All-optical ultrasound generates video-rate images of tissues

11 Jul 2018

UCL system shortens acquisition time to assist image-guided surgeries.

Ultrasound is an established way to image biological tissues, traditionally using arrays of electronic transducers to both generate and receive the ultrasonic signals.

An all-optical variant of the technique, where ultrasound waves are generated by an incident laser pulse and fiber-optic sensors pick up the resulting signal, is a potentially attractive alternative. Removing the electronic components usually required would help to combat electromagnetic interference, and could allow the procedure to be compatible with MRI scanning.

To date, however, the all-optical approach has required long acquisition times to generate a video-rate 2D or 3D image of the kind needed in real-world medical use, making the procedure impractical for clinicians.

A team at University College London (UCL) has now developed a possible solution, by designing and testing an all-optical ultrasound platform capable of real-time video-rate 2D imaging of biological tissue. The work was reported in Biomedical Optics Express.

In trials, the prototype platform was used to provide static ex vivo images of the head of a zebra fish, and dynamic ex vivo images of swine carotid artery. A sustained frame rate of 15 Hz was achieved, along with a penetration depth of at least 6 mm and a resolution of 75 by 100 microns. This allowed the dynamics of the pulsating ex vivo artery to be captured, according to the team.

"These results were achieved through a combination of innovations and optimizations, and each was essential to to achieving video-rate image acquisition," Erwin Alles of UCL commented to Optics.org.

"From a hardware point of view, both an efficient optical ultrasound generator and highly sensitive optical ultrasound detector were required, and within our lab we have extensive, possibly unique, experience in fabricating both. Without either of these, achieving high signal-to-noise ratios is challenging and signal averaging is typically required, which decreases the frame rate."

Based on experience with polymer nano-composites as optical ultrasound generators, UCL was able to generate a clean ultrasound source signal optoacoustically, by focusing 1064-nanometer excitation light onto a thin freely-suspended 5 x 3 centimeter membrane of optically absorbing material. A galvo mirror was used to move the focal spot across the membrane, effectively creating an acoustic source aperture and allowing different source geometries to be synthesized.

A fiber-optic acoustic receiver interrogated by a tuneable continuous-wave laser was used to record the returning signal, while advances in ultrasound signal processing helped to reduce artifacts in the final resulting images.

Miniaturization for clinical use

At present the performance of the imaging system is not limited by the light sources, but by other factors such as the damage threshold of the optical ultrasound generator, and the rates of data acquisition and processing. All the lasers used in the published work were commercially available models.

"The excitation light source is a compact Q-switched pulsed laser with pulse durations of a few nanoseconds, and the interrogation laser is a commercial wavelength-tuneable laser routinely used in the telecom industry," said Alles. "In future work, different light sources that offer control over the pulse duration and shape would allow for control over the generated acoustic bandwidth, enabling dynamic control over the trade-off between acoustic resolution and penetration. This will greatly improve the versatility of all-optical ultrasound compared to its electronic counterpart."

The team noted that the dynamic range achieved with its all-optical ultrasound is at present lower than that of electronic imaging probes, but commented that it was making significant progress in this area. Further improvements in image reconstruction algorithms and related deep learning operations will also assist the system's performance.

Clinical usability for many applications will ultimately require a hand-held probe or invasive device, along with miniaturization of the galvo mirrors and optics, and perhaps a different data acquisition paradigm. This could involve spatially separating the optical switching techniques from the imaging aperture, or having the light delivered through a bundle of miniature optical fibers.

Alles said that both these options are now being explored at UCL, with the goal of achieving sufficient miniaturization for an in vivo demonstration of the platform within 2 to 3 years.

"Given the low price of the materials used, the ease of fabrication of the optical ultrasound generator, and a lack of electronics, we expect future all-optical ultrasound imaging probes to be similarly priced to electronic imaging probes, while offering MRI compatibility, resilience to electromagnetic interference, and increased versatility," he commented. "All-optical ultrasound imaging probes have the potential to revolutionize image-guided interventions."