Chapter 9

Question 1

_ or _is the location

where the beam is the narrowest

Answer 1

Focus or focal point

Question 2

For a disc-shaped crystal, the width of the

beam at the focus is _

Answer 2

½ the width of the

beam as it leaves the transducer

Question 3

Near zone AKA

Answer 3

Fresnel zone

Question 4

The region from the transducer to the focus

Answer 4

Near zone/ fresnel zone

Question 5

The beam _ within the near zone

Answer 5

gradually narrows

Question 6

For a continuous wave, disc-shaped
crystal, the diameter of the sound beam
as it leaves the transducer is

Answer 6

the same as

the diameter of the active element.

Question 7

At the end of the near zone, the beam _

Answer 7

narrows to only ½ the width of the active

element

Question 8

The focus is located

Answer 8

at the end of the near zone

Question 9

The distance from the transducer to the

focus

Answer 9

Focal length

Question 10

Focal length AKA

Answer 10

Focal depth or near zone length

Question 11

The region that starts at the focus and

extends deeper

Answer 11

Far zone

Question 12

Far zone AKA

Answer 12

Fraunhofer zone or far field

Question 13

Within the far zone, the beam

Answer 13

diverges

spreads out

Question 14

At the beginning of the far zone, the

beam is

Answer 14

only ½ as wide as it is at the

transducer.

Question 15

When the beam is two near zone lengths

from the transducer, the beam is

Answer 15

the same size as the active element.

Question 16

At depths more than 2 near zone lengths,

the beam is

Answer 16

wider than the active

element.

Question 17

The region around the focus where the

beam is relatively narrow

Answer 17

Focal zone

Question 18

Reflections arising from the focal zone

create images that are

Answer 18

more accurate

than those from other depths.

Question 19

The _ is the distance from the
transducer to the narrowest part of the
beam (the focus)

Answer 19

focal depth

Question 20

With a fixed focus transducer, two factors

combine to determine the focal depth:

Answer 20

1. Transducer diameter

2. Frequency of sound

Question 21

Relationship between transducer diameter and focal depth

Answer 21

Directly

Question 22

Relationship between frequency and focal depth

Answer 22

Directly

Question 23

Shallow focus:
_ diameter PZT
_ frequency

Answer 23

Smaller

Lower

Question 24

Deep focus:
_ diameter PZT
_ frequency

Answer 24

Larger

Higher

Question 25

Higher frequency sound creates a _ focus

Answer 25

Deeper

Question 26

Higher frequency sound creates a deeper focus. Transducer manufacturers are aware of this and overcome it by

Answer 26

creating very | small diameter, high frequency crystals

Question 27

Focal depth=

Answer 27

diameter^2 x frequency/6

Question 28

The gradual spread of the ultrasound | beam in the far field.

Answer 28

Beam divergence

Question 29

Two factors combine to determine beam | divergence:

Answer 29

1. Transducer diameter | 2. Frequency of sound

Question 30

Relationship between crystal diameter and beam divergence

Answer 30

Inversely

Question 31

Relationship between frequency and beam divergence

Answer 31

Inverse

Question 32

Less divergence: _ diameter _ frequency

Answer 32

Larger | Higher

Question 33

More diveregence: _ diameter _ frequency

Answer 33

Smaller | Lower

Question 34

Sound waves produced by very small sources (tiny pieces of PZT) diverge in

Answer 34

the shape of a V

Question 35

The v-shaped wave is created when the sound source is

Answer 35

about the size of the sound’s wavelength

Question 36

Spherical wave AKA

Answer 36

Diffraction patterns | Huygens' wavelets

Question 37

US transducers with large PZT crystals create | sound beams shaped like

Answer 37

an hourglass

Question 38

Small sound sources create beams that are

Answer 38

V shaped

Question 39

Huygens’ Principle state

Answer 39

that a large active element may be thought of as millions of tiny, distinct sound sources. Each of these tiny particles is a Huygen’s source and creates a Huygen’s wavelet with a V-shape.

Question 40

The hourglass shape produced by a large | crystal is the result o

Answer 40

f interference of the many Huygens’ sound waves emitted from these numerous sound sources.

Question 41

Huygens principle: Some of these wavelets are in phase and | interfere _, creating _

Answer 41

Constructively | an hourglass shaped sound beam.

Question 42

Huygens principle: | Destructive interference occurs where

Answer 42

the wavelets are out-of-phase | and the sound beam is cancelled.