Artifacts and Hemodynamics Flashcards Preview

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Flashcards in Artifacts and Hemodynamics Deck (39):
1

Errors in imaging

  • not real
  • missing reflectors
  • improper brightness
  • improper shape
  • improper size
  • improper position

2

causes of artifacts

violation of assumptions (acoustic artifacts)

equipment malfunction or design

physics of ultrasound

interpreter error

operator error

3

six basic assumptions of imaging systems

  1. sound travels in a straight line
  2. sound travels directly to a reflector and back
  3. sound travels in soft tissue at exactly 1540 m/s
  4. reflections arise only from structures postitioned in the beams main axis
  5. the imaging plane is very thin
  6. the strength of a reflection is related to the characteristics of the tissue creating the reflection

4

Reverberations

multiple echoes appearing on the display as a result of US ping-poning between two reflectors.  Looks like a ladder or a venetian blind.

multiple, equally spaced, parallel to the sound beam, and deeper and along a straight line

5

Comet tail or ring down

appears as a soild line directed downward, merged reverberation

signle soild hyperechoic line, long echo, parallel to the sound beam, a reverb with the space squeezed out

6

Shadowing

US beam unable to pass through a structure because the structure has a higher than usual attenuation.

hypo- or anechoic postior shadowing, prevents visualization of true anatomy on the scan

7

Edge shadow or shadowing by refraction

refraction at the edge of a circular structure can also create an artifact.

Hypoechoic, due to refraction and beam spreading, absence of true anatomy 

8

Enhancement

occurs when the medium through which the sound travels has a lower attenuation rate than soft tissue.  the correct number of anatomic reflectors appear on the image.

parallel to the sound beam, hyperechoic

9

mirror image

Sound may bounce off of a strong reflector and be redirected.  Ultrasound systems assume that sound travels directly to a reflector and back to the transducer.

A second copy of a true reflector, the artifact appears deeper than the true reflector, true reflector and artifact are equal distances from the mirror.

10

Propagation speed errors (range error artifact)

If US doesnt propagate at 1550 m/s, assume relationship between the distance and time is invalid.

correct number of reflections on scan, improper depths

11

When the propagation speed is greater than 1540 m/s, the reflector will be placed too ______.

When the speed is less than soft tissues, the reflector will be placed too ______ on the display.

Shallow

Deep

12

refraction artifact

Sound changes direction striking a boundary: obliquely and when the media have different propagation speeds.

Second copy of reflector

Artifact appears side-by--side with the true anatomic structure

13

What tyoe of resolution does refraction artifact degrade?

lateral resolution

14

Side lobes and grating lobes

Extra acoustic energy may be transmitted in directions other tha  the beams main axis.  

Second copy of the true reflector

artifact appears side-by-side with the true anatomic structure

15

Transducer associated with side lobes?

Mechanical or single crystal transducers create side lobes

16

Grating lobes

Arrays create grating lobes

17

Subdicing

grating lobe artifact can be reduced or cured by dividing each element into even smaller, miniature pieces

18

Apodization

Grating lobes are further reduced by exciting the subdiced elements with different voltagesSubelements closer to the center of the sound beam are excited with higher voltages, further away from the center of the beam are excited with lower voltages

19

Lobe artifact degrades what type of resolution

lateral

20

The assumption that reflections arise from the beams main axis is violated by what artifact?

lobe artifact

21

Slice Thickness

occurs when beam has a greater width than the reflector

22

what type of resolution is associated with slice thickness

elevational 

23

Reflections produced by structures above or below the ideal imaging plane appear in the image?

partial volume artifact or section thickness artifact

cured with 1 1/2 dimensional array

24

Speckle

Grainy appearance not directly from reflections from tissues.

Created by interference effects of scattered sound, both constructive and destructive.

25

Range ambiguity artifact

Caused when very deep reflections arrive at transducer after the next pulse was created.

Places late arriving reflection too shallow on the image

26

How can range ambiguity artifact be cured?

by lowering the PRF because the PRF is too high

27

Artifacts are created when the sound beam is _____ than the reflector?

larger

28

Laminar flow are aligned and parallel and may have what tyoe of patterns?

Parabolic or plug

29

When small regions of turbulent flow have vastly different velocities?

Doppler Spectral Broadening

30

Blood flows when the total fluid energy at one location differs from the total fluid energy at another location?

Energy gradient

31

Units of viscosity

poise

32

Stenosis causes:

  • changes in flow direction
  • increased velocity, highest at the point of max narrowing
  • turbulence at exit
  • pressure gradient across the stenosis
  • arterial gradient across the stenosis
  • arterial flow loses its pulsatile nature and becomes more continuous

33

When velocity at the point of max narrowing.  Pressure is the lowest here.

Bernoulli's principle

34

Factors that determine resistance

radius of lumen, length, viscosity of fluid

35

pressure measured=

circulatory pressure + hydrostatic pressure

36

Coaptation

vessel collapse

37

Inhale

diaphragm moves down

venous flow from legs decreases

venous return to the heart increases

38

Exhale

Diaphragm moves up

Venous flow from legs increases

Venous return to heart decrease

39

Hydrostatic pressure

-50 mmHg at fingertip with hand elevated

-30 mmHg at head

0 mmHg at heart

15 mmHg at knee

100 mmHg at ankle