Silicon
chip (approx. 3 mm x 6 mm) with multiple detectors. The fine black engravings
on the surface of the chip are the photonics circuits interconnecting the
detectors (not visible with bare eyes). In the background a larger scale
photonics circuit on a silicon wafer. Courtesy: © Helmholtz Zentrum Muenchen /
Roman Shnaiderman
Researchers
at Helmholtz Zentrum München and the Technical University of Munich (TUM) have
developed the world’s smallest ultrasound detector. It is based on miniaturized
photonic circuits on top of a silicon chip. With a size 100 times smaller than
an average human hair, the new detector can visualize features that are much
smaller than previously possible, leading to what is known as super-resolution
imaging.
Since the
development of medical ultrasound imaging in the 1950s, the core detection
technology of ultrasound waves has primarily focused on using piezoelectric
detectors, which convert the pressure from ultrasound waves into electric
voltage. The imaging resolution achieved with ultrasound depends on the size of
the piezoelectric detector employed. Reducing this size leads to higher
resolution and can offer smaller, densely packed one or two dimensional
ultrasound arrays with improved ability to discriminate features in the imaged tissue
or material. However, further reducing the size of piezoelectric detectors
impairs their sensitivity dramatically, making them unusable for practical
application.
Using
computer chip technology to create an optical ultrasound detector
Silicon
photonics technology is widely used to miniaturize optical components and
densely pack them on the small surface of a silicon chip. While silicon does
not exhibit any piezoelectricity, its ability to confine light in dimensions
smaller than the optical wavelength has already been widely exploited for the
development of miniaturized photonic circuits.
Researchers
at Helmholtz Zentrum München and TUM capitalized on the advantages of those
miniaturized photonic circuits and built the world’s smallest ultrasound detector:
the silicon waveguide-etalon detector, or SWED. Instead of recording voltage
from piezoelectric crystals, SWED monitors changes in light intensity
propagating through the miniaturized photonic circuits.
This is
the first time that a detector smaller than the size of a blood cell is used to
detect ultrasound using the silicon photonics technology, says Rami
Shnaiderman, developer of SWED. If a piezoelectric detector was miniaturized
to the scale of SWED, it would be 100 million times less sensitive.
Super-resolution
imaging
“The
degree to which we were we able to miniaturize the new detector while retaining
high sensitivity due to the use of silicon photonics was breathtaking,” says
Prof. Vasilis Ntziachristos, lead of the research team. The SWED size is about
half a micron (=0,0005 millimeters). This size corresponds to an area that is
at least 10,000 times smaller than the smallest piezoelectric detectors
employed in clinical imaging applications. The SWED is also up to 200 times
smaller than the ultrasound wavelength employed, which means that it can be
used to visualize features that are smaller than one micrometer, leading to
what is called super-resolution imaging.
Inexpensive
and powerful
As the
technology capitalizes on the robustness and easy manufacturability of the
silicon platform, large numbers of detectors can be produced at a small
fraction of the cost of piezoelectric detectors, making mass production
feasible. This is important for developing a number of different detection
applications based on ultrasound waves. We will continue to optimize every
parameter of this technology — the sensitivity, the integration of SWED in
large arrays, and its implementation in hand-held devices and endoscopes, adds
Shnaiderman.
Future
development and applications
The
detector was originally developed to propel the performance of optoacoustic
imaging, which is a major focus of our research at Helmholtz Zentrum München
and TUM. However, we now foresee applications in a broader field of sensing and
imaging, says Ntziachristos.
While the
researchers are primarily aiming for applications in clinical diagnostics and
basic biomedical research, industrial applications may also benefit from the
new technology. The increased imaging resolution may lead to studying
ultra-fine details in tissues and materials. A first line of investigation
involves super-resolution optoacoustic (photoacoustic) imaging of cells and
micro-vasculature in tissues, but the SWED could be also used to study
fundamental properties of ultrasonic waves and their interactions with matter
on a scale that was not possible before.