Chapter 9: Sound Beams

What shape is diagnostic ultrasound imaging active element NOT and how does it NOT operate

disc shaped, does not operate in continuous wave mode

How does understanding disc-shaped unfocused PZT crystal operating in continuous wave mode benefit a sonographer

it provides the foundation for understanding the shape/behavior of sound beams used in clinical imaging

what happens to the width of the sound beam as it travels

The width changes. It starts with the same width as the transducer diameter then narrows like a funnel until it reaches its smallest diameter, then expands/diverges

what are the 5 regions of a sound beam

Focus, near zone, focal length/near zone length, far zone, and focal zone

What is the focus (or focal point)

The location where the beam is the narrowest

Width of sound beam at focus for disc-shaped crystal

½ the width of the beam as it leaves the transducer

What is the near zone (or near field or fresnel zone)

The region from the transducer to the focus

What happens to the beam width in the near zone for a continuous wave, disc-shaped crystal

It gradually narrows/converges in this zone

It starts with a width thats the same diameter as the active element, and by the end it’s only ½ the width of the active element

What is at the end of the near zone

The focus

What is the focal length (or focal depth or near zone length)

The distance from the transducer to the focus

What is the far zone (or far field of Fraunhofer zone)

The region that starts at the focus and extends deeper

What happens to the beam width in the far zone

It diverges/spreads out.

At beginning, it is ½ as wide as it is at the transducer, and once it’s at 2 near zones away from the transducer, it is again the same width as the active element

What is at the beginning of the far zone

The focus

What happens to the beam width at a depth more than 2 near zones away from transducer

The beam is wider than the active element

What is the focal zone

Region around the focus where the beam is relatively narrow.

Half of this zone is in near field and half is in far field

What zones does reflections arising from create the most accurate images

reflections from focal zone create the most accurate images compared to other depths

What are adjustable focus systems called

phased array

What characteristics of the active element would determine the focal depth

• transducer diameter

• frequency of sound

How are transducer diameter and focal depth related

they are directly related.

larger diameter = deeper focus

smaller diameter = shallow focus

How is frequency and focal depth related

they are directly related.

high frequency = deeper depth

low frequency = shallow depth

2 factors that create shallow focal depth

• smaller diameter of PZT

• lower frequency

2 factors that create deep focal depth

• larger diameter of PZT

• higher frequency

how can a higher frequency sound create a deeper focus when higher frequency probes are used for more superficial structures

rule of physics is that higher frequencies create deeper focus, but manufacturers overcome this by making a small diameter, high frequency crystal. even with the high frequency, the extremely small diameter displaces the focus shallower, making s clinically useful transducer

Describing the relationship between frequency and 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))

equations are more for understanding the concept, not mathematical application

What is beam divergence

it describes the gradual spread of ultrasound beam in far field

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

• transducer diameter

• frequency of the sound

How is transducer diameter and beam divergence related in far field

they are inversely related.

small diameter crystals spread more in far field

large diameter crystals diverge less in far field

Does larger or small diameter crystals improve lateral resolution in far field and why

larger diameter because they diverge less

How is frequency and beam divergence in far field related

they are inversely related.

lower frequency = more divergence

higher frequency = less divergence

does high or low frequency sound improve lateral resolution on far field

higher frequency

describing the effect of transducer diameter and frequency on beam divergence

sin divergence angle = 1.85 / (diameter (mm) x frequency (MHz))

sin divergence angle = (1.2 x wavelength) /diameter

2 factors that create less divergence in far field

• large diameter

• higher frequency

2 factors that create more divergence in far field

• smaller diameter

• lower frequency

How are V shaped waves formed (aka spherical waves, diffraction patter, or Huygens wavelet)

when sound waves are produced by very small sources, which are usually about the size of the sound wavelength

What are Huygens sources

small sound sources that create V shaped sound waves

What does the Huygen principle state

It explains the shape of imaging transducer emitted sound beam based upon in-phase and out-of-phase wavelets interfering with each other.

that large active elements are millions of tiny distinct sound sources, and each one is a Huygen source that creates a Huygen wavelet with a V shape. the hourglass shape produced by large crystal is a result of interface of many Huygens wavelets emitted from numerous sources. some re in-phase and interfere constructively, creating the hourglass appearance to sound beam. Destructive interference occurs when wavelets are out-of-phase and sound beam is cancelled