The Noncontact Laser Ultrasound (NCLUS) system provides images of interior body features such as organs, fat, muscle, tendons, and blood vessels. The system also measures bone strength and may have the potential to track disease stages over time.
"Our patented skin-safe laser system concept seeks to transform medical ultrasound by overcoming the limitations associated with traditional contact probes," explains Robert Haupt, a senior staff member in Lincoln Laboratory's Active Optical Systems Group. Haupt and senior staff member Charles Wynn are co-inventors of the technology.
Medical ultrasound in practice
If your doctor orders an ultrasound, you can expect a highly trained sonographer to press and manipulate an array of transducers, set in a handheld device, onto your body.
As the sonographer pushes the transducer probe across your skin, high-frequency acoustic waves (ultrasound waves) penetrate and propagate through your body tissue, where they "echo" off different tissue structures and features.
The probe receives the returning echoes, which are assembled into representational images of the body's internal features.
Specialised processing schemes (synthetic aperture processing) are used to construct the shapes of the tissue features in 2D or 3D, and these constructions are then displayed on a computer monitor in real time.
Using ultrasound, doctors can noninvasively "see" inside the body to image diverse tissues and their geometries. It is used routinely to evaluate and diagnose a variety of health conditions, diseases, and injuries.
Ranging from handheld devices on an iPhone to cart-based systems, ultrasound is highly portable, relatively inexpensive, and widely used in point-of-care and remote-field settings.
Limitations of ultrasound
However, though state-of-the-art medical ultrasound systems can resolve tissue features within fractions of a millimetre, the technique has some limitations.
Freehand manipulation of the probe by sonographers to obtain the best viewing window into the body interior leads to imaging errors.
The image distortion and positional reference uncertainty are significant enough that ultrasound cannot resolve with sufficient confidence, for example, whether a tumour is getting larger or smaller and precisely where the tumour is located in the host tissue.
Furthermore, this uncertainty will vary upon repeat measurement, even for the same sonographer trying to retrace their steps. This is more severe when different sonographers attempt the same measurement, leading to inter-operator variability.
Because of these drawbacks, ultrasound is often restricted from tracking cancerous tumours and other disease states. Instead, methods such as magnetic resonance imaging (MRI) and computerized tomography (CT) are mandated to track how diseases progress – even with their vastly higher cost, greater system size and complexity, and imposed radiation risk.
By fully automating the process for acquiring ultrasound images, NCLUS has the potential to reduce the need for a sonographer and to mitigate operator variability.
The laser positioning can be accurately reproduced, thus eliminating variability across repeated measurements. Because the measurement is noncontact, no localised tissue compaction or its related distortion to image features occurs.
Moreover, similar to MRI and CT, NCLUS provides a fixed-reference-frame capability using skin markers to reproduce and compare repeat scans over time.
To support such tracking capabilities, the laboratory team developed software that processes ultrasound images and detects any changes between them.
Requiring neither manual pressure nor coupling gels (as required by contact probes), NCLUS is also ideal for patients with painful or sensitive body areas, in fragile states, or at risk of infection.
Light-induced ultrasound waves
NCLUS employs a pulsed laser that transmits optical energy through the air to the skin surface, where the light is rapidly absorbed once in the skin.
The optical pulse yields sufficient ultrasound power with frequencies comparable to that of practiced medical ultrasound, while causing no sensation on the skin.
The ultrasound echoes returning from the tissue interior emerge at the skin surface as localised vibrations, which are measured by a highly sensitive, specialised laser Doppler vibrometer.
"With an appropriate laser transmit-and-receive implementation, any exposed tissue surfaces can become viable ultrasound sources and detectors," Haupt explains.
Advances toward a clinically operational system
In the latest NCLUS system, the lasers that pulse and scan are 500 times faster than the researchers' previous system, thus reducing the entire image-data acquisition time to less than a minute. Future NCLUS prototypes will involve faster acquisition times of less than one second.
The new clinical system also operates at much higher ultrasound frequencies, enabling resolution down to 200 microns, which is comparable to the resolution of state-of-the-art medical ultrasound.
The moveable armature enables many degrees of freedom to view the various regions of the body. Inside the optical head are also programmable fast-steering mirrors that automatically position the source and receive laser beams to establish the ultrasound array precisely.
A 2D lidar is used to map the patient's skin surface topography; a high-frame-rate short-wave-infrared camera records the laser source and receiver projected locations on the skin, providing the array parameters necessary for constructing ultrasound images.
The skin-surface topography mapping and laser-position recordings are registered by using natural skin features, such as freckles. In this way, a fixed reference frame is established for performing precise repeat scans over time.
The NCLUS clinical system generates fully automated and registered ultrasound images via synthetic aperture processing.
With NCLUS, emergency medical technicians, paramedics, and medical staff without specialised sonography training might be able to perform ultrasound imaging outside of a hospital – in a doctor’s office, at home, or in a remote battlefield setting.
"With further development, NCLUS has the potential to be a transformative technology: an automated, portable ultrasound platform with a fixed-reference-frame capability similar to that of MRI and CT," Anthony Samir, Associate Chair of Imaging Sciences at MGH Radiology.