Module 3 : Ultrasonic Field

Question 1

beam uniformity - near fields

Answer 1

• 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

Question 2

beam uniformity - far field

Answer 2

• far fields have uniform beams

- mostly wave form diverging

Question 3

beam uniformity - intensity

Answer 3

• 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

Question 4

beam shape - grating lobes

Answer 4

• off axis beams in array probes

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

Question 5

beam shape - side lobes

Answer 5

• off axis beams in mechanical probes

Question 6

beam shape - main beam

Answer 6

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

Question 7

2 zones in beam

Answer 7

• fresnel

- Fraunhofer

Question 8

fresnel zone

Answer 8

• near field

- constant beam width

Question 9

Fraunhofer zone

Answer 9

• far field

- diverging beam width

Question 10

beam shape - natural focus

Answer 10

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

Question 11

usable beam shape

Answer 11

• length of beam is equal to 2 near zone lengths

- narrow enough to provide reasonable lateral resolution

Question 12

near zone length equation

Answer 12

• NZL = D^2 / 4 wavelength

+ d = diameter

Question 13

Near zone length equation for soft tissue

Answer 13

• NZL = D^2 x f / 6

Question 14

near zone length relation ships in soft tissue

Answer 14

increase frequency = increase near zone length

increase diameter = increase near zone length

Question 15

near zone length relation ships

Answer 15

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

Question 16

far field divergence equation

Answer 16

sin 0 = 1.22 x wavelength / D

+ d= diameter

Question 17

Fairfield relationships

Answer 17

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

Question 18

beam shape - array probe

Answer 18

• diameter controlled by aperture

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

Question 19

focal zone and near zone length relationship

Answer 19

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

Question 20

focusing

Answer 20

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

Question 21

two main types of focusing

Answer 21

• mechanical

- electronic

Question 22

mechanical focusing

Answer 22

• internal and external

- focus is fixed has 3 focal lengths

Question 23

internal mechanical focusing

Answer 23

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

Question 24

external mechanical focusing

Answer 24

• accomplished with acoustic lens or mirror

- fixed

Question 25

mechanical focal lengths

Answer 25

- short - medium - long

Question 26

short focal length

Answer 26

1-4cm | weak

Question 27

medium focal length

Answer 27

4-10cm

Question 28

long focal length

Answer 28

7-19cm | strong

Question 29

electronic focusing

Answer 29

- in array probes - variable and operator controlled - TRANSMIT FOCUSING

Question 30

transmit focusing

Answer 30

- 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

Question 31

large delays

Answer 31

- sharper but less NZ

Question 32

disadvantage to transmit focus

Answer 32

- divergence in far field increases - near zone length decreased - these are over come by dynamic aperture and frequency

Question 33

dual focusing

Answer 33

- 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

Question 34

elevational plane

Answer 34

- z axis | - mechanical lens focuses this

Question 35

multiple focus

Answer 35

- 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

Question 36

receive focus

Answer 36

- 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

Question 37

slice thickness

Answer 37

- 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

Question 38

effective beam shape

Answer 38

- 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

Question 39

controls of effective beam shape

Answer 39

- determine the sensitivity of the system - gain - power - suppression (reject)