Anatomy of the Sound Beam

Anatomy of the Sound Beam

• The sound beam follows a curved pathway.
• It concentrates at a point called the focus.
• It diverges in the far field.

Sound Beam Creation

• Sound beam is created by a single disc (old-fashioned concept).
• Modern probes use about 200 crystals.
• The active element is not typically active 100% of the time.
• The shape of the sound beam changes as it travels.

Beam Width

• At the starting point, the beam width is the same as the transducer diameter.
• The beam progressively gets narrower to a point called focus.
• After the focus, it diverges.
• It converges to focus and diverges in the far field.

Key Terms

• Five terms describe the shape and region of the sound beam:
  • Focus
  • Near zone
  • Far zone
  • Focal region
  • Focal zone

Near Zone (Fresnel Zone)

• From the transducer to the focus.
• Also known as the Fresnel zone.
• The distance from transducer to focus is called near zone length or focal length.
• At one focal length, the sound beam converges to focus.
• At two focal lengths, the diameter of the sound beam is the same as the diameter of the transducer.

Far Zone (Fraunhofer Zone)

• From the focus and beyond.
• Also known as the Fraunhofer zone.
• After focus, the beam continues to get bigger (beam divergence).
• Beam diverges in the far field.
• Beam divergence is a key concept for the far field.

Focus

• Located where the beam diameter is narrowest.
• At the focus, the diameter of the beam is half the width as it leaves the transducer.

Focal Length

• Distance from the transducer to the focus.
• Characteristics of the crystal determine the focal length.
• Focal length is the measurement from the transducer to the focus.

Focal Zone

• Area around the focus where the beam is relatively narrow.
• Reflections arising from the focal zone create more accurate images.
• Structures within the focal zone provide better image quality.
• Keeping the structure of interest in the focal zone yields good resolution, while being precisely at the focuses it yields the best resolution.

Resolution

• Resolution is a heavily tested topic (SPI ARDMS exam):

  • Axial resolution
  • Lateral resolution
  • Temporal resolution

• The resolution includes axial, lateral and temporal.

• This is the highest tested chapter in the whole exam.

• Other heavily tested chapters:

  • Doppler
  • Artifacts
  • Transducer
  • Pulse wave parameters

Image quality

• Image quality is superior in the focal zone.
• Image quality at the focus is the "best".
• Focal zone location:
  • Half in the near field
  • Half in the far field