Artifacts

Question 1

definition of artifacts

Answer 1

artifacts are structures and features (echoes) that appear on the image but do not have a one-to-one correspondence to an anatomical structure.

they can be useful when assisting in diagnosing, may need to be ignored or eliminated.

Question 2

What do we assume of the ultrasound machine?

Answer 2

1) sound travels in a straight line
2) sound travels directly to and from the reflector
3) the imaging plane is thin
4) sound always travels at 1.54 mm/µsec
5) reflections are produced by structures along the main axis of the sound beam
6) the intensity of an echo corresponds to a reflector’s scattering strength

Question 3

definition of axial resolution

Answer 3

ability to distinguish two structures that are in close proximity to each other - front to back - anterior to posterior

(on top of each others on ultrasound image)

Question 4

the backing layer does what?

Answer 4

stops the ringing in the probe - which allows it to have shorter pulse lengths - which increases the axial resolution

Question 5

definition of lateral resolution

Answer 5

minimum distance between two side-by-side structures that can be separated and still produce two distinct echoes

Question 6

What are some causes of artifacts?

Answer 6

• faulty equipment
• improper equipment operation
• improper settings of controls (ex. gain settings)

Question 7

what is acoustic speckle? what does it look like? what causes it?

Answer 7

a one-to-one correspondence between image brightness and scanned objects does not exist.

a granular appearance of images and spectral display.

caused by: interference of echoes from the distribution of scatters in tissue

Question 8

where does acoustic speckle occur?

Answer 8

near the face of the transducer

Question 9

What is section thickness? and what else is it called?

Answer 9

• is the thickness of scanned tissue volume perpendicular to the scan plane
• slice thickness, out-of-plane focusing, elevational resolution or width focusing

Question 10

What is temporal resolution?

Answer 10

the ability to precisely position a moving structure

Question 11

what does low frame rates result in?

Answer 11

less accurate positioning of reflectors

Question 12

reverberation artifact

Answer 12

• harmonics (double the frequency)

- appear as multiple, equally spaces reflections on the image, decreasing in intensity

Question 13

comet-tail artifact

Answer 13

• form of reverberation artifact
• caused by metal object(s)
• surgical clips, prosthetic valves, bullet etc
• appears as multiple small bands

Question 14

ring-down artifact

Answer 14

• type of reverberation
• produced by small gas bubbles
• appears as a long, single, strong echo behind the reflector

Question 15

mirror-image artifact

Answer 15

-occurs when a structure is located in front of a highly reflective surface causing the anatomy to be reproducr=ed or duplicated on the other side of the interface

Question 16

where does the duplicated or mirrored image appear at?

Answer 16

ALWAYS deeper than the true anatomical structure

Question 17

examples of strong reflectors

Answer 17

diaphragm, pleura and bowel

Question 18

multipath artifact

Answer 18

• path to and from a reflector that are not the same

- results in: incorrect axial location of an interface due to an increase in time and improper brightness

Question 19

refraction artifact

Answer 19

• produces shadowing at the edge of structures that are curved
• heart, fetal head, or a cross-section of a vessel
• rectus abdominal muscles (duplicated structures, ghost image artifact)
• ex: two gestational sacs, two CAs, two AO valves

Question 20

side lobes

Answer 20

• single crystal transducer
• energy off the main axis
• the reflection is at the correct depth - but LATERAL from true anatomy

Question 21

grating lobes

Answer 21

• array transducers
• reflection at the correct depth - but LATERAL from true anatomy
• **corrected by sub-dicing and apodization - during manufacturing process

Question 22

what is range ambiguity

Answer 22

it is the misplacement of an interface when the assumption that each echo is derived from the most recent pulse is violated

-the time between the transmitted pulse and the detected echo are not measured correctly

Question 23

propagation speed errors

Answer 23

-causes the improper location of interfaces

Question 24

If the mass has a speed of sound GREATER than 1540 m/sec - what will happen to the reflector?

Answer 24

the reflector will be placed closer to the transducer - it information got back faster

Question 25

If the mass has a speed of sound LESS than 1540 m/sec - what will happen to the reflector?

Answer 25

the reflector will be placed deeper and further from the transducer - it took longer for the information to get back * a posterior displacement* ex: a fatty tumor (propagation speed of fat is slower than ST)

Question 26

What is acoustic shadowing?

Answer 26

the absence or reduced intensity of echo information, distal to a reflector

Question 27

what causes acoustic shadowing?

Answer 27

what the sound beam intersects a highly attenuating (highly reflective) structure ex: calcium stone / bone - most or the entire sound beam is redirected to the transducer = black space behind the strong echo structure

Question 28

What is a useful artifact?

Answer 28

***acoustic shadowing ***enhancement

Question 29

What is enhancement?

Answer 29

an artifact that occurs when the sound beam intersects a very low attenuation (weak reflector) structure i.e., fluid

Question 30

what is focal banding?

Answer 30

an artifact that occurs when focusing characteristics of the transducer, creating a region of increased brightness. - caused by greater intensity in the focal zone - improper TGC settings

Question 31

What does reverberation cause? how can you correct it?

Answer 31

* *near field artifact - seen in the anterior walls of organs that are close to the skin surface - correct by using harmonics

Question 32

What causes an image to appear too bright or too dark?

Answer 32

improper gain or TGCs - operator dependent

Question 33

what happens with excessive reject?

Answer 33

all the low-level echoes are eliminated - operator dependent

Question 34

examples of electrical interference

Answer 34

patient noise - snoring, talking, coughing etc | patient rooms - machines and monitors

Question 35

what is aliasing and how does it occur?

Answer 35

- it is a color artifact - in PW and color Doppler | - occurs when the Doppler shift frequency exceeds half the PRF - pulse repetition frequency

Question 36

What is the Nyquist limit?

Answer 36

it is the limit of aliasing where the Doppler shift frequency exceeds 1/2PRF

Question 37

What does aliasing look like? how do you correct it?

Answer 37

- when using color Doppler, aliasing is a wrap-around of the display = apparent reversal of flow direction - increase the PRF

Question 38

with mirrow imaging or ghosting: what is the relationship between the true anatomical structure vs artifact?

Answer 38

the true, anatomincal vessel, is closer to the trasnducer. The duplicated, artificat, vessel is below it.

Question 39

What artifact happens when the Doppler gain is set too high or the Doppler angle is close to 90 degrees? What does it look like?

Answer 39

- mirror imaging or crosstalk | - the spectral Doppler shows waveform above and below the baseline

Question 40

What is color ghosting or flash? What causes it? How can it be corrected?

Answer 40

- is is when color is being improperly assigned. The sudden burst of color that encompasses the frame. - any movenment causes it: pt breathing, talking etc - can correct it by increasing the color filter, decreasing the persistence, and reducing the width of the color field of view

Question 41

what is color bleeding or blossoming? | how do you eliminate it?

Answer 41

- it is the extension of color beyond the region of flow to the adjacent tissue - decrease the overall color gain or the transmit power (ALARA, which we will not do)

Question 42

What is color noise/clutter? | how can it be eliminated?

Answer 42

- a random variation in signal detection which causes area with no flow to be color encoded - by using filter, threshold, wall filter to eliminate low level echoes

Question 43

What ultrasound Doppler is angle dependent?

Answer 43

- color Doppler | - pulse wave Doppler

Question 44

a feature that appears on the image but does not correspond to an anatomical structure is called ____________?

Answer 44

an artifact

Question 45

which artifact interferes with the ability to detect a low-contrast object?

Answer 45

acoutstic speckle

Question 46

what artifact occurs from the thickness of the scanned tissue volume, perpendicular to the scan plane?

Answer 46

elevational resolution

Question 47

definition: multiple, equally-spaced reflections on the image, decreasing in intensitiy is called ____________

Answer 47

reverberations

Question 48

what is it called when the acoustic energy is emitted in a direction different than from the main axis of the soud beam?

Answer 48

side lobes

Question 49

the extension of color beyond the regoin of flow, to the adjacent tissue is called:

Answer 49

blossoming

Question 50

when dealing with acoustic speckle - what do the dots/pixel represent?

Answer 50

each dpt or pixel represents a collection of scatterers

Question 51

name some types of artifacts

Answer 51

- not real - missing - improperly located - improper brightness - improper shape and size

Question 52

- define interference | - what factors are at play?

Answer 52

-a group of very closely spaced reflectors are positioned in the path of the beam - the average signal obtained from these reflectors depends on many factors * **1.acoustic impedance differences between scatteres and surrounding medium 2. frequency of insonation 3. scatter size 4. number of scatters per unit volume

Question 53

What is referred to as the "dead zone"

Answer 53

the area close to the trasnducer face - where is there poorest resolution

Question 54

a shallower depth has what relationship to PRF

Answer 54

an increased PRF (not far into the body)