How MEMS Ultrasound Imaging turned Ultra Small

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A startling change in medical ultrasound is working its method by way of hospitals and physicians’ places of work. The long-standing, state-of-the-art ultrasound machine that’s pushed round on a cart, with cables and a number of probes dangling, is being wheeled apart completely in favor of handheld probes that ship photos to a cellphone.

These units are sufficiently small to slot in a lab coat pocket and versatile sufficient to picture any a part of the physique, from deep organs to shallow veins, with sweeping 3D views, all with a single probe. And the AI that accompanies them could quickly make these units operable by untrained professionals in any setting—not simply educated sonographers in clinics.

The first such miniaturized, handheld ultrasound probe arrived in the marketplace in 2018, from
Butterfly Network in Burlington, Mass. Last September, Exo Imaging in Santa Clara, Calif., launched a competing model.

Making this attainable is silicon ultrasound know-how, constructed utilizing a sort of microelectromechanical system (MEMS) that crams 4,000 to 9,000 transducers—the units that convert electrical indicators into sound waves and again once more—onto a 2-by-3-centimeter silicon chip. By integrating MEMS transducer know-how with subtle electronics on a single chip, these scanners not solely replicate the standard of conventional imaging and 3D measurements but in addition open up new functions that have been not possible earlier than.

How does ultrasound work?

To perceive how researchers achieved this feat, it’s useful to know the fundamentals of ultrasound know-how. Ultrasound probes use transducers to transform electrical power to sound waves that penetrate the physique. The sound waves bounce off the physique’s delicate tissue and echo again to the probe. The transducer then converts the echoed sound waves to electrical indicators, and a pc interprets the info into a picture that may be seen on a display.

Conventional ultrasound probes include transducer arrays constituted of slabs of piezoelectric crystals or ceramics comparable to lead zirconium titanate (PZT). When hit with pulses of electrical energy, these slabs develop and contract and generate high-frequency ultrasound waves that bounce round inside them.

Black and white photo of a man taking an ultrasound image of a baby in a crib using old, bulky ultrasound equipment

Ultrasound know-how has traditionally required cumbersome equipment with a number of probes. Julian Kevin Zakaras/Fairfax Media/Getty Images

To be helpful for imaging, the ultrasound waves have to journey out of the slabs and into the delicate tissue and fluid of the affected person’s physique. This isn’t a trivial job. Capturing the echo of these waves is like standing subsequent to a swimming pool and attempting to listen to somebody talking below the water. The transducer arrays are thus constructed from layers of fabric that easily transition in stiffness from the laborious piezoelectric crystal on the middle of the probe to the delicate tissue of the physique.

The frequency of power transferred into the physique is decided primarily by the thickness of the piezoelectric layer. A thinner layer transfers greater frequencies, which permit smaller, higher-resolution options to be seen in an ultrasound picture, however solely at shallow depths. The decrease frequencies of thicker piezoelectric materials journey additional into the physique however ship decrease resolutions.

As a consequence, a number of varieties of ultrasound probes are wanted to picture varied elements of the physique, with frequencies that vary from 1 to 10 megahertz. To picture giant organs deep within the physique or a child within the womb, physicians use a 1- to 2-MHz probe, which might present 2- to 3-millimeter decision and might attain as much as 30 cm into the physique. To picture blood circulate in arteries within the neck, physicians usually use an 8- to 10-MHz probe.

How MEMS reworked ultrasound

The want for a number of probes together with the shortage of miniaturization meant that standard medical ultrasound methods resided in a heavy, boxy machine lugged round on a cart. The introduction of MEMS know-how modified that.

Over the final three a long time MEMS has allowed producers in an
array of industries to create exact, extraordinarily delicate elements at a microscopic scale. This advance has enabled the fabrication of high-density transducer arrays that may produce frequencies within the full 1- to 10-MHz vary, permitting imaging of a variety of depths within the physique, all with one probe. MEMS know-how additionally helped miniaturize further elements in order that every little thing suits within the handheld probe. When coupled with the computing energy of a smartphone, this eradicated the necessity for a cumbersome cart.

The first MEMS-based silicon ultrasound prototypes emerged within the mid-Nineties when the joy of MEMS as a brand new know-how was peaking. The key factor of those early transducers was the vibrating micromachined membrane, which allowed the units to generate vibrations in a lot the identical method that banging on a drum creates sound waves within the air.

Ultrasound probe held against a man's chest.

TThe oval-shaped inner membrane of a PMUT ultrasound probe. Exo Imaging developed a handheld ultrasound machine utilizing piezoelectric micromachined ultrasonic transducer (PMUT) know-how.Exo Imaging

Two architectures emerged. One of them, known as the
capacitive micromachined ultrasonics transducer, or CMUT, is called for its easy capacitor-like buildings. Stanford University electrical engineer Pierre Khuri-Yakub and colleagues demonstrated the primary variations.

The CMUT relies on electrostatic forces in a capacitor fashioned by two conductive plates separated by a small hole. One plate—the micromachined membrane talked about earlier than—is made from silicon or asilicon nitride with a metallic electrode. The different—usually a micromachined silicon wafer substrate—is thicker and extra inflexible. When a voltage is utilized, putting reverse prices on the membrane and substrate, engaging forces pull and flex the membrane towards the substrate. When an oscillating voltage is added, that adjustments the drive, inflicting the membrane to vibrate, like a struck drumhead.

When the membrane is involved with the human physique, the vibrations ship ultrasound frequency waves into the tissue. How a lot ultrasound is generated or detected depends upon the hole between the membrane and the substrate, which must measure at about one micrometer or much less. Micromachining methods made that type of precision attainable.

The different MEMS-based structure known as the
piezoelectric micromachined ultrasonic transducer, or PMUT, and it really works like a miniaturized model of a smoke alarm buzzer. These buzzers encompass two layers: a skinny metallic disk fastened round its periphery and a skinny, smaller piezoelectric disk bonded on high of the metallic disk. When voltages are utilized to the piezoelectric materials, it expands and contracts in thickness and backward and forward. Because the lateral dimension is far bigger, the piezo disk diameter adjustments extra considerably and within the course of bends the entire construction. In smoke alarms, these buildings are usually 4 cm in diameter, they usually’re what generates the shrieking sound of the alarm, at round 3 kilohertz. When the membrane is scaled all the way down to 100 μm in diameter and 5 to 10 μm in thickness, the vibration strikes up into megahertz frequencies, making it helpful for medical ultrasound.

Honeywell within the early Eighties developed the primary micromachined sensors utilizing piezoelectric skinny movies constructed on silicon diaphragms. The first PMUTs working at ultrasound frequencies didn’t emerge till
1996, from the work of supplies scientist Paul Muralt on the Swiss Federal Institute of Technology Lausanne (EPFL), in Switzerland.

Early years of CMUT

A giant problem with CMUTs was getting them to generate sufficient stress to ship sound waves deep into the physique and obtain the echoes coming again. The membrane’s movement was restricted by the exceedingly small hole between the membrane and the substrate. This constrained the amplitude of the sound waves that might be generated. Combining arrays of CMUT units with totally different dimensions right into a single probe to extend the frequency vary additionally compromised the stress output as a result of it diminished the probe space out there for every frequency.

Ultrasound probe held against a boy's back and the multi-layered inner membrane of a CMUT ultrasound probeButterfly Network developed a handheld ultrasound machine utilizing capacitive micromachined ultrasonic transducer (CMUT) know-how.Butterfly

The answer to those issues got here from Khuri-Yakub’s lab at Stanford University. In
experiments within the early 2000s, the researchers discovered that rising the voltage on CMUT-like buildings precipitated the electrostatic forces to beat the restoring forces of the membrane. As a consequence, the middle of the membrane collapses onto the substrate.

A collapsed membrane appeared disastrous at first however turned out to be a method of constructing CMUTs each extra environment friendly and extra tunable to totally different frequencies. The effectivity elevated as a result of the hole across the contact area was very small, rising the electrical area there. And the stress elevated as a result of the massive doughnut-shaped area across the edge nonetheless had an excellent vary of movement. What’s extra, the frequency of the gadget might be adjusted just by altering the voltage. This, in flip, allowed a single CMUT ultrasound probe to supply the complete ultrasound frequency vary wanted for medical diagnostics with excessive effectivity.

Inside Butterfly Network’s CMUT ultrasound probe, the membrane collapses onto the substrate, producing an acoustic wave.Butterfly Network

From there, it took greater than a decade to know and mannequin the difficult electromechanical conduct of CMUT arrays and iron out the manufacturing. Modeling these units was tough as a result of hundreds of particular person membranes interacted in every CMUT array.

On the manufacturing facet, the challenges concerned discovering the precise supplies and growing the processes wanted to supply easy surfaces and a constant hole thickness. For instance, the skinny dielectric layer that separates the conductive membrane and the substrate should face up to about 100 volts at a thickness of 1 μm. If the layer has defects, prices could be injected into it, and the gadget can brief on the edges or when the membrane touches the substrate, killing the gadget or at the least degrading its efficiency.

Eventually, although, MEMS foundries comparable to Philips Engineering Solutions in Eindhoven, Netherlands, and Taiwan Semiconductor Manufacturing Co. (TSMC), in Hsinchu, developed options to those issues. Around 2010, these firms started producing dependable, high-performance CMUTs.

Early improvement of PMUTs

Early PMUT designs additionally had bother producing sufficient stress to work for medical ultrasound. But they might bang out sufficient to be helpful in some shopper functions, comparable to
gesture detection and proximity sensors. In such “in-air ultrasound” makes use of, bandwidth isn’t crucial, and frequencies could be under 1 MHz.

In 2015, PMUTs for medical functions bought an sudden enhance with the introduction of enormous 2D matrix arrays for fingerprint sensing in cell phones. In the primary demonstration of this strategy, researchers on the University of California, Berkeley, and the University of California, Davis, related round 2,500 PMUT parts to CMOS electronics and positioned them below a silicone rubberlike layer. When a fingertip was pressed to the floor, the prototype measured the amplitudes of the mirrored indicators at 20 MHz to differentiate the ridges within the fingertip from the air pockets between them.

This was a powerful demonstration of integrating PMUTs and electronics on a silicon chip, and it confirmed that giant 2D PMUT arrays may produce a excessive sufficient frequency to be helpful for imaging of shallow options. But to make the soar to medical ultrasound, PMUT know-how wanted extra bandwidth, extra output stress, and piezoelectric skinny movies with higher effectivity.

Help got here from semiconductor firms comparable to
ST Microelectronics, primarily based in Geneva, which found out easy methods to combine PZT skinny movies on silicon membranes. These movies require additional processing steps to take care of their properties. But the development in efficiency made the price of the additional steps worthwhile.

To obtain a bigger stress output, the piezoelectric layer wanted to be thick sufficient to permit the movie to maintain the excessive voltages required for good ultrasound photos. But elevated thickness results in a extra inflexible membrane, which reduces the bandwidth.

One answer was to make use of an oval-shaped PMUT membrane that successfully mixed a number of membranes of various sizes into one. This is much like altering the size of guitar strings to generate totally different tones. The oval membrane supplies strings of a number of lengths on the identical construction with its slender and broad sections. To effectively vibrate wider and narrower elements of the membrane at totally different frequencies, electrical indicators are utilized to a number of electrodes positioned on corresponding areas of the membrane. This strategy allowed PMUTs to be environment friendly over a wider frequency vary.

From academia to the actual world

In the early 2000s, researchers started to push CMUT know-how for medical ultrasound out of the lab and into industrial improvement. Stanford University spun out a number of startups aimed toward this market. And main medical ultrasound imaging firms comparable to GE, Philips, Samsung, and Hitachi started growing CMUT know-how and testing CMUT-based probes.

But it wasn’t till 2011 that CMUT commercialization actually started to make progress. That yr, a crew with semiconductor electronics expertise based Butterfly Network. The 2018 introduction of the IQ Probe was a transformative occasion. It was the primary handheld ultrasound probe that would picture the entire physique with a 2D imaging array and generate 3D picture knowledge. About the dimensions of a TV distant and solely barely heavier, the probe was initially priced at US $1,999—one-twentieth the price of a full-size, cart-carried machine.

Around the identical time, Hitachi in Tokyo and Kolo Medical in Suzhou, China (previously in San Jose, Calif.), commercialized CMUT-based probes to be used with standard ultrasound methods. But neither has the identical capabilities as Butterfly’s. For instance, the CMUT and electronics aren’t built-in on the identical silicon chip, which implies the probes have 1D arrays fairly than 2D. That limits the system’s means to generate photos in 3D, which is important in superior diagnostics, comparable to figuring out bladder quantity or simultaneous orthogonal views of the guts.

Exo Imaging’s September 2023 launch of its handheld probe, the Iris, marked the industrial debut of PMUTs for medical ultrasound. Developed by a crew with expertise in semiconductor electronics and integration, the Iris is about the identical measurement and weight as Butterfly’s IQ Probe. Its $3,500 worth is akin to Butterfly’s newest mannequin, the IQ+, priced at $2,999.

The ultrasound MEMS chips in these probes, at 2 by 3 cm, are a number of the largest silicon chips with mixed electromechanical and digital performance. The measurement and complexity impose manufacturing challenges by way of the uniformity of the units and the yield.

These handheld units function at low energy, so the probe’s battery is light-weight, lasts for a number of hours of steady use whereas the gadget is related to a cellphone or pill, and has a brief charging time. To make the output knowledge appropriate with cellphones and tablets, the probe’s foremost chip performs digitization and a few sign processing and encoding.

To present 3D info, these handheld probes take a number of 2D slices of the anatomy after which use machine studying and AI to assemble the mandatory 3D knowledge. Built-in AI-based algorithms can even assist medical doctors and nurses exactly place needles in desired places, comparable to in difficult vasculature or in different tissue for biopsies.

The AI developed for these probes is so good that it might be attainable for professionals untrained in ultrasound, comparable to nurse midwives, to make use of the moveable probes to find out the gestational age of a fetus, with accuracy much like that of a educated sonographer, based on
a 2022 research in NEJM Evidence. The AI-based options may additionally make the hand-held probes helpful in emergency medication, in low-income settings, and for coaching medical college students.

Just the start for MEMS ultrasound

This is just the start for miniaturized ultrasound. Several of the world’s largest semiconductor foundries, together with TSMC and ST Microelectronics, now do MEMS ultrasound chip manufacturing on 300 and 200 mm wafers, respectively.

In truth, ST Microelectronics not too long ago fashioned a devoted “Lab-in-Fab” in Singapore for thin-film piezoelectric MEMS, to speed up the transition from proofs of idea to quantity manufacturing.
Philips Engineering Solutions affords CMUT fabrication for CMUT-on-CMOS integration, and Vermon in Tours, France, affords industrial CMUT design and fabrication. That means startups and educational teams now have entry to the bottom applied sciences that can make a brand new degree of innovation attainable at a a lot decrease price than 10 years in the past.

With all this exercise, business analysts count on ultrasound MEMS chips to be built-in into many various medical units for imaging and sensing. For occasion, Butterfly Network, in collaboration with
Forest Neurotech, is growing MEMS ultrasound for brain-computer interfacing and neuromodulation. Other functions embrace long-term, low-power wearable units, comparable to coronary heart, lung, and mind screens, and muscle-activity screens utilized in rehabilitation.

In the subsequent 5 years, count on to see miniature passive medical implants with ultrasound MEMS chips, by which energy and knowledge are remotely transferred utilizing ultrasound waves. Eventually, these handheld ultrasound probes or wearable arrays might be used not solely to picture the anatomy but in addition to learn out important indicators like inner stress adjustments as a consequence of tumor progress or deep-tissue oxygenation after surgical procedure. And ultrasound fingerprint-like sensors may at some point be used to measure blood circulate and coronary heart price.

One day, wearable or implantable variations could allow the technology of passive ultrasound photos whereas we sleep, eat, and go about our lives.

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