chap 9: sound beams

What is the beam width at the starting point?

Exactly the same as the transducer diameter

Focus

AKA focal point. Location where the beam is the narrowest. It is ½ the width of the beam as it leaves the transducer.

• is also the end of the near zone, beginning of far zone, and middle of focal zone.

Near zone

AKA near field / Fresnel zone. The region from the transducer to the focus. Extends from starting point of beam → middle of focal zone (focus).

• diameter of sound beam as it leaves the transducer = diameter of the active element

• at the end of near zone, beam narrows to ½ width of active element

Focal length

AKA focal depth / near zone length. Distance from the transducer → focus

Far zone

AKA far field / Fraunhofer zone. The region that starts at the focus and extends deeper; the beam diverges (spreads out).

At the beginning, the beam is ½ as wide as it is at the transducer then it continues to diverge. When the beam is 2 near zone lengths, the beam is again the same width as the active element. More than 2 = beam wider than active element (PZT)

Focal zone

Region around the focus where the beam is relatively narrow. Reflections arising from focal zone create more accurate images than from other depths.

A region on either side of the focal point (focus) where the beam is relatively narrow, resulting in superior image detail.

Beam diameter at the transducer…

Equals transducer diameter

Beam diameter at the focus…

Is ½ transducer diameter

Beam diameter at the 2 near zone lengths…

Equals transducer diameter

Beam diameter deeper than 2 near zone lengths…

Is wider than transducer diameter

Phased Array

Adjustable focus system where the sonographer changes the location of the focus (found in modern ultrasound systems).

• in the past, focal depth was fixed and could not be changed. the characteristics of the active element (PZT) determine the depth of the focus.

What characteristics of a fixed focus transducer determine the focal depth?

Transducer diameter & frequency of sound

What kind of relationship do transducer diameter & focal depth have?

Directly related

• larger diameter = deeper focus

What kind of relationship do frequency & focal depth have?

Directly related

• higher frequency = deeper focus

What results in a shallow focus?

Smaller diameter PZT & lower frequency

What results in a deep focus?

Larger diameter PZT & higher frequency

What is the formula that describes the mathematical relationship between diameter, frequency, and focal depth in soft tissue?

If the rule of physics (higher frequency sound creates a deeper focus) is “bad physics”, how do manufacturers overcome it?

By making very small diameter, high frequency crystals

Beam divergence

The gradual spread of the ultrasound beam in the far field

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

Transducer diameter & frequency of sound

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

Smaller diameter crystals produce beams that spread out (diverge) more in the deep far zone.

What is the relationship between crystal diameter & beam divergence

Inversely related

• smaller diameter crystals = more beam divergence

• larger diameter crystals = less beam divergence

What affect do larger diameter crystals have on lateral resolution in the far field?

Since larger diameter crystals create sound beams that diverge less than the far field, larger diameter, crystals improve lateral resolution in the far field.

What is the relationship between frequency and beam divergence?

Inversely related

• lower frequency sound beams spread out (diverge) more in the deep far zone

What affect do higher frequency sound beams have on lateral resolution in the far field?

Since high frequency sound beams diverge less in the far field, higher frequency sound improves lateral resolution in the far field

What is the formula that describes the mathematical relationship between the effect of transducer diameter and frequency on beam divergence?

Spherical waves

AKA diffraction patterns / Huygens’ wavelets. V shaped waves that are produced by tiny pieces of PZT. It’s created when the source is about the size of the sound’s wavelength. The small sources of found producing these wavelets are known as Huygens’ sources.

Huygens’ Principle

Explains the shape of an imaging transducer’s emitted sound beam based upon in-phase & out-of-phase wavelets interfering with each other.