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US Physics > Week 1: B-mode imaging > Flashcards

Week 1: B-mode imaging Flashcards

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1
Q

What is B-mode imaging?

A

Cross sectional image of organ & tissue boundaries, constructed from echoes, generated by reflection of US at tissue boundaries & scattering from irregularities within tissues

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2
Q

What are the B-Mode imaging variables

A
  1. Range of the target

2. Direction of the target from active area of transducer (position and orientation)

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3
Q

What is US?

A
  • high frequency (f) sounds waves (MHz)
  • mechanical vibration through a medium
  • internal structure, function & amplitude; motion (BF)
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4
Q

How is an US image produced?

A
  1. US pulses are transmitted into the body
  2. US is reflected or scattered back by surfaces in the body (structures) –> US waves returning detecter by the probe.
  3. Time taken for pulse to go & return = depth of target ( assuming C is constant)
  4. Brightness of image –> relates to amplitude (Z) or returning US
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5
Q

Transducer f changes (dependent on?)

A
  • depth

* properties I.e. density and stiffness

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6
Q

What’s the relationship in f and ATTENUATION

A
  • Lower f US waves penetrate further due to attenuation

* Higher f waves attenuated more quickly than low f

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7
Q

What’s the relationship with ATTENUATION and AMPLITUDE. And what ways is it presented

A

Attenuation- a reduction in amplitude if signal
ABSORPTION –> as waves pass through, some is converted to heat
SCATTERING –> when waves come into contact w/ a smaller target - waves become scattered over large angle e.g. blood

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8
Q

What is Attenunation?

A

US is attenuated when passing through a medium w/ several mechanisms, a reduction in amplitude due to distance/depth

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9
Q

How is attenuation measured?

A

dBcm-1 MHz-1

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10
Q

When echoes from small scatters with sample volume come together to crease random fluctuation in the image brightness. What is this..?

A

Speckle

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11
Q

Constructive interference

A

2 seperate waves in phase meeting at the crest, creating a larger resultant wave w/ > amplitude

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12
Q

Destructive interference

A

when the 2 waves are out of phase i.e. the crest of 1 meets with trough of the other. Producing reduced amplitude or cancel each other out.

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13
Q

What does brightness at certain points relate to?

A
  • boundaries & amplitude; types of material

- amplitude of the returning echo

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14
Q

What is intensity of a reflected wave dependent on?

A
  • (@boundary) acoustic impedance (z) of material p/side
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15
Q

What is acoustic impedance (Z)?

A
  • property of the tissue (dependent on C)

- number of resistance on US beam encounters w/ passing through a medium or tissue

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16
Q

What is Z dependent on?

A
  • density of tissue (kg. m) K.

- C (m/s)

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17
Q

What does returning echo amplitude dependent on?

A
  • change in Z at interface
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18
Q

What US controls govern the brightness of image point

A
  • Gain
  • TGC
  • output power
  • DR/compression
  • Grey scale map
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19
Q

What is Pulse repetition frequency (PRF)?

A

of pulses transmitted p/sec

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20
Q

Why do we only want 1 pulse in flight at a time for PRF

A

To avoid range ambiguity

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21
Q

PRF equation

A

PRF max = C/ 2R

C = Speed of sound
( R = depth)

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22
Q

What is Frame rate (FR)?

A

of images created p/sec

23
Q

Max FR given by:

A

FR = C/ 2RN PRF/N

```
N = # of scan lines
(R = depth)
~~~

24
Q

What happens if theres double scan lines

A

Reduced FR (halved)

25
Q

When looking at moving structure. What do you need to do to FR?

A

Increase it.

26
Q

Resolution

A

Identify 2 objects close to eachother - axial and lateral

27
Q

Contrast Res

A

Able to differentiate 2 objects w/ similar reflective properties

28
Q

Temporal Res

A

Identify 2 events at a time

29
Q

What does sensitivity relate to?

A

Penetration

30
Q

How can the user optimise these features?

A
  • compromise between axial res and penetration - selected f
  • compromise between lateral res and FR - # scan lines
  • selection of appropriate gain & amplitude compression
31
Q

Describe this picture….

A
  • Transmitted US is back scattered from moving blood cells

* The recieved US is unchanged due to velocity of blood cells

32
Q

Intensity (I) or reflected wave at a boundary depends on…… each side of the boundary

A

Acoustic impedance (z) of the material

33
Q

What is the attenuation coefficient for muscle?

A

0.57 dB cm-1 MHz-1

34
Q

What property has the greatest attentunation coefficient?

A

Bone (22dBcm-1 MHz -1)

35
Q

Describe the image….

A
  • US transmitted is back scattered from moving blood cells

* Recieved US is however, changed due to the velocity of the blood cells.

36
Q

Why is resolution important?

A

For high quality image production

37
Q

What information does Colour flow imaging (CFI) show?

A
  • BF direction, mean velocity in relation to US beam
38
Q

In CFI. What does red and blue colours indicate?

A

RED/YELLOW -blood travelling towards the transducer

BLUE - blood is travelling away

39
Q

Detected Doppler Shift Frequency (Fd) = …… and received frequency (Fr)

A

transmitted frequency (Ft)

40
Q

What is the Doppler Shift Frequency dependent of?

A
  • frequency (Ft)
  • C (as it passes through tissue)
  • Velocity of blood
  • angle of insonation (variation of orientation of vessel or probe)
41
Q

Explain this picture…

A
  • Scan lines are sampled several times (sent and recieved)
  • Recieved echoes are displayed at increased depth - moving further away from the transducer.
  • Vertical line corresponds with fixed depth
  • Amplitude changes when crossing the vertical line
  • The change in amplitude of recieved echoes contains the doppler shift and is extracted by demodulation
42
Q

3-16 pulses

A

number of pulses required with CFI

43
Q

What does the displayed colour depend on?

A

angle of insonation

44
Q

Alternation angle of insonation. Can be altered by adjusting the orientation of the probe to get highest Doppler Shift Frequecny.

A

How would you get angle of insonation on higher or lower body limbs that are parallel to skin

45
Q

When would you get the highest Doppler shift f?

A

When vessel and beam are aligned.

46
Q

Whay governs for velocity?

A

Angle of insonation between beam path and BF direction

47
Q

What’s the true and no velocities values?

A

True = 0 degree angle

No velocity detected = 90 degree angle

48
Q

Angle of insonation of US beam relative to ROI, are greatest when echoes produced with ….. approaching ….

A

Angle of incidence

Angle of reflection

49
Q

What happens if angle of insonation is more than 60 degree angle?

A

More chance for velocity error measurements

50
Q

What does Angle correction cursor do in relation to velocity measures?

A

The correct placement is important enabling measurements of angle of insonation so velocity measurements can be obtained

51
Q

At what percentage on the graph are velocity error measurements likely to occur?

A

5% error in velocity measurement ( ~10-20) angle of insonation

52
Q

When would the transducer f change?

A

with;

1) depth
2) tissue properties e.g. density and stiffness

53
Q

In CFI with the principle of angle of insonation, when are echoes greater relave to the tissues and organs?

A

When angle of incidence approaches the angle of reflection

54
Q

What unit is Z presented?

A

kg m−2 s−1 or Rayl

US Physics (6 decks)

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