Chapter 06: Artifacts in Scanning

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1

aliasing

improper Doppler-shift information from a pulsed wave Doppler or color Doppler instrument when the true Doppler shift exceeds one half the pulse repetition frequency

2

anechoic

echo-free

3

baseline shift

movement of the zero Doppler-shift frequency or zero flow speed line up or down on a spectral display

4

comet tail

a series of closely spaced reverberation echoes

5

cross-talk

leakage of strong signals in one direction channel of a Doppler receiver into the other channel; can produce the Doppler mirror-image artifact

6

enhancement

increase in echo amplitude from reflectors that lie behind a weakly attenuating structure

7

hypoechoic

having relatively weak echoes; opposite of hyperechoic (having relatively strong echoes)

8

mirror image artifact

an artifactual gray-scale, color flow, or Doppler signal appearing on the opposite side (from the real structure or flow) of a strong reflector

9

multiple reflection

several reflections produced by a pulse encountering a pair of reflectors; reverberation

10

Nyquist limit

the Doppler-shift frequency above which aliasing occurs; one half the pulse repetition frequency

11

range ambiguity

an artifact produced when echoes are placed too close to the transducer because a second pulse was emitted before they were received

12

resonance

the condition where a driven mechanical vibration is of a frequency similar to a natural vibration frequency of the structure

13

reverberation

multiple reflection

14

ring-down artifact

an artifact resulting from a continuous stream of sound emanating from an anatomic site

15

section thickness

thickness of the scanned tissue volume perpendicular to the scan plane; also called slice thickness

16

shadowing

reduction in echo amplitude from reflectors that lie behind a strongly reflecting or attenuating structure

17

speckle

the granular appearance of images and spectral displays that is caused by the interference of echoes from the distribution of scatterers in tissue

18

speed error

propagation speed that is different from the assumed value (1.54 mm/µs)