Ultrasound physics

Question 1

Rarefaction

Answer 1

Low pressure of a sound wave

opposite of Compression

Question 2

Period in US

Answer 2

peak to peak (micro second)

Question 3

Frequency in US

Answer 3

number of events per second (Hz)

Question 4

Wavelength

Answer 4

peak to peak (distance; mm)

Question 5

Speed of sound in medium

Answer 5

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

v = frequency x wavelength

Question 6

Amplitude in US

Answer 6

average to peak value

Strength of wave

Question 7

Power in US

Answer 7

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

Question 8

Intensity in US

Answer 8

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

1) not uniform
2) temporal variation and spacial variation

Question 9

Relationship of wavelength, period and frequency

Answer 9

Frequency ~ 1/period or 1/wavelength

Question 10

Relationship of power, intensity and amplitude

Answer 10

Intensity ~ amplitude^2

Power ~ amplitude ^2

Question 11

Spatial peak-temporal average (SPTA)

Answer 11

Measure peak spatial and average time intensity

Question 12

Propagation speed

Answer 12

Distance sound wave travels through medium in 1 second

Question 13

Speed of sound in body

Answer 13

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

Question 14

Attenuation

Answer 14

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)

Question 15

Attenuation coefficient

Answer 15

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

Question 16

Anisotropy

Answer 16

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

Question 17

Types of scatter

Answer 17

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

Question 18

Acoustic impedance

Answer 18

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

Question 19

Refraction define

Answer 19

Reflection with a bend

Question 20

Absorption

Answer 20

When acoustic energy converted to heat

Increases with higher frequency and scanning depth

Question 21

Pulse duration

Answer 21

Time from start of purse to end of pulse

Question 22

Receive time

Answer 22

Time in between pulses

Question 23

Spatial pulse length

Answer 23

length of each pulse

Question 24

Pulse repetition period

Answer 24

Start of 1 pulse to the start of the next pulse

Question 25

Pulse repetition frequency

Answer 25

Number of pulses in 1 second

Question 26

Function of transducer

Answer 26

Converts electrical energy to acoustic energy and vice versa

Question 27

Components of an ultrasound

Answer 27

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

Question 28

Receiver functions

Answer 28

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)

Question 29

Aperture

Answer 29

When sound beam leaves transucer it is the same diameter (near zone) it then gets smaller (focal) then widens again (far zone)

Question 30

Phased-array transducers

Answer 30

Transducers with adjustable focus

Question 31

Resolution

Answer 31

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

Question 32

Ways to define resolution

Answer 32

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

Question 33

Display modes of US

Answer 33

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

Question 34

Artifacts violated 6 assumptions of an imaging system

Answer 34

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

Question 35

Reverberation artifact

Answer 35

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

Question 36

Comet tail artifact

Answer 36

Type of reverberation artifact in thoracic ultrasound

Question 37

Shadowing

Answer 37

Hypoechoic region along edge of curved reflector

Question 38

Mirror artifact

Answer 38

Reflect off strong reflector then towards second structure | Creates replica of structure that is deeper

Question 39

Doppler shift equation

Answer 39

Doppler shift = 2vFCostheta / propagation speed

Question 40

Two most common types of Doppler

Answer 40

1) Continuous wave doppler: good for high velocities | 2) pulsed doppler: can identify exact location of a moving RBC