Point-of-Care Lung Ultrasound in Adults: Image Acquisition

Summary

Point-of-care ultrasound (POCUS) of the lungs provides quick answers in rapidly changing clinical scenarios. We present an efficient and informative protocol for image acquisition for use in acute care settings.

Abstract

Consultative ultrasound performed by radiologists has traditionally not been used for imaging the lungs, as the lungs' air-filled nature normally prevents direct visualization of the lung parenchyma. When showing the lung parenchyma, ultrasound typically generates a number of non-anatomic artifacts. However, over the past several decades, these artifacts have been studied by diagnostic point-of-care ultrasound (POCUS) practitioners, who have identified findings that have value in narrowing the differential diagnoses of cardiopulmonary dysfunction. For instance, in patients presenting with dyspnea, lung POCUS is superior to chest radiography (CXR) for the diagnosis of pneumothorax, pulmonary edema, lung consolidations, and pleural effusions. Despite its known diagnostic value, the utilization of lung POCUS in clinical medicine remains variable, in part because training in this modality across hospitals remains inconsistent. To address this educational gap, this narrative review describes lung POCUS image acquisition in adults, including patient positioning, transducer selection, probe placement, acquisition sequence, and image optimization.

Introduction

Over the past several decades, bedside decision-making and treatment have increasingly been augmented by point-of-care ultrasound (POCUS). POCUS is the use of ultrasound for diagnostic or procedural guidance by a patient's primary treatment provider. This is in contrast to consultative ultrasound, where the ultrasound exam is requested by a patient's primary treatment provider but is performed by a separate specialist team¹.

This narrative review focuses on diagnostic POCUS of a specific organ system: the lungs. Diagnostic POCUS of the lungs has proven useful in the acute care setting, allowing the diagnosis of potentially life-threatening conditions in scenarios of respiratory failure, shock, trauma, chest pain, and other situations². Further, procedural lung POCUS is being used to guide needle placement in percutaneous thoracentesis³ and lung recruitment maneuvers⁴. However, despite its clinical significance, lung POCUS proficiency among physicians is variable⁵, limiting the appropriate use of this modality. The purpose of this review is to describe a time-efficient yet thorough image acquisition protocol for diagnostic lung POCUS in adults and to illustrate abnormal findings commonly found in clinical practice. The method described herein is not suitable for newborns and small infants. For information regarding lung POCUS imaging techniques and interpretation in this particular age group, the reader is invited to refer to specific literature^6,7.

There are multiple imaging protocols described in the literature, varying from four-point to 28-point exams depending on how much time is available and what questions the exam is seeking to answer⁸. While the diagnostic accuracy for certain pathologies might be higher when more points are scanned, a focused six-point protocol offers a reasonable trade-off between efficiency and diagnostic accuracy^2,9,10,11,12.

Protocol

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

1. Instrument settings and probe selection

NOTE: Lung POCUS can be performed with a multitude of transducers depending on what question needs to be answered.

1. Superficial lung investigation
   1. For the evaluation of abnormalities that have manifestations superficially (e.g., pneumothorax or pleural line abnormalities), perform lung POCUS using a linear high-frequency (5-10 MHz) probe, with the focal zone set at the pleural line. If a linear high-frequency probe is unavailable, perform superficial lung ultrasound using a low-frequency probe (see section 1.2), although the spatial resolution will be lower, which increases the chances of ambiguous or difficult-to-interpret findings.
2. Deep lung investigation
   1. Use a low-frequency (≤5 MHz) ultrasound probe for the evaluation of anything deeper than the interface of the visceral and parietal pleurae. Ensure that the low-frequency probe has a footprint small enough to fit in between the rib spaces (e.g., a convex array, a micro-convex array, or a linear phased-array sector arc probe).
      NOTE: The linear phased-array sector arc probe is often colloquially referred to as a "phased-array probe". However, this term is misleading, because all modern ultrasound transducers (including linear high-frequency probes) use phasing to steer the ultrasound beam^13,14. For the sake of brevity, the linear phased-array sector arc probe is referred to as a "sector probe."
   2. Preset the machine as follows: abdomen (or lung if there is no abdomen option), varying depth (6-20 cm, depending on the object of interest), harmonic imaging disabled, and indicator to the left of the screen. Perform most of the study in a two-dimensional (2D), grayscale mode called brightness mode (B-mode).
      ​NOTE: Other ultrasound modes such as motion mode (M-mode) and color doppler (CD) can occasionally provide additional information and may be used when screening for certain pathologic states.

2. Patient positioning

1. Supine versus sitting
   1. Perform the studies with the patient sitting up or supine.
2. Delimitation of the imaging regions
   1. Divide each hemi-thorax into three regions, reflecting the anatomic segmentation of the lungs (Figure 1¹⁵). In the left chest, treat the lingula as the left-sided analog of the right middle lobe.

3. Scanning technique

1. Apply ultrasound gel to the transducer.
2. Scanning the right hemithorax
   1. R1: right upper lobe (anterior lung zone) (Figure 2¹⁵)
      1. Place the probe in the mid-clavicular line in the 1st-3rd intercostal spaces (ICSs). Position the probe in the parasagittal orientation, with the indicator mark pointing cranially.
      2. Axis: Center on the pleural line so that the cranial and caudal rib shadows are visible on the edges of the images.
      3. Depth: If the dominant pattern is A-lines (see "Normal lung ultrasound findings" in the representative results section) with ≤ two B-lines (see "Pathologic lung POCUS findings" in the representative results section), decrease the depth so that only a single A-line is visible. If there are >three B-lines, increase the depth until at least three A-lines are visible.
         NOTE: B-lines are vertical hyperechoic artifacts that arise from the pleural line, become wider from superficial to deep, reach the deepest visible portion of the ultrasound screen, and efface the A-lines where the two intersect.
      4. Overall gain: Adjust the gain until the pleural line and A-lines are visible as distinctly echogenic (bright) lines and the spaces between the pleural line and A-lines are hypoechoic (dark).
      5. Click on acquire.
   2. R2: right middle lobe (antero-lateral lung zone) (Figure 3¹⁵)
      1. Place the probe in the anterior axillary line in the 4th-5th ICSs. Position the probe midway between the parasagittal and coronal orientations, with the indicator mark pointing cranially.
      2. Axis: See step 3.2.1.2.
      3. Depth: See step 3.2.1.3.
      4. Overall gain: See step 3.2.1.4.
      5. Click on acquire.
   3. R3: right lower lobe (posterior-lateral lung zone) (Figure 4¹⁵)
      1. Place the probe in the mid-to-posterior axillary line in the 5th-7th ICSs. Position the probe in the coronal plane with the indicator mark pointing cranially.
      2. Axis: Center on the diaphragm such that both the sub-diaphragmatic and supra-diaphragmatic structures are visible at the same time.
      3. Depth: Increase the depth until the sub-diaphragmatic spine is visible.
      4. Overall gain: Increase the gain until the liver/spleen appears slightly hyperechoic.
      5. Click on acquire.
3. Scanning the left hemithorax
   1. L1: left upper lobe (anterior lung zone)
      1. Probe positioning: See step 3.2.1.1.
      2. Axis: See step 3.2.1.2.
      3. Depth: See step 3.2.1.3.
      4. Overall gain: See step 3.2.1.4.
      5. Click on acquire.
   2. L2: lingula of the left upper lobe (lateral lung zone)
      1. Probe positioning: See step 3.2.2.1.
      2. Axis: See step 3.2.1.2.
      3. Depth: See step 3.2.1.3.
      4. Overall gain: See step 3.2.1.4.
      5. Click on acquire.
   3. L3: left lower lobe (postero-lateral lung zone)
      1. Probe positioning: See step 3.2.3.1.
      2. Axis: See step 3.2.3.2.
      3. Depth: See step 3.2.3.3.
      4. Overall gain: See step 3.2.3.4.
      5. Click on acquire.

Results

Normal lung ultrasound findings (Video 1, Video 2, Video 3, Video 4, Video 5, Video 6, and Supplementary File 1)
Due to the marked discrepancy in acoustic impedance between the air in the lungs and the superficial tissues, normally all the ultrasound energy that reaches the interface of the parietal and visceral pleurae is immediately reflected back to the ultrasound transducer. As a result, at the depth of the lung parenchyma, the image seen on the screen of an ultrasound machine normally sho...

Discussion

Diagnostic POCUS is the use of ultrasound at the bedside by a patient's primary treatment provider to answer clinical questions. The questions most amenable to diagnostic POCUS are those that are qualitative or binary in nature and that need to be answered faster than would be possible or practical with consultative ultrasound services.

A few steps are critical for image acquisition. The first one is probe selection. The authors recommend that the initial assessment be performed using the ...

Materials

Name	Company	Catalog Number	Comments
Edge 1 ultrasound machine	SonoSite	n/a	Used to obtain two of the abnormal images/clips (Figures 11 and 12)
Affiniti ultrasound machine	Philips	n/a	Used to obtain all normal and all abnormal images/clips except for Figures 11 and 12