Module 3 : Ultrasonic Field Flashcards
beam uniformity - near fields
- 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
beam uniformity - far field
- far fields have uniform beams
- mostly wave form diverging
beam uniformity - intensity
- 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
beam shape - grating lobes
- off axis beams in array probes
- result of length and width vibration of crystal resulting in cross talk
beam shape - side lobes
- off axis beams in mechanical probes
beam shape - main beam
- contains most of the energy and the grating lobes are weaker
- result of radial mode vibration in the single disc probe
2 zones in beam
- fresnel
- Fraunhofer
fresnel zone
- near field
- constant beam width
Fraunhofer zone
- far field
- diverging beam width
beam shape - natural focus
- natural narrowing occurs at one near zone length NZL
- point is called the transition zone
- beam is 1/2 crystal diameter at this point
usable beam shape
- length of beam is equal to 2 near zone lengths
- narrow enough to provide reasonable lateral resolution
near zone length equation
- NZL = D^2 / 4 wavelength
+ d = diameter
Near zone length equation for soft tissue
- NZL = D^2 x f / 6
near zone length relation ships in soft tissue
increase frequency = increase near zone length
increase diameter = increase near zone length
near zone length relation ships
increase frequency = decrease wavelength = decrease SPL = increase NZL
= increased axial resolution
far field divergence equation
sin 0 = 1.22 x wavelength / D
+ d= diameter
Fairfield relationships
- increasing frequency or diameter = decrease angle of divergence = better lateral resolution
beam shape - array probe
- diameter controlled by aperture
- as depth of focus increases the aperture increases to maintain and relatively consistent beam width at the focus
focal zone and near zone length relationship
- as we push the near zone length deeper aperture/diamter gets larger
- lateral resolution takes a hit at the probe
focusing
- prime resin for focusing is to improve our lateral resolution
- want to decrease beam width
- improve sensitivity
two main types of focusing
- mechanical
- electronic
mechanical focusing
- internal and external
- focus is fixed has 3 focal lengths
internal mechanical focusing
- curve applied to crystal itself
- curved crystal will help focus the sound
- concave curve
- crystal thickness 1/2 wavelength
external mechanical focusing
- accomplished with acoustic lens or mirror
- fixed
mechanical focal lengths
- short
- medium
- long
short focal length
1-4cm
weak
medium focal length
4-10cm
long focal length
7-19cm
strong
electronic focusing
- in array probes
- variable and operator controlled
- TRANSMIT FOCUSING
transmit focusing
- 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
large delays
- sharper but less NZ
disadvantage to transmit focus
- divergence in far field increases
- near zone length decreased
- these are over come by dynamic aperture and frequency
dual focusing
- 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
elevational plane
- z axis
- mechanical lens focuses this
multiple focus
- 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
receive focus
- 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
slice thickness
- 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
effective beam shape
- 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
controls of effective beam shape
- determine the sensitivity of the system
- gain
- power
- suppression (reject)
