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Can We Use Ultrasound to Monitor and Diagnose Lung Diseases?1
Marie Muller and Libertario Demi
   Challenges of Lung Ultrasound1
Ultrasound imaging is highly effective in the medical inves- tigation of most human organs, and, naturally, doctors and scientists have attempted to apply it to lung assessment. The first studies on lung ultrasound date back to the 1960s with work by Floyd Dunn (O’Brien, 2018). In this ear- lier work, researchers tried to characterize lung tissue by means of standard acoustic properties such as the speed of sound and attenuation. However, these approaches showed a large variability in the reported values (Dunn and Fry, 1961; Dunn, 1998), severely affecting the clinical applicability of this type of characterization.
The reason behind this variability has to do with the most important difference between the lungs and soft tissues: the presence of air. Although all soft tissues share very similar acoustic properties, air stands out significantly. And the lung is filled with it. As an example, speed of sound values in soft tissues are in the 1,500 m/s range, whereas air shows speed of sound values in the 300 m/s range (Wong, 1986).
The Lung
The lungs are formed by a distribution of many tiny air sacs, the alveoli, embedded in soft tissue. To picture the structure of the lung, one could use the analogy of a bunch of grapes immersed in water where the grapes would be filled with air and the surrounding water would be soft tissue. In this analogy, the grapes represent the 600 million lung alveoli, which have a diameter on the order of 280 μm.
It is easy to understand how big of an approximation it would be to attribute macroscopic acoustic properties
1 For additional information on lung ultrasound, see the special issue of The Journal of the Acoustical Society of America at
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to such a complex, heterogeneous medium. Moreover, the ratio of lung volume occupied by soft tissue to that occupied by air is not the only parameter that matters in lung ultrasound propagation. The shape, dimension, and spatial distribution of the air spaces will also strongly influence ultrasound propagation (Soldati et al., 2016). Moreover, all of these parameters change during breath- ing as the air spaces expand and contract with inspiration and expiration, thereby adding extra issues when consid- ering imaging the lung with ultrasound. To make things even more challenging, such a large difference in acoustic properties between air and soft tissues implies that ultra- sound waves are essentially fully reflected by the alveoli because the transmission coefficient is approximately zero (Szabo, 2004).
The first important lesson to learn is that the lungs cannot be modeled as soft tissue.
Conventional Ultrasound Imaging
Let us first review the basics of ultrasound imaging. To reconstruct an ultrasound image, acoustic waves in the megahertz range are transmitted into the body by an ultrasound probe. These waves are partly reflected back to the probe by acoustic interfaces within the tissue (dis- continuities in acoustic properties). If the speed of sound was constant and known, as it is in most soft tissues, it would be possible to convert the time traveled by the wave to a specific depth to reconstruct an image repre- senting the volume of interest (Szabo, 2004).
Conventional ultrasound imaging therefore relies on two main hypotheses. First, an effective speed of sound in the imaging volume must be assumed and known. Second, one must assume that higher order scattering can be neglected. This means that significant echoes are generated only from the interaction between the incident field (transmitted by
  Volume 17, issue 4 | Winter 2021 • Acoustics Today 29

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