Ultrasound Beams: Near Zone and Focus

• A special note, this section discusses a single disc-shaped, unfocused PZT crystal operating in continuous wave mode but in diagnostic US, crystal elements are not typically disc-shaped nor operate in CW.
• Sound beams are hourglass-shaped, with narrower beams producing better images.
• The width of the beam changes as the sound travels.
• The beam starts at the transducer's diameter, narrows, and then widens.

Terms

• Converge- to come together or narrow.
• Diverge grow apart or widen.

Near Zone

• Also known as the Fresnel zone.
• The region stretches from the transducer to the focus.
• In the near zone, the beam gradually narrows.
• For a continuous wave disc-shaped crystal, the diameter of the sound beam is equal to the active element's diameter as it leaves the transducer.
• At the end of the near zone, the beam's width is half of the active element's width.

Focus

• Focus or Focal Point is the location where the sound beam reaches its minimum diameter.
• Reflections from this zone are more accurate than those from other zones.
• Focal Depth is the distance from the transducer face to the focus and is also called focal length or near zone length.
• Focal depth is determined by properties of the crystal.
• Focal Zone is the region where the beam is narrowest, resulting in the best picture.

Determining Focal Depth

• Focal depth is determined by two factors: transducer diameter and ultrasound frequency.

• Diameter and focal depth are directly related.

• Frequency and focal depth are directly related.

• Beams with have lower intensity with deep focus than beams with beams with shallow focus.

• Most modern ultrasound machines allow alteration of the focal depth using phased array transducers.

• Early ultrasound development featured fixed focal depths, determined by PZT characteristics.

• Manufacturers use very tiny diameter crystals in transducers with high frequencies, displacing the focus and creating a shallow focus.

Formula

• In soft tissue, the relationship between frequency, focal depth, and diameter is: focal depth (mm) = (Diameter (mm)² x Frequency (MHz)) / 6
• focal depth (mm) = Diameter (mm)² / (4 x Wavelength (mm))

Far Zone

• The Far Zone also called the fraunhofer zone or the far field, is the region deeper than the focus, where the beam diverges.
• The beam begins to diverge at the focus, becoming the same size as the PZT again at two near zone lengths.
• After that point, the beam diverges and is wider than the PZT.

Sound Beam Divergence

• Describes the sound beam spread in the deep far zone.
• Determined by transducer diameter and sound frequency.
• In the far field, the beam is narrowest when using a large diameter, high-frequency transducer.
• Conversely, the beam is widest when using a small diameter, low-frequency transducer.

Summary of sound divergence

• With a Large Diameter Crystal/High Frequency there is a deeper focus and will result in less divergence in the far field
• Conversely when using a Small Diameter Crystal/Low Frequency there is a shallow focus and will result in more divergence in the far field

Spherical Waves

• The information above refers to disc-shaped crystals.
• When the PZT is not necessarily disc-shaped, the sound beam is V-shaped.
• This occurs when the sound source and the wavelength of sound are close to the same size.
• Spherical waves are also known as Diffraction Patterns or Huygen's Wavelets.

Huygens's Principle

• Small sound sources create “v” shaped wavelets.
• Ultrasound transducers have many tiny elements, not one large round element.
• The hourglass shape of sound beams results from interference between these tiny wavelets.

Explanation of ultrasound beams, which have an hourglass shape. The beam narrows in the near zone (Fresnel zone) from the transducer to the focus. The beam's width is half the active element's width at the end of the near zone.