Ultrasound: Near Zone and Focus

• The section discusses a beam created by a single disc-shaped, unfocused PZT crystal operating in continuous wave mode.
• In diagnostic ultrasound (US), the crystal element is not typically disc-shaped and does not operate in continuous wave (CW).
• Sound beams are shaped like an hourglass.
• Narrow beams create higher quality images.
• Sound beam width changes as it travels.
• Beams start at the same diameter as the transducer, narrow, and then widen.
• Converge: to come together or narrow.
• Diverge: to grow apart or widen.

Near Zone

• It is also called the Fresnel zone.
• The near zone is the region from the transducer to the focus.
• The beam gradually narrows in the near zone.
• For a continuous wave disc-shaped crystal, the sound beam diameter as it leaves the transducer is the same as the active element diameter.
• At the end of the near zone, the beam narrows to ½ of the active element width.

Focus

• The focus, or focal point, is where the sound beam reaches its minimum diameter.
• Reflections from the focal point create enhanced images.
• Focal Depth is the distance from the transducer face to the focus, it is also called focal length or near zone length.
• Focal depth is determined by the crystal's characteristics.
• Focal Zone is where the beam is narrowest, producing the best image quality.

Determining Focal Depth

• Focal depth is determined by transducer diameter and ultrasound frequency.
• Diameter and focal depth are directly related.
• Frequency and focal depth correlate directly.
• Beams with a deep focus have lower intensity due to attenuation.

Focal Depth Influences

• Large Diameter PZT equals Deep Focus/Focal Depth.
• Small Diameter PZT equals Shallow Focus/Focal Depth.
• High Frequency equals Deep Focus/Focal Depth.
• Lower Frequency equals Shallow Focus/Focal Depth.
• Most modern ultrasound machines allow for altering the focal depth using phased array transducers.
• In early ultrasound development focal depth was fixed, determined by the characteristics of the PZT, and unchangeable by the sonographer.

The Imaging Conundrum

• High frequency sound creates a deeper focus, but superficial imaging requires higher frequencies.
• Manufacturers use very tiny diameter crystals in high-frequency transducers, displacing the focus to achieve a shallow focal depth.

The Math of the Focus

• In soft tissue, the equation that describes the relationship between frequency, focal depth and diameter is focal depth (mm) = (Diameter (mm)² x frequency (MHz)) / 6
• A simplified equation is focal depth (mm) = Diameter (mm)² / (4 x Wavelength (mm))

Far Zone

• Also known as the fraunhofer zone or far field, is the region deeper than the focus, the beam diverges in the far zone.
• The beam diverges at the focus and reaches its original PZT size after two near zone lengths.
• Beyond this, the beam is wider than the PZT and diverges

Sound Beam Divergence

• Describes the sound beam's spread in the deep far zone.
• Sound beam divergence is determined by transducer diameter and frequency.
• The beam is narrowest in the far field when using a large diameter, high-frequency transducer.
• The beam is widest in the far field when using a small diameter, low-frequency transducer.

Sound Beam Divergence Influences

• Larger diameter transducer equals Less Divergence.
• Smaller diameter transducer equals More Divergence.
• High frequency equals Less Divergence.
• Low frequency equals More Divergence.

Summary

• Large Diameter Crystal and High Frequency creates deeper focus and less divergence in the far field.
• Small Diameter Crystal and Low Frequency creates shallow focus and more divergence in the far field.
• Spherical Waves, aka Huygen's Wavelets or Diffraction Pattern, the information presented has been in regard to a disc shaped crystal.
• When a very tiny PZT piece, not necessarily a disc shape, is used, the sound beam takes on a V shape.
• This occurs when the sound source mirrors the sound wavelength size wise.

Huygens's Principle

• Small sound sources create "v" shaped wavelets.
• Ultrasound transducers use multiple tiny elements instead of one large, round element.
• The hourglass shape of sound beams is created by interference between the tiny wavelets.

This section describes beam creation using a disc-shaped PZT crystal in continuous wave mode. It details the near zone (Fresnel zone), where the beam narrows, and the focus (focal point), where the beam reaches its minimum width. It also covers the concepts of convergence and divergence of sound beams.