Fundamentals

Objectives

To reach the objectives for this module read the following text, watch the videos and finish the QUIZ.

• Understanding the basic physics of an ultrasound machine and how the image is acquired
• Being able to explain and interpret common artifacts
• Knowing how to use the basic configurations of the ultrasound machine
• Understanding and knowing how to use color doppler
• Understanding the difference between the most common probe types and how to switch between them
• Knowing how to take care of the ultrasound machine

1. Introduction: Asking Questions

Emergency bedside ultrasound is all about asking focused questions. Is there free fluid in the abdomen post trauma? Or is there pericardial fluid present in the patient post heart-surgery? As we become more proficient practitioners of bedside ultrasound we’ll be able to answer more complex questions such as: does the pericardial fluid cause right ventricular strain and an imminent tamponade?

However, when learning bedside ultrasound, especially in the beginning, the aim is to practice answering simple and focused questions. We can rule in free fluids in the thorax and/or abdomen but never rule it out with bedside ultrasounds. While there now is some diagnosis, such as pneumothorax *, where the consensus among emergency physicians (mainly in the US) at present is that it can be ruled out using bedside ultrasound, the aim for this course is to adhere to the principle of ruling in, and never ruling out, when it comes to bedside ultrasound.

But before we can get to the clinical questions follows an introduction of what ultrasound is, how to interpret the image acquired, how to optimize the image, and how to maintain the machine.

2. Anatomy of a Medical Ultrasound Machine

– Somewhere on the machine there are also buttons – either on the touchscreen, physical, or both, or in the case of handheld devices with built in processors that lack screens and connect (either via wire or bluetooth) to your phone or tablet: in the software & the connected devices touchscreen.
[Image credit: pocusbasics.org] 

Most standard ultrasound machines consist of four crucial parts. The probe (also called the transducer), a wire that connects the probe to the machine, a processor that processes the input from the probe, and a screen that displays the generated image. Somewhere on the machine you’ll also find an on/off button and depending on model either actual physical buttons for image management, or touch-screen buttons, or a combination of both.

Newer portable devices can operate with just a wire, or in some cases without a wire via bluetooth, using your phone or other mobile screen of choice as your operating screen. In other words, the minimal requirement for an ultrasound set-up is an ultrasound probe, a screen, and software.

Handling & Care

Be careful when handling the probe, making sure you don’t drop them since the probe contains sensitive, and expensive, material that could damage the quality of the image.

The probes are the most expensive part, however the probe wire comes in a close second as these contain hundreds of sensitive connections carrying the signal from the probe to the machine. Make sure to never run over or damage the wires in any way.

When cleaning the probes after an exam always use a wet cloth with surface disinfectant. Hand disinfectant often contains moisturiser that is harmful for the probes sensitive surface, therefore we only use surface disinfectant (Yt-des). Also avoid first wiping the ultrasound gel off the probes with dry cloth as the surface of dry cloth is too rugged, instead use cloth that is wet with surface disinfectant, even though it might take several wipes.

3. How Medical Ultrasound Works

Video on Ultrasound Physics & Knobology (42 min):

Physics Live November 2012 from Harvard EUS Course from Mike Stone on Vimeo.

[Video credit: Mike Stone. 2012.*]

An electric signal is applied through the cord that connects the ultrasound machine with the probe. Most modern ultrasound probes contain piezo-electric crystals that vibrate when the electric signal is applied. The vibration of the piezo-electric crystals creates a pressure wave, i.e. ultrasound, the ultrasound waves interact with soft tissue and return as echos. These echos are registered by the piezo-electrical crystals and turned into returning electric signals that the computer converts to points of brightness on the screen and: an ultrasound image is acquired. The vibration of the piezo-electric signals sending out pressure waves and receiving echos happens approximately a thousand times a second.

4. Understanding the Physics of How the Image Produced

In order to understand how the image you see on the screen is acquired, it’s also necessary to understand the basic physics of sound waves.

Properties of Sound Waves

A sound wave can be broken down to amplitude, velocity, frequency, and wavelength. Amplitude is the peak of the wave, velocity is its speed, frequency is the number of times the wave is repeated per unit time, and wavelength is the distance the wave travels in a single cycle. Wavelength is inversely related to frequency, since velocity must remain constant in a given medium (velocity = frequency x wavelength). 

Amplitude and Echogenicity

Ultrasound machines measure the intensity (amplitude) of the returning echoes and translate it to different color gradients on the black and white spectrum, strong echos become bright/white (hyperechoic) and weak returning echoes become dark/black (anechoic). When interacting with tissue, bone and air become hyperechoic while fluids become anechoic, and soft tissue ends up somewhere in between displayed as shades of grey.

Velocity and Depth

The velocity, or speed, of sound in air (at 20C) is 343 m/s while it’s 1431 m/s in water and 1540 m/s in soft tissue. By measuring the time it takes for echos to return back to the probe the machine can calculate the distance/depth of a structure.  

Frequency, Wavelength and Resolution 

Ultrasound are soundwaves with frequencies higher than the upper audible limit of human hearing (approximately 20 kHz). Medical ultrasound probes typically range in the frequencies of 2 to 12 mHz, allowing for resolution of small internal details in structures and tissues due to the shorter wavelength. Higher frequency probes decreases wavelength which results in higher resolution but less penetration, and vice versa when it comes to lower frequency probes. 

5. Probes & Movement

Probes

The three most common probes used by the emergency and intensive care physician are the phased array (heart), curvilinear (abdominal), and linear (vascular/musculoskeletal) probes. The phased array and curvilinear probes are short frequency probes that have higher depth but also less resolution, while the linear probe is a high frequency probe with higher resolution but less depth than the previous two mentioned.

Movement

[Image credit: Bahner et al. ‘Language of Transducer Manipulation’. 2016.*]

There is a nomenclature for probe movements that helps communication when teaching and learning bedside ultrasound (see image above). One of the terms that is less self-explanatory and that we will use a lot during this curse is “fanning” (top left in the image above). Fanning, sometimes also called tilting, or perhaps “solfjädra”/”fläkta” in Swedish, is when you hold the probe still to the surface and gently tilt/fan it towards either side on the x-axis.

6. Modes & Protocols

B-mode (2D mode)

The B-mode is the most common ultrasound setting and produces a two-dimensional (2D) image cross section of the area that is examined. In most machines there’s a B-mode or 2D mode button that you can use to return the image to its basic settings. In some machines the B-mode button is also a wheel-button that regulates gain when turned in either direction.

M-mode

M-mode creates a single beam in the ultrasound scan that produces a picture with a motion signal. The moving structure of interest is then (often) depicted in a wavelike matter.

Protocols

Most modern ultrasound machines have different preset protocols depending on what kind of exam you want to perform. Protocols like: lung, abdomen, and cardiac can help optimize the image for the desired exam. However, when for example scanning the lung a protocol or machine setting that is too optimized to remove artifacts can make the evaluation more difficult as many of the significant findings in lung ultrasound are artifacts.

7. Artifacts

Video on Ultrasound Artifacts (11 min):

[Credit: Sachita Shah et al. University of Washington School of Medicine. 2017. *]

Acustic Shadowing

Occurs when the waves encounter a structure that strongly absorbs or reflects the waves, creating a shadow posterior in the image to that structure. This happens most notably with structures that are dense like bone or stones, such as ribs and gallstones *.

Reverberation

Occurs when sound waves encounter two strong parallel reflectors, causing the ultrasound beam to reflect back and forth between the reflectors (“reverberates”) and interprets the returning sound waves as deeper structures since they’ve taken longer to return. A-lines and B-lines are examples of reverberating artifacts *.

Mirroring

When sound waves encounter a highly reflective structure (such as the diaphragm) and bounces back to the original structure, at an altered angle, before returning to the probe it can cause the screen to create a mirror image of that structure on the screen. This can happen with booth the spleen and liver in the RUQ/LUQ interface with the diaphragm, creating a mirroring image of the organs on the other side of the diaphragm *.

Refraction

Occurs when sound waves encounter an interface at a non-perpendicular angle, causing the image to be displayed at an incorrect location *.

Edge Shadowing artifact

When sound waves from the probe encounter a cystic wall, like the gallbladder, or a curved surface at a tangential angle the waves are scattered and refracted, leading to energy loss and the formation of a shadow at the edges *.

8. Doppler

When using color doppler, the image superimposes a color-coded map onto a B-mode ultrasound, resulting in vascular structures portraying the color red or blue or a mixture of the two. A mnemonic for color doppler is BART: Blue Away Red Towards. When the probe is angled towards the blood flow of a vascular structure the color red will appear, and when it is angled in the same direction of the flow the color will be blue indicating a blood flow away from the probe.

9. Important buttons

Except for the on/off switch, and learning how to change probes and protocols,  there’s just three other buttons you will be working in most of your exams at the beginning.

Gain

Gain increases the brightness of the image but it does not improve resolution. Too much gain and there will be a lot of noise making the image more difficult to asses, too little gain and the image will be too dark. A rule of thumb is to turn up the gain no more than making vascular structures, like the inside of the ventricles of the heart, anechoic/black.

Depth

Self-explanatory button, as a rule of thumb the image of interest should be in the center of image on screen as the ultrasound machines algorithm is often created to optimize that part of the image.

Freeze & Save/Measure

Freezing images or videoclips allows for measuring when needed, it also helps to review the images later on for help with assessment from more experienced colleagues. It is a great way to help beginners improve as they can ask for feedback after the exam.

10. Heating & Mechanical Effects

While not producing any ionised radiation and generally considered less harmful than CT:s, ultrasound waves do generate heat and higher frequencies generate more heat than lower frequencies.

A modern ultrasound machine displays two standard indices: thermal and mechanical. The thermal index (TI) is defined as the transducer acoustic output power divided by the estimated power required to raise tissue temperature by 1°C. The mechanical index (MI) is equal to the peak rarefactional pressure divided by the square root of the center frequency of the pulse bandwidth. TI and MI indicate the relative likelihood of thermal and mechanical hazard in vivo, respectively. Either TI or MI greater than 1.0 is hazardous.

In general ultrasound imaging is considered safe, however caution is advised when examining neonatal patients as they (and in particular because of developing bones as bones absorb more ultrasound waves than soft tissue) can be more susceptible to the increase of thermal heat. Caution is also advised when it comes to examining sensitive structures like the eyes.

More still/stationary ultrasound modes like doppler and M-mode require higher frequencies and therefor produce more heating. In The Use of Ultrasound in Medical Diagnosis from 2010 the authors concluded that: “Simple grey scale B-mode imaging is not capable of producing harmful temperature increases in tissue.” *

11. Summary

Bedside ultrasound is based on asking focused clinical questions not making detailed examinations of organ parenchyma. For this course we will be using ultrasounds machines that come equipped with three kinds of probes: the high frequency linear probe (MSK/lungs) with high resolution but less depth, and the lower frequency curvilinear (abdomen) and phased array (cardiac) probes with less resolution but better depth.

The ultrasound waves interact with tissue as they propagate back and fourth, producing a greyscale image in B-mode that helps us differentiate between the different structures. Artifacts are often produced due to these interactions and can, in many cases, be of clinical importance and help us in assessing our focused question(s).

Ultrasound produces thermal and mechanical effects, and should be used with more cation in neonatal/younger patients and in cases with more sensitive structures like the eye.

12. Quiz