# Module 3 : Ultrasonic Field Flashcards

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1
Q

beam uniformity - near fields

A
• non uniform beams as the result of the interference between the wavelets
• many frequencies emitted from low Q probes also make near field less uniform
• all interference
2
Q

beam uniformity - far field

A
• far fields have uniform beams

- mostly wave form diverging

3
Q

beam uniformity - intensity

A
• can represent beam uniformity to intensity changes
• near field
+ less uniform intensity
• far field
+ intensity levels out and drops off do to attenuation
+ more uniform intensity
4
Q

beam shape - grating lobes

A
• off axis beams in array probes

- result of length and width vibration of crystal resulting in cross talk

5
Q

beam shape - side lobes

A
• off axis beams in mechanical probes
6
Q

beam shape - main beam

A
• contains most of the energy and the grating lobes are weaker
• result of radial mode vibration in the single disc probe
7
Q

2 zones in beam

A
• fresnel

- Fraunhofer

8
Q

fresnel zone

A
• near field

- constant beam width

9
Q

Fraunhofer zone

A
• far field

- diverging beam width

10
Q

beam shape - natural focus

A
• natural narrowing occurs at one near zone length NZL
• point is called the transition zone
• beam is 1/2 crystal diameter at this point
11
Q

usable beam shape

A
• length of beam is equal to 2 near zone lengths

- narrow enough to provide reasonable lateral resolution

12
Q

near zone length equation

A
• NZL = D^2 / 4 wavelength

+ d = diameter

13
Q

Near zone length equation for soft tissue

A
• NZL = D^2 x f / 6
14
Q

near zone length relation ships in soft tissue

A

increase frequency = increase near zone length

increase diameter = increase near zone length

15
Q

near zone length relation ships

A

increase frequency = decrease wavelength = decrease SPL = increase NZL
= increased axial resolution

16
Q

far field divergence equation

A

sin 0 = 1.22 x wavelength / D

+ d= diameter

17
Q

Fairfield relationships

A
• increasing frequency or diameter = decrease angle of divergence = better lateral resolution
18
Q

beam shape - array probe

A
• diameter controlled by aperture

- as depth of focus increases the aperture increases to maintain and relatively consistent beam width at the focus

19
Q

focal zone and near zone length relationship

A
• as we push the near zone length deeper aperture/diamter gets larger
• lateral resolution takes a hit at the probe
20
Q

focusing

A
• prime resin for focusing is to improve our lateral resolution
• want to decrease beam width
• improve sensitivity
21
Q

two main types of focusing

A
• mechanical

- electronic

22
Q

mechanical focusing

A
• internal and external

- focus is fixed has 3 focal lengths

23
Q

internal mechanical focusing

A
• curve applied to crystal itself
• curved crystal will help focus the sound
• concave curve
• crystal thickness 1/2 wavelength
24
Q

external mechanical focusing

A
• accomplished with acoustic lens or mirror

- fixed

25
Q

mechanical focal lengths

A
• short
• medium
• long
26
Q

short focal length

A

1-4cm

weak

27
Q

medium focal length

A

4-10cm

28
Q

long focal length

A

7-19cm

strong

29
Q

electronic focusing

A
• in array probes
• variable and operator controlled
• TRANSMIT FOCUSING
30
Q

transmit focusing

A
• if all elements in an array are excited at the same time then act as single flat disc
• we can apply a delay to crystal to steer the probe but delays can also focus
• when delays are added they can converge at a focal point
• increasing delays will increase focus
31
Q

large delays

A
• sharper but less NZ
32
Q

A
• divergence in far field increases
• near zone length decreased
• these are over come by dynamic aperture and frequency
33
Q

dual focusing

A
• refers to the use of both mechanical and electronic focusing in probe
• beam is 3D so we need to focus in z-axis ro elevational plane
34
Q

elevational plane

A
• z axis

- mechanical lens focuses this

35
Q

multiple focus

A
• possible to have more than one transmit focus on image
• multiple focus expands the focal region creating a long focus
• requires multiple pulses per scan line with each pulse focused at different depth
• frame rate reduced but resolution optimized
36
Q

A
• time delays applied to the recited echo to allow for constructive interference
• does not effect frame rate and is not operator controlled
• done dynamically as echoes come back from deeper depth
• goal is to bring echoes into phase so don’t cancel
• DYNAMIC RECIEVE FOCUS
37
Q

slice thickness

A
• another way to describe elevational plane
• depend on beam width perpendicular to image plane
• cystic structure smaller than slice thickness demonstrate false debris from echoes in off axis beam
• fixed and requires a curved element or lens to help reduce thickness at a fixed depth
38
Q

effective beam shape

A
• most effective = in NZL and central
• next effective = in far field but central
• next effective = in far field but off axis
• next effective = of central beam shape in far field
• least effective = deeper than far field and off axis
39
Q

controls of effective beam shape

A
• determine the sensitivity of the system
• gain
• power
• suppression (reject)