DMS 240 Sound Beams

Page 1

  • Title: Sound Beams DMS 240

  • Author: Pascual Nava, RDMS, RVT

Page 2

Spherical Waves

  • Sound waves produced by PZT diverge in a V-shape.

  • Created when the source's size matches the sound's wavelength.

  • Known as Huygens' wavelets.

  • Huygens' Principle: A large active element can be seen as many tiny sound sources, creating individual wavelets.

  • Interference of wavelets results in an hourglass-shaped sound beam; Huygens' sound wavelets can be in-phase (constructive interference).

Page 3

Shape of Sound Beam

diameter and sound beam directly related

  • The width of the beam changes as sound travels.

    • Starts equal to transducer diameter, narrows to smallest point, then expands.

  • Five terms describing the shape and regions:

    • Focus most narrow

    • Near zone (Fresnel zone) near the transducer

    • Focal length (near zone length)

    • Far zone (Fraunhofer zone)

    • Focal zone

Focus

  • Focus: The location where the sound beam is narrowest.

  • Width at the focus is half the width as the beam leaves the transducer.

Near Zone

  • Also referred to as the near field or Fresnel zone.

  • Focus is situated at the end of the near zone.

Focal Length

  • Known as focal depth or near zone length.

  • This is the distance from the transducer to the focus.

Far Zone

  • Known as the far field or Fraunhofer zone.

  • Extends from the focus deeper into the body.

  • At the beginning of the far zone, the beam is ½ as wide as at the transducer.

  • The beam diverges further, equaling the transducer diameter when at two zone lengths from the transducer.

Focal Zone

  • The region around the focus where the beam remains relatively narrow.

  • Characterized by superior image detail.

Page 9

Sound Beam Diameters Location

  • Beam Diameter Locations:

    • At the transducer: Equal to transducer diameter.

    • At focus: ½ the transducer diameter.

    • At 2 near zone lengths: Equal to transducer diameter.

    • Deeper than 2 near zone lengths: Wider than transducer diameter.

Page 10

Transducer Diameter and Focal Depth

  • Larger diameter results in deeper focus.

  • Direct relationship between transducer diameter and focal depth.

Page 11

Transducer Frequency and Focal Depth

  • Higher frequency leads to a deeper focus.

  • Directly related: Frequency and focal depth.

Page 12

The Issue with High Frequency Sound Beams

  • High frequency transducers are used to image shallow structures but have drawbacks.

  • Manufacturers use small, high-frequency crystals.

  • Mathematical relationship in soft tissue:

    • Focal depth (mm) = (diameter (mm)² x frequency (MHz)) / 6

    • Focal depth (mm) = (diameter (mm)²) / (4 x wavelength (mm))

Page 13

Sound Beam Divergence

  • Describes the gradual spread of the sound beam.

  • Two factors determining beam divergence:

    • Transducer diameter

    • Frequency of the sound

  • Smaller diameter crystals diverge more in the far zone; larger diameters enhance lateral resolution.

  • Lower frequency sound shows more divergence.

Page 14

Sound Beam Divergence Cont.

  • Mathematical relationships describing divergence:

    • Sin divergence angle = 1.85 / (diameter (mm) x frequency (MHz))

    • Sin divergence angle = 1.2 x wavelength / diameter.

  • Divergence relationships:

    • Less divergence: Larger diameter, Higher frequency

    • More divergence: Smaller diameter, Lower frequency