200 to 300 µm and therefore are not visible on routine radiologic imaging (Fig. 1.5). The pulmonary acinus and secondary pulmonary lobule are the smallest components of the lungs visible on radiologic imaging, and it is therefore essential that radiologists be familiar with their appearance in health and disease. The pulmonary acinus is defined as the portion of the lung that is distal to the terminal bronchiole and is composed of a respiratory bronchiole (or bronchioles) and the associated alveolar ducts and sacs. The pulmonary acinus is, therefore, the largest lung unit in which all the airways perform gas exchange.1 Each acinus contains ˜50 to 400 alveoli. The pulmonary acinus ranges in size from 6 to 10 mm in adolescence and adulthood and can be visualized on high-resolution CT (HRCT).2,3 The acinus is smaller in younger children, ranging in size from 1 to 2 mm in children under 1 year of age and therefore may not be visible.
FIGURE 1.3. Schematic diagram of the lower airway branches. (Adapted from Nath J. A Short Course in Medical Terminology; 2018. Figure 8. © Wolters Kluwer, with permission.)
FIGURE 1.6. Schematic diagram of the normal secondary pulmonary lobule. (Reprinted from Elicker BM, Webb RW. Fundamentals of High-Resolution Lung CT: Common Findings, Common Patterns, Common Diseases, and Differential Diagnosis; 2013. Figure 1.11. © LWW/Wolters Kluwer, with permission.)
FIGURE 1.10. Schematic diagram of thoracic bronchial arteries. (Reprinted from Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 7th ed.; 2013:117. © LWW/Wolters Kluwer, with permission.)
TABLE 1.1 Prenatal Lung Development
barrier makes survival outside the uterus feasible by the end of this phase. However, capability for gas exchange remains quite immature, and survival of extremely premature infants with underdeveloped lung function depends on neonatal intensive care with surfactant replacement therapy. Largely owing to pulmonary disease, survival rates for preterm infants born at 23, 24, and 25 weeks of gestational age are low at 11%, 26%, and 44%, respectively.13
35 weeks, the lung produces surfactant in smaller amounts than at term, and this surfactant is more susceptible to inactivation.24 By 35 weeks, the amount of surfactant is increased and the chemical composition is mature, containing more saturated phosphatidylcholine, phosphatidylglycerol, and surface proteins, and less phosphatidylinositol.21 The process of surfactant maturation can be slowed by insulin and can be accelerated by glucocorticoids and thyroid hormone. This hormone-mediated acceleration of surfactant maturation can be manipulated clinically, and mothers are routinely given glucocorticoids (e.g., betamethasone) 24 to 48 hours prior to preterm delivery. This reduces the incidence of the most significant complication of surfactant deficiency in preterm infants, SDD. When a preterm child is born with SDD, exogenous surfactant can be administered via an endotracheal tube to supplement the small amount of endogenous premature surfactant that is being produced. Although premature birth is the most common clinical scenario in which surfactant deficiency is seen, rare genetic mutations can also be causal. These include mutations in surfactant proteins (SPs) SP-B and SP-C as well as the intracellular transporter ABCA3.21
FIGURE 1.14. A 6-year-old boy with an inferior accessory fissure detected on a CT obtained for abdominal pain. Axial lung window CT image shows normal bilateral major fissures (black arrows) and an inferior accessory fissure (white arrow).
FIGURE 1.15. A 7-year-old boy with an azygos fissure. A: Frontal chest radiograph shows an azygos fissure (arrow) and lobe. B: Axial lung window CT image demonstrates an azygos fissure (arrow) and azygos lobe (asterisk). T, Trachea.
alternatives, which may involve less or no ionizing radiation. For example, US and MRI are being utilized with greater frequency to assess disorders of the lung and should be employed when possible. CT remains the preferred imaging modality for cross-sectional assessment of most pulmonary parenchymal disorders due to acoustic shadowing from osseous structures and aerated lung on US and issues related to motion artifact and signal dephasing on MRI. When CT is indicated, it is imperative to utilize optimized low-dose pediatric protocols, which decrease tube current or milliamperage (mA) and kilovoltage peak (kVp) based on patient size, ideally in conjunction with tube current modulation.52,56,57,58
selected cases.63,64,65,66,67,68,69,70,71 Techniques utilizing inhaled hyperpolarized gases can increase signal within the lungs and improve image quality, although these techniques are still largely research based and not available in most clinical settings.72,73,74,75 Recently, Fourier decomposition pulmonary MRI, which is a noninvasive method for assessing ventilation and perfusion-related information, has been identified as a promising new MRI technique; however, its use is currently limited to research applications.76
59.1% born at 25 weeks, 75.3% born at 26 weeks, 93.6% born between 27 and 31 weeks, and 98.9% born between 32 and 34 weeks survived to discharge.85 SDD is twice as common in males and white patients.86
FIGURE 1.16. Male neonate born at 27 weeks of gestation who presented with respiratory distress due to surfactant deficiency disorder. Frontal chest radiograph shows diffuse granular opacities in both lungs with slightly decreased lung volumes.
fine granularity is thought to correspond to excessive lung fluid. Patients with this pattern of disease do not fulfill clinical criteria for respiratory distress syndrome, and this condition has been termed “immature lung.”90 Despite the lack of chest radiographic findings, affected patients frequently have significant left-to-right shunts across the patent DA, sepsis, bradycardia, and apnea that frequently necessitates mechanical ventilation.87 At days 2 and 3 of life, the chest radiograph typically worsens, and there is development of coarse, irregularly distributed lung disease that frequently persists and evolves into a pattern of chronic lung disease.86,87
therapy, it is now uncommon for SDD to develop in infants born at more than 30 weeks of gestation or with birth weight of >1,200 g.93 With improvements in treatment, a larger number of extremely low-birth-weight infants (<1,000 g) are surviving and eventually developing chronic lung disease. However, these infants often have different clinical and radiographic characteristics than the infants described by Northway. These infants frequently do not demonstrate the typical radiographic findings of RDS, but rather develop a pattern termed “immature lung,” which was described in the previous section (Fig. 1.20). Moreover, lung disease that occurs in this patient group may not be the result of high supplemental oxygen concentrations or positive pressure ventilation87 but may be
due to arrested acinar and vascular maturation instead.94 This entity has been called the “new BPD” in order to distinguish it from the classic BPD described by Northway.92,94
FIGURE 1.22. A 3-month-old boy born at 26 weeks of gestation with chronic lung disease of infancy. Frontal chest radiograph shows bilateral increased coarse opacities, atelectasis (arrow), cyst-like lucencies, and hyperinflation (H).
behind fetal lung fluid resorption, but this actually appears to play a very small role.106 The majority of lung fluid is cleared as a result of epithelial sodium channel activity, which is inactive in the fetal lung until adrenergic stimulation occurs at birth.107,108,109,110
temperature and hypothermia.116 As symptoms are nonspecific, it is often difficult to distinguish neonatal pneumonia from other causes of respiratory distress based on clinical findings.
FIGURE 1.24. A 1-day-old full-term boy who presented with respiratory distress due to meconium aspiration. Frontal chest radiograph shows coarse opacities in both lungs. The endotracheal tube tip terminates in low position, just above the carina.
FIGURE 1.25. Newborn full-term boy who presented with respiratory distress due to meconium aspiration. Frontal chest radiograph shows small bilateral pneumothoraces (arrows) and clear lungs.
FIGURE 1.26. A 1-day-old full-term girl who presented with decreased oxygen saturation due to neonatal pneumonia. Frontal chest radiograph shows bilateral symmetric alveolar opacities in both lungs.
FIGURE 1.27. A 4-day-old full-term boy who presented with respiratory distress due to neonatal pneumonia. Frontal chest radiograph demonstrates bilateral asymmetric alveolar opacities, which are more severe on the right.
is radiographically indistinguishable from meconium aspiration. There have been numerous reports of group B streptococcal neonatal pneumonia and sepsis associated with late-onset right-sided congenital diaphragmatic hernia.117 The appearance of the diaphragmatic defect at surgery is identical to that in cases without associated pneumonia. The mechanism for the association is currently unclear.
FIGURE 1.28. A 3-day-old boy who presented with fever and respiratory distress due to group B streptococcal pneumonia. Frontal chest radiograph shows airspace opacities (asterisks) in both lower lobes and a small right effusion (arrow).
it can be a harbinger of other air leak complications including pneumothorax. Acknowledgment of this finding often prompts a change to HFOV.123 The majority of cases of PIE resolve spontaneously, with fewer going on to develop PPIE. First-line management of PPIE is nonoperative, although surgical resection may be required in cases of hyperexpansion with significant mass effect and/or respiratory compromise.123 When surgery is considered, CT is often useful to define the lobar distribution.123,124,125
arteriovenous malformation [AVM]) on the other end. These malformations can be further divided into three groups: pure airway and parenchymal lesions without vascular abnormalities, pure vascular lesions, and malformations of both lung parenchyma and the vasculature.
typically located within the middle mediastinum and, less commonly, within the lung parenchyma (Fig. 1.36A). On CT, foregut duplication cysts are typically circumscribed, round, or ovoid nonenhancing lesions with homogeneous fluid attenuation between 0 and 20 Hounsfield units and a thin wall, although density can be higher if there are proteinaceous contents61 (Fig. 1.36B). On MRI, foregut duplication cysts are typically high in signal on T2-weighted MR images, variable in intensity on T1-weighted MR images depending on the nature of the cyst contents, and lacking enhancing solid components142 (Fig. 1.36C and D). While mediastinal bronchogenic cysts rarely communicate with the airway, intrapulmonary bronchogenic cysts often do communicate with the airway, increasing the risk of superimposed infection.150 In cases of superinfection, air-fluid levels may develop and the wall may become thickened, with associated hyperenhancement and adjacent inflammatory changes135 (Fig. 1.37).
soon after birth.133,144,155 Smaller type 1 CPAMs may only become symptomatic later in life if superimposed infection occurs or in rare cases of associated malignancy.155 Type 2 CPAMs account for ˜20% of all CPAMs, and ˜50% of type 2 CPAMs are accompanied by additional congenital anomalies, including cardiovascular malformations, extralobar pulmonary sequestration, tracheoesophageal fistula, renal agenesis, intestinal atresia, and congenital diaphragmatic hernia, leading to poorer prognosis than type 1 CPAMs.155,161 Type 3 CPAMs account for ˜5% to 10% of all CPAMs and typically present as a solid mass involving an entire lobe. Type 3 CPAMs may be associated with polyhydramnios, fetal hydrops, and significant respiratory symptoms soon after birth.162 The entity described as type 4 CPAM, which has significant overlap with type I PPB, accounts for ˜10% of CPAMs and may be symptomatic in the newborn period or may present with pneumothorax in older children, a feature which is unusual for other CPAMs.155
corresponding to Stocker type 2 lesions.135,144 In lesions without visible cysts, as in some Stocker type 2 lesions and all Stocker type 3 lesions, the hyperechoic, hyperintense lung lesion may be indistinguishable from other congenital lung lesions such as bronchopulmonary sequestration and CLE.144 CPAMs may be large and can exert mass effect on the adjacent lung and mediastinum.144 CPAMs most commonly affect a single lobe, although multilobar and bilateral lesions rarely occur.155 Prenatally diagnosed CPAMs are usually first detected in the 2nd trimester, typically increase in size between 20 and 25 weeks of gestation, and stabilize in size by the end of the 2nd trimester.144 Twenty to fifty percent decrease in size during the 3rd trimester, but complete involution is rare.144,163,164
been reported165,166,167,168; however, it has been proposed that these cases represent low-grade PPBs rather than CPAMs with malignant degeneration.128
FIGURE 1.39. A 1-month-old boy with type 2 congenital pulmonary airway malformation in the right lung. Axial lung window CT image shows two multicystic lung lesions (arrows).
categorization, alternative classification systems have been proposed. For example, Clements and Warner created a comprehensive system, using the term “pulmonary malinosculation” to describe “a congenitally abnormal connection or opening of one or more components of the bronchopulmonary vascular complex.”126 Another approach described by Langston points out that many different lung malformations may have a systemic arterial supply; on the basis of this fact, the author considers that abnormal systemic arterial supply is an attribute rather than a defining characteristic and encourages use of a descriptive approach.128,131 Despite the existence of valid and widely cited competing classification systems, the terms “intralobar” and “extralobar” pulmonary sequestration seem to be deeply entrenched, and familiarity with the classic definitions and their limitations is essential. As with all congenital lung lesions, the radiologist is advised to focus on imaging features, including location, vascular supply, and appearance of the lesion, and avoid descriptors that may be unclear.
Although many lesions seem to disappear during the 3rd trimester, complete regression is very unusual and followup postnatal cross-sectional imaging is recommended in all cases because radiographs may not detect small lesions.144,177
infection, findings may include consolidation, air-fluid levels, and increased cyst formation in chronically infected lesions.61
in children under the age of 5, ˜20 million of which lead to hospitalization.183 Acute lower respiratory infection, defined as infection that affects the airway below the glottis, is a rare cause of mortality in Western countries (<1 per 1,000 per year)184 but is the leading cause of childhood mortality in the world overall.183
from as early as 1958.207 HMPV causes a lower respiratory tract infection with highly similar clinical manifestations to RSV. HMPV is primarily an infection of children, with 77% of the population becoming seropositive by age 5, 91% by age 10, and 96% by age 20.208 Like RSV, infections from HMPV are usually self-limited, although severe symptoms in a subset of pediatric patients can require hospitalization. As in RSV, children with comorbidities including chronic lung disease, prematurity, and immunodeficiency are more likely to have severe disease207 (Fig. 1.46).
FIGURE 1.45. A 4-month-old girl with respiratory syncytial virus infection who presented with cough and wheezing. Frontal chest radiograph shows perihilar interstitial opacities with peribronchial cuffing and hyperinflation.
Streptococcus pneumoniae or Staphylococcus aureus infection.228 Seasonal influenza epidemics typically occur annually, for instance, usually between October and May in the United States.229 In recent years, several contagious outbreaks of infection by novel influenza strains have resulted in high mortality and morbidity. A highly virulent avian-origin virus, H5N1, crossed over to humans in 1997, Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) resulted in a worldwide outbreak in 2002, a swine-origin H1N1 virus caused a pandemic in 2009, and Middle East Respiratory Syndrome Coronavirus (MERS-CoV) spread to 27 countries in 2016. These variants are discussed further below.
FIGURE 1.49. A 4-year-old boy with rhinovirus lower respiratory tract infection who presented with fever and asthma exacerbation. Frontal chest radiograph shows prominent interstitial opacities and atelectasis (asterisk) in the left lower lobe.
FIGURE 1.50. A 5-year-old boy with influenza B lower respiratory tract infection who presented with fever and cough. Frontal chest radiograph shows bilateral peribronchial thickening and atelectasis (asterisk) in the left lower lobe.
FIGURE 1.51. A 15-year-old boy with avian-origin influenza (H5N1) virus infection who presented with severe respiratory distress requiring intubation. Frontal chest radiograph shows diffuse bilateral airspace opacities with air bronchograms.
pneumonitis with respiratory failure, diffuse alveolar damage (DAD), septic shock, and multiorgan failure.238 Children are more likely to have mild disease, with cough and fever being the most commonly reported symptoms.238 Mortality is much less common in children than in adult patients, except in cases in which there is an underlying medical condition.238
FIGURE 1.54. A 10-year-old girl with fever and worsening cough due to swine-origin influenza A (H1N1) virus infection. Frontal chest radiograph shows bilateral, multifocal consolidations (asterisks).
and gastroenteritis but may also cause keratoconjunctivitis, cardiac infection, genitourinary infection, and lymphatic infection. Adenovirus can cause a variety of respiratory tract infections, including otitis media, pharyngitis, tonsillitis, croup, bronchiolitis, bronchitis, pneumonia, and pleural effusion. Up to 20% of pneumonias occurring in children <5 years of age are due to adenovirus.246 Types 3, 7, and 21 have been associated with the most severe disease, and fatalities have been reported with these types and several others.247
FIGURE 1.56. A 5-year-old girl with measles who presented with fever, cough, and maculopapular rash. Axial lung window CT image shows multiple small bilateral pulmonary nodules.
FIGURE 1.57. An 8-month-old boy with adenovirus lower respiratory tract infection who presented with fever and cough. Frontal chest radiograph shows perihilar interstitial and hazy opacities.
bilateral pleural effusions266,267 (Fig. 1.63). Imaging findings in immunocompromised older patients with HSV pneumonia include patchy segmental and subsegmental consolidation, ground-glass opacities, atelectasis, centrilobular nodules, and pleural effusion.268,269,270 HSV-1 pneumonias are frequently associated with coexisting bacterial pneumonia.260
FIGURE 1.61. A 40-year-old female with a prior history of varicella pneumonia. Frontal chest radiograph shows multiple small, calcified pulmonary nodules in both lungs.
canals of Lambert, and resultant centrifugal spread of infection within the lung leads to spherical consolidation with sharp margins. The mean age for round pneumonia is 5 years, with 75% occurring before 8 years of age and 90% occurring before 12 years of age.271
disproven, but the organism was given the species name of influenzae to reflect this historical association. H influenzae type B (HiB) is a particularly virulent strain and was the most common cause of bacterial meningitis in children under 5 years in the United States and a common cause of several other infections including pneumonia, empyema, epiglottitis, pericarditis, cellulitis, and septic arthritis before 1990.284 Since the introduction of a conjugate vaccine in 1988, the incidence of HiB infection in the United States has fallen by more than 95%.285 Before widespread vaccination, children with underlying medical conditions, including SCD, asplenia, HIV infection, and malignancy, were at increased risk. H influenzae infection is now relatively rare in countries with widespread vaccination but can still be seen in nonimmunized children and in children with poor antibody response to vaccination, including those with underlying immunodeficiency.286
FIGURE 1.66. A 4-year-old boy who presented with fever and cough due to Streptococcus pneumoniae pneumonia. A: Frontal chest radiograph obtained at initial presentation shows consolidation (asterisk) in the left lower lobe. B: Frontal chest radiograph obtained 4 days later demonstrates a cystic cavity (black arrowhead) within the left lower lobe due to necrosis. A chest tube (black arrow) was placed for treatment of a complicated effusion. C: Axial enhanced CT image obtained 10 days after initial presentation shows a loculated empyema with pleural enhancement (white arrows) and multiple cystic cavities (white arrowheads) in the left lower lobe due to necrosis.
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