How MEMS Ultrasound Imaging grew to become Extremely Small

A startling change in medical ultrasound is working its method by hospitals and physicians’ workplaces. 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 primary such miniaturized, handheld ultrasound probe arrived in the marketplace in 2018, from
Butterfly Community in Burlington, Mass. Final September, Exo Imaging in Santa Clara, Calif., launched a competing model.

Making this doable is silicon ultrasound expertise, 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 expertise with refined electronics on a single chip, these scanners not solely replicate the standard of conventional imaging and 3D measurements but additionally open up new functions that had been not possible earlier than.

How does ultrasound work?

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

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

Ultrasound expertise has traditionally required cumbersome equipment with a number of probes. Julian Kevin Zakaras/Fairfax Media/Getty Photos

To be helpful for imaging, the ultrasound waves must journey out of the slabs and into the tender 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 exhausting piezoelectric crystal on the middle of the probe to the tender tissue of the physique.

The frequency of vitality transferred into the physique is decided primarily by the thickness of the piezoelectric layer. A thinner layer transfers increased 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.

Because of this, 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 massive organs deep within the physique or a child within the womb, physicians use a 1- to 2-MHz probe, which may present 2- to 3-millimeter decision and might attain as much as 30 cm into the physique. To picture blood move in arteries within the neck, physicians sometimes use an 8- to 10-MHz probe.

How MEMS remodeled ultrasound

The necessity for a number of probes together with the dearth of miniaturization meant that standard medical ultrasound techniques resided in a heavy, boxy machine lugged round on a cart. The introduction of MEMS expertise modified that.

Over the past three many years 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 expertise additionally helped miniaturize further elements in order that the whole lot matches within the handheld probe. When coupled with the computing energy of a smartphone, this eradicated the necessity for a cumbersome cart.

The primary MEMS-based silicon ultrasound prototypes emerged within the mid-Nineteen Nineties when the joy of MEMS as a brand new expertise was peaking. The important thing ingredient 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.

Exo Imaging developed a handheld ultrasound machine utilizing piezoelectric micromachined ultrasonic transducer (PMUT) expertise.Exo Imaging

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

The CMUT is predicated 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 manufactured from silicon or asilicon nitride with a metallic electrode. The opposite—sometimes a micromachined silicon wafer substrate—is thicker and extra inflexible. When a voltage is utilized, putting reverse prices on the membrane and substrate, enticing 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 in touch 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 form of precision doable.

The opposite MEMS-based structure is named the
piezoelectric micromachined ultrasonic transducer, or PMUT, and it really works like a miniaturized model of a smoke alarm buzzer. These buzzers include 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. As a result of the lateral dimension is way bigger, the piezo disk diameter adjustments extra considerably and within the course of bends the entire construction. In smoke alarms, these constructions are sometimes 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 Nineteen Eighties developed the primary micromachined sensors utilizing piezoelectric skinny movies constructed on silicon diaphragms. The primary PMUTs working at ultrasound frequencies didn’t emerge till
1996, from the work of supplies scientist Paul Muralt on the Swiss Federal Institute of Know-how Lausanne (EPFL), in Switzerland.

Early years of CMUT

A giant problem with CMUTs was getting them to generate sufficient strain 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 could possibly be generated. Combining arrays of CMUT units with totally different dimensions right into a single probe to extend the frequency vary additionally compromised the strain output as a result of it diminished the probe space accessible for every frequency.

Butterfly Community developed a handheld ultrasound machine utilizing capacitive micromachined ultrasonic transducer (CMUT) expertise.Butterfly

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

A collapsed membrane appeared disastrous at first however turned out to be a method of creating 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 subject there. And the strain elevated as a result of the massive doughnut-shaped area across the edge nonetheless had a superb vary of movement. What’s extra, the frequency of the machine could possibly be adjusted just by altering the voltage. This, in flip, allowed a single CMUT ultrasound probe to supply the whole ultrasound frequency vary wanted for medical diagnostics with excessive effectivity.

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

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

On the manufacturing aspect, the challenges concerned discovering the correct supplies and creating 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 may be injected into it, and the machine can quick on the edges or when the membrane touches the substrate, killing the machine or a minimum of degrading its efficiency.

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

Early growth of PMUTs

Early PMUT designs additionally had bother producing sufficient strain to work for medical ultrasound. However they may 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 important, and frequencies may be under 1 MHz.

In 2015, PMUTs for medical functions received an surprising increase with the introduction of huge 2D matrix arrays for fingerprint sensing in cellphones. Within the first demonstration of this method, researchers on the College of California, Berkeley, and the College of California, Davis, linked round 2,500 PMUT components 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 tell apart the ridges within the fingertip from the air pockets between them.

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

Assist got here from semiconductor firms comparable to
ST Microelectronics, based mostly in Geneva, which found out how one can combine PZT skinny movies on silicon membranes. These movies require further processing steps to keep up their properties. However the enchancment in efficiency made the price of the additional steps worthwhile.

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

One resolution was to make use of an oval-shaped PMUT membrane that successfully mixed a number of membranes of various sizes into one. That is much like altering the size of guitar strings to generate totally different tones. The oval membrane offers 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 method allowed PMUTs to be environment friendly over a wider frequency vary.

From academia to the actual world

Within the early 2000s, researchers started to push CMUT expertise for medical ultrasound out of the lab and into industrial growth. Stanford College spun out a number of startups geared toward this market. And main medical ultrasound imaging firms comparable to GE, Philips, Samsung, and Hitachi started creating CMUT expertise and testing CMUT-based probes.

But it surely wasn’t till 2011 that CMUT commercialization actually started to make progress. That 12 months, a crew with semiconductor electronics expertise based Butterfly Community. The 2018 introduction of the IQ Probe was a transformative occasion. It was the primary handheld ultrasound probe that might picture the entire physique with a 2D imaging array and generate 3D picture information. Concerning the dimension 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.

Across 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 techniques. However 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 moderately than 2D. That limits the system’s means to generate photos in 3D, which is critical in superior diagnostics, comparable to figuring out bladder quantity or taking a look at 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 dimension and weight as Butterfly’s IQ Probe. Its $3,500 value is corresponding to Butterfly’s newest mannequin, the IQ+, priced at $2,999.

The ultrasound MEMS chips in these probes, at 2 by 3 cm, are among the largest silicon chips with mixed electromechanical and digital performance. The scale 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 machine is linked to a cellphone or pill, and has a brief charging time. To make the output information suitable with cellphones and tablets, the probe’s major chip performs digitization and a few sign processing and encoding.

To offer 3D info, these handheld probes take a number of 2D slices of the anatomy after which use machine studying and AI to assemble the required 3D information. Constructed-in AI-based algorithms may also assist docs and nurses exactly place needles in desired areas, comparable to in difficult vasculature or in different tissue for biopsies.

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

Only the start for MEMS ultrasound

That is solely the start for miniaturized ultrasound. A number of 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 actual fact, ST Microelectronics just lately 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 Options affords CMUT fabrication for CMUT-on-CMOS integration, and Vermon in Excursions, France, affords industrial CMUT design and fabrication. Meaning startups and tutorial teams now have entry to the bottom applied sciences that may make a brand new stage of innovation doable 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. As an example, Butterfly Community, in collaboration with
Forest Neurotech, is creating MEMS ultrasound for brain-computer interfacing and neuromodulation. Different functions embrace long-term, low-power wearable units, comparable to coronary heart, lung, and mind screens, and muscle-activity screens utilized in rehabilitation.

Within the subsequent 5 years, count on to see miniature passive medical implants with ultrasound MEMS chips, through which energy and information are remotely transferred utilizing ultrasound waves. Ultimately, these handheld ultrasound probes or wearable arrays could possibly be used not solely to picture the anatomy but additionally to learn out important indicators like inner strain adjustments as a result of tumor progress or deep-tissue oxygenation after surgical procedure. And ultrasound fingerprint-like sensors may sooner or later be used to measure blood move and coronary heart fee.

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