Image Artifacts

Question 1

What are the four main reasons that artifacts occur?

Answer 1

Ultrasound machine not behaving as as assumed (acoustic artifacts)

Equipment settings are inappropriate

Equipment is faulty

Electrical interference

Question 2

What are the 6 main assumptions of the ultrasound machine?

Answer 2

The transmitted ultrasound beam is narrow and straight

Ultrasound travels along this straight line and does not deviate from it

The ultrasound pulse always travels directly from the transducer to a given reflector (or scatter) and from that object back to the transducer

The propagation speed is 1540m/s in all tissues

The attenuation is the same in all tissues

All echoes from the transducer and cause by the most recent transmit pulse

Question 3

List the types of attenuation artifact?

Answer 3

Shadowing
Enhancement
Edge shadowing

Question 4

What is shadowing artifact?
How should you optimise it for benefit?

Answer 4

A darkened line cast behind a region of tissue with more attenuation than surrounding tissues

The transmitted intensity deep to the region is reduced compared tot he intensity at other parts of the image

It occurs parallel to the ultrasound beam

The beam should be focussed at the level of the object causing shadowing to maximise the effect it has on intensity beyond

Question 5

What is enhancement artifact?
How should you optimise it for your benefit?

Answer 5

An area of increased brightness behind a rehion of tissue with lower attenuation than surrounding tissues

The beam should be focussed at the level of the object causing shadowing to maximise the effect it has on intensity beyond

Question 6

What is edge shadowing artifact?

Answer 6

Occurs when the ultrasound beam strikes the edge of a curved structure, causing a combination of reflection and refraction
Results in defocussing of the ultrasound beam beyond the edge and a shadow is displayed in the image, parallel to the beam.

Not a particularly useful artifact and can be ignored

Question 7

List the types of depth artifact

Answer 7

Propagation speed artifact
Reverberation artifact
Ring-down artifact
Comet-tail artifact
Range ambiguity artifact

Question 8

What is propagation speed artifact?

Answer 8

When ultrasound passes through tissues with propagation speed different to the assumed speed (1540m/s), the echoes are not displayed at the correct depth

Question 9

What is reverberation artifact?

Answer 9

Reverberation is the reflection of waves between parallel surfaces

Occurs when ultrasound strikes the tissues at 90’
E.g. layers of the abdominal wall

Question 10

What is ring-down artifact?

Answer 10

A special case of reverberation artifact, occuring within small gas bubbles
Multiple echoes return to the transducer as ultrasound reverberates back and forth among the bubbles
Reflection by gas bubbles is highly efficient so echoes remain strong for some time
Results in a bright line of echoes, perpendicular to the probe, extending a considerable depth

Question 11

What is comet-tail artifact?

Answer 11

A shorter-lived version of ring-down artifact
Mostly associated with small calcifications or other crystalline structures
The ultrasound reverberates between these structures, producing a series of echoes
Energy is lost quickly and so the artifact fades quickly with depth

Question 12

What is range ambiguity artifact?

Answer 12

Occurs when the machine transmits a new pulse before all echoes from the previous pulse have been received
Can occur when scanning through liquid, since attenuation is lower than expected (higher PRF selected incorrectly)

Question 13

List the types of beam dimension artifacts

Answer 13

Beam width artifact
Sidelobe artifact
Slice thickness artifact
Speckle

Question 14

What is beam width artifact

Answer 14

Machine displays any object as though it was in the centre of the beam
Given that the beam has width (mm), a number of lines of sight will “see” an object
Therefore the object is shown as a line, equal to the width of the beam

Lateral resolution is maximised when tissues are scanned at 90’

Question 15

What is sidelobe artifact?

Answer 15

Sidelobes are unwanted low-intensity beams which exist symmetrically lateral to the main beam
The sidelobes can “see objects” which are displayed as though they were seen within the main beam

Apodisation

Question 16

What is the technique of apodisation

Answer 16

Reduction of the amplitude of sidelobes at the expense of increasing beamwidth

Transmit pulse and receive gain are reduced for the outer elements of the aperture

Question 17

What is speckle?

Answer 17

Speckle describes the echo texture typically seen in soft tissue
Occurs because scattered echoes from small structures add randomly
Makes it more difficult to see small structures and subtle variation in soft tissue echogenicity and echotexture

Question 18

List the techniques used to reduce speckle

Answer 18

Frame averaging (persistence)
Compound imaging
Image processing

Question 19

List the types of beam path artifacts

Answer 19

Mirror image artifact
Refraction artifact

Question 20

List the main categories of acoustic artifacts

Answer 20

Attenuation artifacts
Depth artifacts
Beam path artifacts

Question 21

What is mirror image artifact

Answer 21

Reflection occurs when ultrasound waves strike a reflector (smooth surface) with significant difference in acoustic impedence

If this reflected wave then travels to an object, the object is displayed as beyond the initial reflector, rather than in its true position

Question 22

Define “artifact”

Answer 22

Any appearance in an image that does not accurately represent the anatomy that is being scanned

Question 23

Discuss the challenges with accuracy of measurement