Lecture Notes: Beam Anatomy

Introduction

• Focus on beam anatomy using a single element transducer.
• Discussion will be simplified for initial understanding, with more advanced concepts later.
• Focus on continuous wave single element transducer.

Ultrasound Beam Shape

• Changes shape as it moves away from the transducer.
• Begins as element size, converges to a natural focus, then diverges indefinitely until attenuated.
• Key Concepts:
  • Names of beam areas.
  • Relationships between regions.
  • Width of the beam at specific areas.
  • Diameter vs. divergence, frequency vs. divergence, diameter vs. focal depth, frequency vs. focal depth.

Section 9.1: Sound Beam Regions

1. Near Zone (Fresnel Zone)

   • Closest to transducer, ends at focus.
   • Width starts equal to transducer diameter (aperture).
   • Near Zone Length: Distance from transducer to focus.
   • Directly related to frequency and diameter.
   • Formula: Near zone length = (diameter^2 x frequency) / 6 (for soft tissue).

2. Focus (Focal Point)

   • Thinnest part of the beam.
   • Middle of the focal zone.
   • Beam width at focus = half of the diameter.

3. Far Zone (Fraunhofer Zone)

   • Starts at focus, continues to diverge.
   • At two near zone lengths, beam returns to element diameter size.
   • Formula for divergence angle: sine of angle = 1.85 / (diameter x frequency).

4. Focal Zone

   • Area around the focus with the narrowest beam.
   • Extends equally into the near and far zones.
   • Important for detail resolution.

Practice Problems

• Use transducer element width and frequency to explore beam characteristics.

Section 9.2: Focal Depth

• Relationship:
  • Frequency and diameter both directly affect focal depth.
  • Higher frequency/diameter = deeper focus.

Section 9.3: Beam Divergence

• Relationships:
  • Diameter and frequency inversely related to divergence.
  • Lower frequency = more divergence.
  • Diameter = more effect on near zone length than frequency.

Section 9.4: Review of Concepts

• Increased Frequency/Diameter:
  • Deep focal depth, less divergence.
• Decreased Frequency/Diameter:
  • Shallow focal depth, more divergence.

Section 9.5: Clinical Discussion

• Modern ultrasound beams more complex than simple models.
• Focus Control:
  • Not just natural focus; affected by lenses, curved elements, and electronic focusing.
  • Huygens' Principle: Explains natural focus through interference of wavelets.
• Intensity Variation:
  • Narrowest beam at focus has highest intensity, but attenuates with distance.
• Pulse Wave vs. Continuous Wave:
  • Similar beam shape, but pulse needs time to form and travel.

Conclusion

• Understanding beam anatomy is crucial for optimizing image quality.
• Next steps: Explore multi-element transducers and image creation.
• Review workbook activities and nerd check questions for better understanding.