New Roles For Ultrasound
Once upon a time, ultrasound was a technology used primarily for pregnant mothers and prenatal checkups.
That was then, and now ultrasound is becoming increasingly valuable for a wider assortment of medical applications — even pulmonary disease.
Ultrasound waves are night frequency sound waves with frequencies ranging from 20 kilohertz to hundreds of megahertz. As a point of reference, humans can’t hear frequencies about 20 kilohertz while dogs can, and bat signals can go as high as 200 kHz.
Recently Marie Muller, associate professor in NC State’s Department of Mechanical and Aerospace Engineering and an expert in the physics of sound shared her views on ultrasound being used in assessing lung health.
It’s important to note that lungs contain millions of air-filled alveoli, and these pockets of air scatter ultrasound waves in random directions. It doesn’t travel in a straight line and it’s difficult to know which path the ultrasound pulse will take, which makes imaging very difficult.
That said, Muller said ultrasound is a great imaging technology because it’s non-evasive, non-ionizing and great for monitoring a patient’s response overtime. You can repeat ultrasound assessments multiple times without exposing patients to dangerous radiation, so it’s ideal for monitoring chronic conditions. You can also use ultra sound to observe dynamic phenomena in real time to guide surgery. Ultrasound is also relatively inexpensive and widely available, so any imaging innovation has the potential to affect millions of patients — even in rural areas and developing countries.
Because of COVID-19, there’s more interest in finding ways to perform quick, easy assessments of lungs. And with thousands of people presenting with COVI0-related pneumonia, CT scanning isn’t an ideal option.
Italy was where the first wave of the pandemic hit, and ultrasound imaging is prevalent there. As a result, we’re seeing a large number of studies presenting results of ultrasound imaging in patients with COVID-19. However, it’s difficult to use conventional ultrasound to get good images of the lung. Instead, they are looking for phenomena that appear in ultrasound images (artifacts) that can be used to help diagnose specific conditions.
Muller said, “For example, if there is pneumonia, there will be areas of the lung filled with fluid. Ultrasound waves cannot travel straight through the packets of air in the lung, but those waves are able to travel through the parts of the lung filled with fluid. In other words, for critically ill patients, the room full of broken mirrors has areas without mirrors. This will show up on the ultrasound image as a vertical bright line, called a “B-line.” Recent studies have used the presence of B-lines to assess the severity of COVID-19.”
“Unfortunately, there is a big limitation to this approach: it is qualitative and not quantitative,” she continued. “It will tell you that a patient is not doing well, but it won’t say how bad things are. It is also not specific: the artifacts have the same appearance whether the patient has COVID, congestive heart failure, or lung fibrosis. Finally, we know that this approach is highly operator dependent. Depending on who is holding the probe and how, those B-lines may or may not appear.”
Muller is working on trying to make lung ultrasound quantitative, specific and operator-independent in her lab, and to do that, she has to leverage knowledge of wave physics. Muller’s lab is developing specific ultrasound algorithms to detect and quantify pulmonary fibrosis and pulmonary edema, which both can change the microstructure of the lung. They are relying on the physics of wave propagation in complex media to extract microstructural properties of the lunch.
She said, “Pulmonary fibrosis and pulmonary edema (associated with congestive heart failure) affect millions. We aim at developing technologies that can be both specific and quantitative. This will allow them to be deployed widely and to be used often in private practices, for screening, diagnosis and monitoring. Monitoring is especially important in the context of pulmonary fibrosis, because new treatments have emerged in the past few years. We will want to follow these patients to see how they respond to these new treatments.”
Muller’s work might also be used in a surgical context. She reports they have developed methods for the real time localization of pulmonary nodules, based on the physics of wave propagation in complex media. These nodules are detected by CT scanning and, if peripheral enough, are removed using Video Assisted Thoracic Surgery (VATS). During surgery, surgeons use surgical staplers to cut out these lesions. If they are cancerous, it’s critical that no part of the tumor be left behind. However, this isn’t easy, since the surgeon can’t see the tumor during surgery.
She said, “Surgeons sometimes try to palpate the nodule with their finger, but bad surprises are common. Either surgeons can’t find the tumor, or they do find it but are unable to remove it with safe margins. This is why a real time imaging modality that could be used during surgery is needed. CT imaging is great at detecting lesions, but it does not work in real time. The technology we have developed relies on the principle that lung tissue is a highly complex medium from the point of view of ultrasound waves, but that the nodule is not. By mapping the complexity of the tissue, we obtain a map of the tissue, and that, in turn, allows us to accurately detect and localize the nodule.”
Muller says the biggest challenge in lung ultrasound is that it’s not currently quantitative and not based on first principles. They are based on the assumption of a fairly homogenous medium, but these assumptions don’t hold because the lung is so complex.
This makes the uniformization of the processes very challenging, and makes quantification of lung damage impossible,” she said. “Interpreting lung ultrasound is currently a highly subjective business. We are working on improving this, with a group of lung ultrasound experts worldwide. We have developed a series of statements related to the topic, which will be submitted to a much larger community of experts. The statements will be graded using a Delphi method in order to reach a consensus. But it is also critical that this new consensus not be taken at face value, and that the community keep an open mind as research progresses and new tools become available.”
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