Intermediate Physics for Medicine and Biology: Pocket Ultrasound

Friday, November 22, 2019

Pocket Ultrasound


The November 4, 2019 issue of TIME magazine, about health innovation.

For decades I’ve been a loyal subscriber to TIME magazine. I read it—more or less cover-to-cover—every week. Most issues have little overlap with Intermediate Physics for Medicine and Biology, but the November 4, 2019 issue is an exception; it focuses on health innovation.

My favorite of the featured innovators is Jonathan Rothberg, who’s developed a handheld ultrasound imager. His device looks like an electric shaver and would fit in your pocket. Don Steinberg writes in TIME

Jonathan Rothberg, a Yale genetics researcher and serial entrepreneur, figured out how to put ultrasound technology on a chip, so instead of a $100,000 machine in a hospital, it’s a $2,000 go-anywhere gadget that connects to an iPhone app.

On his website, Rothberg says his aim is to make healthcare accessible to everyone around the world. A noble goal, but what’s unique about the physics of Rothberg’s invention? In Chapter 13 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss traditional ultrasound transducers.

Ultrasound is typically produced using a piezoelectric transducer. A piezoelectric material converts a stress (or pressure) into an electric field, and vice versa. A high-frequency oscillating voltage applied across a piezoelectric material creates a sound wave at the same frequency. Conversely, an oscillating pressure applied to a piezoelectric material creates an oscillating voltage across it. Measurement of this voltage provides a way to record ultrasonic waves. Thus, the same piezoelectric material can serve as both source and detector.

According to an article by Eliza Strickland in IEEE Spectrum, Rothberg’s ultrasound device, called the Butterfly iQ, does away with the piezoelectricity.

Today’s ultrasound systems use piezoelectric crystals, which convert electrical energy into vibrations in the form of ultrasonic waves. A typical system has a display screen on a bulky cart with several wands for imaging at different depths within the body. These machines can cost upwards of $100,000…

Developing the iQ’s chip-based technology was a two-step process. First, Butterfly’s engineers replaced the piezoelectrics with a micromachine that acts like a tiny drum to generate vibrations. Inside this “capacitive micromachined ultrasound transducer” (CMUT), an applied voltage moves a membrane to send ultrasonic waves into the body. The waves that bounce back from various body tissues move the membrane and are registered as an electric signal, which creates the image…

Rothberg explains that typical ultrasound systems require separate probes for different clinical applications because the crystals have to be tuned at the time of manufacture to produce the right type of ultrasonic wave for imaging at a particular depth. But the Butterfly iQ can be tuned on the fly. “We have 10,000 of these micromachine transducers on a probe, and that gives us a monster dynamic range,” he says. "We can make them buzz at 1 megahertz if we want to go deep, or 5 megahertz if we want to go shallow.”

The second innovation was to do away with the wiring that connects a typical piezoelectric probe to the electronic controls and displays. Butterfly’s micromachines are attached directly to a semiconductor layer that contains all the necessary amplifiers, signal processors, and so on.

I’m not expert enough to judge how revolutionary this device really is, but it sure sounds cool! One thing I know for certain: when you mix physics with medicine, the results can be astounding.

The three issues I need to read to catch up with TIME magazine, superimposed on Intermediate Physics for Medicine and Biology.

Three issues behind in reading TIME magazine;
I've got my work cut out for me.

Thank goodness Christmas break is approaching, because I’m three issues behind with TIME. I try to keep up, I really do; it’s a challenge. But you never know what you’ll find when you open a new issue. Sometimes you find physics applied to medicine and biology!