Sound Beams chapter 9

Sound Beams Anatomy of a Sound Beam

1. Focus

  • Focal Point: The location where the beam is the narrowest.

    • The width of the beam at focus is one-half the width of the beam as it leaves the transducer.

2. Near Zone (Nearfield/Fresnel Zone)

  • Description: The region from the transducer to the focus.

  • Features:

    • The end of the near zone is defined by the beam narrowing to one-half the width of the active element.

3. Focal Length/Focal Depth

  • Definition: The distance from the transducer to the focus, also referred to as near zone length.

4. Far Zone (Farfield/Fraunhofer Zone)

  • Description: This region starts at the focus and extends deeper.

  • Characteristics:

    • In the far zone, the beam diverges and spreads out. Initially, the width at this stage is one-half the diameter of the active element before it continues to expand back to the original width of the active element.

5. Focal Zone

  • Definition: A region around the focus where the beam remains narrow.

  • Importance:

    • Reflections from the focal zone yield images with better resolution.

    • Consists of a narrow region on either side of the focal point leading to better image detail.

  • Alternate Definitions:

    • The focus can be described as the focal point, the end of the near zone, the beginning of the far zone, or the middle of the focal zone.

Beam Profile

  • General Overview: The profile details the sound beam as it exits the probe and enters the near Fresnel zone.

  • Beam Diameter Characteristics:

    • At the Transducer: The beam diameter equals the transducer diameter.

    • At Focus: The beam diameter is one-half the transducer diameter.

    • At 2 Near Zone Lengths: The beam diameter again equals the transducer diameter.

    • Deeper than 2 Near Zone Lengths: The beam diameter is wider than the transducer diameter.

Focal Depth & Focal Length

  • Definitions:

    • Focal Depth/Focal Length (Near Zone Length): The distance from the transducer to the narrowest part of the beam.

Phased Array Adjustable Focus Systems

  • Characteristics of Fixed Focus Transducer:

    • The focal depth is determined by the transducer diameter and frequency of the sound.

    • Focal depth and transducer diameter are directly related, while frequency and focal depth are also directly related.

Factors Affecting Focal Depth

  1. Shallow Focus:

    • Smaller diameter piezoelectric crystals (PZT).

  2. Deep Focus:

    • Larger diameter PZT and lower frequency.

  3. Higher Frequency:

    • Higher frequency sound creates a deeper focus if the diameter is very small.

  4. Standard Measurements:

    • Focal depth in mm vs. crystal diameter in mm.

    • Frequency in MHz impacting focal depth.

Sound Beam Divergence

  • Definition: Beam divergence is the spread of the ultrasound beam in the far field.

  • Factors Determining Beam Divergence:

    • Transducer diameter and frequency of the sound.

    • The relationship between crystal diameter and beam divergence is inversely related:

    • Larger diameter crystals result in improved lateral resolution in the far field.

    • Higher frequency also improves lateral resolution in the far field.

Factors Affecting Beam Divergence in the Far Field

  • Less Divergence:

    • Larger diameter and higher frequency produce beams with less divergence.

  • More Divergence:

    • Smaller diameter and lower frequency increase divergence.

  • Divergence Angle Formula:

    • \text{Divergence Angle} = \frac{1.85 \cdot \text{diameter (mm)}}{\text{Frequency (MHz)}}

Spherical Waves

  • Definition of Wave Types:

    • Spherical waves are characterized by a diffraction pattern similar to Huygen's wavelets.

  • Huygen's Principle:

    • Small sources of sound create wavelets that shape the sound beam, exhibiting inconsistencies between large and small sound sources due to phase differences between various emitted wavelets.

  • Impact on Imaging:

    • The shape of an imaging transducer is influenced by whether wavelets are in-phase or out-of-phase, affecting the quality of emitted sound beams.