ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)

Acute respiratory distress syndrome (ARDS) is the term applied to a spectrum of conditions with different etiologies sharing common clinical-pathological features, including:

• Increased permeability of the alveolo-capillary membrane, resulting in inflammatory edema
• Increased non-aerated lung tissue, resulting in higher lung elastance (lower compliance)
• Increased venous admixture and dead space, which result in hypoxemia and hypercapnia

Table 1: ARDS New Global Definition 2023 - Radiological features

There are several risk factors that may end up developing an acute respiratory distress syndrome:

Table 2: ARDS Risk factors

Radiological findings (see also Table 2):

• Bilateral heterogeneous ground-glass opacities and consolidations
• Interlobular septal thickening
• Symmetric or asymmetric lung involvement
• Pleural effusions less common than in cardiogenic pulmonary edema

Fig 1: ARDS - Both lungs images display heterogeneous diffuse ground-glass areas of increased attenuation with a gravitational anteroposterior gradient (red arrows). Thickening of interlobular septa (black arrows) combined with ground glass opacities forms the pattern known as "crazy paving". Mild pleural oedema could also be seen on left picture (triangle) .

Fig 2: ARDS - CT images in a patient with ARDS obtained 7 days after intubation show bilateral heterogeneous opacities, where intense parenchymal opacification in non-dependent areas (blue arrows) merges with ground-glass opacifications in the non-dependent lung (green arrows). We could also see areas of preserved parenchyma (red arrows).

Fig 3: ARDS - Images show pneumothoraces due to "Barotrauma", a well known complication of invasive mechanical ventilation in SDRA. In the left image it can be seen a thoracic tub (red arrow) and in the right an aerial cavity.

ACUTE CARDIOGENIC PULMONARY EDEMA

Accumulation of excessive fluid in the alveolar walls and alveolar spaces of the lungs caused by elevated pulmonary capillary pressure originating from a cardiac impairment.

Pathophysiology: ↑ pressure/fluid accumulation → ↑ pulmonary venous pressure → ↑ pulmonary capillary pressure → fluid in interstitial spaces → ↑ pressure in interstitial spaces → fluid in alveoli

Clinical manifestations:

Characteristics of left, right or simultaneous ventricular failure:

• Dyspnea, orthopnea and paroxysmal nocturnal dyspnea
• Tachypnea and tachycardia
• Jugular venous distention and peripheral edema
• Pink frothy sputum
• Angina
• Cool and clammy skin

Radiological findings:

• Central ground-glass opacities with peripheral area preservation ("bat wing" alveolar edema)
• Inter and intralobar septal thickening
• Peribronchial cuffing
• Cardiomegaly
• Bilateral pleural effusions and thickening of fissures
• Consolidation

Fig 4: CPE - CT scan shows images of two patients with cardiogenic pulmonary oedema with fluid overload and cardiac failure. The CT on the left (image A) shows a severe bilateral pleural effusion (red arrows). CT scan (image B) demonstrates the bat wing alveolar oedema with a central distribution and with lung cortex sparing. Cardiomegaly suggests ventricular failure.

Fig 5: CPE - The CT scans of three patients with CPE shows diffuse ground-glass areas of increased attenuation with inter- and intralobar septal thickening (green arrows) predominating in the anterior and upper portion of the right lung field (image A and B) with some peribronchial cuffing (blue arrows) and increased in the vascular diameter (orange arrows). Pleural effusion and fissures thickening (red arrows) are other features that could be found.

Fig 6: CPE - These two CT scans show a combination of alveolar edema (blue arrow) manifested as centrilobular ground glass opacities and interstitial edema manifested as smooth thickening of interlobular septa (green arrows).

PULMONARY HEMORRHAGE

It represents blood extravasation into alveoli due to multiple pathologies. Can be diffuse or localized.

Distribution may be:

- Diffuse: Vasculitis, Goodpasture syndrome, coagulation disorders, hemosiderosis, etc.

- Localized: Vasculitis, pulmonary infarction, infectious, traumatic, neoplastic, AVM, iatrogenic, etc.

Clinical manifestations:

• Hemoptysis
• Complementary tests: hypoxemia

Radiological findings:

Diffuse:

• Widespread ground-glass opacification with septal thickening (crazy paving pattern)
• May associate with areas of consolidation

Localized:

• Ground-glass opacity limited to an area of lung parenchyma

Fig 7: P. Hemorrhage - CT scans of 2 different patientes with hemoptysis and a crazy paving pattern localized or diffuse. The images (C, D and E) show a localized ground-glass area with interlobular septa thickening. The other CT scans (images A and B) show diffuse ground-glass areas combined with interlobular septa thickening.

PNEUMOCYSTIS JIROVECI

Pneumocystis jirovecii pneumonia is a potentially fatal opportunistic fungal disease commonly affecting immunocompromised patients (HIV, immunosuppressants, biological agents, corticosteroids, hematologic malignancies, solid tumors, etc).

Clinical manifestations:

• Dyspnea
• Non-productive cough
• Fever
• Complementary tests: hypoxemia, PCR (+), BAL (cysts), CD4 lymphocyte count < 200 mm3

Radiological findings:

• Ground-glass pattern (main finding)

• • Perihilar or upper/mid-zones (main involvement)
  • Peripheral area preservation (~40%) 
  • Bilateral or asymmetric opacities

• Septal thickening or reticular opacities ("crazy paving" pattern)
• Pneumatoceles (~30%) predominantly involving the upper lobes. Higher risk of spontaneous pneumothorax and pneumomediastinum
• Atypical features: consolidation, nodules and "tree-in-bud" opacities suggesting an intercurrent infectious bronchiolitis, pleural effusion and adenopathy

Fig 8: PCP pneumonia - The CT scans above display the main radiological pattern in patients with PCP pneumonia, diffuse bilateral ground-glass opacities predominantly involving the upper lobes with a central distribution with relative peripheral sparing.

Fig 9: PCP pneumonia - In the following images we can observe an asymmetric lung involvement with asymmetric ground glass opacities (blue arrows), areas of preserved parenchyma (green arrows) and an area of consolidation (red star)."

Fig 10: PCP pneumonia - A CT scan of a patient with PCP pneumonia shows pulmonary cysts of varying shape, size, and wall thickness, predominantly involving the upper lobes.

Fig 11: PCP pneumonia - CT scans above show pulmonary cysts which are associated with an increased frequency of spontaneous pneumothorax and pneumomediastinum (green arrows).

BACTERIAL AND VIRAL PULMONARY INFECTIONS

Pulmonary infection caused by microorganisms (bacteria or viruses) presenting a diverse array of clinical manifestations and varying degrees of seriousness

Clinical manifestations of bacterial infection:

• Fever (>38°C)
• Productive cough with purulent sputum
• Dyspnea, tachypnea and increased respiratory effort
• Complementary tests: hypoxemia, leukocytosis, urine antigen test (+), blood/respiratory cultures (+)

Radiological findings of bacterial infections:

• Consolidation (heterogeneous density) associated with air/fluid bronchogram
• Ground-glass opacity
• Nodules: diffuse or patchy tree-in-bud suggestive of bronchiolitis
• Abscesses
• Empyema: infected purulent and often loculated pleural effusion

Fig 12: Bacterial pneumonia - The four CT scans of patients with bacterial pneumonia show consolidations of heterogeneous density that do not uniformly enhance and are associated with air bronchograms. There is no loss of volume.

Fig 13: Bacterial pneumonia - In these two CT images, smooth thickening with enhancement of the pleural leaves is observed, with content of heterogeneous density consistent with an empyema.

Clinical manifestations of viral infection:

• Low-grade fever
• Flu-like symptoms or upper respiratory tract infection signs
• Non-productive cough
• Dyspnea, tachypnea and increased respiratory effort
• Complementary tests: hypoxemia, PCR tests (+)

Radiological findings of viral infections:

• Altered lung parenchymal attenuation with heterogeneously patchy areas (mosaic pattern)
• Ground-glass opacities
• Interlobular septal thickening (crazy paving pattern)
• Nodules/micronodules and tree-in-bud opacities
• Thickening of bronchial/bronchiolar walls
• Consolidation and fibrotic pattern evolution

Fig 14: Viral pneumonia - CT scans at the top (images A and B) show a patchwork of regions with differing pulmonary attenuation (mosaic attenuation pattern), associated with viral pneumonias. The CTs below (images C and D) display multiple well and ill-defined patchy areas of ground-glass opacities (red arrows) with interlobular septal thickening, “tree-in-bud” opacities (green arrow) and bronchial wall thickening (blue arrow).

Fig 15: Viral pneumonia CT images of three patients with COVID and different manifestations. The CT at the left (image A) shows patchy peripheral subpleural consolidations with ground-glass opacities and an architectural distortion with interlobular septal thickening, all consistent with a fibrotic pattern evolution. Irregular areas of consolidation (posterior regions) and diffuse ground-glass opacities with interlobular septal thickening are seen in the middle CT image (image B). CT in the right (image C) displays diffuse ill-defined patchy ground glass opacities with interlobular septal thickening (crazy paving pattern) in both lungs.

ACUTE ASPIRATION PNEUMONIA / PNEUMONITIS

Lung injury associated with the aspiration of solid and/or liquid materials into the airways and lungs. Clinical manifestations and radiological findings will depend on the type of content and the severity of the aspiration.

Clinical manifestations:

• Cough
• Purulent sputum
• Hemoptysis
• Fever

Radiological findings:

Posterior segments of the upper lobes and superior segments of the lower lobes are frequently involved when patients aspirate in recumbent position, whereas the bibasilar segments, right middle lobe, and lingular segment are affected in erect patients.

• Centrilobular nodules with tree-in-bud pattern → Impaction of aspirated particles in distal airways.
• Ground-glass opacities (parenchymal inflammation)
• Central airway obstruction
• Segmental or lobar atelectasis
• Consolidations which can associated air/fluid bronchogram

Fig 16: Aspiration pneumonia - Posterior segments of the upper lobes and superior segments of the lower lobes are commonly involved in aspiration pneumonia, as it is shown in the CT scans. Common radiological findings in the CT are areas of airspace consolidation (posterior regions) surrounded with ground-glass opacities, “tree-in-bud” opacities (green arrows) and bronchial obstruction (blue circle).

Fig 17: Aspiration pneumonitis - CT images show extensive bilateral areas of ground-glass opacities predominantly in the upper lobes, the right middle lobe and the apical regions of the lower lobes, compatible with chemical pneumonitis.

DRUG-INDUCED LUNG DISEASE

Lung injury resulting from different types of agents (antineoplastics, anti-inflammatories, cardiovascular agents, antibiotics, etc.), presenting with various clinical manifestations and severity.

It is a diagnosis of exclusion.

Clinical and radiological improvement may occur upon discontinuation of therapy.

Nonspecific clinical manifestations:

• Dyspnea
• Non-productive cough
• Fever
• Complementary tests: hypoxemia, eosinophilia

Radiological findings:

• Ground-glass alveolar or interstitial opacities (reticular and/or nodular opacities).

Fig 18: Drug-induced lung disease - CT scans exhibit different radiological findings due to drug-induced lung disease. CT (Image A) shows diffuse areas of alveolar ground glass opacities with interlobular septal thickening (green circle) in a patient with PD-L1 immunotherapy therapy. CT scans (image E and F) displays an interstitial ground-glass opacities (red arrows) in a patient undertaking Folfox treatment. CT scans (images G and H) show intersticial nodular opacities (blue circle/arrow) in a patient treated with Pembrolizumab. Patchy areas of ground glass opacities are seen in the upper right CTs (images B, C and D) in a patient with Metotrexate treatment (orange arrows).

CRYPTOGENIC ORGANIZING PNEUMONIA (COP)

Cryptogenic organizing pneumonia (COP) is a form of diffuse idiopathic interstitial lung disease resulting from alveolar injury triggering an inflammatory and fibroproliferative process, characterized by fibrotic plugs within alveoli obstructing alveolar and bronchiolar lumens. Interstitial inflammation and fibrosis are minimal or absent.

The idiopathic form has no identifiable cause, meanwhile, the secondary form can be caused by a wide range of infectious/non-infectious causes.

Reversible with immunosuppressive or anti-inflammatory therapy.

Clinical manifestations:

• Acute dyspnea (rapidly progressive forms)
• Non-productive cough
• Dry crackles
• Fever
• Complementary tests: hypoxemia, increased inflammatory markers (ESR, C- reactive protein and leukocytes)

Radiological findings:

• Peripheral or peribronchial bilateral patchy migratory consolidations and ground-glass opacities
• Atoll or reversed halo sign: ground-glass opacities surrounded by a peripheral consolidation
• Bronchial dilatation (it may reproduce an air bronchogram sign within a consolidation or ground-glass opacity) 
• Perilobular pattern: bowed or polygonal opacities with poorly defined margins bordering the interlobular septa (specific finding)
• Nodular or reticular opacities
• Fibrotic pattern

Fig 19: COP - CT images depict the typical COP pattern with multifocal and asymmetrical parenchymal consolidations (red arrows), with peripheral distribution which tend to migrate, disappearing spontaneously and appearing in different sites.

Fig 20: COP - CT scans show that consolidations may be associated with ground-glass opacities (red arrows) and they usually reproduce an air bronchogram sign in the context (green arrows).

Fig 21: COP - CT scans of two patients with COP show areas of ground-glass opacities surrounded by a ring- or a crescent-shaped consolidation known as “Atoll or Reverse halo sign” (red arrows), which are depicted in the upper lobes of both patients.