Artifacts - Beam Path

Artefacts - Beam Path

Beam path artefacts occur when assumptions about ultrasound beams are violated:

  • The ultrasound beam travels in a straight line and reflects just once.

  • The propagation velocity of sound in soft tissue is 1540m/s1540 m/s.

These artefacts can cause echoes to be displayed in the wrong place or cause duplication of structures.

Types of Beam Path Artefacts

Common beam path artefacts include:

  1. Reverberation artefacts

  2. Refraction artefacts

  3. Mirror artefacts

  4. Multipath artefacts

Reverberation Artefacts

  • Multiple reflections occur at the interface between two or more strong reflectors positioned in close proximity.

  • The amount of ultrasound energy reflected depends on the acoustic impedance mismatch at the interfaces.

  • Reverberations may occur if the acoustic mismatch is large and the interface is perpendicular to the direction of propagation.

  • Typically, they occur between:

    • The transducer and a strong reflector (e.g., transducer-skin interface).

    • Two strongly reflective anatomic surfaces (e.g., soft tissue-gas interface).

    • Two strongly reflective artificial surfaces.

Reverberation Artefact Explained

  1. A strong reflection is created when the ultrasound beam encounters the transducer-skin interface.

  2. This strong reflection returns to the transducer:

    • Some energy is received and registered as an echo.

    • Some energy is re-reflected back into the body from the skin-transducer interface, undergoing a second reflection.

  3. The second echo returns to the transducer, and the sequence repeats.

  4. The ultrasound beam effectively "ping pongs" back and forth between the transducer and the skin.

  5. This artefact occurs only when the ultrasound beam is perpendicular to the involved interface.

Range Equation

Time is converted to distance using the range equation:

D=V×t2D = \frac{V \times t}{2}

Where:

  • DD = depth (m)

  • VV = velocity of sound in tissue (assumed 1540m/s1540 m/s)

  • tt = round trip time (s)

Violated Assumption

  • The ultrasound beam travels in a straight line and reflects just once.

Locations where Reverberation Artefacts Occur:

  • Anteriorly in fluid-filled objects

  • Fluid - soft tissue interface

  • Fat-muscle interface

  • Transducer-skin interface

  • Transducer - gas interface

  • Soft tissue - pericardial interface

  • Prosthetic heart valves

  • When using needles and catheters

Other Types of Reverberation Artefacts

  • Ring down

  • Comet tail

Ring Down Artefacts

  • Caused by a resonance phenomenon associated with a collection of gas bubbles.

  • Results in a continuous streak of ultrasound.

Comet Tail Artefacts

  • Occurs when there are two highly reflective, closely spaced interfaces such as a surgical clip, bullet, or metal support of a prosthetic heart valve.

  • Multiple reverberations merge together into a streak of ultrasound that rapidly diminishes in intensity over depth.

Avoiding Reverberation Artefacts

  • Reposition the transducer so that the beam is not perpendicular to the involved interfaces.

  • Try a different transducer frequency.

  • Scan from different angles.

  • Try a different acoustic window.

  • Adjust the TGC (Time Gain Compensation) - decrease initial gain or use delay to decrease effect at the origin.

  • Beware: Do not simply turn the gain right down as you may miss "real" echoes from pathology.

Usefulness of Reverberation Artefacts

  • Yes - they can help identify the tips of needles in biopsies.

  • No - generally these echoes are problematic because they can mask anatomical structures or lesions.

Refraction Artefacts

  • Refraction occurs when there is a change of direction or "bending" of the ultrasound beam.

  • Normally, when an ultrasound beam strikes an interface that is perpendicular to the beam, a percentage of the beam is reflected back to the transducer, and the rest is transmitted into the second medium without a change in direction.

  • If the beam strikes an interface at an angle other than 90∘90^{\circ} and the propagation velocity on either side of the interface is different, the transmitted beam will be refracted or bent away from the straight line path.

Requirements for Refraction

  1. There is an oblique angle of incidence between the sound beam and an interface.

  2. There are different propagation velocities on either side of the interface.

Snell's Law

Refraction of ultrasound obeys Snell's law:

Sinθ<em>1Sinθ</em>2=V<em>1V</em>2\frac{Sin \theta<em>1}{Sin \theta</em>2} = \frac{V<em>1}{V</em>2}

Where:

  • θ1\theta_1 = incident angle

  • θ2\theta_2 = transmitted angle

  • V1V_1 = propagation velocity in the first medium

  • V2V_2 = propagation velocity in the second medium

  • SinSin = trigonometric function that varies between 0 and 1 as θ\theta varies between 0∘0^{\circ} and 90∘90^{\circ}

Beam Bending Based on Velocity

  • When the propagation velocity in the incident medium is less than in the transmitted medium, the beam bends away from normal (θ<em>1V</em>2\theta<em>1 V</em>2).

Violated Assumptions

  • The ultrasound beam travels in a straight line and reflects just once.

Appearances and Examples

  • Echoes in the wrong place - may cause reflector to be misplaced

  • Added echoes in the wrong place - may produce duplicated structures

  • Improper brightness or missing - may cause shadowing at edges of structures that are large and curved (edge shadowing)

  • Refraction of the ultrasound beam can also lead to duplication of a single structure

Avoiding Refraction Artefacts

  • To avoid duplication of a single structure, try changing the transducer position so that individual beams travel through one rectus muscle only.

  • Avoid perpendicular angle of incidence.

  • Try another acoustic window.

Usefulness of Refraction Artefacts

  • No. In fact, these artefacts can be quite misleading. For example, if "two" gestational sacs are imaged when only one is real, a twin pregnancy may be misdiagnosed.

Mirror Artefact

  • The mirror image artefact is similar to the reverberation artefact.

  • When there is a high acoustic impedance mismatch between an interface and a specular reflector within the scan plane, the specular reflector acts like a mirror (near-total reflection occurs).

Violated Assumption

  • The ultrasound beam travels in a straight line and reflects just once.

Appearances

  • Added echoes

  • Two images of the same structure are displayed on the image (duplicated).

  • The structure that acts as the mirror always lies between the two duplicated images.

  • The mirror image always appears deeper than the image from the real structure (placed on the other side of the mirror).

  • The distance of placement of an echo is determined by the time taken for the echo to return to the transducer.

Example Locations

  • Pleura - lung interface

  • Diaphragm - lung interface

  • Pericardial-myocardial interface

  • Other strong reflecting interfaces

Avoiding Mirror Artefacts

  • Change the angle of interrogation so that the specular reflector is not perpendicular to the transducer.

  • Angle away from the "mirror".

Usefulness of Mirror Artefacts

  • No. These artefacts can be very misleading!

  • It may be difficult to differentiate real pathology from artefactual echoes.

Clues for Identifying Mirror Artefacts

  • If pathology is real, the specular interface will not be bright (no longer a high acoustic impedance mismatch).

  • If echoes are artefacts, they will have exactly the same appearance as the organ from which they have arisen.

Multipath Artefacts

  • An artefact that occurs when the ultrasound beam strikes multiple interfaces before returning to the transducer.

  • Produced when multiple surfaces or curved interfaces are encountered by the ultrasound beam.

  • If the ultrasound beam strikes a large curved interface or an interface at an angle, the beam may be reflected from a second (or third) interface before returning to the transducer.

Multipath Explanation

  • The ultrasound beam travels via multiple, different paths to and from the transducer.

  • The echo returning to the transducer has traveled via multiple paths:

    • It takes a longer time to return to the transducer. Based on the Range equation, the echo is placed at an incorrect depth (positioned deeper than the actual reflector position).

    • It is weaker than if it traveled via a direct path straight back to the transducer.

Violated Assumption

  • The ultrasound beam travels in a straight line and reflects just once.

Appearances and Examples

  • Echoes in the wrong place with improper brightness.

  • Any concave specular reflecting surface can produce this artefact.

  • Common in the pelvis, diaphragm, and bladder.

Avoiding Multipath Artefacts

  • Change the orientation of the ultrasound beam.

Miscellaneous Artefacts
"Main Bang" Effect (Dead-Zone)

  • A problem with mechanical probes and annular arrays in which the near field is obscured due to an initial large pulse from the transducer.

  • This effect may mask superficial structures.

  • To reduce this effect:

    • Try a higher frequency transducer.

    • Try using a stand-off.

    • Try decreasing initial gain on the TCG.

Acoustic Speckle

  • The detailed echo pattern often is not directly related to the scattering properties of tissue.

  • Acoustic speckle may occur within the imaging field of view.

  • This speckle occurs in a region of tissue where scatterers are spaced less than the axial resolving distance.

  • Results in an interference pattern resulting from constructive and destructive interference of echoes returning simultaneously from many individual scatterers within the ultrasound beam.

  • Acoustic speckle may result in the loss of small or low contrast objects amid the background of speckle.

Incorrect Image Optimisation Settings

  • "Noise" created by excess gain settings

  • If gain settings are too low, weak echoes may be lost

  • Reducing the dynamic range to improve contrast between low intensity (blood) and high intensity (myocardium) structures may eliminate important low intensity structures

Conclusions

  • Imaging artefacts vary in their appearance and causes.

  • Recognition of these artefacts coupled with an understanding of their origins may allow suppression of the artefact and result in the accurate display of the structure being imaged.

  • Although many artefacts detract from the presentation, it is also important to remember that not all artefacts are bad.

  • Some artefacts offer useful diagnostic information, for example, acoustic enhancement in identifying cystic structures.

General Rule

  • Artefacts are not seen in 2 planes

  • Real structures are seen in more than one plane.

  • Therefore to differentiate between real and artefacts: check to see if the object can be reproduced in more than one plane.

Categories of Artefacts - Summary
Attenuation Artefacts

  • Acoustic enhancement

  • Acoustic shadowing

    • Reflective shadowing

    • Attenuation shadowing

  • Edge shadows

Beam Dimension Artefacts

  • Beam width

  • Slice thickness

  • Grating lobes/side lobes

Depth of Origin Artefacts

  • Propagation velocity

  • Range ambiguity

Beam Path Artefacts

  • Reverberation

  • Refraction

  • Mirror

  • Multipath

Miscellaneous Artefacts

  • Main bang

  • Acoustic speckle