A device described as the world’s smallest ultrasound detector has been created by Vasilis Ntziachristos and colleagues at the Technical University of Munich and Helmholtz Zentrum München.
The extremely sensitive device can image structures smaller than individual living cells and is made using inexpensive and readily available silicon-on-insulator technology. With further optimization, the team says their detector could be mass-produced for use in a broad range of imaging applications.
Traditionally, ultrasound detectors use piezoelectric transducers to both broadcast high-frequency sound and also pick up sound that has reflected from target objects – using the reflected signal to create an image. The spatial resolution of an ultrasound image can be improved by shrinking the size of the transducers, but this can drastically lower the sensitivity of the system.
Recently, optical detection techniques have been used to get around this resolution problem. One approach has been to detect changes in the resonant properties of an optical cavity that are caused by ultrasound waves. But so far, even the most advanced miniaturization techniques have not succeeded in confining light to dimensions smaller than about 50 microns, placing a constraint on the resolution that can be achieved.
Ntziachristos’ team has improved on these designs using silicon-on-insulator technology, which can be fabricated through techniques widely used in the semiconductor industry. The researchers developed a “silicon waveguide-etalon detector” (SWED).
The waveguide is contained in a periodic arrangement of Bragg gratings, each separated by spacers; but with one grating replaced by a cavity. A reflective layer of silver is then deposited on the end of the waveguide.
When Ntziachristos and colleagues pumped a continuous-wave laser into the SWED, they found that incident ultrasound waves could induce characteristic intensity variations in the light reflected off the silver layer. Furthermore, the high contrast between the cladding and cavity material enabled far better light confinement than had been achieved previously.
With a sensing area that is 220×500 nm in width, the SWED is a factor of 10 smaller than the diameter of a blood cell; and 10,000 times smaller than previous resonator-based sensors.
The resulting spatial resolution made possible for Ntziachristos’ team to image of structures 50 times smaller than the wavelength of the ultrasound used to obtain the images – a capability called super-resolution imaging. At the same time, the SWED is 1000 times more sensitive than current optical devices; and some 100 million times more sensitive than piezoelectric detectors of the same size.
Such a significant improvement in both sensitivity and resolution mean that the SWED can fit onto a chip just half a micron in size. This opens-up a wealth of opportunities for improvement in both medical and industrial imaging.
With further optimization, the device could soon be integrated into mass-produced, extremely dense ultrasound arrays, capable of picking out ultra-fine details in materials and biological tissues. It could also be used to study the fundamental properties of high-frequency sound waves, and their small-scale interactions with matter.
The new technology is described in Nature.
Ultrasound detectors use high-frequency sound waves to image objects and measure distances, but the resolution of these readings is limited by the physical dimensions of the detecting element. Point-like broadband ultrasound detection can greatly increase the resolution of ultrasonography and optoacoustic (photoacoustic) imaging, but current ultrasound detectors, such as those used for medical imaging, cannot be miniaturized sufficiently.
Piezoelectric transducers lose sensitivity quadratically with size reduction, and optical microring resonators and Fabry–Pérot etalons cannot adequately confine light to dimensions smaller than about 50 micrometres. Micromachining methods have been used to generate arrays of capacitive and piezoelectric transducers, but with bandwidths of only a few megahertz and dimensions exceeding 70 micrometres.
Here we use the widely available silicon-on-insulator technology to develop a miniaturized ultrasound detector, with a sensing area of only 220 nanometres by 500 nanometres. The silicon-on-insulator-based optical resonator design provides per-area sensitivity that is 1,000 times higher than that of microring resonators and 100,000,000 times better than that of piezoelectric detectors.
Our design also enables an ultrawide detection bandwidth, reaching 230 megahertz at −6 decibels. In addition to making the detectors suitable for manufacture in very dense arrays, we show that the submicrometre sensing area enables super-resolution detection and imaging performance. We demonstrate imaging of features 50 times smaller than the wavelength of ultrasound detected. Our detector enables ultra-miniaturization of ultrasound readings, enabling ultrasound imaging at a resolution comparable to that achieved with optical microscopy, and potentially enabling the development of very dense ultrasound arrays on a silicon chip.