Page 39 - Fall2020

(3) Velocity errors occur based on the assumption that the speed of sound is constant even though it changes in different mediums, thereby leading to errors with calculating the distance. A speed displacement arti- fact and refraction are examples of velocity errors where the object is display at the incorrect distance or incorrect location on the image.
(4)Attenuation errors occur from the absorption and scattering of the beam in different mediums where the machine attempts to make the image more homogeneous as compensation for echoes that take varying times to return to the transducer. Acous- tic shadowing occurs when the beam encounters a highly reflective or attenuating medium and the intensity of the beam weakens distal to this encoun- ter, as demonstrated as rib shadowing in a normal lung ultrasound (Figure 3F). The machine interprets that area as having less signal then the highly reflec- tive surface and presents it as hypoechoic or anechoic (gray or black) on the image. Increased through transmission is the opposite of acoustic shadowing in which the beam encounters a lower attenuating medium then the adjacent mediums. The machine interprets this area as having more signal and pres- ents it as hyperechoic (white) on the image.
Applications for POCUS
POCUS can provide bedside diagnostic and procedural information with several different applications, including lung, cardiac, abdominal, head, and procedural ultraso- nography (Figure 1).
Lung Ultrasound
Although respiratory pathology is common in neonates, the etiology and prognosis can be difficult to discern from the clinical history and plain film radiography alone. Lung ultra- sound can be employed to discern the etiology of respiratory distress, predict the need for increased respiratory support and intervention, and assess the risk of long-term morbid- ity in neonates. Lung ultrasound has also been utilized to assess adequate fluid replacement during the management of severe infections in children and may be particularly useful to guide therapeutic options in emergencies (e.g., rapid accumulation of air or fluid between the lungs and chest wall; Lichtenstein, 2012). A recent international task force has evaluated the evidence with neonatal lung POCUS and is in agreement with its use to delineate and diagnose
lung pathology as well as to guide interventions for respira- tory emergencies (Singh et al., 2020).
Lung ultrasound is based on the interpretation of arti- facts rather than the direct visualization of the lung because of beam reflection between the air, pleura (tissue covering the lungs), and fluid around or within the lungs. In a normal lung ultrasound, the skin, muscle, ribs, and surface of the lung are easily identified and normal lung movement with breathing is observed. Normal lung ultrasound in B-mode (Figure 4A) shows reverbera- tion artifacts from the tissue covering the lungs (pleural line). We can also see acoustic shadowing from the ribs that is displayed as hyperechoic, equally displaced, horizontal lines. The pleura slides back and forth as the patient breathes. This process is known as “lung sliding” (see Multimedia 1 at acousticstoday.org/ruossmm). In M-mode, normal lung movement with breathing appears as a stratosphere sign (waves on a beach) where absent lung movement is displayed as a barcode (Figure 4B). Although the presence of lung movement with breath- ing is not the equivalent of a healthy lung, its absence is always pathological. Comet tail artifacts are from increased fluid in the lungs and, as previously described, are vertical, hyperechoic (white), triangular shapes that extend from the pleural line to bottom of the image and move with normal lung movement with breathing. In the neonate, a small number of comet tails can be normal, such as immediately after birth when the lungs still have fluid. However, an increasing number and size of comet tails correlate with disease pathology and severity, with coalesced comet tails being the most severe as in the lung disease seen in premature neonates (Figure 4C).
Lung ultrasound is performed utilizing a high-frequency linear probe with a small footprint (e.g., microlinear). Lung ultrasound has been used to discern the difference between common respiratory illnesses in neonates (Rai- mondi et al., 2019) and has been demonstrated to be a reliable diagnostic tool with high concordance with chest plain film radiography (X-ray) and may be able to predict the need for interventions in neonates born early (Corsini et al., 2020). Lung ultrasound findings in neonates born early include abnormalities of the pleural line (thickened, irregular) resulting in abnormal or absent reverberation artifacts, some comet tails in moderate cases, and “white lung” appearance in severe cases (Copetti et al., 2008).
Fall 2020 • Acoustics Today 39