compared to 7/12 chest radiographs. Chest radiographs remain very useful in suggesting a diffuse process rather than a focal one, and in suggesting that an entity other than diffuse lung disease may be present. The presence of an abnormal chest radiograph remains one of the criteria for a diagnosis of diffuse lung disease. Chest radiographs in children with diffuse lung disease usually show hyperinflation. The most common pattern is an increase in lung markings, often similar to the appearance seen with viral airways disease. An appearance of diffuse hazy increased density can be seen with alveolar proteinosis. If lung volumes are low, atelectasis is a frequent cause of parenchymal opacities. CT scanning is the imaging modality of choice for the evaluation of the lung parenchyma. CT scanning is widely available, but high quality images in children require special expertise [2]. CT scanning uses the same ionizing radiation used for chest radiographs but requires up to 100 times the amount of radiation. CT Technique Different techniques are available depending on the indication. Volumetric scanning moves the patient through the CT scanner as the data is acquired by the rotating Xray tube and detectors. This is also called spiral or helical scanning. Images are usually between 5 and 10mm thick. Contrast material is frequently administered to opacify the vascular system and to evaluate masses for enhancement. The newest CT scanners can use the same data to produce sections as thin a 0.6mm from these data, but the image quality of these thin sections can be limited. Imaging of diffuse lung disease is usually performed using the high-resolution CT technique (HRCT) [3]. HRCT is a confusing term used to describe a sampling technique that images a section approximately 1mm thick at intervals, often 10mm, by moving the patient between successive images. This axial technique provides higher image quality and by imaging only a portion of the lung much less radiation is used. Contrast material is very rarely helpful when using HRCT. Utility of HRCT in Children The roles of HRCT include confirming the presence of disease, suggesting a specific etiology, evaluating disease progression, and localizing a site for biopsy. In one study of 20 children the first choice HRCT diagnosis was correct in 61%. Observers were confident of their diagnosis in 42% of cases; in these the diagnosis was correct in 91%. Interpretations were most accurate in pulmonary alveolar proteinosis, lymphangiectasis, and idiopathic pulmonary hemosiderosis [1]. A second study, also of 20, children found that is was possible to separate the underlying diseases into five groups with little overlap by the dominant HRCT pattern [4]. The groups were airway disease, septal disease, infiltrative lung disease, air space disease, and cystic disease. HRCT Terminology Although the diseases are very different, the terminology used to describe the HRCT appearance in adults works well for children [5]. Consolidation describes complete filling of the airspaces with a resulting CT density that is the same as and so obscures the vessels in the lung. Ground glass opacity describes an increase in the density of the lung parenchyma intermediate between normal lung density and the density of the vessels. In children this observation is often problematic as normal lung has a ground glass appearance if lung volumes are low. Septal thickening describes thickening of the septae surrounding secondary pulmonary lobules. Bronchiectasis is defined as an airway lumen larger than the diameter of an accompanying pulmonary artery. "Tree in bud" describes the appearance of bronchioles and alveolar ducts when they are filled or their walls are thickened. Bronchial wall thickening has been defined, but in practice is highly subjective and therefore a highly variable observation. HRCT Appearance of Pediatric Diffuse Lung Disease None of these appearances are specific to a single disease, but in our experience some patterns can be highly suggestive of certain diseases. The combination of septal thickening and ground glass opacity has been called "crazy paving" and is highly suggestive of alveolar proteinosis. In infants this suggests a diagnosis of surfactant protein abnormality [6]. It is important to recognize that this appearance in surfactant protein abnormality is transient with older children showing a more heterogeneous appearance consistent with the pathologic diagnosis found in these children of nonspecific interstitial pneumonitis [7]. Neuroendocrine cell hyperplasia of infancy (NEHI) [7] has a very characteristic appearance on HRCT with sharply defined areas of ground glass opacity located centrally and particularly in the lingula and right middle lobe with no other airway or parenchymal abnormalities. Bronchiolitis oblitterans is characterized by sharply defined areas of hyperlucent lung with small vessels and bronchiectasis. Lymphangiectasis with marked septal thickening and pleural effusions, and hemosiderosis with heterogeneous ground glass opacity and small nodules were two additional diagnoses felt to have characteristic appearances by Copley, et al. [1]. Conclusion HRCT provides important information about diffuse lung disease in children. Combining the clinical, imaging, and pathology findings is necessary to provide the most accurate diagnosis. Understanding the benefits and limitations of each evaluation should increase the ability of the physicians to best care for children with these diseases. References Copley, SJ, Coren, M, Nicholson, AG, et al., Diagnostic accuracy of thin-section CT and chest radiography of pediatric interstitial lung disease. AJR Am J Roentgenol, 2000. 174(2): p. 549-54. Copley, SJ and Bush, A, HRCT of paediatric lung disease. Paediatr Respir Rev, 2000. 1(2): p. 141-7. Owens, C, Radiology of diffuse interstitial pulmonary disease in children. Eur Radiol, 2004. 14 Suppl 4: p. L2-12. Lynch, DA, Hay, T, Newell, JD, Jr., et al., Pediatric diffuse lung disease: diagnosis and classification using high-resolution CT. AJR Am J Roentgenol, 1999. 173(3): p. 713-8. Austin, JH, Muller, NL, Friedman, PJ, et al., Glossary of terms for CT of the lungs: recommendations of the Nomenclature Committee of the Fleischner Society. Radiology, 1996. 200(2): p. 327-31. Newman, B, Kuhn, JP, Kramer, SS, et al., Congenital surfactant protein B deficiency--emphasis on imaging. Pediatr Radiol, 2001. 31(5): p. 327-31. Fan, LL, Deterding, RR, and Langston, C, Pediatric interstitial lung disease revisited. Pediatr Pulmonol, 2004. 38(5): p. 369-78.