Name the anatomy of a sound beam from closest to furthest

Near Zone (Fresnel)

Focal Depth (Length)

Focal Zone

Focus (Focal Point)

Far Zone (Fraunhofer)

Define:

Focus or Focal Point

The location where the sound beam reaches its minimum diameter (narrowest).

~1/2 width of transducer

Define:

Focal Depth (Length)

The distance from the transducer face to the focal point.

aka: Near Zone Length

Define:

Fresnel Zone

(Near Zone)

The region or zone in between the transducer and the focal point.

The sound beams converge in the near zone.

Define:

Fraunhofer Zone

(Far Zone)

The region or zone deeper than the focus, beyone the near field.

Sound beams diverge in the far zone.

Define:

Focal Zone

The region around the focal point where the beam is relatively narrow. Half of the focal zone is located in the near field and the other half is located in the far field with the focus (focal point) exactly in between the two.

What happens to sound beams in the Fresnel Zone?

They converge

What happens to sound beams in the Fraunhofer Zone?

They diverge

Which zone does this describe?

Starts the same width of the transducer face and is 1/2 that width at its end.

Fresnel Zone

Which zone does this describe?

At the beginning of this zone, the beam is only 1/2 as wide as it is at the transducer. From the focus, the beam continues to diverge. When the beam is again the same size as the active element. At depths more than two near zone lengths, the beam is wider than the active element.

Fraunhofer Zone

How wide is the beam diameter at the transducer (aperture)?

The beam diameter equals the transducer diameter (aperture).

How wide is the beam diameter at the focus?

The beam diameter is 1/2 the transducer diameter (aperture).

How wide is the beam diameter at 2 near zone lengths (NZL)?

The beam diameter equals the transducer diameter (aperture).

How wide is the beam diameter when it is deeper than 2 near zone lengths (NZL)?

The beam diameter is greater than the transducer diameter (aperture).

What two factors determine the Focal Depth (Length)?

1. transducer diameter or aperture

2. frequency of the ultrasound

What is the equation to determine near zone length?

NZL = d²/4λ

ex:

An unfocused transducer with a beam diameter of 20mm operating at 3.08 MHz. What is the NZL?

λ=c/f =1.54mm/μs/3.08 MHz = 0.5mm

NZL = d²/4λ = 202mm²/4(0.5mm) = 400mm/2 = 200mm

or 20cm

What is the equation for the beam diameter at the focal point?

D/2

ex: An unfocused transducer with a beam diameter of 20mm operating at 3.08 MHz. What is the diameter of the focus?

20mm/2 = 10 mm or 1 cm

Define:

Sound Beam Divergence

Describes the spread of the sound beam in the deep far zone.

How does transducer diameter affect the focal depth?

A larger diameter results in a deeper focus; therefore, transducer diameter and focal depth are directly related.

How does frequency affect focal depth?

Higher frequency sound results in a deeper focus; therefore, frequency and focal depth are directly related.

Two probes are identical except the diameter of one active element is larger than the other. Which probe's focus will penetrate deeper?

The beam from the larger diameter active element will have a deeper focus.

Two probes are identical except one emits sound at 8MHz and the other at 4 MHz. Which probe's focus will penetrate deeper?

The 8 MHz beam will have a deeper focus than the 4 MHz.

What characteristics of a transducer determine the spread of the beam in the far field?

1. Transducer diameter

2. Frequency of the sound

How does transducer diameter affect beam divergence in the far field?

Smaller diameter crystals produce beams that spread out or diverge more in the far (Fraunhofer) zone; therefore, crystal diameter and beam divergence are inversely related.

How does frequency alter beam divergence in the far field?

Lower frequency sound beams spread out or diverge more in the deep far (Fraunhofer) zone; therefore, frequency and beam divergence are inversely related.

More or less divergence?

Narrower beam in far field

Less

More or less divergence?

Wider beam in far field

More

More or less divergence?

Larger aperture

Less

More or less divergence?

Smaller aperture

More

More or less divergence?

High frequency

Less

More or less divergence?

Low frequency

More

More or less divergence?

Improved lateral resolution in far field

Less

More or less divergence?

Degraded lateral resolution in far field

More

What is a spherical wave?

aka diffraction patterns, v-shaped wave, Huygens' wavelets

When produced by a tiny source, with a size near the wavelength of the sound, waves will diverge in this shape as they propagate. 

What is Huygens' Principle and how does it present itself?

This principle explains the hourglass shape of an imaging transducer's sound beam.

Each tiny part of the surface of the large transducer face may be considered an individual tiny sound source creating a Huygens' wavelet. The overall hourglass shape of a sound beam is the result of the constructive and destructive interference of the many sound wavelets emitted from these numerous sound sources. It has been found that when all of these multiple wavelets are combined according to Huygens' Principle, an hourglass shaped sound beam is created.