POCUS Basics

What you absolutely must know before doing your first scan.

How to Hold an Ultrasound Transducer

First time holding and ultrasound probe? Here is some help to make you look like you have done it before!

Probe and Image Orientation

Let's put the probe on the patient and figure out how image orientation works.

Depth Optimization

How to set depth correctly to get the best view possible, and avoid imaging artifacts.

Gain

Gain is a setting you can adjust to effect the "brightness" of the image. Adjusting gain does not affect how much energy the probe is outputting, but once the echos return the probe the ultrasound machine amplifies what it "hears" to boost the brightness of the on screen image.

For each image gain should be adjusted to get the best image possible. Within any image there usually should some amount of fluid within it. Gain should be adjusted so that fluid appears as black. Under gained images will appear to dark, and it will be difficult to make out structures within the image. Over-gained images will be too bright, washing out any detail within the image.

Low Gain

High Gain

Balanced Gain

Time Gain Compensation (TGC)

There are two general ways to adjust the gain of an image on an ultrasound machine. On the machine you will find a "Gain" knob or slider that adjusts the gain of the whole image. In addition to this you will also find sliders to adjust the gain a specific depths within the image.

We get the term Time Gain Compensation (TGC) from the machine's use of the Distance = Rate x Time equation. Depth is determined by the amount of time it took for the echo to return probe. The ultrasound machine is able to target the specific time period an echo returned to the probe, and amplify that echo, making the image at that depth brighter or darker as need to improve image quality.

Imaging Modes

Brightness Mode (B-Mode): This is also known as a Sector Scan, and is the imaging mode that people are most familiar with. In this mode a 2-dimensional image is created in grey scale, by the ultrasound machine sending out a pulse of sound, and waiting for any echoes returning to the probe.

The machine uses the Distance = Rate x Time to determine the distance a structure is away from the probe. For Rate the machine uses the average speed of sound of a sound wave through soft tissue (1540m/s).

The "brightness" of the image is determined by how "loud" a returning echo is. How loud an echo is a function of its amplitude which is related to a tissue's echogenicity. The highest amplitude echo returns are plotted as white, where the weakest low amplitude echo returns are plotted as black or dark grey. See the POCUS Physics section that goes more in depth on the nuts and bolts on echogenicity.

M-Mode: Also known as Time-Motion mode. As the name suggests, this mode is used to track motion over time. The graph that produced shows Time on the X-Axis, and depth on the Y-Axis.

The first step to obtaining an M-Mode scan, is to get a B-Mode image of a structure. Next you will place the M-Mode spike over the part of the structure you want to detect motion. The ultrasound machine will focus only on the structures that fall on the line produced by the M-Mode spike.

Structures that are motionless will appear as horizontal lines, where structures have some component of vertical motion will have a wave like appearance. If there is horizontal motion there will be a broken, irregular line appearance in that part of the M-Mode tracing. Three common uses of M-Mode is to assess for Lung Sliding, E-Point Septal Separation, and calculating Fetal Heart Rate.

E-Point Septal Separation

Fetal Heart Rate

Lung Slide (Seashore Sign)

Doppler Effect

The next few imaging modes take advantage of the Doppler Effect so it is worth talking about. The frequency of sound changes as objects moves towards or away from each other. The classic example is ambulance moves towards, then away from an observer. You will hear a higher pitch sound as the ambulance moves towards the observer, and a lower tone as the ambulance moves pass and away from the observer.

The frequency of the siren of the ambulance is fixed and does not change. However, as the ambulance approaches, from the perspective of the observer, the sound waves stack on top of each other being compressed artificially creating a higher frequency sound to the observer. As the source of the sound leaves the sound waves are created further apart from each other pulling them apart, creating a lower frequency sound.

In Ultrasound the frequency produced by the probe is constant. However, the frequency of the returning echo will change depending on whether an object, often a blood cell, is either moving towards or away from the probe.

Color Doppler: is the doppler mode that we are probably most familiar with. This mode shows flow either towards or away from the probe. By convention flow Towards the probe is plotted as Red, and flow away from the probe is plotted as Blue. This can be remembered with the acronym BART: Blue Away Red Toward.

We can make a qualitative assessment of the speed of flow by looking at the brightness of the color as it is represented by the ultrasound machine. The brightest color represents the greatest amount of flow, and darker colors representing lower flow states. When there is flow perpendicular, (90*) to the probe there is no net flow either towards or away from the probe, and these areas will be plotted as black.

Power Doppler: Is another form doppler that is able to detect flow and movement. It differs from Color Doppler, is that it does not detect direction of flow. Its advantage is that higher sensitivity being able to detect lower flow states. The disadvantage of this increased sensitivity is the increased likelihood of Flash Artifact which is erroneous plotting motion. This is often caused by small movements of the probe by the user. Common uses of Power Doppler is an augment to detect lung sliding, called Power Lung Slide, or to look for carotid pulse in hypotensive patients.

Carotid Pulse

Power Lung Slide

Pulse-Wave Doppler: Using color doppler we as able to make qualitative assessment of flow velocity and direction based on how bright the color the color is. With Pulse doppler we are able to make distinct quantitative measurements of flow velocity and direction. This mode requires the user to place a sampling box over a region of interest. The ultrasound machine ignores everything outside this sampling area, so you are able get flow velocity from a specific location. Common uses are to measure cardiac output, or to evaluate the degree of regurgitation or stenosis of heart valves and vascular structures.

Carotid Pulse Wave Doppler

Carotid Pulse Wave Doppler

PSV: Peak Systolic Velocity

EDV: End Diastolic Velocity