Lecture Notes on Beam Anatomy - Unit 9

Introduction

• Lecture by Sano Nerds on Beam Anatomy
• Focus on single-element transducer for ultrasound
• Simplified discussion to be extended with modern transducer considerations

Key Concepts

• Beam Shape Changes:

  • Begins at element size, converges to natural focus
  • Diverges in the far field until attenuation

• Terminology and Relationships:

  • Understand alternative area names and how they relate
  • Beam width description at specific areas
  • Relationships:
    • Diameter & Divergence
    • Frequency & Divergence
    • Diameter & Focal Depth
    • Frequency & Focal Depth

Section 9.1: Sound Beam Regions

• Near Zone:
  • Closest to the transducer, ends at focus
  • AKA: Near Field, Fresnel Zone
  • Widest at element diameter (aperture)
• Focus:
  • Narrowest part of the beam
  • AKA: Focal Point, End of Near Zone, Beginning of Far Zone, Middle of Focal Zone
  • Beam width at focus = 1/2 of element diameter
• Far Zone:
  • Starts at focus, diverges indefinitely
  • AKA: Far Field, Fraunhofer Zone
  • Beam returns to original diameter at two near zone lengths
• Focal Zone:
  • Area around the focus where the beam is relatively narrow
  • Extends equally into near and far zones

Practice Problems

• Calculate beam attributes using frequency and diameter
• Formulas:
  • Near Zone Length: (Diameter^2 * Frequency) / 6
  • Divergence Angle: 1.85 / (Diameter * Frequency)

Section 9.2: Focal Depth

• Factors Affecting Focal Depth:
  • Frequency: Directly related (higher frequency, deeper focal depth)
  • Diameter: Directly related (wider diameter, deeper focal depth)

Section 9.3: Beam Divergence

• Formula:
  • Divergence Angle is inversely related to diameter and frequency
• Clinical Implications:
  • Higher frequencies have less divergence, improving lateral resolution

Section 9.4: Summary of Relationships

• Increase in frequency or diameter results in:
  • Deeper focal depth
  • Less divergence
• Decrease in frequency or diameter results in:
  • Shallower focal depth
  • More divergence

Clinical Discussion

• Unfocused Beams:
  • Natural focus due to diffraction and Huygens' principle
• High Frequency and Shallow Imaging:
  • High frequency transducers have small diameters to achieve shallow focal points
• Pulse vs. Continuous Wave:
  • Pulse wave beams form images similar to continuous wave but in pulses

Modern Transducer Considerations

• Pulse Wave Imaging:
  • Scans lines created over time to form images
• Intensity Variation:
  • Intensity is highest just before focus due to attenuation and beam width

Conclusion

• Review workbook and practice problems
• Focus on understanding relationships from formulas
• Be prepared to label beam anatomy and describe regions

These notes capture the essential details and relationships discussed in the lecture about beam anatomy for ultrasound, providing a study reference for understanding how beam shaping affects imaging and resolution.