[Ultrasound Physics] SOUND BEAM

[Contents]

 

1. Shape of sound beam

2. Parts of sound beam

3. Beam Diameter

4. Determining the Focal Depth

1. Shape of sound beam

1) Sound beam is not uniform as it travels (beam width changes as it travels)

- The beam width is the same as the transducer diameter at the starting point

(beam width=disk diameter)

- The beam narrows as it travels to the focus: smallest diameter at NZL

(beam width= 1/2 disk diameter)

- After the beam reaches the focus it diverges (expands)

(beam width = disk diameter at 2 NZL, then rapidly diverges)

2. Parts of the Beam [★]

 

1) Near Zone (= Fresnel Zone, Near Field)

- Region from the transducer to the focus

- Beam gradually narrows (converges) within the near zone

- At the end of the near zone, the beam narrows to only ½ width of the active element

- Determined by the size and operating frequency of the element

NZL (mm)= Diameter (mm) X Frequency (MHz)

: If aperture increases, near-zone length increases

: If frequency increase, near-zone length increases

 

2) Focal Length/focal Depth

- The distance (length) from the transducer to the focus.

- The length of the Near Zone (Near Zone Length)

 

3) Focus/Focal Point

- Narrowest part of the beam

: Beam width = ½ Disk diameter       

- Located at the end of the near zone

- The starting point of the far zone

- The middle of the focal zone

- point of maximum intensity in a sound beam

 

4) Focal Zone

- Region around the focus

: Region on either side of the focal point where the beam is relatively narrow

- The area that creates more accurate images

 

5) Far Zone (= Fraunhofer Zone, Far Field)

- starts at focus and extends deeper (Region of the beam beyond the NZL)

- Can’t focus in far zone

- At 2 near zone lengths from the transducer: Beam width = Disk diameter

: depth past 2 near zone lengths are wider than disk diameter

3. Beam Diameter

1) Depends on crystal aperture, frequency, and distance from transducer

- Beam diameter = transducer diameter

- Beam diameter = ½ transducer diameter at the focus

- Beam diameter = transducer diameter at 2 near zone lengths

- Beam diameter > transducer diameter deeper than 2 near zone lengths

2) large beam diameter → focus at greater depths 

4. Determining the Focal Depth

1) focal depth [mm] = D²f/6 or D²/4λ

 

2) Transducer/Disk Diameter: directly related to focal depth

- Large diameter = deeper focus  

- small diameter = shallow focus

 

3) Frequency: directly related to focal depth

- Higher frequency = deeper focus

- lower frequency = more shallow focus

 

3) Divergence

- the divergence of the beam in the far field is also determined by disk diameter & frequency

- Disk diameter (inversely proportional to divergence)

: smaller diameters = greater divergence in far field

: large diameters = less divergence in the far field

- Large disk diameter improves Lateral Resolution

- Frequency (inversely proportional to divergence)

: lower frequency = greater divergence in far field

: higher frequency = less divergence in the far field

-High frequency improves Lateral Resolution

Reference

 

* Davies Ultrasound Physics review

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* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler