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Flashcards in Intro to Ultrasound Deck (54):
1

What is ultrasound?

High-frequency sound waves--beyond the range of (human) hearing (20 Hz - 20 kHz)

2

How many MHz is diagnostic ultrasound?

1-30 MHz

3

What tomographic modality is US?

First

4

What are the physics behind ultrasound?

  • Mechanical energy as soundwaves are transmitted through a patient (tissues) and returning echoes are recorded 
  • Assumption: a constant velocity of sound within soft tissues even though differences exist
  • Propagation velocity (1540 m/s)
  • The interface: acoustic impedence (Z) = velocity (v) x tissue density 

5

Quantifying sound

  • Wavelength
  • Frequency
  • Velocity = wavelength x frequency
  • Inverse relationship (important in choosing a transducer

6

Wavelength vs. frequency

  • Wavelength = distance between one peak or trough and the next peak or trough (mm)
    • Frequency = cycles per second (Hz)

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Why are frequency and wavelength important?

  • Resolution
    • Better with higher frequency
    • Small wavelength
  • Penetration
    • Better lower frequency
    • Long wavelength
  • Attenuation
    • Occurs with higher frequency
    • Less returning information 

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8

What happens to the sound as it interacts with tissue in US?

  • Attenuation
    • Reflection
      • Acoustic impedence
    • Refraction 
    • Absorbtion
  • Transmission 

9

What is attenuation? What is it increased with?

  • The loss of ultrasound 
  • Increased with:
    • Increased distance from the transducer
    • Less homogenous medium to transverse due to increased acoustic impedance mismatch
    • Higher frequency (shorter wavelength) transducers

10

Acoustic impedance values

  • Air = 0.0004
  • Bone = 7.80
  • Both are strong interfaces

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Acoustic impedance--general

  • Impedance (Z) is a characteristic of the propagation medium
  • A reflected sound wave is generated at the interface of an impedance mismatch
  • No reflections occur in a homogenous medium (constance impedance)

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12

Principles of impedance (3)

  • Pulse-echo principle
    • Emitting 1% of the time, listening 99% of the time
  • Round-trip transit time is directly related to the depth, i.e. distance of the wave reflection site
  • The amount of reflected sound depends on the acoustic impedance 

13

How deep is the interface (explain reasoning)?

  • U/S assumes speed of sound in tissues is 1540 m/s
  • Sound is sent--timer is started, sound hits interface (time to interface)
  • Sound is reflected, hits transducer--timer is stopped (total round trip time)
  • Total round trip time needs to be divided by 2 to represent interface
  • In one second sound travels 1540 m

14

What assumptions does the U/S machine make?

  • The speed of sound in all tissues is 1540 m/s
  • The U/S beam only travels in a straight line with a constant rate of attenuation
  • The U/S beam is infinitely thin with all echoes originating from its central axis
  • The depth of a reflector is accurately determined by the time taken for sound to travel from the transducer to the reflector and return 

15

T/F: Artifacts are present in only some ultrasonagrams and are never helpful.

FALSE--Artifacts are present in EVERY single ultrasonagram and can be helpful or confusing.

16

What are the 6 artifacts?

  1. Acoustic shadowing
  2. Acoustic enhancement
  3. Edge shadowing
  4. Reverberation artifact
  5. Slice thickness artifact
  6. Mirror image artifact

17

Acoustic shadowing (general)

  • Distal to highly reflective objects (high acoustic impedance mismatch)
    • Bones
    • Air
  • Interface absorbs or reflects entire sound
  • Results in an anechoic area

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Clean acoustic shadowing

  • At the tissue-bone interface
  • Substantial amount gets absorbed
  • Complete absence of reverberation artifacts
  • A "clean" shadow is produced (homogenous anechoic)
  • Ex: FOREIGN BODY

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19

Dirty acoustic shadowing

  • At the tissue-gas interface
  • 99% of the sound wave gets reflected
  • Acoustic shadow is dirty (inhomogenous/reverberation artifact)

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20

Acoustic enhancement

  • Fluid of homogenous acoustic impedance attenuates less sound than the surrounding tissue
  • Machine processing compensates (--> overcompensation)
  • Results in a hyperechoic area distal to the structure in comparison to the surrounding tissue

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Edge shadowing

Small shadow at the edge of round structures

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22

Slice thickness artifact

  • At curved surfaces
    • urinary bladder
    • gall bladder
  • Can mimic sediment
  • Curve of structure gives 2 colors--> machine takes average 

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23

Mirror image artifact

  • At highly reflective air/fluid interfaces
    • Diaphragm-lung
    • Pericard-lung
  • Concave structures
  • False image is produced on other side of the reflector due to its mirror like effect
  • Mirror image is not useful

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Interpretation artifact

  • Highly dependent on viewer
  • Most common artifact for inexperienced viewers

25

What is the enemy of ultrasound?

AIR

26

What is the purpose of coupling gel?

  • Provides a media that conforms to the patient
  • Prevents loss of sound due to the compressibility of air
    • Air acts like a shock-absorber and dampens the ultrasound wave

27

What are the 4 transducer types?

  1. Multi-frequency
  2. Linear
  3. Convex
  4. Concave

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How does the transducer work?

  • Piezoelectric crystals both emit ultrasound and receive it
  • Choose highest frequency that will penetrate to depth of the patient

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29

What are the ultrasound machine controls?

  • Power (intensity, output)
  • Absolute gain (amplification)
  • Time gain/depth compensation
  • Focus
  • Mode
  • Measurement tools
  • Freeze

30

Power and gain settings

  • Overall gain: every signal returned will be reinforced/enhanced
    • Whole image will be homogenously more gray

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31

What does time-gain compensation (TGC) do?

Selectively influences certain areas of the image

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32

Focusing the U/S beam

  • Because it's a wave, U/S can be focused
  • Structures should be investigated near the focal point

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33

What are the different modes of U/S?

  • M-mode
    • Motion
  • B-mode
    • Gray-scale (b = brightness)
  • Doppler
    • Color--blood flow
    • BART = Blue Away, Red Toward

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34

What are the 8 steps in preparation for U/S?

  1. 12 hr fasting
  2. Free access to water
  3. Avoid stress--> aerophagia
  4. Shave fur
  5. Dorsal recumbency 
  6. U/S machine and examiner on left side of table (right side of patient)
  7. Dog's head in direction of machine
  8. Acoustic coupling gel

35

What are the different scanning planes?

  1. Saggital or dorsal
  2. Transverse
  3. Refers to the organ or the animal
  4. Each organ must be examined in 2 planes

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Saggital plane

Transducer cranially oriented

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37

Transverse plane

  • Transducer toward examiner oriented
  • Cross-section

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38

What are the 4 transducer positions?

  1. Turn
  2. Move/slide
  3. Angle/fan
  4. Reposition

39

Echo signs

  • Same as Rontgen signs:
    • Size
    • Shape
    • Number
    • Location
    • Margination
    • Echogenicity 
  • +
    • Homogeneity
    • Texture
    • Compressibility
    • Surrounding tissue
    • Vascularity 
    • Through-transmission
    • Other artifacts

40

Echogenicity

  • Anechoic
  • Hyperechoic
  • Isoechoic 
  • Hypoechoic
  • Normoechoic

41

Anechoic

  • Homogenously black
  • Very low intensity of returning echoes

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Hypoechoic

  • Less echoic than other structures
  • Reference point needed
  • Refers in general to medium-gray tones
  • Low intensity of returning signals

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43

Isoechoic

  • Same echogenicity of another structure
  • Same intensity of returning echoes compared to adjacent tissues

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44

Hyperechoic

  • Higher echogenicity than other structures
  • Needs a reference point
  • Refers in general to white structures
  • High intensity of returning signal

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45

Normoechoic

Returning signal is as expected for a particular organ

46

Surrounding tissue

  • Compressed
  • Invaded
  • Ex: free fluid in abdominal cavity

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47

Vascularity

Doppler (BART)

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48

Through-transmission

  • Acoustic enhancement
  • Acoustic shadowing
  • + other artifacts

49

Scanning technique (direction)

  • Start from left side:
    • Spleen
    • Adjust machine
    • Left kidney
    • Bladder
    • Prostate/uterus 
  • Continue on right side
    • Right kidney
    • Liver
    • GIT
  • Rest of abdomen:
    • Pancreas
    • Adrenals

50

Echogenicity triad

  • My Cat Loves Sunny Places
    • Kidney --> liver --> spleen (most echoic)
  • Use split screen to compare organs in different planes
  • Echogenicities may only be compared at same levels

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51

Free abdominal fluid

  • Earliest accumulation
    • Apex of bladder
    • Between liver lobes
  • Mobility--ballotment
  • Artifact: acoustic enhancement
    • Everything distal to the organ will look brighter
  • Anechoic = transudate
  • Speckled/more echoigenic = exudate, blood, chyle

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52

Biopsy

  • Complicated
    • Sterility
    • Guides
    • Biopsy tools
    • Complications
  • Invasive
    • Large gauge core samples

53

Fine needle aspirate (FNA)

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  • Easy
    • Simple set-up
    • Needle and syringe
  • Non-invasive?
    • Small gauge aspirates
  • Results often diagnositic, but...?

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