Ultrasound modes and exam presets

Today’s talk is going to go over some of the modes and settings you might find on your ultrasound machine. As you can imagine there are multiple ways to use an ultrasound machine. If you’re talking about sending out a sound wave and listening for the echo, you can probably imagine how this could be displayed in different ways. But not only can you change the way you use this technology, you can use it for different reasons as well. Because of this, most modern ultrasound machines have more than one way to interpret the data that has been received and more than one way to display it. These are known as ultrasound modes and exam presets respectively.

Modes

Ultrasound modes consist of mostly of letters and doppler types. Some of them aren’t used very often but will be explained here just for the sake of thoroughness. These are often accessed by a button on the machine.

There are often buttons readily accessible to change modes, exam presets on the other hand are often behind a menu.
There are often buttons readily accessible to change modes, exam presets on the other hand are often behind a menu.

A – mode   Back in the days before computers that could interpret the sound and location data received from ultrasound were plentiful, the echo’s were displayed on oscilloscope screens as a histogram. These displays could show the intensity and depth (pretty accurately in fact) of objects but was a far cry from anything we would call “imaging” by todays standards. A-mode is actually still used in medicine by ophthalmology for very accurate intraocular measurements, but that’s probably the only place you will see it used. In case you were wondering, the A stands for amplitude.

A mode ultrasound image. From http://www.ob-ultrasound.net
A mode ultrasound image. From http://www.ob-ultrasound.net

M – mode    Which stands for motion mode. This is basically an A – mode image viewed from the top and repeated over time. This allows for accurate measurements and visualization of moving objects. In emergency medicine we use this for measuring fetal heart rate and for identifying a pneumothorax among other things. In other branches of medicine it can often be used to visualize valves and calculate EF. Very useful mode as well if you want to print a still picture of a moving process.

M-Mode ultrasound w/ B-mode view at top. Classic
M-Mode ultrasound w/ B-mode view at top. Classic “Waves on a beach” presentation of a normal lung exam. Note the line in the B-mode, this is what is repeated across the bottom like an EKG.

B – mode (2D mode)    This is the most common ultrasound mode, whether you know it or not, this is what you think of when you think ultrasound. B-mode stands for brightness mode, I’m not sure why but it does and I didn’t pick it. B-mode is created basically by sweeping an A-mode image from side to side and then viewing them from the top. Things that reflect a stronger echo show up as brighter (more amplitude) and vice versa.

B-Mode ultrasound. Morrison's pouch view. A common view in FAST and RUSH exams to check for intra abdominal (peritoneal) free fluid.
B-Mode ultrasound. Morrison’s pouch view. A common view in FAST and RUSH exams to check for intra abdominal (peritoneal) free fluid.

Doppler

Just like the weather radar, doppler is used to see things that are moving. Most commonly we see something called duplex doppler where a doppler image is overlaid on top of a B-mode image for better location reference. This is very helpful but causes an issue with doppler imaging, slow frame rate. There are several types of doppler discussed below.

Color doppler    The most common type of doppler. An image is generated showing the rough direction and velocity of motion, most often blood flow. This is what we see with red and blue blobs on the image. The problem here is when you try and use this to identify a vein vs an artery. It’s important to remember that the colors are generated by the direction of flow. So if it’s coming towards the probe it’s red, therefore arteries will only be red if the probe is tilted so that the probe surface is facing proximally. The second problem is if you get the probe surface 90° to the flow, then you won’t see any flow as none of it is coming towards or away from the probe, it’s all moving across the probe.

Color doppler overlaid on B-mode of the heart.
Color doppler overlaid on B-mode of the heart.

Power doppler    A slight variation on color doppler. Rather than displaying color based upon velocity and direction, power doppler displays the intensity of the doppler signal as a variation in color.

Power Doppler. Courtesy Wikipedia
Power Doppler. Courtesy Wikipedia

Continuous wave doppler    In this mode part of the probe is transmitting while part is receiving continuously. You can focus the measurement to just one line and it is measured very accurately even to high flow velocities. The pitfall with this mode is that the flow is summated along the entire line being measured. So you have no idea if the flow is being measured deep or shallow. If you have two arteries that fall along the line being measured, it would be impossible to know the flow of each artery individually. This mode is often used for measuring heart defects and jets.

Continuous wave doppler
Continuous wave doppler

Pulsed wave doppler    A variation of continuous wave doppler that gives up its ability to work at high velocities for the ability to focus along the imaging plane. PWD can differentiate the blood flow in a shallow vs a deep artery along the same plane. By timing it’s transmission and listening, the machine is able to create an ultrasound “window” at a specific depth. This is quite useful and was what we used for transcranial doppler measurement of MCA strokes.

Pulsed wave doppler. Transcranial view of the MCA.
Pulsed wave doppler. Transcranial view of the MCA.

Exam Presets

Presets are different than modes in that they don’t drastically alter the way the system behaves as much as the way the information is displayed. Presets are specific to the manufacturer of the machine and can be custom built in some systems. These are often accessed through a menu.

Abdominal exam  This is probably where the machine will spend most of its time. This is what we often use for the FAST exam. The presets like this will optimize the way the image is displayed for the best picture of abdominal organs.

Cardiac exam   This setting is optimized for imaging moving things. One of the biggest changes however is often the orientation marker flips to the top right of the screen as this is convention for echocardiograms but weird for most ED docs. Just be aware of this. The cardiac exam also opens up a bunch of cardiac calculators and measurements if needed.

OB/Gyn exam   This setting is similar to the abdominal setting except that the calculators and measurements are set up for OB uses.

MSK (musculoskeletal)   Optimized for soft tissue imaging.

TCD (trans-cranial doppler)    This is the highest power setting (TI) in most machines to be able to get through the cranium. This is used for, as the name implies, transcranial doppler imaging.

Other technology

Tissue Harmonic Imaging (THI)    This transmits at one frequency and receives at double that frequency. This reduces the background noise significantly and does so with very little downside. This option should probably stay turned on all the time.

THI button on a Sonosite MMX to toggle the feature.
THI button on a Sonosite MMX to toggle the feature.

Why does this matter? For one you need to know how to get the most out of your machine by utilizing all the different modes as needed. The other though is that most ultrasound manufacturers charge extra money for both modes and presets. Even if the machine has that capability, you must pay money and enter in a serial number to turn it on. The exception is demo machines which have everything turned on. So if you’re demoing a machine be sure to keep track of what you use and don’t use so that you can have a good idea of what to pay for when you by machines. Some of these packages can be in the range of $5,000 for a cardiac calculation package that you may never use in an ambulance. So learn how to use these different modes to your best advantage and if you’re demoing a machine keep in mind that you have to pay for them when you do go get your real machine so make sure you know what you want.


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