Ultrasound Physics

1
Ultrasound Physics
2
Outline

• Understand interference
• Interference and beam formation
• Resolution

3
Interference

• Multiple waves together cause interference.
• Can be constructive and/or destructive
• Can be used to shape and direct beams

4
180 degrees out of phase
No output
5
Different wavelengths
6
Beam forming
Array of Transducers
Wavefronts
7
Directed Beams
If the elements are fired in order, the beam can
be directed
8
Optics of Ultrasound
Focal plane
By using beam forming techniques, a focal plane
can be produced. Here the intensity of the beam
is highest and the clearest picture obtained.
9
Beam characteristics

• Beams are 3 dimensional usually the peak
  intensity is at the centre of the beam
• There are side lobes, which are not on the axis
  of the beam
• Beam focus can be changed.
• There can be more than one focal plane
• If there is more than 1 object in the beam at the
  same depth, they cannot be resolved

10
Resolution

• Spatial (distance) and temporal (time).
• Can be thought of as the point at which two
  objects just merge.
• Axial resolution along the beam i.e. depth
• Lateral resolution across the beam i.e. width.

11
Improving resolution

• Best resolution is at the focus.
• Higher frequencies and shorter pulse length give
  better axial resolution
• Narrower beams give better lateral resolution
  focal point is best and it gets worse with depth.