Collin Arocho
23 March 2021

Researchers at Imec have demonstrated an ultrasound sensor with unprecedented sensitivity. The 20-µm sensor, built on a silicon photonic chip, uses an innovative optomechanical waveguide, which enables the ultra-low detection limit. Compared to a state-of-the-art piezoelectric sensor of identical size, Imec’s sensor is capable of a detection limit two orders of magnitude better. This low sensitivity enables new clinical and biomedical applications of ultrasonic and photoacoustic imaging such as deep-tissue mammography and the study of vascularization or innervation of potential tumorous tissue.

Tomographic ultrasonic and photoacoustic imaging build two or three-dimensional images using an array of ultrasound sensors. Piezoelectric ultrasound sensors, however, have limitations, ie the detection limit depends inversely on the size of the sensors, which is a problem for high-resolution imaging with small acoustic wavelengths. High-resolution images require small piezo-electric sensors, which intrinsically have a higher detection limit resulting in a noisy image. Additionally, piezoelectric sensors rely on their mechanical resonance to enhance signal amplitude, meaning they operate in a small range around the resonance frequency to avoid high detection limits. Finally, matrices of piezoelectric sensors require one wire for each sensor element, hampering application, for instance, in catheter applications.

Imec sensor

Imec’s solution is based on a highly sensitive split-rib optomechanical waveguide fabricated using new CMOS-compatible processing. A low detection limit can improve the trade-off between imaging resolution and depth for ultrasound applications, and is crucial for photoacoustic imaging, where pressures are up to three orders of magnitude lower than in conventional ultrasound imaging techniques. Furthermore, it may enable low-pressure applications like through-skull functional brain imaging, which suffers from the strong ultrasound attenuation of bone.

Finally, a fine-pitched (30 µm) matrix of these tiny (20 µm) sensors can be easily integrated on-chip with photonic multiplexers. This opens the possibility of new applications such as miniaturized catheters because the sensor matrices require only few optical fibers to be connected instead of one electrical connection per element in the case of piezoelectric sensors.

“The sensor we’ve demonstrated will be a game-changer for deep tissue imaging in otherwise non-transparent tissues such as skin or brain,” says Xavier Rottenberg, fellow of wave-based sensors and actuators at Imec. “For applications such as subcutaneous melanoma imaging or mammography, it enables a more detailed view of the tumor and vascularization around, aiding in a more detailed diagnosis.”