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HOW TO SCAN THE LUNGS
To image the lungs, you need to be able to elicit the artefacts. For any ultrasound, imaging is best if the beam of the US is perpendicular to the structure being imaged. To begin, place the transducer on the patient's chest in a longitudinal orientation with the probe marker directed cranially. In the anterior chest,
On the screen you will see subcutaneous tissue deep to the skin. You should try to image two ribs and their posterior shadow. Just beneath the ribs will be the bright white pleura.
Ideal "bat sign" with crisp white pleura and black shadows posterior to ribs
Lichtenstein called this the bat sign. In each lung quadrant, this is the ideal image you should strive for.
For all lung US, orientate the transducer vertically on the chest with the probe marker cranial. Once the transducer is placed on the chest, decrease depth so that the pleura is in the middle of the screen. Adjust the gain so that the pleura is bright white and the rib shadows are black. The angle of the probe face should follow the natural contour of the chest wall so that the US beam is always perpendicular to the pleura.
Off axis imaging will lead to a fuzzy pleural line and lung artefacts which are difficult to interpret.
Pleural line is fuzzy due to off axis imaging.
Diagram of the US beam perpendicular to the pleura (left) and at an oblique angle (right). In the right diagram, the returning US waves miss the transducer and so are not read by the machine.
THE LUNG ZONES
You don't always need to scan the entire chest. Often, placing the transducer at the point of chest pain will show you the pathology. However, where possible we recommend a thorough examination covering most of the lung segments. The two main protocols described to achieve this are the 4 quadrants developed by Volpicelli (1), or the Blue and Plaps points developed by Lichenstein (2).
Volpicelli et al (1) simplified lung US to 8 zones in total as shown below. The patient remains supine for the examination. These areas are described as R1-4 and L1-4, which is convenient nomenclature when saving images. Note that the heart can interfere with imaging zones on the left and the transducer may need to be moved cranially to avoid this.
L1 and R1 are in the midclavicular line superior to the nipple line.
L 2/R2 is in the midclavicluar line inferior to the nipple line
In the lateral chest, the transducer is angled to make the US beam perpendicular with the pleura
L3/R3 is in the midaxillary line superior to the nipple line.
L4/R4 is in the midaxillary line inferior to the nipple line.
To view the spine sign, Increase the depth and angle the trandsducer inferiorly into the bed.
best imaging is when the US beam is perpendicular to the pleura.
In most instances these four quadrants are adequate to make the diagnosis and there is no need to sit the patient up. However, in lung US (as in any other form of US) if you see pathology you should interrogate the area and image it in the best plane possible. For example, sometimes you may see a glimpse of a basal pneumonia from R4, but sitting the patient up and imaging the posterior chest will likely provide a more comprehensive evaluation of this pathology.
The zones of the lung will roughly correlate with the lobes of the lung as shown below:
R1 - anterior upper lobe
R2 - anterior middle lobe
R3 - posterior upper lobe
R4 - posterior lower lobe
L1 - anterior upper lobe
L2 - lingula
L3 - posterior upper lobe
L4 - posterior lower lobe
If you see pathology on any Ultrasound, keep scanning and confirm it in multiple views
COVID19 scanning
CT imaging (3) and lung US (4) in COVID19 shows that the interstitial lung disease caused by COVID19 begins in the posterior lower zones. For this reason, most of the COVID19 specific lung US protocols recommend scanning the posterior chest as well (5,6,7).
Posterior chest zones: nomenclature L5-6 and R5-6
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REFERENCES
1. Lichtenstein DA. BLUE-protocol and FALLS-protocol: two applications of lung ultrasound in the critically ill. Chest. 2015 Jun;147(6):1659-1670.
2. Volpicelli G. Point-of-care lung ultrasound. Praxis (Bern 1994). 2014 Jun 4;103(12):711-6.
3. Dai WC, Zhang HW, Yu J, et al. CT Imaging and Differential Diagnosis of COVID-19. Can Assoc Radiol J. 2020;71(2):195-200.
4. Poggiali E, Dacrema A, Bastoni D, et al. Can Lung US Help Critical Care Clinicians in the Early Diagnosis of Novel Coronavirus (COVID-19) Pneumonia?. Radiology. 2020;295(3):E6.
5. Vetrugno L, Bove T, Orso D, et al. Our Italian experience using lung ultrasound for identification, grading and serial follow-up of severity of lung involvement for management of patients with COVID-19. Echocardiography. 2020;37:625-627
6. Zanobetti M, Poggioni C, Pini R. Can chest ultrasonography replace standard chest radiography for evaluation of acute dyspnea in the ED?. Chest. 2011;139(5):1140-1147.
7. Manivel V, Lesnewski A, Shamim S, Carbonatto G, Govindan T. CLUE: COVID-19 lung ultrasound in emergency department [published online ahead of print, 2020 May 9]. Emerg Med Australas. 2020;10.1111/1742-6723.13546.