Common Artifacts in Neurosonology by Erwin Stolz

Common Artifacts in Neurosonology

by Erwin Stolz

Adjunct Professor of Neurology and Neurological Intensive Care Medicine, Justus-Liebig-University, Gießen, Germany,

Neurological Practice, Juergen-Ponto-Platz 2, D-60329 Frankfurt

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Mirroring artifacts

Amongst the most frequently observed artifacts are mirroring (reverberation) artifacts. They can occur when the propagating ultrasound beam hits two or more parallel, highly reflective, relatively smooth structures. Part of the beam is reflected back to the probe at the first acoustic boundary, part of it penetrates the boundary to be reflected back at the second (or third …) boundary, but is then reflected back into tissue at one of the more proximal acoustic boundaries. Part of the beam may bounce between the boundaries before reaching the probe again. Because the ultrasound system assumes that a signal with a longer time interval between emission and reception has to originate from tissues at a greater depths (propagation time ~ depth), mirror boundaries (virtual images as opposed to the true image) at a constant distance (depending on the distance of the acoustic boundaries) are depicted in the B-mode image.

Frequent sources of mirroring artifacts during extracranial examination are the walls of the jugular veins, or the ventricular system during transcranial examination.

Mirroring artifacts may mimic moving membranes. In color-mode mirroring artifacts regularly arise from the lung apex when examining the subclavian artery or may be caused by atlanto-occipital membrane causing the appearance of two basilar arteries in the picture without being a true doubling.

Mirroring Artifacts - Principle

The depth within an ultrasound image is dependent on the time from emission to reception of the echo, because assuming a constant acoustic velocity within soft-tissues, propagation time is proportional to the distance travelled by the ultrasound beam. This is shown in the upper panel of the figure. However, if the beam bounces back and forth between highly reflective acoustic boundaries the time to reach the probe is prolonged proportional to the distance between these boundaries, and hence appeares in the ultrasound image at a larger depth.

 

Mirroring artifact in the lumen of the carotid artery

The arrow (→) shows the mirroring artifact which must not be mistaken for a floating membrane.

 

Mirroring artifact in the lumen of the carotid artery

An other example of a mirrowing artifact in the lumen of the carotid artery. In total 3 mirror echos can be seen (arrows →).

 

Mirroring artifact in the vertebral artery

The picture shows a large vertebal artery and an acoustic shaddowing caused by the vertebral body. Within this shadow where the ultraound beam cannot pass on, nevertheless bright lines in a fairly constant distance are depicted (→). These are mirror artifacts cause by the highly reflectice acoustic boundary above the vertebral body.

 

Example of mirroring artifacts during transcranial sonography

The figure shows several mirror artifacts in constant intervals (arrow →) cause by a wide anterior horn (double arrow <—>) of the lateral ventricle.

 

Mirroring artifact of the subclavian artery

A common mirrowing artifact occurs during examination of the vertebral artery origin (V0). This image shows V0 of the vertebral artery (VA), the thyreocephalic trunc (THCT), and the sublavian artery (SA) in cross section. Below the echogenic boundary, i.e. the lung apex, marked by the small arrows, another "SA" appears (long arrow →), which is the mirror (virtual) image of the true SA.

 

Mirroring artifact of the basilar artery

This image shows seemingly two basilar arteries emerging from the occipital foramen without beeing a true doublication. The mirror image is presumably produced by the atlanto-occipital membrane. Which of the structures is the true basilar artery and which is the virtual picture cannot be decided.

 

Refraction artifacts

They are caused by ultrasound beams which do not strike an acoustic boundary orthogonally. The part of the beam propagating deeper into tissue is refracted at the acoustic boundary, i.e. deviated from its original course before being backscattered to the probe. However, the ultrasound system assumed the backscattered signal originating from its original course. Refraction artifacts regularly occur during transcranial sonography where the midline structures appear to be slightly arched.

Refraction artifact in transcranial sonography

Please note the slightly curved midline of this normal sonogramm at the level of the anterior horns of the lateral ventricle. The refraction occurs in the skull.

 

Shadowing artifacts

They are caused be highly reflective structures preventing the ultrasound beam from penetrating deeper structures.

Shadowing artifact caused by a echolucent plaque

Shaddowing artifact (arrows →) caused by an echolucent plaque in the lumen of the carotid artery.

 

Soddowing artifact commenly caused by the vertebral bodies

The vertebral bodies commenly cause a shaddowing artifact (→) in the examination of the V2 segment of the vertebral artery.

 

Shadowing artifact caused by a echolucent plaque

Not only B-mode but of course all other ultrasound modes are subject to shaddowing artifacts (→). This example shows shaddowing of a plaque in the proximal internal carotid artery.

 

Amplifying artifacts

When ultrasound penetrates into tissue with lesser absorption than the neighboring tissue this gives the impression of ultrasound amplification, but strictly it is just lesser absorption.

Ultrasound amplifying artifact in the carotid artery.

The cross section through the carotid artery commonly causes an ultrasound amplifying artifact behind the low echogenic lumen of the vessel (double arrow ⇒). Corss section also commonly causes a tangent artifact (simple arrow →, see below). Please also note the echolucent plaque on the left wall of the vessel.

 

Ultrasound amplifying artifact in TCS.

This image shows an ampifying artifact in transcranial sonography caused by lesser ultrasound absorption at a specific spot of the scranial skull (arrows →).

 

Tangent artifacts

When ultrasound hits an oblique acoustic boundary, much more of the beam is scattered sideways into tissue than reaching the back the probe. This causes the impression of shadowing.

 

Tangent artifact in the carotid artery.

The cross section through the carotid artery commonly causes an ultrasound amplifying artifact behind the low echogenic lumen of the vessel (double arrow ⇒). Corss section also commonly causes a tangent artifact (simple arrow →, see below). Please also note the echolucent plaque on the left wall of the vessel. The shadowing on the right side of the vessel (tangent artifact) cannot be caused by any plaque.

 

Ultrasound amplifying artifact in TCCS.

The image shows a transcranial sonography at the level of the anterior horns (double arrows, <—>) of the lateral ventricle in a patient with massive hydrocephalus. The wall perpendicular to the ultrasound beam appears echolucent (⇒) while the wall, although biologically not different, which is hit in an unfavorable angle appears less echogenic (→)

 

Plane artifacts

In case the ultrasound beam is not located longitudinally within a vessel, the beam can touch the posterior vessel wall and give the impression of a plaque/narrowing of the vessel.

 

Plane artifact of the carotid artery.

Upper image: A plane artifact is produced when the beam is not located longitudinally within the carotid mimiking a plaque. Lower image: the plane has been corrected.

 

Artifacts due to tissue pulsation

The pulsating blood in the arteries also causes movements of the surrounding tissue. The backscattered signal from tissue, however, is much stronger than that of the moving erythrocytes producing the Doppler shift frequency. The tissue pulsations not only are stronger than the actual Doppler signal but also overlap with the lower Doppler frequencies. In order to receive an undisturbed Doppler signal the tissue signals are removed using a so called clutter (wall) filter. When the pulse repetition frequency is low, and/or the gain high, and/or the wall filter low then tissue movement may be interpreted as a flow signal and in color mode color patches appear outside the vessel. 

 

Pulsation artifacts.

The image shows pulsation artifacts caused by tissue movements (simple arrows →) and also a shoddowing artifact caused by a plaque (double arrow ⇒).

 

Beam-line artifacts

Beam line artifacts occur when the backscattered signal is so strong that the system is not able to allocate it only to a single beam line, but assumes an origin also from neighboring beam lines. Beam line artifacts commonly occur when using echocontrast agents (“blooming” artifact). 

 

Blooming artifact due to application of an echocontrast agent.

The image shows a blooming artifact after application of an echocontrast agent during transtemporal insonation.