Laser-based system achieves noncontact medical ultrasound imaging

Researchers from the MIT Lincoln Laboratory and their colleagues at the Massachusetts General Hospital (MGH) Center for Ultrasound Research and Translation (CURT) have come up with a brand new medical imaging device called The Noncontact Laser Ultrasound (NCLUS). The laser-driven ultrasound device provides images of body parts inside, like muscles, organs, fat, tendons, and blood vessels. It also monitors bone strength and can analyze different stages of disease as time passes.

“Our patented skin-safe laser system concept seeks to transform medical ultrasound by overcoming the limitations associated with traditional contact probes,” says principal investigator Robert Haupt, a senior Lincoln Laboratory’s Active Optical Systems Group staff member. Haupt and senior member Charles Wynn are co-inventors of the technology, along with group leader assistant Matthew Stowe, providing technical leadership and supervision for the NCLUS program. Rajan Gurjar serves as the lead system integrator along with Jamie Shaw, Bert Green, Brian Boitnott (now at Stanford University), and Jake Jacobsen, collaborating on optical and mechanical engineering as well as the building of the system.

Medical ultrasound used in the practice

If your doctor has ordered an ultrasound, you may expect a highly experienced sonographer to manipulate and press a variety of transducers mounted on a handheld device on your body. When the sonographer pushes the transducer probe across the skin, high-frequency acoustic frequencies (ultrasound waves) travel through the body’s tissues, where they “echo” off different tissue structures and characteristics. Echoes result from sound impedance or change in tissue strength (tissue stiffness or rigidity) as well as from muscle, fat, blood vessels, organs, and bone-deep inside your body. The probe can receive the echos, which are then compiled into pictures of the internal organs and features. Specialized processing algorithms (synthetic aperture processing) are employed to create the shape of the features of the tissue in 3D or 2D, which are displayed on the computer monitor in real-time.

Utilizing ultrasound, doctors can, without invasively, “see” inside the body to visualize various tissues and their geometrical structures. Ultrasound also measures blood flow pulsing through veins and arteries and determines the mechanical characteristics (elastography) of organs and tissues. Ultrasound is commonly used to aid physicians in evaluating and diagnosing various health conditions such as diseases, injuries, and infections. In particular, ultrasound is used to visualize the developing fetus’s anatomy, detect tumors, and determine the extent of leakage or narrowing in the heart valves. From handheld devices like iPhones to handheld devices on iPhones to cart-based devices, ultrasound is highly accessible, cheap, and frequently used in remote-field and point-of-care settings.

The limitations of ultrasonics

While the latest medical ultrasound technology can detect tissue structures within millimeter-sized incrementsthis technique has limitations. The freehand maneuvering of the device by ultrasound technicians in order to get the most effective viewing window into the body’s interior can lead to errors in imaging. In particular, as they apply pressure on the probe through feeling, they can randomly compress the tissue with which the probe comes into contact, which causes unpredictable changes in the properties of tissue that affect the path of travel for ultrasound waves. This compression can alter the appearance of images with a certain amount of uncertainty, which means the shape of features could be more precisely depicted. Furthermore, tilting the probe even just a bit alters the plane of the image view, creating a skew in the image and causing uncertainty about the exact location of features within the body.

The distortion of the image and the positional reference uncertainty are significant enough that ultrasound cannot determine with enough certainty, such as the extent to which a tumor is growing smaller or more significant and the tumor’s location within the host’s tissue. Additionally, the uncertainty in the size, shape, and location will change with each repeated measurement, even with the same ultrasound operator trying to repeat their steps. This uncertainty, referred to as operator variability, becomes more than when several sonographers try the same measure, which can lead to inter-operator variation. Due to these issues, ultrasound is typically unable to detect cancerous tumors and other disease-related conditions. Instead, methods like magnetic resonance imaging (MRI) and computerized tomography (CT) are required to monitor how disease progresses despite their astronomically higher costs, greater system size and complexity, and the imposed radiation risks.

“Variability has been a major limitation of medical ultrasound for decades,” says Anthony Samir, associate chair of Imaging Sciences at MGH Radiology and director of CURT. Samir and his MGH CURT colleagues Kai Thomenius and Marko Jakolvejic bring crucial expertise in medical practice, technical knowledge, and advice on traditional ultrasound devices to the lab team. They also collaborate with them on the NCLUS system’s development.

By fully automating the procedure for taking ultrasound images, NCLUS could decrease the requirement for a sonographer and reduce operator variability. The laser position is accurately replicated and eliminates the possibility of variability between repeated measurements. Because the measurement is noncontact, no localized tissue compression or distortion related to image features will occur. Moreover, like MRI and CT, NCLUS provides a fixed-reference-frame capability using skin markers to reproduce and compare repeat scans over time. To enable tracking, the lab team has developed software that processes ultrasound images and identifies any modifications between the two. Not requiring manual pressure or the use of coupling gels (as is required for probes that contact), NCLUS is also suitable for patients suffering from pain or sensitive body regions or in a fragile state or who are at risk of getting infections.

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