While learning POCUS, I admittedly skipped over most of the "ultrasound physics" chapters and moved right into the various types of scans. I suppose it's the same reason I dropped out of drum lessons as a kid. I wanted to learn actual songs and move on from the mind-numbing paradiddles. I eventually realized that skipping the fundamentals does not make it impossible to learn something, but it can steepen the curve, causing more frustration in the long game.
Learning the basics of ultrasound physics improves your ability to obtain, troubleshoot, and interpret scans to answer specific clinical questions. A strong mental model of how the technology works can prevent you from flying blind as a bat into point-of-care ultrasound...
The bat is a great place to start.
In the late 1700s, Lazzaro Spallanzani conducted experiments to understand how bats flew at night. He suspected it had nothing to do with their eyes, so he tested this theory with little bat blindfolds.
The concept was simple: cover the bat's eyes and see if they can navigate around obstacles. He noted that if the bat was blindfolded, it could still fly and navigate obstacles without issues. However, if that bat was deaf (even in one ear), it could not safely fly in the test environment. This is because bats emit a sound by clicking their tongue and then listen to how that sound wave reflects off objects within a space. This process is called "echolocation" and is similar to how ultrasound works.
Sound waves travel through the room and echo back at different amplitudes depending on the type of medium they bounce off of. The time it takes for a sound wave to bounce off a medium and return to the bat can give our flying rat friend an idea of how far away that medium is.
Fast forward to 1880, and two French brothers, Jacques & Pierre Curie, discovered something they would call “piezoelectricity.” The piezoelectric effect is that when specific types of crystals (like tourmaline, quartz, topaz, cane sugar, and Rochelle) are compressed, they create energy.
The interesting thing about piezoelectric crystals is that the reverse is also true; if you charge a piezoelectric crystal with energy, it will deform its shape, releasing that energy and creating an ultrasonic vibration. This allows the crystals to act as ultrasound emitters and receivers.
The sound waves produced from piezoelectric crystals are considered ultrasound, which means the frequency is so high that they are not detectable by the human ear. Try out this free tone generator and see how high you can get the frequency while still hearing it. My upper limit is around 15,000 Hz.
This online tone generator can reach a maximum frequency of just north of 20,000 Hz, which, while undetectable to most humans, is still lower than the frequency used for diagnostic ultrasound.
I suppose this is a good spot to define frequency. Frequency is measured in hertz (Hz) and is the number of complete sound waves that pass through a single point in one second. The example below shows a high and low frequency relative to each other for illustrative purposes. However, ultrasound is measured in the range of millions of hertz (megahertz, MHz) and I didn't feel like drawing a million of these waves.
Look at how often the high-frequency probe comes into contact with the green column in the illustration compared to the low-frequency. You can see why higher-frequency probes, such as the linear array, can collect more data and, therefore, have a higher resolution. This, however, comes at the expense of depth. The heat loss from contact with more tissues causes the ultrasound wave to attenuate (decrease in strength) faster and lose depth.
This is why higher-frequency probes are used for more superficial scans, such as vascular access. The higher frequency allows excellent resolution when identifying vessels. In contrast, the phased array probe operates at a lower frequency and is used for deeper structures, such as the abdominal organs or heart. This is why you can only hear the bass when your neighbor down the street is drinking garage beers and jamming out to Creed.
Modes
The very first mode of ultrasound imaging was called amplitude mode or "A" mode. It stimulated one transducer element to emit a short pulse of sound waves and then stopped to listen. Listening means stopping the crystals' stimulation so they can be free to deform from the returning vibrations.
The received echoes are plotted as spikes on a linear graph, where the x-axis typically represents time or depth (calculated from the time it takes for the echo to return), and the y-axis represents the amplitude of the received echoes.
The highest amplitude is at the visceral and parietal pleura interface (VPPI), represented by a taller spike. "A" mode can help determine tissue boundary depths, such as in ophthalmology. However, its clinical usefulness is niche and simply plotting a line with spikes representing various reflected amplitudes is not as helpful as an actual image.
So, how do we turn these amplitude spikes into an actual image?
If we were to assign a greyscale or 'brightness' to the various amplitude spikes, we could plot these to start creating something that resembles an image. This is known as "brightness" mode or "B" mode.
In the animation above, only one transducer element is firing. We will need to add quite a few more to see something that looks like an image. Each transducer element sends out a pulse wave and listens for how it is reflected back, generating a real-time image of the tissue boundaries.
The number and frequency of transducer elements determine what type of scan a specific probe is ideal for. The linear array probe comprises around 256 transducer elements that emit a frequency between 5-13 MHz. The transducer elements are fired linearly, creating a square ultrasound beam profile (UBP).
If we curve those transducer elements slightly, the UBP will splay out from the probe, and we will end up with a curvilinear probe. This probe uses a lower frequency to allow the ultrasound to travel deeper into the tissue.
👆Remember, lower frequency means less contact with tissue and less heat lost, allowing it to travel deeper before attenuating. The trade-off is that less contact with tissue means less information coming back from the tissue to the transducer and lower resolution.
The phased array probe, also known as the cardiac probe, is ideal for visualizing the heart through the rib spaces because of its small footprint and low frequency. In contrast to the ~256 transducer elements in the linear probe, the phased array only has around 60-128.
Attenuation
Regardless of the probe being used, the ultrasound wave amplitude will decrease in intensity as it travels through the tissue. This is called attenuation, and while many things can change the degree to which the amplitude is diminished, the three main offenders are:
Absorption is the conversion of the ultrasound wave's energy into heat as it passes through tissue. It's the primary mechanism by which ultrasound waves lose energy within the body, resulting in a decrease in the wave's amplitude.
Reflection: This occurs when the ultrasound waves encounter interfaces between different types of tissues (like between muscle and bone). The difference in acoustic impedance at these interfaces causes some of the ultrasound energy to be reflected rather than continuing into the tissue.
Scattering: This happens when the path of ultrasound waves is randomly redirected as they hit small structures within the tissue. Scattering diffuses the sound wave in various directions, diminishing the direct energy that can be captured to form an image or deliver therapy.
Operator to Scan Artist
The journey through ultrasound physics isn't just an academic exercise; it's a practical guide that sharpens your clinical acumen. By understanding the intricacies of how ultrasound waves interact with different tissues, you gain the ability to make subtle adjustments to the equipment settings or your scanning technique. This finesse enables you to capture precisely the right images needed to address specific clinical questions. Such skills are crucial when clarity and detail are paramount for accurate diagnoses.
Embracing the science behind the scans not only makes you a more competent practitioner but also deepens your appreciation for the delicate interplay between ultrasound waves and biological tissues. This appreciation transforms technical scanning into art, elevating your role from a mere operator to a 'Scan Artist.'
References:
Abu-Zidan, F. M., Hefny, A. F., & Corr, P. (2011). Clinical ultrasound physics. Journal of emergencies, trauma, and shock, 4(4), 501–503. https://doi.org/10.4103/0974-2700.86646
Griffin, D. R. (2001). Return to the Magic Well: Echolocation behavior of bats and responses of insect prey. BioScience, 51(7), 555-556. https://doi.org/10.1641/0006-3568(2001)051[0555:RTTMWE]2.0.CO;2
Grogan, S. P., & Mount, C. A. (2023). Ultrasound Physics and Instrumentation. In StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Available from: https://www.ncbi.nlm.nih.gov/books/NBK570593/
Kamel N. A. (2022). Bio-piezoelectricity: fundamentals and applications in tissue engineering and regenerative medicine. Biophysical reviews, 14(3), 717–733. https://doi.org/10.1007/s12551-022-00969-z
Roche, A., Watkins, E., Pettit, A., Slagle, J., Zapata, I., Seefeld, A., & Lundgreen Mason, N. (2024). Impact of Prehospital Ultrasound Training on Simulated Paramedic Clinical Decision-Making. The western journal of emergency medicine, 25(5), 784–792. https://doi.org/10.5811/westjem.18439
