Imaging the Lung in ARDS: A Primer

Abstract

Despite periodic changes in the clinical definition of ARDS, imaging of the lung remains a central component of its diagnostic identification. Several imaging modalities are available to the clinician to establish a diagnosis of the syndrome, monitor its clinical course, or assess the impact of treatment and management strategies. Each imaging modality provides unique insight into ARDS from structural and/or functional perspectives. This review will highlight several methods for lung imaging in ARDS, emphasizing basic operational and physical principles for the respiratory therapist. Advantages and disadvantages of each modality will be discussed in the context of their utility for clinical management and decision-making.

Keywords: ARDS; chest radiograph; computed tomography; electrical impedance tomography; magnetic resonance imaging; positron emission tomography; ultrasound.

Fig. 1.

Example chest radiographs in a male patient with ARDS, illustrating the evolution of lung injury from hospital admission (A) over the course of 3 days (B–D). From Reference 40 with permission under terms of the Creative Commons Attribution 4.0 International License.

Fig. 2.

A: Illustration of radiographic tube and detector array rotating around subject with the computed tomography gantry. B: As the radiographic tube and detector array rotates, the table moves through the gantry with a constant velocity, resulting in a helical path of projection data around the subject. The helical projection data can then be reconstructed into transverse planes using interpolation, thus yielding a stack of images. From Reference 117 with permission. Copyright © 2002 Wolters Kluwer Health, Inc. All rights reserved.

Fig. 3.

A: Example of a single 1 mm³ voxel from a computed tomography (CT) image in transverse (ie, axial) cross section of the thorax. At this spatial resolution, one voxel contains about 170 alveoli. From Reference 117 with permission. Copyright © 2002 Wolters Kluwer Health, Inc. All rights reserved. B: Given this spatial resolution, 2 different voxels may have the same level of CT grayness (or density) since they have the same air and water contents on average, despite very different alveolar structure. From Reference 38 with permission. Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved. Anesthesiology is an official journal of the American Society of Anesthesiologists. Readers are encouraged to read the entire article for correct content at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692186/. The original authors, editors, and the American Society of Anesthesiologists are not responsible for errors or omissions in adaptations.

Fig. 4.

Illustration of computed tomography (CT) window (W) and level values. The level (L) corresponds to the center of the window, with a narrow window producing a high-contrast image. CT densities below P₁ will appear black, while those above P₂ will appear white. Only CT densities between P₁ and P₂ will vary according to radiographic attenuation. CT = computed tomography. From Reference 117 with permission. Copyright © 2002 Wolters Kluwer Health, Inc. All rights reserved.

Fig. 5.

Comparison of (A) a plain chest radiograph with (B) a computed tomography (CT) scan in ARDS. Images were obtained in the same patient, close together in time. Note the more specific structural information obtained with the CT scan. Whereas both images demonstrate disuse infiltrates, the CT image indicates that these are located primarily in the dorsal, gravity-dependent regions (indicated by the black arrow), with the ventral regions of the lung having greater aeration (indicated by green arrow). From Reference 38 with permission. Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved. Anesthesiology is an official journal of the American Society of Anesthesiologists. Readers are encouraged to read the entire article for correct content at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692186/. The original authors, editors, and the American Society of Anesthesiologists are not responsible for errors or omissions in adaptations.

Fig. 6.

Example density histograms from a small cohort of patients with ARDS, plotting a distribution of the lung volume as a function of Hounsfield density for zero airway pressure, as well as lower and higher PEEPs. For clarity, error bars are omitted. Also shown are compartmentalized aeration ranges, corresponding to hyper-aerated, normally aerated, poorly aerated, and non-aerated regions. Note that as the level of airway pressure increases a greater proportion of total lung volume occurs in these more aerated regions, while conversely the amount of non-aerated lung decreases. From Reference 132 with permission. Copyright © 1999 American Thoracic Society. All rights reserved. The American Journal of Respiratory and Critical Care Medicine is an official journal of the American Thoracic Society. Readers are encouraged to read the entire article for the correct context at https://doi.org/10.1164/ajrccm.159.5.9805112. The original authors, editors, and the American Thoracic Society are not responsible for errors or omissions or adaptations.

Fig. 7.

Examples of lung ultrasound images from a normal lung (A), lungs with decreased aeration due to interstitial syndrome (B), and lungs with complete consolidation (C). In addition to shadows from the underlying ribs, the healthy lung in (A) also shows a stripe corresponding to the pleural surface, with A-line reverberation artifacts. From Reference 38 with permission. Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved. Anesthesiology is an official journal of the American Society of Anesthesiologists. Readers are encouraged to read the entire article for correct content at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692186/. The original authors, editors, and the American Society of Anesthesiologists are not responsible for errors or omissions in adaptations.

Fig. 8.

Determination of regional electrical impedance in the thorax using electrical impedance tomography. In this example, alternating electrical current (I₁) is injected from electrodes 1 and 16, and the corresponding voltage responses are sensed by electrodes 2–15. This cycle is repeated using different electrode pairs (ie, current injection between electrodes 1 and 2, then 2 and 3, etc) until the voltage responses are traced throughout the whole belt. From Reference 38 with permission. Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved. Anesthesiology is an official journal of the American Society of Anesthesiologists. Readers are encouraged to read the entire article for correct content at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692186/. The original authors, editors, and the American Society of Anesthesiologists are not responsible for errors or omissions in adaptations.

Fig. 9.

A: Comparison between a computed tomography (CT) image (upper panel) and an electrical impedance tomography (EIT) image (lower panel). The EIT image depicts regional changes in electrical impedance according to a color scale, with red indicating maximal tidal impedance change and blue indicating no change. B: Corresponding time tracings of regional electrical impedance changes over the course of 1 min. Note that the right dorsal region of the lung is electrically silent in the EIT image, consistent with the complete consolidation of the CT mage in A. From Reference 39 with permission under terms of the Creative Commons Attribution 4.0 International License.

Fig. 10.

Cross-registered computed tomography and [¹⁸F]FDG positron emission tomography (PET) images from a patient with ARDS. The PET image represents average [¹⁸F]FDG concentration during the last 20 min of acquisition (from 37–57 min since administration). Color scale represents radioactivity concentration in units of kBq/mL. Note that [¹⁸F]FDG uptake is high in normally aerated regions (square 1) but is lower in the dorsal, non-aerated regions (square 2). From Reference 111 with permission. Copyright © 2009 Wolters Kluwer Health, Inc. Permission conveyed through Copyright Clearance Center, Inc.