Most important information

Question 1

4 categories of artifacts

Answer 1

1. Propagation
2. Attenuation
3. Spectral Doppler
4. Color Doppler

Question 2

Slice thickness artifact

Answer 2

non-zero beam width perpendicular to the scan plane

Question 3

Speckle artifact

Answer 3

granular appearance of images caused by interference of echoes from distribution of scatterers in the tissue

Question 4

Reverberation artifact

Answer 4

multiple reflections btwn. a structure & the transducer, btwn. structures, or w/in a structure

Question 5

Comet tail artifact

Answer 5

reverberation btwn. closely spaced surfaces

Question 6

Mirror image artifact

Answer 6

duplicates a structure on the opposite side of a strong reflector

Question 7

Refraction artifact

Answer 7

displaces structures laterally from their correct locations

Question 8

Grating lobes artifact

Answer 8

duplicate structures laterally to the true ones (weaker beams)

Question 9

Speed error artifact

Answer 9

displaces structures axially, with slower speeds placing echoes deeper and faster speeds placing echoes closer

Question 10

Range ambiguity artifact

Answer 10

places structures much closer to the surface than they should be b/c all echoes are not received before the next pulse is emitted

Question 11

Shadowing artifact

Answer 11

weakening of echoes distal to a strongly attenuating or reflecting structure or from the edges of a refracting structure

Question 12

Enhancement artifact

Answer 12

strengthening of echoes distal to a weakly attenuating structure

Question 13

Aliasing artifact

Answer 13

• appearance of misplaced Doppler spectral information on the wrong side of the baseline
• caused by under sampling of Doppler shifts
• spatial or temporal

Question 14

Pulse-wave Doppler operation

Answer 14

• generates pulses with several cycles of voltages when driving the transducer
• 5-30 cycles

Question 15

Low-pass filtering

Answer 15

filtering (removes higher frequencies) smooths the sampled result, yields the desired Doppler shift wave form

Question 16

Range Gating

Answer 16

enables depth selectivity & a small Doppler sample volume (based on arrival time w/ 13us/cm rule)

Question 17

Nyquist limit

Answer 17

• highest frequency in a sampled signal represented unambiguously
• equal to one half of PRF
• minimum 2 samples per cycle

Question 18

Doppler-shift frequency exceeds 1/2 PRF…

Answer 18

temporal aliasing occurs

Question 19

What instruments do not experience aliasing?

Answer 19

continuous wave Doppler intruments

Question 20

Methods of correcting or eliminating aliasing

Answer 20

• shift baseline
• increase PRF
• increase Doppler angle
• use lower operating frequency
• use CW mode

Question 21

Continuous wave mode & aliasing

Answer 21

• no aliasing but no depth localization
• large sample window
• don’t know where velocities are recorded

Question 22

Doppler- range ambiguity

Answer 22

• caused by higher PRF, results in multiple sample volumes
• correct by increasing PRF & decreasing baseline

Question 23

Spectral Doppler mirror image artifact

Answer 23

• occurs w/ equipment malfunction & Doppler gain too high
• Doppler angle close to 90
• duplications on opposite side of strong reflector

Question 24

Aliasing & volume depth

Answer 24

• when sample volume is deep, PRF is low & Nyquist limit is low
• velocity is sampled fewer times per second
• system is unable to accurately define the velocities
• results in aliasing
• sampling rate = PRF

Question 25

Aliasing & transducer frequency

Answer 25

- Doppler shift is directly related to transducer frequency - higher freq. transducers create a higher Doppler shift - lower freq. = lower Doppler shift - smaller Doppler shifts from lower freq. transducers are less likely to exceed Nyquist limit

Question 26

5 techniques to avoid aliasing artifact

Answer 26

1. Adjust scale to its maximum (increases NL) 2. Select a new ultrasonic view w/ a shallower sample volume (increases NL) 3. Select a lower freq. (decreases DS) 4. Use baseline shift (aliasing remains but display more appealing) 5. Use continuous wave Doppler (never aliases, but range ambiguity)

Question 27

Multiple angle artifact

Answer 27

- Doppler angel in color-Doppler displays is usually assumed to be zero (unrealistically) - color overlay is coded relative to location of transducer, can lead to confusing displays

Question 28

Ghost artifact (mirror image on color-Doppler)

Answer 28

- duplication of a vessel or Doppler shift on opposite side of string reflector - mirror vessel will demonstrate color & spectral flow

Question 29

Refraction in color-Doppler

Answer 29

- can confuse interpretation of color-Doppler - distorts Doppler-shift regions

Question 30

Clutter or Flash artifact

Answer 30

- sudden burst of color Doppler - typically caused by tissue or transducer motion - demonstrates an extension of color beyond the region of blood flow - usually eliminated by wall filters

Question 31

Color gain artifact

Answer 31

instrument tends to write color noise preferentially in areas where non-Doppler shifted echoes are weak or absent (improved by reducing gain)

Question 32

Twinkling artifact

Answer 32

- presence of more than one strong reflector, such as calcs, surgical clips, or stones