KNOBOLOGY AND IMAGE OPTIMISATION

Each ultrasound machine will have a slightly different setup and configuration. However the functionality between most modern point of care ultrasound machines is relatively similar.

To use and acquire optimal images you need to learn ultrasound KNOBOLOGY. Effectively this extends from turning on the machines, choosing a transducer and preset, using functions to optimise your image (depth, gain, focus, TGC etc), freezing, saving stills/clips, and making measurements.

PHILIPS CX50 ULTRASOUND MACHINE

PHILIPS CX50 ULTRASOUND MACHINE

STARTING A NEW SCAN

TURN THE MACHINE ON

TURN THE MACHINE ON

scan details

When starting a new scan generally the first step should be to enter the scan details. Sometimes in an emergent situation these details may be entered subsequent to the POCUS scan being acquired.

To input these details chose the PATIENT (select new patient) tab on the ultrasound machine. The minimum information should include:

  • Patient first and last names

  • Patient NHI (ID)

  • Simple relevant clinical question (e.g. ? pericardial effusion)

  • User details/name

TRANSDUCER AND SCAN PRESET

Chose you required transducer for the scan you will perform. There will be presets for different scan types under each transducer e.g. Phased transducer - preset ED ECHO

  • These are pre-determined settings loaded into the machine for different scanning applications.

  • The presets are a baseline set at a certain frequency, depth, gain, and focus to start your scan

  • The scan can be further optimised by the user

Sonosite X-Porte Orientation

IMPROVING YOUR IMAGE AND KNOBOLOGY

TOP POCUS TIPS (CORE ULTRASOUND)

After completing the above steps you will find your image of interest using techniques discussed in PROBECRAFT. To improve your image and optimise the view you will need to manipulate your depth, gain functions, and focus.

DEPTH

Ensure your depth is adequate and the area of interest in your image is optimised. Generally starting with a slightly deeper field of view and then reducing the depth is a practical way to ensure you don’t miss anything of interest (e.g. pericardial effusion in cardiac Parasternal-Long-Axis view)

  • Reducing depth:

    • Improves image resolution

    • Makes the area of interest physically easier for you to visualise

    • Helps make any measurements more accurate

  • A general rule is that the area of interest should take up about 80% of your screen

GAIN and time gain compensation(tgc)

Gain effectively changes the brightness of the image. It is a pre-processing function amplifying the received transducer signals. It does not affect power output from the probe, as such there is no change in intensity of tissue exposure for the patient.

Depending on the particular machine the gain can be adjusted in different ways:

  • Overall GAIN

  • Near and Far-field GAIN (Sonosite X-Porte)

  • Time Gain Compensation (TGC)

Overall GAIN should be adjusted so that grey-scale image contrast resolution is optimal giving a smooth echotexture:

  • Under-gained the image will be too black

  • Over-gained the image will be too white

Excessive gain can result in loss of subtle differences in echo texture, and also increased artifactual echoes.

GAIN.png

TGC function changes the gain at varying depths in the field of view.

  • Typically this utilised to increase the receive amplification for weak echoes from depth, and reduce those from strong unwanted echoes that are usually from superficial structures or strong reflectors.

  • It can also be used to reduce posterior acoustic enhancement deep to fluid filled structures

Source: WINFOCUS

Source: WINFOCUS

FOCUS

The focus is where the ultrasound beam is at its narrowest and the best spatial resolution will occur:

  • Some machines have auto focus whilst others will allow the user to manipulate the focal depth. The focus is usually indicated by an arrow or triangle marker on the left of the screen.

  • The focus should be placed at the depth of interest in the image, or just deep to it:

    • spatial resolution will be maximised

    • improved image clarity

    • more accurate caliper measurements

If the focus is too superficial then rapid divergence of the ultrasound beam will occur beyond the focal zone, resulting in poor image resolution at depth.

focal.png

ZOOM

Write zoom (pre-processing function):

  • The write zoom is a dynamic function that allows the user to select an area of interest within the image and zoom in.

  • Enlarges small areas of interest e.g. early pregnancy

  • Makes it easier to measure small luminal distances e.g. CBD

    • Image resolution is maintained

    • Improved frame rate as reduced number of lines of sight

Read zoom (post-processing function):

  • Used to magnify an area of a stored/saved image when reviewing

    • Degrades picture with more prominent pixels

IMAGE ACQUISITION

  • Freeze button freezes a frame that the user chooses in order to review, make measurements, or save the image.

  • Cine loop is acquired by the machine when an image has been frozen. The user can scroll back through a number of frames to find the images required.

CINE LOOP SONOSITE X-PORTE

CINE LOOP SONOSITE X-PORTE

  • Acquire/save/store button

    • when image is frozen using this button will save a still image

    • when used on an active image a video clip will be saved

  • Video clips

    • Typically the machine is set to acquire a clip prospectively

    • The user can change to retrospective video clip capture

    • Clip time can be altered but typically clips are taken at 2-3 s duration

MEASUREMENTS AND CALCULATIONS

Caliper/distance measure is used to measure required dimensions

  • To store any required measurement the save button should be pressed after each measurement

  • The most accurate measurements are made when:

    • the ultrasound beam is at perpendicular incidence to the object measured

    • the highest viable frequency is utilised

    • the depth is reduced as much as possible

    • focus is set at the level of interest

    • write zoom is used for small objects (e.g. crown rump length, CBD diameter)

Calculations (CALC Button)

  • Under different presets a variety of machine calculations will be available, the user is required to accurately take and input particular data measurements to complete the calculations. Some examples include:

    • ECHO - large number of quantitative calculations e.g EF, SV, CO etc.

    • Volume measurements


ADDITIONAL FUNCTIONS

Tissue Harmonic Imaging (THI) and Compound imaging are tools used by the ultrasound machine to improve image resolution, improve imaged structure boundary definition, and reduce artifact. Different makers will use a variety of names for compound imaging functions:

  • Philips = sonoCT

  • Sonosite = sonoMB

  • GE = CrossXBeam

Typically these functions will be active unless the user turns them off. Generally they help to improve our image quality and optimisation.

Turning these functions can be useful in some circumstances when loooking for diagnostic artifacts. For example deactivating THI and compound imaging helps to accentuate diagnostic posterior acoustic shadowing from small objects, such as biliary calculi and renal stones.

IMAGING MODES

2D or B mode

  • Standard grey-scale ultrasound imaging used for most POCUS

M mode

  • Motion mode is a single line of sight grey-scale display of motion versus time

  • Typical uses include:

    • Early pregnancy fetal heart beat measurement

    • Pleural assessment for pneumothorax (see Lung)

    • EPSS and TAPSE measurements in beside echo (see FELS)

Doppler modes (See physics)

  • Spectral Doppler

    • Produce spectral waveform from measured Doppler shifts

    • Pulsed wave (PW) used to determine velocity at a single point e.g LVOT VTi

    • Continuous wave (CW) used to measure very high velocities eg. TR max

  • Colour Doppler detects flow and direction

  • Power Doppler detects Doppler shift amplitude