US FUNDAMENTALS: B-MODE AND DOPPLER

MACHINE SETTINGS

MSUS BASIC

US: 2-20 MHz

US: emission and reception of mechanical sound waves with a frequency greater than the human hearing frequency range (15-20 MHz)

Sound waves produced and captured by piezoelectric material located inside the transducer or probe

MSUS BASIS

The degree of reflection of an ultrasound wave depends on the body tissue being assessed. Each tissue has its own acoustic properties (density and resistance) which determine its ‘acoustic impedance’. Reflections occurs at interfaces between tissues of different acoustic interfaces.

MSUS BASIS

Gray scale (GS) ultrasound refers to the black and white images that are generated. Gray scale defines the morphology of a tissue. The whiter a structure appears, the more reflective it is. e.g. bone or calcification will appear white. The darker a structure is, the less reflective it is. As water is the best transmitter of sound, fluid e.g. blood, synovial fluid, cysts generally appear black on the image as sound is not reflected back and registered on the screen.

BoneFluid

MSUS BASICDoppler technique detects movement of red blood cells

in vessels

The Clinical Application Specialist from the ultrasound company for your machine will assist in setting up appropriate settings for the assessment of synovitis (and other joint structures) as these will vary from machine to machine

MACHINE SET UP

• The choice of transducer frequency is dependant on the joint, body habitus and age of a patient

• A higher frequency transducer (e.g. 10-18 MHz) is preferable for the assessment of superficial lying anatomical structures e.g. for the MCPJ. Generally > 18MHz used for skin and very superficial structures.

• A lower frequency transducer (e.g. 5-10 MHz) is preferably for deeper lying structures e.g. for the shoulder

FREQUENCY OF TRANSDUCER

The footprint of the transducer is the surface area of thetransducer that is physically in contact with the skin.

An image this is acquired should contain all the informationrequired to make a valued assessment

For small structures e.g. MCPJ, this may be achieved with a smaller footprint (e.g. hockey stick style transducer) whilst larger structures e.g. shoulder and hip may require a larger footprint to capture the full field of view.

Small transducers may be useful if there are deformities or for guiding interventional procedures

FOOTPRINT OF TRANSDUCER

IMAGE OPTIMIZATION: GENERAL

Grey scale

Use a minimal number of focal points (on average 2 are used)

Ensure focus points are at level of region of interest

Adjust the depth so that region of interest at least half way down screen

Adjust B-mode or 2D gain to optimize image brightness

There are other modifiable parameters on some machines such as dynamic range. This ‘contrast control’ may be adjusted for personal preference but we would advise discussing this (and others) with clinical applications specialist according to your machine

Doppler

Most scans for the assessment of joint inflammation are performed using Power Doppler or Colour Doppler displays

After choosing either option, the Doppler box should be adjusted to cover the whole joint (or specific region of interest). It should encompass the joint proximally and distally and also reach the skin surface.

GENERAL CONSIDERATIONS

Recommended settings for colour and power Doppler in rheumatology

Doppler frequency Lowest or highest depending on machine

Pulse repetition frequency Lowest possible*

Colour priority All priority to colour

Wall filter Lowest possible*

Persistence Lowest possible

Gain On the threshold to noise

Focus Placed where highest sensitivity is required

*Lowest possible where motion artifacts are avoided most of the time.

From: Torp-Pedersen ST, Terslev L Settings and artefacts relevant in colour/power Doppler ultrasound in rheumatology. Ann Rheum Dis. 2008;67:143-9.

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– Always observe the full extent of joint space (capsule) proximally and distally

IMAGE PRESENTATION

– Ensure colour box captures whole joint and that box reaches skin surface

IMAGE PRESENTATION

– Always ensure pathology observed in both longitudinal and transverse views

SCORING PATHOLOGY