Chapter 01: Foundations of Sonography

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1

acoustic impedance

measure of a material’s resistance to the propagation of sound; expressed as the product of acoustic velocity of the medium and the density of the medium

2

amplitude

strength of an ultrasound wave measured in decibels

3

angle of incidence

angle at which the ultrasound beam strikes the interface between two types of tissues

4

angle of reflection

angle at which the beam of the sound is reflected from an interface; the angle of reflection equals the angle of incidence

5

attenuation

reduction in the amplitude and intensity of a sound wave as it propagates through a medium; attenuation of ultrasound waves in tissue is caused by absorption and by scattering and reflection

6

axial resolution

refers to the minimum distance between two structures positioned along the axis of the beam where both structures can be visualized as separate objects

7

compression

region of increased particle density

8

crystal

special material in the transducer that has the ability to convert electrical impulses into sound waves

9

cycle

a sequence of events occurring at regular intervals. Sound waves are generated over a period of time; the time required to produce each cycle depends on the frequency of the transducer

10

decibel (dB)

unit used to quantitatively express the ratio of two amplitudes or intensities; decibels are not absolute units, but express one sound level or intensity in terms of another or in terms of a reference (e.g., the amplitude 10 cm from the transducer is 10 dB lower than the amplitude 5 cm from the transducer)

11

dynamic range

ratio of the largest to smallest signals that an instrument or component of an instrument can respond to without distortion

12

focal zone

region over which the effective width of the sound beam is within some measure of its width at the focal distance

13

frame rate

rate at which images are updated on the display; dependent on frequency of the transducer and depth selection

14

Fraunhofer zone

the field farthest from the transducer during formation of the sound beam

15

frequency

number of cycles per second that a periodic event or function undergoes; number of cycles completed per unit of time; the frequency of a sound wave is determined by the number of oscillations per second of the vibrating source

16

Fresnel zone

the field closest to the transducer during formation of the sound beam

17

gain

measure of the strength of the ultrasound signal; can be expressed as a simple ratio or in decibels; overall gain amplifies all signals by a constant factor regardless of the depth

18

gray scale

B-mode scanning technique that permits the brightness of the B-mode dots to be displayed in various shades of gray to represent different echo amplitudes

19

hertz (Hz)

unit for frequency, equal to 1 cycle per second

20

intensity

power per unit area

21

interface

surface forming the boundary between media having different properties

22

kilohertz (kHz)

1000 Hz

23

lateral resolution

the minimum distance between two objects where they still can be displayed as separate objects

24

megahertz (MHz)

1,000,000 Hz

25

piezoelectric effect

generation of electric signals as a result of an incident sound beam on a material that has piezoelectric properties; in a reverse piezoelectric effect, an element exposed to an electric shock will begin to vibrate and transmit a sound wave

26

power

rate of energy flow over the entire beam of sound; in general terms it is the rate at which energy is transmitted and is often measured in watts (W) or milliwatts (mW)

27

pulse duration

the time interval required for generating the transmitted pulse; it is calculated by multiplying the number of cycles in the pulse by the period

28

pulse repetition frequency (PRF)

in pulse-echo instruments, it is the number of pulses launched per second by the transducer

29

real-time

ultrasound instrumentation that allows the image to be displayed many times per second to achieve a “real-time” image of anatomic structures and their motion patterns

30

refraction

change in the direction of propagation of a sound wave transmitted across an interface where the speed of sound varies

31

resolution

ability of the transducer to distinguish between two structures adjacent to one another

32

slice thickness

thickness of the section in the patient that contributes to echo signals on any one image

33

temporal resolution

ability of the system to accurately depict motion

34

time gain compensation (TGC)

also referred to as depth gain compensation (DGC); ability to compensate for attenuation of the transmitted beam as the sound wave travels through tissues in the body; usually, individual POT (potentiometer) controls allow the operator to manually change the amount of compensation necessary for each patient to produce a quality image

35

transducer

any device that converts energy from one form to another

36

velocity

speed of the ultrasound wave; determined by tissue density

37

wave

propagation of energy that moves back and forth or vibrates at a steady rate

38

wavelength

distance over which a wave repeats itself during one period of oscillation

39

DOPPLER KEY TERMS

...

40

aliasing

technical artifact occurring when the frequency change is so large that it exceeds the sampling view and the pulse repetition frequency

41

color flow

velocity in each direction is quantified by allocating a pixel to each area; each velocity frequency is allocated a color; flow toward the transducer may be red; flow away from the transducer may be blue

42

continuous wave (CW) Doppler

one transducer continuously transmits sound, and one continuously receives sound; used in higher-velocity flow patterns

43

Doppler angle

the angle that the reflector path makes with the ultrasound beam; the most accurate velocity is recorded when the beam is parallel to flow

44

Doppler shift

the difference between the receiving echo frequency and the frequency of the transmitted beam

45

frequency shift

amount of change in the returning frequency compared with the transmitting frequency when the sound wave hits a moving target, such as blood in an artery

46

gate

the sample site from which the signal is obtained with pulsed Doppler

47

laminar

normal pattern of vessel flow; flow in the center of the vessel is faster than it is at the edges

48

Nyquist sampling limit

in pulsed Doppler, the Doppler signal must be sampled at least twice for each cycle in the wave if the Doppler frequencies are to be detected accurately

49

pulsed wave (PW) Doppler

sound is transmitted and is received intermittently with one transducer

50

rarefaction

region of decreased particle density

51

resistance

passive force in opposition to another active force; occurs when tissue exerts pressure against the flow

52

spectral analysis

analysis of the entire frequency spectrum

53

spectral broadening

change in the spectral width that increases with flow disturbance