Trainees new to POCUS are often focused on learning to apply this powerful tool; how to identify important structures, spot pathology, and tweak the gain and depth to get the perfect image. This knowledge is essential for anyone hoping to use POCUS effectively in a clinical setting. But have you ever thought about what is under the hood of these devices?

Whether you are a seasoned POCUS user or a trainee, chances are there is a lot about these little devices you may not be aware of. Together, we will explore the fundamentals of POCUS devices, specifically what is in them and how they work. The goal is not to turn you into a POCUS engineer. Instead, my hope is you will be better able to answer patient questions, impress attendings on the wards, dazzle at your graduate school defense, or just enjoy learning something new.

Gross Anatomy of a POCUS Device

Before we begin, it is important to orient ourselves with some terminology. Most POCUS devices contain the following components:

• Transducer
• Control Panel
• Display
• Central Processing Unit
• Power Source
• Data Storage

While some of these are self-explanatory (I am sure you know what a power source is and how to charge a battery), others deserve a bit of explanation.

With that said, we are finally ready to dive into today’s main topic: transducers!

Transducers – Probe-ably the Most Important Part

At first glance, the transducer (probe) may not seem like much. The transducer is covered in a protective hard housing, designed to serve as an ergonomic handle where most people intuitively wrap their fingers. Inside this, a casing provides both acoustic and electromagnetic insulation, protecting the probe from external interference and helping to retain heat. This insulation is critical for improving image quality. Unless you have a Bluetooth-capable device, a cable that carries information from the transducer to the CPU connects at the base of the handle at a strain reliever. As you probably guessed, this component helps protect the cable from fraying due to bending. Finally, the head is at the tip of the probe where contact with the patient is made.

A Look Inside

Let’s pry open that casing and look at some transducer anatomy, from head to base.

1. Acoustic Lens

The acoustic lens focuses and shapes ultrasound beams, enabling enhanced image quality and resolution by converging sound waves to a specific point. This is critical for reducing the scatter of sound waves and resulting noise in the image. Different types of probes exist for imaging different structures in the body for this reason. So called array types will be discussed in a future post.

The lens also serves a few additional purposes. First, it acts as an electrical insulator to protect patients from a light zap of electricity they would otherwise feel. It also protects the ultrasound from fluids, particularly the acoustic gel.

2. Piezoelectric Crystals

Commonly made of lead zirconate titanate (also known as PZT), these crystals are where the real magic of ultrasound occurs. Piezoelectric crystals convert electrical energy into sound waves and vice versa, relying on what is known as the Piezoelectric Effect.

When an electric voltage is applied, these crystals physically deform due to realignment of the electric dipoles of the molecules. This results in mechanical vibrations that generate sound waves. Conversely, upon receiving sound waves reflected from the body, the crystals again deform resulting in the formation of an electric charge across their surface. It is this electric charge that causes the formation of new current in the adjacent wires, relaying information about the soundwaves to the POCUS device. It’s all about the interplay between electric fields and mechanical stress!

3. Matching Layer(s)

Every material has a different acoustic impedance, a measure of how much the material resists the passage of sound waves. The density of each material influences the impedance by determining the velocity of particle movement and pressure required to move those particles to maintain a sound wave. To summarize some tricky physics, it is because of impedance and wave properties that some sound waves pass through, while others are reflected when sound hits the boundary of two different materials. This phenomenon is exactly what allows POCUS to differentiate different tissues in the body, each with slightly different impedance properties that alter how sound waves behave at their boundaries.

The human body, however, has significantly lower impedance than the piezoelectric crystals in the transducer. This is a major concern as the majority sound waves from the transducer would reflect and never reach the patient, thus making POCUS impossible. To address this issue, engineers place matching layers (normally 2-3) between the piezoelectric crystals and the lens. These materials gradually change impedance to better match that of the skin, improving the efficiency of sound wave transmission to the patient. The matching layers also work in reverse, aiding movement of reflected sound waves from the body to reach the piezoelectric crystals again.

4. Backing Layer

While incredibly useful, the piezoelectric crystals also cause two problems. First, the vibrating crystals create both forward (towards the body) and backward (towards the transducer housing) traveling sound waves. Undesirable backward traveling sound waves could interfere with electrical signals being produced, degrading image quality. Second, after initial excitation by an electric pulse, the crystals continue to vibrate for some time due to rapidly shifting electric dipoles from mechanical deformation. This phenomenon is known as ringing. To reduce noise in the image, the duration of vibrations must be precisely controlled.

The backing layer, also known as a dampening layer, directly addresses these challenges. Made of materials that readily absorb sound waves and positioned in direct contact with the piezoelectric crystals, this layer physically suppresses unwanted vibrations and absorbs backward travelling sound waves. By dampening residual oscillations, the ultrasound pulse is shortened, improving axial resolution and allowing better separation of closely spaced structures. Additionally, the backing layer increases the transducer’s bandwidth, allowing it to generate a broader range of frequencies, enhancing the ability to differentiate tissues at varying depths.  

5. Conductive Elements and Wiring

Small metal electrodes are attached directly to the piezoelectric crystals. These carry an electric signal from the POCUS device to the transducer and receive the electric signal from returning sound waves. Electric signals are sent through the wiring to the CPU for interpretation and translation into images.

Go Forth and Scan, Expert

By now, I hope it’s clear that when you grab a POCUS probe, you’re not just holding a piece of plastic. Transducers are precision tools, built on decades of incredible discoveries and taking advantage of multiple physics principles to do the “simple” job of making, receiving, and interpreting sound waves. You should not be surprised to find out that we have just scratched the surface of what is under the hood of these powerful little devices.

Until next time. May your images be crisp and your gel always warm.