Teaching Medicine - Tutorial: Ultrasound Physics without Physics

Ultrasound

Level 1

Tutorial: Ultrasound Physics without Physics

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Tutorial: Ultrasound Physics without Physics

This module will explain how ultrasound works in simple terms.

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Tutorial: Ultrasound Physics without Physics The knobs

Lessons

Times Practiced

1284

Cases Completed

1h 24m

Total Time spent

1m 24s

Average Time

Progress

Accuracy

Efficiency

Accuracy

Efficiency

	1	Tissue Properties Tissue Properties
	2	Pick Your Probe Pick Your Probe
	3	The knobs The knobs
	4	Artifacts Artifacts

The knobs

There are many knobs and buttons on the ultrasound machine. Every machine is a little bit different. This lesson will describe the function of the most common knobs and buttons.

Gain
In ultrasound, gain = sound volume, plain and simple. Think of a stereo where the volume is too loud. The music gets distorted and you can't really hear the music very well. If the volume is too low, you can't hear it. Too loud and too quiet are both not very good.

Let's put this into "ultrasound" perspective. When the ultrasound probe hears it's own echo (the reflected sound), it will produce a white image on the screen. If it cannot hear anything, the screen is all black. If the volume of the echo is too loud, then the screen is too white and the features gets distorted; the greys and blacks get turned into white and everything is white. Another easy way to think of gain is brightness.

So quick summary:

if gain is too high, the image is too white (over gain)
if gain is too low, the image is too black (under gain)
best gain is set to show white, grey and black images clearly

Gain is appropriate

Gain is too high (too white)

Gain is too low (too dark)

You have nothing to gain by turning the gain up too far!

Some machines have a few additional settings for gain:

Near gain knob will adjust only the top half (the half that is nearest the probe)
Far gain adjusts the bottom half (that is furthest from the probe)

Near and far gain are useful because sometimes the deeper (far) parts of the image are darker because of attenuation, so being able to brighten up only the deep parts of the image is helpful.

Here are some terms that are important about how black or how white the image is on the screen:

echogenic means a structure will show up white on ultrasound
anechoic means that the structure will be totally black (like liquid)
hyperechoic means the structure will be extra white (like bone)
hypoechoic means the structure will show up, but be more black

Depth
Depth defines how many centimeters (cm.) deep the image is. There is a depth indicator on the side of the screen so you can see exactly how many cm. deep a structure is. The following images are intended to visualize the artery (red arrow) and vein (blue arrow). The first image shows optimal depth: the entire structure(s) of interest are 100% in view and there is not excess depth below the structures.

Depth is appropriate

Depth too deep (images too small)

The vessels are quite small on the screen. If the depth was reduced, the vessels would appear larger on the screen.

Too shallow (vessels not completely seen)

The artery and vein are both partly cut off by the bottom of the screen.

There are other buttons on the ultrasound machine that you can probably ignore for now if this is your very first time playing with the machine. However, so that you don't feel like we are leaving you out, here is a quick description of some of these other buttons and knobs:

Time-Gain Compensation: this is another form of gain. It is like the near and far gain we already told you about but instead of dividing the screen into 2 sections, it is divided into multiple sections and you can control each section. You probably will never need to adjust the screen in this much detail, but it can be helpful if a monkey was playing with the ultrasound machine before you used it and all the TGC buttons are pushed all the way to the left (too black) or right (too white), you can be smart enough to put them back in the middle.

All in the middle. Looks good!
TGC settings

TGC on far left. Image too black.
TCG all to the left

TGC on far right. Image too white.
TCG all to the right

Remember we talked about attenuation? This is loss of energy of your ultrasound signal as it penetrates into deeper tissue. If you are finding that your far field image is too black, then you could adjust lower TGC buttons in the following way to make the deeper structures more white:
TCG higher for deeper levels

Here is an example where someone came along and moved one of the TGC settings to the right, creating a white band. Can you see it?

Focus: do you remember in the last lesson we told you about axial resolution? If you forgot, axial resolution differentiates 2 structures that are 2 different distances away from the probe. In contrast, lateral resolution differentiates 2 images that are the same distance from the probe. The focus feature helps to optimize lateral resolution. However, there is a limitation: you can only set the focus at a single depth. You cannot "focus" on the entire image (otherwise we would just do that all the time and not build in a knob for it).

Reminder that axial resolution depends on frequency (previous lesson), but lateral resolution depends on the focus function.