Ultrasound from a plaster
Ultrasound scanning via a plaster or catheter: Michiel Pertijs sees real possibilities for reducing the size of ultrasound equipment to millimetre format. And it's all thanks to smart chip technology.
Your very first photograph as a baby was probably an ultrasound image. A black and white picture on which, with a little imagination, you can recognise a baby in the womb. Ultrasound scanning is a cost-effective and safe imaging method for viewing inside the body. It is not only suitable for pictures of the unborn child, but for all kinds of organs, as long as they are made of soft tissue.
There are good reasons why doctors have long dreamt of probes that are so small that they can enable real-time imaging during minimally-invasive surgery, such as an angioplasty or the replacement of a heart valve. Or how about a wireless incontinence plaster that monitors bladder volume and sets off an alarm when it’s time to urinate? All of this is achievable within the foreseeable future – in around 5 to 10 years, if you ask Michiel Pertijs.
Together with its TU Delft partners, Michiel Pertijs’ research group is working on the ‘miniaturisation’ of ultrasound scanning. This is because the technology needs to become considerably smaller before a doctor can fit a patient with an ultrasound catheter. Pertijs: “For ultrasound scanning, you need a probe with thousands of transducers that transmit and receive ultrasound. Currently, this kind of probe is connected by a thick cable to a box next to the bed. This is packed with electronics: a piece of electronics to transmit and receive signals for every single element. However, passing thousands of cables through a catheter of 3 mm in diameter is simply impossible.”
Our collaborative partnerships enable us to make more progress towards a medical application. I find that extremely motivating.
The challenge facing Pertijs and his research group is to miniaturise the functionality of this large box of electronics and make the connection between the probe and the monitor as efficient as possible. Chip technology plays a role in this. “We can use it to make smart switches at micrometre level”, explains Pertijs. “Small enough to be able to control those thousands of transducers from the probe.” Thanks to smart switches, his chips can already process some of the signals before they are sent through the cables. “If you can already combine the signals of groups of elements on the chip, you only need to transmit the sum of those signals rather than the signals from all those individual elements. This alone reduces the number of cables by a factor of 10.”
Pertijs believes that he can make the chips even smarter. He is working on chips that digitise the ultrasound signals first, potentially making the flow of information even more efficient. “You can compare it to the development of telephony: we used to make calls using analogue telephones. That telephone conversation is now digital. In the past, only this telephone signal passed through the same cable, but now we also receive broadband internet and around a hundred television channels directly into the home. Possibilities like that also apply in this field. Ultrasound probes are the old-fashioned analogue telephones. In medical equipment, the digitisation of data is still in its infancy.”
All of that miniaturisation and reducing the number of cables: surely these chips could also communicate wirelessly? Pertijs: “That depends on how much data you need to transmit. An incontinence plaster only needs to transmit an estimate of the bladder volume every ten minutes. That can easily be done wirelessly. But with real-time imaging, you’re talking around tens of gigabits per second. That’s a completely different order of magnitude.” In addition, wireless ultrasound can sometimes be counter-productive. “A catheter always has a guide wire around it to push and pull it through the bloodstream. Some cables could easily run through a wire like that, making a wireless signal unnecessary. However, probes that the doctor holds by hand will certainly be wireless in the future. Our chips will be able to make an important contribution to that development.”
Ultrasound probes are the old-fashioned analogue telephones. In medical equipment, the digitisation of data is still in its infancy.
But there is a downside to these increasingly smarter, smaller chips: processing all of the data streams on a mini-surface consumes a lot of energy. And this needs to go somewhere, in the form of heat. Although this is not a problem with large ultrasound equipment, it is not ideal in or on the patient. “Heat can result in tissue damage”, says Pertijs. “This is why we are working with our partners on energy-efficient, safe chips.”
It is through collaboration with partners that Michiel Pertijs achieves the greatest progress. “We are often working on the same problem from different perspectives. This means that we reach solutions that we would not have come to individually.” Using chip technology to create superfast ultrasound images is an example of this. This in turn can be used for new medical diagnostics. “There is cross-fertilisation that you only find in this type of project. The bigger picture it gives us enables us to make more progress towards the medical application. I find that extremely motivating.”
About ultrasound scanning
Ultrasound is known from the animal world: bats also use it. They emit ultrasound that reflects off their prey. That reflection – or echo – returns to the bat after a time. The time between transmitting and receiving is a measure of the distance between the bat and its prey. This principle can be translated to ultrasound scanning: a probe with a sound transmitter and receiver, a transducer, emits ultrasound. When the ultrasound signal hits a surface between two tissues, it reflects back. The transducer receives the echo and transmits it to the ultrasound device, which combines several echoes into an image, for example of a foetus in the womb.
An individual echo is comparable to a depth measurement from a boat: if you throw a lead weight on a rope into the water, you know how deep the water is by measuring the length of rope. By doing the same kind of measurement from several boats, you gain an image of the river or seabed. This is similar to a two-dimensional ultrasound image of an object in the body: the boat is the transducer and the rope is the route taken by the sound.
About Michiel Pertijs
Michiel Pertijs is associate professor in the Electronic Instrumentation Lab in TU Delft’s Microelectronics department. He studied Electrical Engineering at TU Delft and obtained his doctorate in smart temperature sensors. He then became a product designer at the chip company National Semiconductor and a senior researcher at the Holst Centre in Eindhoven. After returning to research, he had the opportunity of a position as associate professor back at TU Delft. In it, he combines his expertise as a chip designer with ultrasound research.
Text: Koen Scheerders