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FUS 1.3 The basics of how ultrasound works

Core Underlying Theory of Ultrasonography:

Ultrasonography is based on the principle that a transducer emits high-frequency sound waves, ranging from 3 to 25 MHz (Maximum 20 MHz in a portable Ultrasound as of 2023), which bounce off the acoustic interface of the tissues being examined and are received back by the transducer as echoes.
In B-Mode (Brightness Mode) the amount of echos bouncing off a structure and returning to the probe is quantified and given a black and white shade. The whiter the object, the more echos are originating from that structure. This information is used to create black and white pixels on a screen to generate an image.
Check different tissues and structures have different degrees of echogenicity (the ability to reflect sound back at the probe), and abilities for sound to pass through the tissue. The more sound which is reflected back at the probe, the whiter the image appears. This is how ultrasonography generates images.
Air the enemy of ultrasound! They are scattered ultrasound waves resulting in echoes not being returned to the probe. For this reason, any place where an ultrasound wave encounters air will appear black. This is the reason why we use ultrasound gel to create an interface betweenCosts the probe and the skin. Any air between the probe and the skin will greatly diminish the quality of the image.

[insert modified https://www.scienceabc.com/wp-content/uploads/2016/08/How-does-an-ultrasound-work.jpg]

[tissue pennitration]

Echogenicity:

When the probed tissues produce images that are similar in appearance to the surrounding structures, they are described as isoechoic.

On the other hand, when no echoes are reflected from the tissue, they appear black and is this area of an image is called anechoic.

When a structure creates some echos but less than the surrounding tissue, it is said to be hypoechoic. It will appear darker than the surrounding tissues.

When a structure produces a lot of echos it appears white and is called hyperechoic.

[insert echogenicity png]

[real examples of US echogenicity]

A core principle ultrasound imaging is that the transducer emits sound waves above what the ear can hear from 3 to 25 MHz and receives reflected sound off of various internal structures.

When the probed tissues produce similar images as the surrounding structures, they are referred to as isoechoic. When no echoes are reflected from the tissue, they are dark images and are called anechoic. When lots of echos are reflected, the structure appears white, this is called hyperechoic. There is a spectrum between black and white along the grey scale, which reflects the amount of echos a probe is receiving.

Ultrasonography frequency In the relationship to resolution and penetration:

When the US wave frequency is high, the spatial resolution is bigger, and the penetration depth of the tissue is smaller. Resolution is the distance apart two separate objects can be seen as distinct and not blended together.

[resolution graphic]

[resoloution vs pretitration graphic]

Frequency of the transducer is very important in facial ultrasound as probes that are not high frequency will not be able to resolve the small anatomical features of the face, and a penetration depth of beyond 3 cm is rarely needed when imaging the face.

[compare low MHz US image to high MHz image of same location]

Gain:

Gain is the white balance of the image. This changes the way the computer in the probe processes the data collected from the sound waves. It does not change the actual sound waves or echos received.

Turning up the gain makes darker areas appear more white across the entire image. Turning the gain down makes whiter areas appear more dark across the entire image. Turning up gain can create artifacts, and turning down gain can result in a loss of data acquired by the probe.

[image of high vs low gain]

Probe angle:

The angle of the probe relative to the anatomical structure of interest has a significant influence on how the waves penetrate and reflect off of tissues. The highest resolution is when the probe is held 90 degrees to the target structure. As the angle of the probe to the target moves away from 90 degrees, the resolution decreases. Therefore, whenever possible attempt to hold the probe at a right angle with the anatomical area of interest.