Researchers extend capabilities of ultrasound with new tech to dynamically shape ultrasound waves

Scientists from the Max Planck Institute have developed a chip-based technology that generates sound profiles with high resolution and intensity, opening up new possibilities for ultrasound therapy.

The research team, led by Prof. Dr Peer Fischer from the Max Planck Institute for Intelligent Systems and the University of Stuttgart has developed a projector that flexibly modulates three-dimensional ultrasound fields with comparatively little technical effort. This enables dynamic sound profiles to be generated and tailored to individual patients.

Ultrasound is widely used as a diagnostic tool, however it can also be used therapeutically. For example, in the US high-powered ultrasound is used to treat tumours of the uterus and prostate. The ultrasound destroys the cancer cells by specific heating of the diseased tissue.

Researchers around the world are also looking at using ultrasound to treat tumours and other pathological changes in the brain.

Dr Fischer explained: “In order to avoid damaging healthy tissue, the sound pressure profile must be precisely shaped.” However, tailoring an intensive ultrasound field to diseased tissue is somewhat more difficult in the brain. This is because the skullcap distorts the sound wave.

However, Dr Fischer’s team may have found a solution to this. The Spatial Ultrasound Modulator (SUM) developed by the researchers allows the three-dimensional shape of even very intense ultrasound waves to be varied with high resolution – and with less technical effort than is currently required to modulate ultrasound profiles.

Conventional methods vary sound fields with several individual sound sources, the waves of which can be superimposed and shifted against each other. However, because the individual sound sources cannot be miniaturized at will, the resolution of these sound pressure profiles is limited to 1000 pixels. The sound transmitters are then so small that the sound pressure is sufficient for diagnostic but not therapeutic purposes. With the new technology, the researchers first generate an ultrasonic wave and then modulate its sound pressure profile independently, essentially killing two birds with one stone.

“In this way, we can use much more powerful ultrasonic transducers,” explained postdoctoral fellow Kai Melde, who is part of the team that developed the SUM. “Thanks to a chip with 10,000 pixels that modulates the ultrasonic wave, we can generate a much finerresolved profile.”

Zhichao Ma, a post-doctoral fellow in Fischer’s team, added: “In order to modulate the sound pressure profile, we take advantage of the different acoustic properties of water and air. While an ultrasonic wave passes through a liquid unhindered, it is completely reflected by air bubbles.”

To implement this, the research team constructed a chip the size of a thumbnail on which they can produce hydrogen bubbles by electrolysis (i.e. the splitting of water into oxygen and hydrogen with electricity) on 10,000 electrodes in a thin film of water. The electrodes each have an edge length of less than a tenth of a millimetre and can be controlled individually.

If you send an ultrasonic wave through the chip with a transducer (a kind of miniature loudspeaker), it passes through the chip unhindered. But as soon as the sound wave hits the water with the hydrogen bubbles, it continues to travel only through the liquid. Like a mask, this creates a sound pressure profile with cut-outs at the points where the air bubbles are located. To form a different sound profile, the researchers first wipe the hydrogen bubbles away from the chip and then generate gas bubbles in a new pattern.

The researchers demonstrated how precisely and variably the new projector for ultrasound works by writing the alphabet in a kind of picture show of sound pressure profiles. To make the letters visible, they caught micro-particles in the various sound pressure profiles. Depending on the sound pattern, the particles arranged themselves into the individual letters.

Using the ultrasound projector, the Stuttgart team is able to generate a new sound profile in about 10 seconds.

The technique could be used not only for diagnostic and therapeutic purposes but also in biomedical laboratories. For example, to arrange cells into organoid models. “Such organoids enable useful tests of active pharmaceutical ingredients and could therefore at least partially replace animal experiments,” added Dr Fischer.