Ultrasound Beam: Near Zone & Focus

• This section covers beams created by a single disc-shaped, unfocused PZT crystal operating in continuous wave mode.
• In diagnostic ultrasound, the crystal element is not typically disc-shaped, nor does it typically operate in continuous wave mode.
• Sound beams are shaped like an hourglass.
• Narrow beams create better images.
• As sound travels, the beam's width changes.
• The beam begins with the same diameter as the transducer, narrows, and then widens.
• Converge means to come together or narrow.
• Diverge means to grow apart or widen.

Near Zone (Fresnel Zone)

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

Focus (Focal Point)

• The location where the sound beam reaches its minimum diameter.
• Reflections from the focal point are more accurate.

Focal Depth

• The distance from the transducer face to the focus, it is also called focal length or near zone length.
• Focal depth is determined by crystal characteristics.
• The focal zone is the region where the beam is narrowest, and the picture is best.

Determining Focal Depth

• Focal depth depends on the transducer diameter and ultrasound frequency.
• Diameter and focal depth are directly related.
• Frequency and focal depth are directly related.
• Beams with deep focus have lower intensity because of attenuation.
• Larger diameter PZT correlates with deeper focus.
• High frequency correlates with deeper focus.
• Small diameter PZT correlates with shallow focus.
• Lower frequency correlates with shallow focus.
• Modern ultrasound machines allow you to alter the focal depth using phased array transducers.
• Early ultrasound machines had a fixed focal depth that could not be changed and was determined by PZT characteristics.
• Manufacturers utilize a smaller diameter crystal in high frequency transducers, displacing the focus and creating a shallow focus.

Equations for Focal Depth in Soft Tissue

• focal depth (mm) = (Diameter (mm)² x frequency (MHz)) / 6
• focal depth (mm) = Diameter (mm)² / (4 x Wavelength (mm))

Far Zone (Fraunhofer Zone or Far Field)

• The region deeper than the focus.
• The beam diverges in the far zone.
• The beam begins to diverge at the focus.
• It reaches the same size as the PZT after two near zone lengths.
• Beyond that, the beam is wider than the PZT.

Sound Beam Divergence

• Describes the spread of the sound beam in the deep far zone.
• Divergence depends on transducer diameter and sound frequency.
• In the far field, a large diameter, high-frequency transducer produces the narrowest beam.
• In the far field, a small diameter, low-frequency transducer produces the widest beam.

Summary

• Large Diameter Crystal/High Frequency correlates with Deeper focus and Less divergence in the far field.
• Small Diameter Crystal/Low Frequency correlates with Shallow focus and More divergence in the far field.

Spherical Waves (Diffraction Pattern, Huygen's Wavelets)

• Previous information pertained to a disc-shaped crystal.
• A tiny piece of PZT that is not necessarily disc-shaped will project a V-shaped sound beam.
• This occurs when the sound source and the wavelength of sound are nearly the same size.
• Small sound sources create "v" shaped wavelets.
• Ultrasound transducers contain many tiny elements, not one large round element.
• Interference between these wavelets produces the hourglass shape of sound beams.