Overview
This lecture explains the anatomy of the ultrasound beam, focusing on the regions of a single-element transducer beam, key formulas, relationships among diameter, frequency, focal depth, and divergence, and clinical implications. Examples are included to illustrate key points.
Sound Beam Regions
- The ultrasound beam has five main regions: near zone, near zone length (focal depth), focus, far zone, and focal zone.
- Near zone (near field/Fresnel zone): Extends from the transducer to the focus. Its maximum width equals the transducer element diameter (aperture).
- Example: If the element diameter is 8 mm, the near zone starts at 8 mm wide.
- Near zone length (focal length/focal depth): The distance from the transducer face to the focus.
- Example: For an 8 mm diameter, 12 MHz transducer, near zone length = (8² × 12) / 6 = 128 mm.
- Focus (focal point): The narrowest part of the beam, located at half the element diameter.
- Example: With an 8 mm diameter, the focus is at 4 mm width.
- Far zone (far field/Fraunhofer zone): Begins at the focus, where the beam diverges.
- Example: At two near zone lengths (256 mm in the above example), the beam widens back to 8 mm.
- Focal zone: Surrounds the focus and extends equally into the near and far zones; the beam remains relatively narrow here.
- Example: If the focal zone starts at 9.8 cm and the focus is at 12.8 cm, it ends at 15.8 cm.
Key Formulas & Relationships
- Near zone length (soft tissue):
Near zone length (mm) = (diameter² × frequency [MHz]) / 6
- Example: 8 mm diameter, 6 MHz: (8² × 6) / 6 = 64 mm.
- Beam width at focus:
Beam width = ½ transducer diameter.
- Example: 8 mm diameter → 4 mm at focus.
- Beam diameter at two near zone lengths:
Equals the element diameter again.
- Example: After 2 × 128 mm = 256 mm, beam is 8 mm wide.
- Divergence angle:
sin(θ) = 1.85 / (diameter × frequency)
- Example: 4 mm diameter, 1 MHz: sin(θ) = 1.85 / (4 × 1) = 0.4625.
- Relationships:
- Diameter and frequency are directly related to near zone length (increasing either increases focal depth).
- Diameter and frequency are inversely related to beam divergence (increasing either decreases divergence).
Clinical Relevance & Practical Examples
- Narrow beams (higher frequency or larger diameter) improve lateral resolution, especially in the focal zone.
- Example: A 12 MHz, 8 mm transducer has a deeper focus and less divergence than a 6 MHz, 8 mm transducer.
- High frequency transducers are made with smaller diameters to keep focal zones shallow for superficial imaging.
- Example: A 17 MHz, 1 mm transducer has a focal depth of 2.8 cm, suitable for imaging shallow structures.
- The most intense point of the beam is just above the focus due to the balance of narrowing and attenuation.
- The system's focus control moves the focal zone, allowing optimization for the area of interest.
- Example: Placing the focus just below a structure of interest improves image detail.
Q&A & Practice Highlights
- Use the largest frequency and diameter for best lateral resolution in the far field.
- Example: 12 MHz, 10 mm transducer provides better lateral resolution than lower frequency or smaller diameter options.
- Low frequency or small diameter transducers result in shallow focus and more divergence.
- Example: 1 MHz, 3 mm transducer has a shallow focus and wide divergence.
- High frequency or large diameter transducers yield deeper focus and less divergence.
- Example: 12 MHz, 19 mm transducer has a deep focus and narrow far field.
- One beam forms one scan line in imaging; modern transducers use multiple elements to create a full image.
Key Terms & Definitions
- Near Zone (Fresnel zone): Region from transducer to focus, where the beam converges.
- Far Zone (Fraunhofer zone): Region beyond focus, where the beam diverges.
- Aperture: Diameter of the transducer element.
- Focal Depth/Length: Distance from transducer to focus.
- Focus (Focal Point): Narrowest part of the beam.
- Focal Zone: Area around the focus with relatively narrow beam width.
- Divergence Angle: The angle at which the beam widens in the far field.
Action Items / Next Steps
- Complete workbook practice problems and review all calculations.
- Review and label diagrams of beam anatomy regions.
- Answer open-ended "nerd check" questions to reinforce learning.