Overview
This lecture covers the anatomy of the ultrasound beam, outlining its primary regions, key formulas, and important relationships between transducer properties, beam shape, and image resolution.
Sound Beam Regions
- The sound beam is divided into five regions: near zone, far zone, focus, focal zone, and focal length.
- The near zone (also called near field or Fresnel zone) extends from the transducer to the focus.
- The diameter of the beam at the transducer equals the element's diameter (aperture).
- Focal length, near zone length, and focal depth all refer to the distance from the transducer face to the focus (narrowest part).
- The focus (focal point) is where the beam is narrowest and equals half the transducer diameter.
- The far zone (far field or Fraunhofer zone) starts at the focus, where the beam diverges (widens).
- At two near zone lengths, the beam regains the original diameter and widens further beyond.
- The focal zone extends equally into near and far zones and is centered on the focus; it provides the best lateral resolution.
Mathematical Relationships & Formulas
- Near zone length (mm) = (diameter² × frequency) / 6 (for soft tissue).
- At the focus, beam width = 0.5 × diameter.
- At two near zone lengths from the transducer, beam width = original diameter.
- Beam divergence angle: sin(θ) = 1.85 / (diameter × frequency).
- Diameter and frequency are directly related to near zone length and focal depth.
- Diameter and frequency are inversely related to beam divergence.
Practice Problems & Examples
- Use provided formulas to calculate near zone length, focus depth, and beam widths at various points.
- To find the focal zone endpoints: calculate the distance equally on both sides of the focus.
- Beyond two near zone lengths, beam width is greater than the transducer diameter.
Key Relationships
- Higher frequency and larger diameter yield deeper focal depth and less divergence.
- Lower frequency and smaller diameter yield shallower focus and more divergence.
- Better lateral resolution in the far field is achieved with high frequency and large diameter transducers.
Clinical Considerations & Modern Application
- Natural focus occurs due to Huygens' principle, not electronic focusing.
- High-frequency transducers are manufactured with small diameters for shallow imaging.
- Continuous and pulsed beams have similar shapes over time.
- Beam intensity is highest just above the focus due to the balance of attenuation and narrowing.
- Multiple scan lines (each beam) create the image in modern ultrasound; modern transducers use multiple elements.
Key Terms & Definitions
- Aperture — the diameter of the transducer element.
- Near zone (Fresnel zone) — region from transducer to focus where beam converges.
- Far zone (Fraunhofer zone) — region beyond the focus where beam diverges.
- Focus (focal point) — narrowest part of the beam.
- Focal length/near zone length/focal depth — distance from transducer face to focus.
- Focal zone — area around focus where beam is narrowest and resolution is best.
- Divergence — widening of the beam after the focus.
- Lateral resolution — ability to distinguish structures side by side.
Action Items / Next Steps
- Complete workbook activities and review practice calculations.
- Review and label diagrams of beam regions.
- Answer Nerd Check questions for self-assessment.