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Ultrasound basics > Ultrasound physics > Flashcards

Ultrasound physics Flashcards

1
Q

Rarefaction

A

Low pressure of a sound wave

opposite of Compression

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2
Q

Period in US

A

peak to peak (micro second)

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3
Q

Frequency in US

A

number of events per second (Hz)

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4
Q

Wavelength

A

peak to peak (distance; mm)

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5
Q

Speed of sound in medium

A

1540 m/s (1 mile per second)
1MHz frequency = wavelength 1.54 mm

v = frequency x wavelength

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6
Q

Amplitude in US

A

average to peak value

Strength of wave

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7
Q

Power in US

A

WATTS
rate at which work is performed
Increase power increases image brightness
(gain)

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8
Q

Intensity in US

A

Concentration of energy in the cross-section of the sound beam
power / area

1) not uniform
2) temporal variation and spacial variation

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9
Q

Relationship of wavelength, period and frequency

A

Frequency ~ 1/period or 1/wavelength

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10
Q

Relationship of power, intensity and amplitude

A

Intensity ~ amplitude^2

Power ~ amplitude ^2

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11
Q

Spatial peak-temporal average (SPTA)

A

Measure peak spatial and average time intensity

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12
Q

Propagation speed

A

Distance sound wave travels through medium in 1 second

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13
Q

Speed of sound in body

A

500-4000 m/s depending on medium
solid > liquid > gas

Soft tissue 1540 m/s
Lung 500 m/s
Bone 3500 m/s

Speed ~ stiffness / density

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14
Q

Attenuation

A

Weakening of waves as it goes through objects
Higher frequencies weaken more
Longer distances weaken more
Unit = Decibel (relative measure of intensity on a logarithmic scale)

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15
Q

Attenuation coefficient

A

Intrinsic properties of each tissue with effect on attenuation
In soft tissue this is half transducer frequency (0.5 dB/cm/MHz)

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16
Q

Anisotropy

A 
Directional dependence of the fibers on the attenuation
Muscle fibers (perpendicular > parallel) intensity
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17
Q

Types of scatter

A

Specular reflector - on smooth surface; organized reflection
Diffuse reflectors - rough surface; scattered
Rayleigh scattering - when surface much smaller than wavelength; rock in pond, for RBC’s

18
Q

Acoustic impedance

A

Property intrinsic to each tissue that affects the reflected sound
Only factor in perpendicular incidence (difference between impedance of the 2 media)

19
Q

Refraction define

A

Reflection with a bend

20
Q

Absorption

A

When acoustic energy converted to heat

Increases with higher frequency and scanning depth

21
Q

Pulse duration

A

Time from start of purse to end of pulse

22
Q

Receive time

A

Time in between pulses

23
Q

Spatial pulse length

A

length of each pulse

24
Q

Pulse repetition period

A

Start of 1 pulse to the start of the next pulse

25
Q

Pulse repetition frequency

A

Number of pulses in 1 second

26
Q

Function of transducer

A

Converts electrical energy to acoustic energy and vice versa

27
Q

Components of an ultrasound

A

1) Transducer
2) Master synchronizer
3) Pulser
4) Receiver

28
Q

Receiver functions

A

1) amplification (brighter)
2) compensation (time gain compensation)
3) compression (reduce signal to 2D image)
4) demodulation (corrects for negative voltages)
5) rejection (eliminate low level noises)

29
Q

Aperture

A

When sound beam leaves transucer it is the same diameter (near zone)

it then gets smaller (focal) then widens again (far zone)

30
Q

Phased-array transducers

A

Transducers with adjustable focus

31
Q

Resolution

A

ability to distinguish two objects that are spacially close to each other

32
Q

Ways to define resolution

A

1) distance of two objects
2) spatial (in space: axial/parallel/depth or lateral/perpendicular/side)
3) temporal
4) shades of gray (contrast)

33
Q

Display modes of US

A

A-mode (amplitude): used in ophthalmology for precise measurement
B-mode (brightness): varying amplitude converted to dots of varying intensity
M-mode (movement)

34
Q

Artifacts violated 6 assumptions of an imaging system

A

1) sound travels straight
2) sounds travels directly to reflector and back
3) sound in soft tissue is exactly 1540 m/s
4) reflections arise only from structures in main axis
5) imaging plane is thin
6) strength of reflection is related to characteristics of the tissue

35
Q

Reverberation artifact

A

1) equally spaced echos
2) sound bounce between two strong reflectors
3) first two real, the rest not

36
Q

Comet tail artifact

A

Type of reverberation artifact in thoracic ultrasound

37
Q

Shadowing

A

Hypoechoic region along edge of curved reflector

38
Q

Mirror artifact

A

Reflect off strong reflector then towards second structure

Creates replica of structure that is deeper

39
Q

Doppler shift equation

A

Doppler shift = 2vFCostheta / propagation speed

40
Q

Two most common types of Doppler

A

1) Continuous wave doppler: good for high velocities

2) pulsed doppler: can identify exact location of a moving RBC

Ultrasound basics (1 decks)

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