Chapter 3 Transducers - quick Notes

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1

thickness

thickness = c/(2*fo)

2

In pulsed mode what determines the operating frequency?

fo = c/(2*thickness)

3

Pd

Pd = Period * n

4

NZL Formula

(D2 * f) / 6

5

Axial Resolution

Axial R = 1/2 SPL

6

Lateral Resolution

Lateral R = Beamwidth

7

Focus width

1/2 element thickness

8

SPL

SPL = nc/f

9

Wavelength

λ = c/f

10

fractional bandwidth

fractional bandwidth = bandwidth/operating frequency

11

Q factor

Q factor = operating frequency/bandwidth

12

Bandwidth

bandwidth = max frequency - min frequency

13

man made piezoelectric material

lower impedance
improved bandwidth, sensitivity, resolution

14

How many cycle pulses does one cycle of alternating current produce?

2-3

15

How many cycles of alternating current are needed to produce CW?

5 - 30

16

Operating frequency equals

the freqency of the sound produced

17

What determines te operating frequency

thickness and speed of material

18

Thickness of the element equal

1/2 wavelength

19

Typical thickness and speed of element

1 - 2 mm thick

4 - 6 mm/μs speed

20

What effect does thinner element have on frequency?

Higher frequencies which create better detail resolution

21

What is the importance of wide-bandwidth transdcers?

the transducer is driven at 1 - 3 selectable frequencies.

higher frequencies create better resolution
but if the depth is not suffient a low frequency can be chosen

make harmonics possible - low frequency goes in higher frequency returns

22

Pulse Repetition Voltage

PRV

Equal PRF

The number of voltage pulses sent to the transducer each second

23

Pulse duration formula

Pd = Period * n

24

What is damping material made from?

epoxy and plastic

25

WHat does damping reduce?

SPL
Pd
Q-factor

26

WHat does damping increase

frequency
bandwidth

RESOLUTION

27

WHat is the reason for using backing material?

improve sound transmission across the element/tissue boundary

transducer elements are a solid with high impedance

impedance 20 times higher than skin, with out it 80% of intensity would be reflected at the skin.

28

How many layers of backing are used?

typically 2

multiple layers are best

29

What is the purpose of Coupling medium?

eliminate the air between the transducer and skin

30

Why does little Francus like Ducks?

Wide divergence

low frequency

low diameter

31

NZL formula

(D2 * f) / 6

32

What happens to near zone length with increasing frequency or aperture?

increases

33

Define convergence

decrease of size with increasing depth

Fresnel

34

Define divergence

increase of size with increasing depth

fraunhofer zone

35

What happens during focusing?

the end of the near zone is moved closer to the transducer and beam is narrowed.

36

What is the Focal length

card image

depth of field

37

How is focusing complished?

lens
curved footprint
Phasing

38

Define array

made up of many elements

an element has been sliced

39

Define linear and describe the signifgance of linear transducers.

straight line

Linear transducers produce images that are rectangle

40

Define sequencing

card image

applying voltage to groups of elements in succession

Think of taking a group of photos in a line to be spliced togething in PS for panaramic.

Shoot slide shoot slide shoot slide shoot

41

Explain Linear squenced Array

element in a straight line
squenced - pulses originating at different points but all traveling in the same vertical direction

42

Define convex and describe the signifgance of convex transducers.

curved line (bowed outward)

Convex transducers produce sector-like images
pizza w/bite

Sequencing - pulses originating at different points and traveling out in different directions.

43

Define Phasing

Electrical steering

voltage is sent to most or all elements with small time differences so that the resulting pulse is sent out in a specific direction

44

How do you determine the direction of the steered beam?

voltage applied from left to right will go right

45

Explain electronic focus.

accomplished with a curved pattern of phasing.

Greater curvature places the focus closer to the transducer

46

Explain multi foci

multiple pulses per scan line each focused at a different depth

47

What are the advantages and disadvantages of multi foci

improves detail resolution

degrades temporal resolution

48

Variable Aperture

to maintain the same beam width at increasing focal lengths

larger groups are used for deeper focal lengths

49

Explain section thicnkess focus

With multiple rows of element phasing can be applied to focus the slice thickness

50

What is a grating lobe?

additional beams that go off in a direction out of the scan plane.

Cause artifacts

Array transducers only

*side lobes are produced from single element transducers

51

How are grating lobes reduced?

Apodization

lowering amplitude of outer elements

dynamic - changed on the fly

52

Explain the signifigance of Vector Arrays

Phasing is applied to each element group during sequencing to steer pulses in various directions.

Produced a vector shape image

53

How is a parallelogram image produced?

when phasing is applied to linear sequenced arrays

each pulse travels in the same direction but not straight down.

54

Explain dynamic reception focus

reception focus depth changed continually as the transmitted pulse travels through tissue and the echoes arrive from deeper and deeper locations

55

What is dynamic aperture?

aperture increases to maintain a constant focal width with changing focus

56

List the transducers

Linear Array - phased and sequenced

Convex Array - phased and sequenced

Phased Array - Phased

Vector Array - Phased and sequenced (linear - small)

Annular

57

What are the three aspects of Resolution?

Detail
*Lateral
*Axial

Contrast

Temporal

58

What is Axial resolution?

resolution along the direction of the beam.

59

What determines Axial Resolution?

SPL
*Frequency
*Speed
*# of pulses

60

Axial Resolution formula

1/2 SPL

61

What is the trade off for improving Axial / Lateral resolution by raising the frequency?

Depth is lost

62

What is Lateral resolution?

resolution across the beam

63

What determines Lateral Resolution?

beam width

64

How can Lateral Resolution be improved?

focusing - decreasing beam width

Increasing frequency

65

What is the useful U/S range of Frequency

2 MHz to 15 Mhz

Lower than 2 MHz and resolution sucks
Higher than 15 MHz and Depth is insufficient

66

What is the relationship between Axial Resolution and # of pulses?

indirect

67

What is the relationship between Axial / Lateral Resolution and Frequency?

direct

*note
The numeral value of Resolution goes down when frequency goes up but the numberal value is best at low numbers. Therefore making this a direct relationship.