Sound Beams

Chapter 9

Sound beam:

a beam created by a single, disc shaped, unfocused PZT crystal operating in the continuous wave mode

Active element is _____ in the shape of a disc

not typically

The active element does not operate in the ______ when used to create anatomic images

continuous wave mode

In the starting point of a sound beam, the beam width is…

the same as the transducer diameter

A sound beam ____ progressively until it reaches its smallest diameter

narrows

At the end, a sound beam will…

diverge

The anatomy of a sound beam includes…

near zone, near zone length (or focal depth), focus, focal zone, far zone

Near zone (or Fresnel zone):

the region from the transducer to the focus

Near zone length (or focal depth):

the distance from the transducer to the focus

Focus:

point in the beam where the beam diameter is the narrowest

The focal point can be found at the end of the ___ zone, beginning of the ___ zone, and the middle of the ___ zone

near, far, focal

For a disc shaped crystal, the width of the sound beam at the focus is ____ the width of the beam as it leaves the transducer

½

Focal zone:

the region around the focus where the beam is relatively narrow

Reflections arising from the _____ zone create images that are more accurate than those from other depths

focal

____ the focal zone is located in the near field, the other in the far field

½

Far zone (or Fraunhofer zone):

starts at the focus & extends deeper

In what zone will the beam diverge & spread out?

far zone

At the beginning of the far zone, the beam is ___ as wide as it is at the transducer

½

When the beam is ___ near zone lengths from the transducer, the beam is the same size as the active element

2

At depths more than ____ near zone lengths, the beam is wider than the active element

2

How does the beam profile change as it exits the transducer?

sound beam exits the probe & enters the near zone

beam diameter or width is exactly the same as that of the transducer

sound beam gets progressively narrower in the near zone

beam narrows until it reaches the narrowest point, the focus

What marks the end of the near zone?

focus

The distance from the transducer to the focus is called…

focal length, focal depth, or near zone length

Focus marks the beginning of the ___ zone

far

Adjustable focus systems are called…

phased array

Initially focal zones were fixed, the characteristics of the active element determined the ______

depth of focus

Focal depth:

the distance from the transducer to the narrowest part of the beam

The sonographer can change the…

focus

What characteristics of a fixed focus transducer determine focal depth?

transducer diameter & frequency of the sound beam

How does transducer diameter affect the focal depth?

increasing diameter results in a deeper focus

What is the relationship between transducer diameter & focal depth?

directly related

How does frequency affect focal depth?

higher frequency sound results in a deeper focus

What is the relationship between frequency & focal depth?

directly related

Focal depth is directly related to…

transducer diameter & transducer frequency

High frequency sound beam creates a _____ focus

deeper

High frequency sound beam does not…

penetrate far

Sound beam divergence:

describes the gradual spread of the ultrasound beam in the far field

At depths greater than the focus, the sound beam ____ or ____

spreads out, diverges

What kind of beam diameter is better for deep imaging?

larger

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

transducer diameter & frequency of sound beam

What is the relationship between the crystal diameter & beam divergence?

inversely related

_____ diameter crystals produce beams that diverge more in the far zone

Smaller

_____ diameter crystals produce beams that diverge less in the far field

Larger

What do large diameter crystals improve in the far field?

lateral resolution

What is the relationship between frequency & beam divergence?

inversely related

____ frequency sound beams spread out or diverge more in the far zone

Lower

What do higher frequency sound beams improve in the far field due to less divergence?

lateral resolution

Beam divergence is inversely related to…

transducer diameter & transducer frequency

Spherical waves:

sound waves produced by very small sources diverge in the shape of a V

When is the V shape wave created?

when the source is about the size of the sound’s wavelength

Spherical waves are also known as…

diffraction patterns or Huygens’ wavelets

The small sources of sound producing wavelets are known as…

Huygens’ sources

Huygen’s principle:

a large active element may be thought of as millions of tiny, distinct sound sources

When wavelets are in-phase & interfere constructively…

an hourglass shaped beam is created

When wavelets are out-of-phase and destructive interference occurs…

the sound beam is cancelled

What does Huygens’ principle explain?

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