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## Speed of Sound In Different Mediums

Now the “speed of sound” is also often referred to with ultrasound. So why is the speed or velocity of sound so important?

Well, the exact speed of sound in specific tissue does not actually mean much to you clinically. However, the change in speed between two different mediums is extremely important. This is the essence of how ultrasound waves reflect and refract to create important ultrasound artifacts. So while you don’t need to know the exact speed of sound in certain tissue you do need to understand how the speed of sound changes between different mediums such as soft tissue, fluid, air, and bone.

The average speed/velocity of sound in all mediums is 1540 cm/s. However, depending on what medium the sound waves travel through, it can drastically change the propagation speed of sound as it passes through.

Two of the factors that affect the speed of sound are the stiffness and density of the material it is traveling through. The stiffer the medium, the faster the sound waves will travel and that is why sound waves travel faster in solids than in liquids or gases. So the ultrasound propagation speed from slowest to fastest is: Lung (air) << Fat < Soft tissue << Bone. This happens because stiffer mediums have tighter particles to propagate the ultrasound wave and therefore the velocity is greater.

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## Frequency and Wavelengths

Now I’m sure you’ve heard the word “Frequency” a lot when it comes to ultrasound transducers. Such as high versus low frequency ultrasound probes. But what exactly does that mean? Okay let’s get some definitions out of the way:

Wavelength = length or distance of a single cycle of a wave.

Frequency = the number of sound wave cycles per second.
The equation for Frequency = Speed of sound wave/Wavelength

So you can see from the equation, as wavelength increases, frequency decreases (and vice versa). This is because Frequency is inversely related to wavelength. The SHORTER the wavelength the HIGHER the frequency and the LONGER the wavelength the LOWER the frequency.

This is why higher frequency ultrasound probes will give you better resolution compared to a lower frequency probe. A high-frequency ultrasound probe will emit shorter wavelengths, so tissues will receive more ultrasound “waves” per unit of time with a high-frequency probe. However, the trade-off with high-frequency probes is decreased penetration because the piezoelectric crystal can only send so many ultrasound waves out before the waves dissipate.

Here is a graph showing the relationship between the frequency of an ultrasound probe and the resolution versus penetration it is able to achieve.

• Phased array probe: great penetration, okay resolution
• Curvilinear probe: good penetration, good resolution
• Linear probe: poor penetration, great resolution
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## What Ultrasound Physics do you Actually Need to Know?

There are a few simple ultrasound physics principles that you will need to know in order for you to optimize your use of ultrasound and to understand ultrasound artifacts. I’ll also introduce some important ultrasound physics formulas and equations to help you grasp the concepts such as artifacts and Doppler (no need to memorize this stuff). Just invest a little time into learning these basic ultrasound physics concepts and it will help you tremendously.

Just think of Ultrasound in terms of “Waves”

An ultrasound device creates images, simply by sending short bursts of “waves” into the body. Understanding how these waves behave will be helpful in understanding how to optimize your ultrasound settings and images. I’ll make it as simple as possible for you and just go over the things I have found to be most relevant to be able to use the ultrasound machine.

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## Ultrasound Physics Table

Here is an important ultrasound physics table you can reference that goes over the speed, density, acoustic impedance, and attenuation of ultrasound relative to specific tissue types. You’ve may recognize it from other resources but never understood how to use it.

Don’t attempt to memorize this table, just look at the trends. This will help you understand why certain tissues look brighter (echogenic) compared to others, why ultrasound waves get reflected/refracted, and how ultrasound artifacts are formed. We will go over the importance of the findings of this table throughout the post.

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## How Ultrasound Creates a Picture – The Piezoelectric Effect

Next let’s go over how an ultrasound device uses ultrasonic waves to create pictures on the screen for you.

It traditionally does this by using an effect called the “Piezoelectric Effect.” This is simply the vibration of a piezoelectric crystal at the tip of the transducer that generates a specific ultrasonic frequency to create ultrasound waves. (FYI These crystals are easily broken and cost thousands of dollars to replace. Think about that each time you drop a probe. Yikes!)

These ultrasonic waves can then penetrate through the body’s soft tissue and return to the transducer as reflected ultrasound waves. These returning waves are then converted into an ultrasound image on the screen for you to view.

Therefore, all ultrasound principles are based on the physics of “waves” and if you can understand some basic physics principles that pertain to waves, you can derive exactly how ultrasound images are formed, ultrasound artifacts are created, and even how to use more advanced ultrasound applications such as Doppler.

(Note: Many of the newer handheld ultrasound devices do not use the traditional piezoelectric effect to create ultrasound images, and instead use silicon chips. However, the concepts of waves still apply)

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## What is the Definition of Ultrasound?

The definition of “ultrasound” is simply the vibration of sound with a frequency that is above the threshold of what humans can hear. The frequency of ultrasound is by definition, any frequency greater than 20,000 Hz. However, ultrasound used in medical practice is typically 1,000,000 Hz (1 Megahertz) or greater.

So the next time you pick up an ultrasound probe or transducer just notice what “Frequency” the probe is. It will usually range (termed bandwidth) between 2 Megahertz to 10 Megahertz. For example, 2.5-3.5 MHz for general abdominal imaging and 5.0-10 MHz for superficial imaging.