US Physics Wk 6 Quiz

at the starting point, the beam width is exactly the same as the transducers

diameter

transducer diameter, AKA

aperture

a sound beam narrows progressively until it reaches the smallest diameter and then it

diverges

there are ____ terms to describe the anatomy of a sound beam

5

- focus

- near zone

- focal length

- far zone

- focal zone

transducer anatomy terms

the location where the beam is the narrowest

focus

for a disc shaped crystal, the width of the beam at the focus is _______ the width of the beam leaving the transducer

1/2

AKA near field or fresnel zone

near zone

the region from the transducer to the focus

near zone

the beam gradually narrows, or ______ within the near zone

converges

in the near zone, the diameter as the sound beam leaves the transducer is _______ as the diameter of the active element

the same

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

1/2

the ______ is at the end of the near zone

focus

AKA focal depth or near zone length

focal length

the distance from the transducer to the focus

focal length

AKA far field or Fraunhofer zone

far zone

the region that starts at the focus and extends deeper

far zone

at the beginning of the far zone, the beam is only _____ as wide as the transducer

1/2

when the beam is ____ near zone lengths from the transducer, it is again the same size as the active element

2

at depths more than 2 near zone lengths, the beam is _____ that the active element

wider

the region around the focus where the beam is relatively narrow

focal zone

reflections arising from the _____ create images that are more accurate than reflections from other depths

focal zone

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

focal depth

___ factors combine to determine focal depth

2

1. transducer diameter

2. frequency of sound

factors that determine focal depth

transducer diameter and focal depth are ______ related

directly

a smaller diameter will have a ________ focus

shallower

frequency and focal depth are _____ related

directly

a lower frequency will have a _______ focus

shallower

_______ frequency creates a deeper focus

higher

focal depth =

diameter (mm)^2 x frequency (MHz) / 6

the gradual spread of the US beam in the far field

beam divergence

___ factors combine to determine beam divergence

2

1. transducer diameter

2. frequency of sound

(same as focal depth)

factors that determine beam divergence

crystal diameter and beam divergence are _______ related

inversely

a smaller diameter will have _____ divergence in the far field

increased

frequency and beam divergence are ________ related

inversely

lower frequency will have _____ divergence

increased

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

V

a V shaped wave is created when the sound source is about the size of the sounds

wavelength

- diffraction patterns

- Huygens waves

AKA for spherical waves

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

hourglass

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

Huygen's principle

Huygens wavelets interfere both __________ and __________

constructively , destructively

any device that converts one form of energy into another

transducer

ultrasound transducers perform ____ functions

2

electrical energy from the system is converted into acoustic sound during

transmission

the reflected sound pulse is converted into electricity during

reception

the property of certain materials to create a voltage when they are mechanically deformed or when pressure is applied to them

the piezoelectric effect

a process in which piezoelectric materials change shape when a voltage is applied to them

reverse piezoelectric effect

piezoelectric effect happens during _______ (transmission/recection)

reception

reverse piezoelectric effect happens during ______ (transmission/reception)

transmission

materials that convert sound into electricity

piezoelectric materials

piezoelectric materials, AKA

ferroelectric

the synthetic material used in transducers

lead zirconate titanate (aka PZT)

- the ceramic

- active element

- crystal

AKAs for PZT

protects the internal components of the transducer from damage and insulates the patient from electrical shock

transducer case

transducer cases are made up of

metal or plastic

thin metallic barrier lining the inside of the transducer case

electrical shield

prevents electrical signals that are unrelated to diagnostic information from entering the transducer

electrical shield

thin barrier of cork or rubber that isolates the internal components of the transducer from the case

acoustic insulator

prevents vibrations in the case from inducing a electrical voltage in the PZT of the transducer

acoustic insulator

in a simple probe, the PZT is shaped like a

coin

the characteristics of the sound beam emitted by the transducer are relation to the dimensions of the

active element

the PZT is ______ a wavelength thick

1/2

provides an electrical connection between the PZT and the ultrasound system

wire

during _________, the voltage from the US system causes the crystal to vibrate and produce and ultrasonic wave

transmission

during ________, the crystals vibration produces a voltage that must return to the system for processing into an image

reception

increases the efficiency of sound energy transfer between the active element and the body

matching layer

protects the active element

matching layer

the matching layer is _____ of a wavelength thick

1/4

the impedance of PZT is about ____ greater than the impedance of skin

20 x

the ________ is designed with an impedance in between that of the PZT and the skin

matching layer

the impedance of _____ is between that of the matching layer and the biologic media

gel

multiple matching layers of different impedances further increase the % of sound _________

transmission

usually around ___ matching layers (with different impedances) are used in a transducer

2

PZT > matching layer > gel > skin

decreasing order of impedance

bonded to the back of the PZT

backing material

reduces the "ringing" of the PZT

backing material

when an electrical signal excites the PZT, the backing material restricts the amount of PZT deformation. This dampens the emitted sound pulse, making it ____ in duration and length

short

backing material improves _______ resolution

axial

characteristics of backing material:

____ degree of sound absorption

acoustic impedance similar to ______

high

PZT

backing material is made of

metal powder and epoxy resin

there are ____ consequences of using backing material in transducers

3

backing material __________ sensitivity

decreases

backing material creates a _______ bandwidth

wide

backing material causes a _____ quality factor

low

the backing material decreases vibration of the PZT during transmission, but also during __________. This makes the returning signal low

reception

during reception, transducers with _________ are less able to convert low-level sound reflections into meaningful electrical signals

damping material

the range of frequencies in the pulse; the difference between the highest and lowest frequencies

bandwidth

continuous waves will have a _______ bandwidth due to no backing material

narrow

a single frequency produced

resonant frequency

AKA the main frequency

resonant frequency

the main frequency of a transducer divided by the bandwidth

quality factor

units for quality factor

unitless

quality factor is _______ related to bandwidth

inversely

wide bandwidth probes have a ____ Q factor

low

narrow bandwidth probes have a ____ Q factor

high

because imaging probes use backing material and have a wide bandwidth, they are often referred to as

Low-Q

shorter pulse = ______ Q factor

lower