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Ultrasonic Holograms: Who Knew Acoustics Could Go 3D?

Imaging and other medical applications waiting in the wings

2 min read
Two 3D-printed ultrasonic hologram lenses (lens for hole hologram, left; lens for conventional hologram, right).
Two 3D-printed ultrasonic hologram lenses (lens for hole hologram, left; lens for conventional hologram, right).
Photo: Xidian University/IEEE

Although its origins trace back to the 1960s and ‘70s, acoustic holography as a field has been hampered by rampant noise created by widespread reflection and scattering. (Lasers tamp this problem down somewhat for conventional optical holograms; ultrasound as yet offers no such technological quick fix.) 

But in a recent study published last month in IEEE Sensors Journal, a group of researchers report an improved method for creating acoustic holograms. While the advance won’t lead to treatment with acoustic holograms in the immediate future, the improved technique yields higher sensitivity and a better focusing effect than previous acoustic hologram methods.

There are a number of intriguing possibilities that come with manipulation using sound, including medical applications. Ultrasound can penetrate human tissues and is already used for medical imaging. But more precise manipulation and imaging of human tissues using 3D holographic ultrasound  could lead to completely new therapies—including targeted neuromodulation using sound.

The nature of sound itself poses the first hurdle to be overcome. “The medical application of acoustic holograms is limited owing to the sound reflection and scattering at the acoustic holographic surface and its internal attenuation,” explains Chunlong Fei, an associate professor at Xidian University who is involved in the study.

Photograph of the experimental setup of pulse-echo test. Setup for an initial test run the Xidan University group performed of their ultrasonic hologram apparatusPhoto: Xidian University/IEEE

To address these issues, his team created their acoustic hologram via a “lens” consisting of a disc with a hole at its center. They placed a 1 MHz ultrasound transducer in water and used the outer part of the transducer surface to create the hologram. By creating a hole in the center of the lens, the center of the transducer generates and receives soundwaves with less reflection and scattering.

Next, the researchers compared their new disc approach to more conventional acoustic hologram techniques. They performed this A vs. B comparison via ultrasound holographic images of several thin needles, 1.25 millimeters in diameter or less.

“The most notable feature of the hole-hologram we proposed is that it has high sensitivity and maintains good focusing effect [thanks to the] holographic lens,” says Fei. He notes that these features will lead to less scattering and propagation loss than what occurs with traditional acoustic holograms.

Fei envisions several different ways in which this approach could one day be applied medically, for example by complementing existing medical imaging probes to achieve better resolution, or for applications such as nerve regulation or non-invasive brain stimulation. However, the current set up, using water, would need to be modified to be more suitable for medical setting, along with several next steps related to characterizing and manipulating the hologram, says Fei.

The varied design improvements Fei’s team hopes to develop match the equally eclectic possible applications of ultrasonic hologram technology. In the future, Fei says they hope acoustic holographic devices might achieve super-resolution imaging, particle trapping, selective biological tissue heating—and even find new applications in personalized medicine.

Correction: The first examples of acoustic holography date back to the 1960s, according to this 1971 IEEE review of the field—not 1985, as a previous version of this article stated. 

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Europe Expands Virtual Borders To Thwart Migrants

Our investigation reveals that Europe is turning to remote sensing to detect seafaring migrants so African countries can pull them back

14 min read
A photo of a number of people sitting in a inflatable boat on the water with a patrol ship in the background.

Migrants in a dinghy accompanied by a Frontex vessel at the village of Skala Sikaminias, on the Greek island of Lesbos, after crossing the Aegean sea from Turkey, on 28 February 2020.

ASSOCIATED PRESS

It was after midnight in the Maltese search-and-rescue zone of the Mediterranean when a rubber boat originating from Libya carrying dozens of migrants encountered a hulking cargo ship from Madeira and a European military aircraft. The ship’s captain stopped the engines, and the aircraft flashed its lights at the rubber boat. But neither the ship nor the aircraft came to the rescue. Instead, Maltese authorities told the ship’s captain to wait for vessels from Malta to pick up the migrants. By the time those boats arrived, three migrants had drowned trying to swim to the idle ship.

The private, Malta-based vessels picked up the survivors, steamed about 237 kilometers south, and handed over the migrants to authorities in Libya, which was and is in the midst of a civil war, rather than return to Malta, 160 km away. Five more migrants died on the southward journey. By delivering the migrants there, the masters of the Maltese vessels, and perhaps the European rescue authorities involved, may have violated the international law of the sea, which requires ship masters to return people they rescue to a safe port. Instead, migrants returned to Libya over the last decade have reported enslavement, physical abuse, extortion, and murders while they try to cross the Mediterranean.

If it were legal to deliver rescued migrants to Libya, it would be as cheap as sending rescue boats a few extra kilometers south instead of east. But over the last few years, Europe’s maritime military patrols have conducted fewer and fewer sea rescue operations, while adding crewed and uncrewed aerial patrols and investing in remote-sensing technology to create expanded virtual borders to stop migrants before they get near a physical border.

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