Chapter 9: Sound Beams

What is the shape of a sound beam as it travels?

The beam starts the same width as the transducer, narrows to a focus, then diverges.

What are the five key terms that describe the shape and regions of a sound beam?

Focus, near zone, focal length, far zone, focal zone.

What is the focus in ultrasound physics?

The location where the beam is the narrowest.

What is the near zone also known as?

The near field or Fresnel zone.

What happens to the beam in the near zone?

The beam gradually narrows or converges.

What is the focal length?

The distance from the transducer to the focus.

What is the far zone also referred to as?

The far field or Fraunhofer zone.

What occurs in the far zone?

The beam diverges or spreads out.

What is the focal zone?

A region around the focus where the beam is relatively narrow, resulting in better image detail.

At what point does the sound beam start to diverge?

At the focus, marking the beginning of the far zone.

What is the beam diameter at the transducer?

The beam diameter equals the transducer diameter.

What is the beam diameter at the focus?

The beam diameter is one-half the transducer diameter.

What happens to the beam diameter at a depth of two near zone lengths?

The beam diameter equals the transducer diameter.

What is the beam diameter at depths greater than two near zone lengths?

The beam diameter is wider than the transducer diameter.

How is the focal zone defined in relation to the focus?

It is the region around the focus, with half in the near field and half in the far field.

What is the significance of the focal zone?

Reflections from the focal zone create more accurate images.

What is the relationship between the focus and the near zone?

The focus marks the end of the near zone.

What happens to the beam diameter as it exits the transducer?

It is the same as the diameter of the active element.

At what depth is the focus located?

The focus is located at the end of the near zone.

What is the diameter of the sound beam at a depth of 8 cm?

At the end of the near zone, the beam diameter is one-half the transducer diameter.

At which depth does the beam begin to widen?

In the far zone, starting at the focus.

What depth marks the beginning of the focal zone?

It begins at a depth 2 cm shallower than the focus.

What depth marks the end of the focal zone?

It ends at a depth 2 cm deeper than the focus.

At which depth is the beam narrowing?

At 6 cm, which is within the near zone.

At which depth is the beam widening?

At 9.375 cm, which is within the far zone.

What is the best estimate of the diameter of the sound beam as it exits the transducer?

12 mm, which is the same as the diameter of the active element.

What depth is the beam most likely to have the same diameter as it has at a depth of 11 cm?

5 cm, as the beam is symmetrical around the focus.

Which term does not belong: focus, end of the near zone, end of the Fraunhofer zone, middle of the focal zone?

End of the Fraunhofer zone, as it is the very end of the sound beam.

What factors determine the focal length?

Transducer diameter and frequency.

How does higher frequency affect near zone length?

It increases the near zone length, making the near field longer.

What is the relationship between frequency and beam divergence?

Higher frequency sound beams have less divergence in the far zone.

What determines beam divergence in the far zone?

Transducer diameter and frequency.

What happens to the beam area as it enters the focal zone?

It reaches its minimum size before beginning to diverge.

How are active element diameter and near zone length related?

Directly related.

How are transducer frequency and near zone length related?

Directly related.

How are wavelength and near zone length related?

Inversely related.

What equation describes focal depth in terms of diameter and frequency in soft tissue?

focal depth = (diameter² × frequency) / 6

What equation describes focal depth in terms of diameter and wavelength?

focal depth = diameter² / (4 × wavelength)

How do manufacturers create a shallow focus with high frequency sound?

By using extremely small diameter crystals.

What is Huygens' Principle?

States that large active elements are considered as millions of tiny, distinct sound sources.

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

Larger diameter transducers result in less beam divergence in the far zone.

What is another name for the far zone?

Fraunhofer zone.

What happens to the focal depth if the transducer diameter is doubled?

The focal depth is quadrupled.

What is the near zone length also called?

Focal depth or near field length.

What are spherical waves also known as?

Diffraction patterns or Huygens' wavelets.

What size is a sound source that produces a spherical wave?

About the size of the sound's wavelength.

What are the small sources of sound that produce Huygens' wavelets called?

Huygens' sources.

Who explained the apparent inconsistency between large and small sound sources and their beams?

Christiaan Huygens.

According to Huygens' Principle, how may a large active element be thought of?

As millions of tiny, distinct sound sources.

At what location is the sound beam diameter three times greater than the transducer diameter?

At a depth equal to four focal lengths.

What is the shape of a sound beam created by a tiny piece of PZT?

V-shaped, also called a diffraction pattern or a Huygens wavelet.

Which principle explains why a sound beam created by a disc-shaped crystal is hourglass shaped?

Huygens' Principle.

Which of the following locations is the deepest: end of the Fresnel zone, end of the focal zone, end of the Fraunhofer zone, or end of the near zone?

The end of the Fraunhofer zone.

Which of longitude locations is the most shallow: beginning of the far zone, beginning of the focal zone, focal depth, or beginning of the Fraunhofer zone?

The beginning of the focal zone.