ultrasound image and doppler artefacts amanda watson€¦ · reflection of ultrasound several times...

FRCR /MSc - Artefacts AJW - 2014

Ultrasound Image and Doppler Artefacts

Amanda Watson

[email protected]


Definition:

artefact /‘a:tı,fækt/ n. (also artifact )

…a feature not naturally present, introduced during preparation or investigation ….


Ultrasound Imaging Assumptions

The ultrasound travels on a straight line along the beam and returns along that same straight lineUltrasound travels at a constant speed of sound of 1540ms-1

The range equation applies in positioning the echo i.e. d=(c x t)/2the intensity of the displayed echo relates to the acoustic properties of the tissue in that regionAttenuation is uniformAll the echoes arise from the centre of the beam –narrow beam, uniform width

Violation of any of these assumption gives rise to the potential to cause artefacts.


Reflection Artefacts

Reflection effects, particularly those involving a strongly reflecting interface, are responsible for a group of artefacts including:

� mirror image

� reverberation


Mirror Image ArtifactsA strong reflecting interface in an object can cause duplicate images of features in that object to appear beyond the interface.

The ultrasound system assumes the received echoes are in a straight line and places them according to their timings.


Mirror Image Artefacts


Reverberation Artefacts

Artefact caused by the reflection of ultrasound several times back and forth between two closely spaced interfaces.

Also referred to as "ring-down artefact" and "comet tail artefact". Reverberation artefacts are commonly seen at soft tissue-to-gas interfaces e.g. Needle tip during Bx.



On a smaller scale, multiple reverberations within very small structures can cause bright “streaks” of reverberations in the image. These may so close together that it is difficult to identify separate reverberations.

Reverberation artefacts from very small air or liquid structures are usually referred to as ring-down

Reverberation artefacts from very small foreign body, metallic or calcified structures are usually referred comet tail


Comet Tail


Ring Down


Speed of sound artefacts

Material Speed of Sound (m/s)

Air 330 Water 1480

Fat 1460 Liver 1555 Blood 1560 Kidney 1565 Bone 4080

Average Soft Tissue

1540

The ultrasound system assumes a speed of sound of 1540ms-1.

Large differences in the speed of sound can affect the placement of objects in the image.


Speed of Sound Artifacts

The wrong speed of sound can cause misplacement artifacts


Speed of Sound Artifacts - refraction

Changes in the speed of sound at non-perpendicular interfaces can cause the ultrasound beam to bend.

Objects can be missed or misplaced.



sinθ2/sinθ1 = C2/C2

θ2

θ1


Shadowing and Enhancement

Shadowing following high reflections and enhancement following low attenuation can be useful artifacts.


Shadowing and Enhancement

If the strength of an echo from a target is sufficiently different from the echoes around the target, then the appearance of the image below that target will be affected.


Critical Angle Shadowing or Edge Shadowing


Beam Width Artefacts

We know that the beam is not an ideal narrow line but that it has a variable volume.

Focussing - can be controlled by the user but there are limits. Limited focussing and beam interpolation can distort small targets.


Slice ThicknessSlice thickness limitations can also cause “fill in” artifacts


Feldman M K et al. Radiographics 2009;29:1179-1189©2009 by Radiological Society of North America

Grating Lobes and Side Lobes

Side lobes are additional off axis beams of energy generated by the transducer.

Grating lobes can occur with arrays due to the evenly spaced nature of the elements.

Most scanners have features designed to minimise this effect. However, the combination of high amplitude transmit beams and strong receive echoes can cause “ghost” images to appear due to the presence of significant side or grating lobes.


Doppler Artefacts


Doppler Artefacts

Many Doppler artefacts represent limitations of the system, although they can be minimised by careful adjustment of the system controls.

Artefacts that appear in the B-Mode image e.g. reflection effects, shadowing etc. can also affect the Doppler signal.


Colour Noise or False Flow

Mirror Image Artefacts - as for B-mode imaging, strongly reflecting interfaces can cause mirror image artefacts so that a duplicated image of a vessel may appear.

Note that a spectral signal will also arise from this position.



Doppler Clutter - -may give the appearance of flow where there is one. Clutter can be minimised by careful use of gain and persistence controls as well as filters. Common clutter artefacts include:

• Respiratory or general patient motion.

•The effect of adjacent strongly pulsatile vessels (e.g. giving “pulsatile”venous signals.

•Transducer movement.



Twinkling Artefact - -this is a multicoloured mosaic appearing in areas of high b-mode signal (e.g. calcifications) - often associated with the comet-tail artefact.


Twinkling Artefact

The twinkling artefact is thought to be caused by noise introduced by phase (or clock) jitter in the Doppler circuitry system. Because rates of change in phase represent Doppler shifts in colour and spectral Doppler signals, if there is a fluctuation in the clock sampling of a signal, there will be a perceived Doppler shift even if there is no true motion.

Normally these small fluctuations would be suppressed by the filters and colour-write controls.

However because the B-mode echoes in that region are so high, the “jitter” signal can “break through”.


Pictorial Essay: US Diagnosis of Urinary Tract Calculi in Children Using the 'Twinkling Sign‘

Darge, Kassa; Heidemeier, Anke

Ultrasound, Volume 14, Number 3, August 2006, pp. 167-173(7)


No Flow

Where the Doppler signal is weak, it sometimes cannot be detected e.g. for:

� Deep vessels� Small Vessels� Slow Flow

Adjustment of control settings may help - but, as always, there are limits.


Misleading Flow Patterns

Two common effects that can give misleading flow information are:

� Angular effects� Aliasing

Both of these can me minimised by careful adjustment control settings -but,again, there are limits.


Misleading Flow Patterns - Angular

Remember the angular term in the Doppler equation?

c

vCosff

oD

θ2=

Angles greater than 60°can give large errors


Misleading Flow Patterns - Angular

No Flow ?


Misleading Flow Patterns - Aliasing

Remember aliasing?

It’s easy to identify in spectral Doppler - but can be misleading in colour ...


Misleading Flow Patterns - Aliasing


Web Resources

http://radiographics.rsna.org/content/29/4/1179.full

http://web.archive.org/web/20060111060500/http://www1.stpaulshosp.bc.ca/stpaulsstuff/USartifacts.html

http://www.meduniwien.ac.at/zbmtp/people/kollch1/artefakt.htmlclick on “test objects” for some images

http://www.frca.co.uk/article.aspx?articleid=374

http://radiographics.rsnajnls.org/cgi/reprint/12/1/35.pdf -Doppler

ultrasound image and doppler artefacts amanda watson€¦ · reflection of ultrasound several times...

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