November 2023

By Dr. Christopher Kennedy, DACVECC, DECVECC


In this VETgirl online veterinary continuing education blog, Dr. Christopher Kennedy, DACVECC, DECVECC discusses ultrasound knobs – the science behind all those buttons and dials on the ultrasound machine, otherwise known as “knobology.” Knobology is super fun and will help you optimize the image you see on the ultrasound screen. Knowing which buttons do what can help you go from a Focused Cardiac Ultrasound (FCU) – sometimes called cardiac point of care ultrasound (cardiac POCUS) – rookie to an FCU elite literally with the touch of a button! Read on if you need a refresher on what all of those knobs do!

When performing an FCU exam, remember to start with a right parasternal long axis 4-chamber view (PLAX4). This view is obtained by placing the probe on the right thoracic wall over the apex beat and angling the probe such that the ultrasound beam longitudinally dissects the heart. If the orientation marker on the screen is on the right side, then the orientation marker on the probe should be angled cranio-dorsally to obtain this view. You might need to adjust the angle and/or slightly rotate the ultrasound probe to appropriately view all four cardiac chambers. See our previous blog called The Focused Cardiac Ultrasound (FCU) exam for more information HERE.

By Dr. Christopher Kennedy, DACVECC, DECVECC


The relevant knobs are labeled on the ultrasound in the image below (See Figure 1). You may need to zoom in to see the words written by each knob. NOTE: The location of each exact knob will vary based on brand of ultrasound.

An image of the Ultrasound keyboard with all knobs and buttons labeled with a number that corresponds to the blog.

Figure 1. Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC

Major Buttons (i.e., The important buttons!)

Depth (knob 1): Ultrasound waves penetrate the body. Depth lets you select how deep you want to see. For Focused Cardiac Ultrasound, the heart should fill the screen. A common mistake is not reducing your depth.

Video 1: Parasternal long axis view (PLAX4) with the depth set too deep.

I often scan too shallow, which means I might miss things. To avoid this, the pericardium should be visible in the far field.

Video 2: Parasternal long axis view (PLAX4) with the depth set too shallow.

Sector size/width (knob not shown): This is the width of the image displayed on the screen. Some probes allow width adjustment. For Focused Cardiac Ultrasound, we tend to keep the width wide.

Video 3: Parasternal long axis view (PLAX4) sector too narrow:

Gain (knob 2): By increasing the gain the machine amplifies reflected echoes detected by the transducer. This provides a brighter image (See video 4 below). This can be helpful in a bright room, like an Emergency Room. Over-gaining (over-amplification) leads to noise (like a microphone turned up too loud). Over-gaining results is an overly bright image, with low contrast and low spatial resolution (low ability to differentiate between structures in proximity). Some machines have an “auto” button, which quickly sets a usually appropriate level of gain.

Video 4: Parasternal long axis view (PLAX4) over-gained.

Video 5: Parasternal long axis view (PLAX4) under-gained.

Pro tip: Rather than cranking up the gain, try turning down the room lights.

Time Gain Compensation (knob 3):  These nifty dials allow you to adjust the gain at specific depths. This is useful to temper over-gaining in the near field and improve under-gaining in the far field (See Figure 2a and 2b below).

Figure 2a: Extreme examples of over-gained near field with under-gained fair field; Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC

Figure 2b: extreme examples of under-gained near field with over-gained fair field; Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC

Pro tip: Time Gain Compensation does not accommodate for poor image acquisition – did you shave the hair, wet the skin with alcohol, use coupling gel and align the cardiac structures appropriately in your sector?

Focus/Focal zone(s) (knob 4): This selects a depth to focus the ultrasound waves, improving lateral resolution (i.e., the ability to see two things lying side-by-side at the same depth). For Focused Cardiac Ultrasound, we usually use one focal zone set at edge of the heart in the far field (See video 6).

Video 6: PLAX 4 with the focal zone set in an appropriate location (the blue arrowhead on the right of the sector).

Capture and measure
Freeze (knob 5):  This button freezes the image. You can scroll backward to review the image.

Image capture (knob 6): Saves a still image.

Cine loop capture (knob 7): This allows you to record a cine loop of several seconds. Typically, you press freeze then the designated cine loop button for a retrospective cine loop and simply just press the cine loop button if you want to capture prospectively. Prospectively means the machine records real-time what is displayed on the screen, so you need to ensure you maintain image quality and alignment.

Callipers: Digital callipers allow you to make measurements (See Figure 3).

Figure 3. Digital calliper measurement of the left atrial diameter, minor axis in a dog; Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC

B-mode & M-mode
B-mode (knob 2): Brightness or 2d, the default mode. Pressing this button clears advanced settings and returns you to basic B-mode. Try pressing it if you get lost in the advanced settings (like when your cardiologist pops in and activates the Doppler). On the image below, B-mode and gain are the same knob: you push for B-mode and turn for gain.

M-mode (knob 8):  (Time-)Motion or sometimes “1d” (See Figure 4). Set your cursor through a specific structure and activate this mode: A cool display graphs movement across this line against time (on the x-axis). M-mode provides excellent resolution.

Figure 4: PLAX4 with M-mode activated; Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC

Other buttons
Sweep speed (knob not shown): This is how many seconds are shown on the x-axis (See Figures 5). In M-mode, you can stretch the image (fewer seconds on the x-axis) or compress the image (more seconds on the x-axis). Stretching can be useful to make measurements.

Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC

Figures 5: M-mode of the PSAX-pap view in the same dog recorded using two different sweep speeds; Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC

Probe select (knob 9): This allows you to change probes.

Probe frequency (knob 10): Some probes allow you to change the frequency of ultrasounds emitted. Higher frequencies provide better resolution, but poor depth. For smaller animals, use higher frequencies; for big dogs you will need lower frequencies.

Flip the screen (knob not shown): This flips the side of the display that the marker sits on (video 8) This is good to know, as cardiologists and radiologists scan in opposite directions!

Video 7: the image display has been flipped. Note the orientation marker is on the left now. You can achieve the same result by simply rotating the probe 180o.

Zoom/magnify (knob 11): This zooms in on a selected region (Video 8). Typically, we do not use it in Focused Cardiac Ultrasound – turn it off, as it can be confusing.

Video 8: the zoom has been activated.

Further reading:
1. Enriquez JL and Wu TS. An introduction to ultrasound equipment and knobology. Crit Care Clin. 2014 Jan;30(1):25-45, v. doi: 10.1016/j.ccc.2013.08.006.
2. Wiafe YA and Badu-Peprah. The Influence of Ultrasound Equipment Knobology in Abdominal Sonography. In: Essentials of Abdominal Ultrasound, Gamie SAA and Foda EM (ed.) [online] 2019. DOI: 10.5772/intechopen.83713
3. Dinh V. Ultrasound Machine Basics-Knobology, Probes, and Modes. Pocus 101 [online]

Only VETgirl members can leave comments. Sign In or Join VETgirl now!