Ultrasound physics, artifacts and knobs

Learning objectives

Define what POCUS is
Describe the basic physical principles by which ultrasound images are acquired
Operate the ultrasound machine, identify its controls and show proper transducer selection, technique and manipulation
Understand how ultrasound artifacts are produced and how to interpret them

Key points

"Ultrasound" (US) refers to sound waves at a frequency above the normal human audible range (>20 kHz). Frequencies used in ultrasonography range from 2 to 18 MHz
Understanding basic ultrasound physics and artifacts is essential to acquire and interpret images accurately
Higher-frequency transducers (linear array probes) produce higher-resolution images but penetrate shallower
Lower-frequency transducers (phased array or curvilinear probes) produce lower-resolution images but penetrate deeper
Basic modes of ultrasound include two-dimensional ("B-mode"), M-mode, and Doppler ("color")

Point-of-care ultrasonography (POCUS) is defined as ultrasonography brought to the patient's bedside and performed by the clinician in real time.

US penetrates well through fluid and solid organs (e.g., liver, spleen); it does not penetrate well through bone or air, limiting its usefulness in the skull and areas of the abdomen where bowel gas obscures the image.

Fluid (e.g, blood, urine, ascites, pleural effusion) is completely anechoic and appears black. This makes US particularly useful for detecting fluid in areas where it's not supposed to be and for differentiating cystic or vascular areas from solid structures.

A brief history of the evolution of transducers:

Early transducers (i.e. probes) used a single crystal to create a one-dimensional image known as A-mode. The standard images that machines now generate is known as B-mode (also called two-dimensional (2D) or gray-scale ultrasonography) and is created by an array of crystals across the face of the transducer.

These “crystals” (made out of quartz or other piezoelectric material) generate sound waves when an electric current is applied and is what conventional transducers still use. Some newer transducers are using "ultrasound-on-chip" technologies that allow for more portability and can emulate different transducer types in a single probe (i.e linear and phased array).

The principle remains the same as the probes are able to transmit and receive sound. When the sound wave returns, the material in turn generates a current that is then converted to an image.

Transducer selection

The proper technique for holding the probe is known as the pencil hold and except for a few cases (like when doing the subxiphoid view) this how the probe should always be held .

Rest the palm on the patient's body and make small and controlled adjustments.

Knobology

It is important that you familiarize yourself with the controls beforehand. Below is an example guide for the Sonosite. Most machines have similar and equivalent functions.
There are many different brands/models of ultrasound machines, some of the most common at MSM/MSW are: Sonosite, Mindray and Sparq. Below is a sample picture from a Sonosite (Taken from the http://www.slredultrasound.com/ website).

A: Patient: Start every study by pressing this button. This is where you will type the patient’s MRN in the “ID” box.

B: Select: Use it like the clicker on a laptop trackpad.

C: Trackpad: Like that of a laptop. Moves the cursor.

D: Exam: Optimizes the frame rate and grayscale for different applications; e.g. the phased array probe has settings here for cardiac, abdominal, or obstetrical studies. E Freeze: Freezes whatever is on screen; you can swipe the trackpad to rewind or advance.

F: Save: Saves a still image of whatever is on screen.

G: Clip: Saves a short video clip (default, 6 seconds) of whatever is on screen. When it “beeps,” the clip has stopped recording.

H: Depth: Increasing the depth will bring deeper structures into view; decreasing it will do the opposite. Use this to bring whole structures of interest into the screen area (pericardial sac), or specifically focus on a superficial structure (peripheral vein).

I: Gain: Increases/decreases the overall brightness of the picture. “Near” and “Far” adjust the gain for the superficial and deep field of view, respectively.

J: M-mode: Motion mode. Press once to generate a line or “icepick” over the structure of interest. Press a second time to generate the M-mode image.

K: Calc: Used to calculate fetal heart rate.

L: Caliper: Used to measure the distance between 2 points. The trackpad moves each caliper; pressing select switches which caliper you can move.

M: Color: Generates a color doppler field, which can be used to observe blood flow and direction.

N: Soft keys You will see functions along the bottom of the screen in line with these top buttons. To choose one, press the corresponding button. The ones you should know for now are:

*Guide: Generates a guide marker line to assist with procedures such as an IV or central line. Mainly used with the linear probe. Turn it off for any nonprocedural applications (such as the FAST exam).

*New/End: Press this every time you BEGIN and END a study. Before you begin putting in patient info, press this button. Then, after you’re done with the study, go BACK to the patient window and press this button again.

O: Text: Press this button and start typing on the keyboard to generate text. You should use this to label or add a description of the image you are capturing and the orientation, e.g. “R KID LONG” or “GB TRANSVERSE”

P: 2D: This is like an “escape” key. Pressing this will always bring you back to the probe’s basic view.

Ultrasound physics, artifacts and knobs

Transducer selection

Knobology

Ultrasound basics and ultrasound artifacts videos:

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