Noncontact laser ultrasound could yield significant impact for medical imaging

Optical approach acquires interior body imagery without patient contact

Ultrasound is an important tool used by physicians to image and diagnose disease, state of health, injury, and other conditions in patients. However, it suffers from limitations resulting from operator variability—the differences caused by hand pressures in the subtle angle planes applied to the transmit-sensing head in contact with the patient's skin. For example, this variability creates significant enough distortion to and reduced confidence in the resultant image that ultrasound is not used for tracking changes in tumors. Rather, computerized tomography (CT) and magnetic resonance imaging (MRI) are the preferred methods for monitoring cancer tumors and other diseases, but these methods come with large, expensive, and complex machinery and some adverse health effects, such as radiation exposure in CT.

Robert Haupt and Charles Wynn from MIT Lincoln Laboratory have been developing the concept of an optically based noncontact laser ultrasound (NCLUS) that is designed to acquire anatomical images of the body's interior and in vivo tissue mechanical properties (elastography). NCLUS is being designed to mitigate operator variability and to thus open up the range of applications for which ultrasound can be used in medical imaging.
 
The phenomenology behind NCLUS is that it employs short optical pulses to convert near-infrared wavelength energy to heat at a very localized spot via absorption in the skin. This heating, which causes no sensation in and no harm to the patient, causes a rapid strain deformation in the skin and launches high-frequency ultrasonic waves into the body that are comparable to those used in practiced medical ultrasound. The ultrasound waves travel several inches into the body, reflecting off various internal features, and gather image signals before returning to the surface of the skin. These signals are then measured by a highly sensitive laser Doppler vibrometer developed specifically for NCLUS.

Lincoln Laboratory researchers (from left to right) Rajan Gurjar, Robert Haupt, and Charles Wynn demonstrate the NCLUS system operation. A pulsed fiber laser (held by Wynn) sends the pulsed optical beam that converts light into ultrasonic waves to the tissue sample (in this picture a 3D-printed phantom femur bone is shown). The laser Doppler vibrometer (pictured near Gurjar and Haupt) receives and measures the returning ultrasonic vibration signal from the bone interior at its surface. Fast-scanning mirrors (not shown) are used to scan the laser excitation and receive beams over the target surface within a few seconds to acquire the data needed to construct a full ultrasonic image.

To date, Haupt and Wynn have demonstrated that small objects (on the order of 1 mm) embedded in muscle mass can be resolved and imaged by using the NCLUS system. The NCLUS system is currently being refined to operate with a fixed reference system. Small laser spots that generate and measure ultrasonic waves that travel into and out of the body can be consistently located to the same set of positions for different times (for example, measurements taken a week or month apart use the same reference system). Because the optical beams do not impart pressure or change angle for a given location over time, they mitigate operator variability. As a consequence, these ultrasonic images can be compared or subtracted to quantify differences over time with much greater accuracy than can the images from practiced contact ultrasound.

"Such a system can enable ultrasound to acquire imagery with resolution and quality comparable to MRI and CT, but at much lower cost, with significantly less system complexity, and with no adverse biohazards," says Haupt. "NCLUS is being designed to be about the size of a shoebox in the near term and to be a handheld system in the future, so it is very portable." The team hopes that NCLUS will be further developed and tested so that it can be used to detect and track the progression of injuries and diseases, e.g., concussion trauma, internal bleeding and organ damage, the healing rate of stress fractures, or the growth rate of a tumor.

A complementary set of benefits to using NCLUS to acquire ultrasonic images in patients is its noncontact capability that avoids painful contact with patients who are experiencing traumas, such as burned skin, serious contusions, or diseases of the skin. NCLUS can acquire ultrasonic data from a 1-inch to 10 m standoff from the patient and is unlikely to interfere with other medical procedures. This standoff capability is particularly attractive for crowded surgical settings because NCLUS only requires a line of sight to the area of the body to be imaged. The line of sight can be a simple straight line or could involve mirrors to direct the optical beams around bends and corners for more complex paths.

NCLUS also does not require skin treatment or preparation to yield the image, unlike contact ultrasound that requires the application of gels to the skin to enhance signal coupling and transmission into the body. These gels can lead to infection if they are leaked into the body through open wounds or during surgery.

So far, the research team has tested NCLUS on beef tissue, bone, and other biomaterials. Strong image quality with submillimeter resolution was obtained by using optical powers that were eye and skin safe. Moreover, selected excitation and sensing wavelengths that were used are commercially available for the necessary optical components and are anticipated to minimize costs.

NCLUS produced image of metal sliver in beef tissueThe left panel shows the NCLUS-produced image for a 2.54 cm thick beef muscle sample laid on a metal tabletop. The middle panel displays a metal sliver that is embedded 1.27 cm below the muscle surface without disturbing the muscle. The rightmost panel shows the wound image with the metal sliver removed. The measurement is made at a 1 m standoff distance from the muscle specimen.

Testing has revealed that several types of elastic waves can be generated from an optical pulse converted in skin to ultrasound. Traditional ultrasound typically generates compression waves that are designed to measure depth within the body and to indicate anatomical features. "NCLUS can capture the whole family of elastic waves to analyze injury and disease," says Haupt. For example, shear waves, which are more dispersive than compressional in the human body, show a strong dispersion change as blood begins to coagulate. Because coagulating blood exhibits a viscosity different from that of healthy tissue, the shear wave can react to this difference and disclose possible hematomas. Surface waves, including shear, compressional, Rayleigh, Love, and Lamb waves, are affected by and interfere with surface changes, which can reveal conditions such as bone cracks and fractures.

Significant technology gaps have made it difficult to provide rapid, portable, and accurate imaging tools that help visualize, detect, and diagnose many internal injuries. The NCLUS optical approach may bridge some critical gaps and yield a useful tool for medical diagnosis not only in hospitals but also at remote locations. "It is our objective to bring the NCLUS concept to the civilian population and military community," says Wynn. "We can envision its use on ambulances, at military field hospitals, and even on sports fields to provide timely evaluation of internal trauma or musculoskeletal injuries."

The research team has recently begun a partnership with Dr. Anthony Samir of Massachusetts General Hospital and Dr. Brian Anthony of MIT Medical Device Realization Center to develop a prototype tomographic imaging NCLUS device. The device will be used to create prosthetic fittings well-matched to an amputee’s residual limb. In this application, NCLUS will measure both compressional and shear waves to determine the mechanical strength distribution in the residual limb and then use that distribution to make a uniquely matched socket that mounts to the prosthetic limb.

The current process for fitting prosthetics typically takes doctors up to three years of trial and error to establish a good limb fit and creates a uniform property mold of the patient’s limb and then a socket. Because the process fails to account for differential stiffness and compliance distributions of the patient's residual limb, it often leads to poorly distributed loading on patients that causes horrible sores, wear zones, and pain in their residual limbs. According to Haupt, using a NCLUS device in the fitting process would improve prosthesis performance, lower costs incurred by rounds of prosthetic modifications, and alleviate patient discomfort associated with ill-fitting prosthetics. Moreover, the optical approach is more suitable for measuring the socket fit for the prosthetic while it is in use and under more realistic loading conditions, such as standing and walking.

Haupt is hopeful that this new research partnership will lead to NCLUS's becoming a mainstream device. "The research team is in the process of filing several patents, so commercial applications will be coming along soon."

Posted May 2016

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