Ultrasound Physics 1

Front 1

Acoustic variables (term)

Back 1

Changes that happen in the medium as waves are propagating

Front 2

Describe the acoustic variables

Back 2

1. Pressure - a concentration of force, or force per area. (in compression- pressure increases, in rarefaction- pressure decreases)

2. Density – periods of higher particle concentrations (high density) and lower particle concentrations

3. Temperature- Celsius and Fahrenheit (1. 8 or temp in F=1.8 x temp in C

4. Particle motion- the particles vibrate back and forth on their original location, allowing the concentration of energy to propagate along the wave path

Hint 2

Ppdt

Front 3

Describe what type of wave ultrasound is

Back 3

Mechanical wave- requires physical interaction. Must have a medium in order to exist!!

- the medium affects the wave and the wave affects the medium

Front 4

Describe the other type of way and why isnt ultrasound considered this type of wave?

Back 4

Electromagnetic waves- can travel thru a medium and thru a vacuum

Front 5

Describe how propagation ultrasound waves exhibit

Back 5

Longitudinal- propagates by a series of compressions and rare factions back and forth in the same direction as the wave propagation direction

-sound is longitudinal

Front 6

How does the energy in an ultrasound wave travel thru the body?

Back 6

The source of the wave relays energy to the molecules of the medium in contact w/ the source. This collision happens they start to move. 2 responses:

1. some energy of the higher energy molecules is now transferred to the neighbouring lower energy particles.

2. From the collision, the higher energy molecules reflect back towards the wave source.

-this collision process continues until all of the energy of the wave dissipates.

Front 7

Equation for density

Back 7

Density (P) = mass (kg) / volume (m^3)

Front 8

Constructive interference

Back 8

In phase waves- both waves reach peaks and cross zeros at the same time

- the 2 waves interact to produce 1 larger wave

Front 9

Destructive interference

Back 9

Out of phase waves- positive peaks of one wave align w the negative peaks of the other wave

-2 waves cancel each other out

Front 10

Medium properties that affect prop vel

Back 10

Elasticity- the ability of a solid object to return to it's original shape after distortion by force

Compressibility- measure of how much vol. Of the material the same given force

(High compressibility can be compressed to a smaller vol than low compressibility)

Stiffness- implies the inverse of elasticity or compressibility

Bulk modulus- the decrease ratio of the stress to strain

Stress= change in pressure

Strain= %change in vol

-bulk modulus is related to stiffness - low compressibility has a high bulk modulus

Hint 10

Becs

Front 11

What's the relationship btwn each medium property and the prop vel?

Back 11

High density = low prop vel

-> c ○< 1/density (P)High compress. = low prop vel

-> c ○< 1/compressibilityHigh bulk modulus = high prop. Vel

Front 12

Why does bone have a higher prop vel than lung tissue?

Back 12

Bone is more dense and more stiff

- stiffness has a much larger effect on prop vel than density

Front 13

Why do we care abt the amplitude of an ultrasound wave?

Back 13

1. A higher power is related to a stronger ultrasound signal (improved signal resulting in a better image)

2. A higher power is also related to an increase in risk of bio effects (tissue dmg)

Front 14

Relationships btwn amplitude, power and intensity

Back 14

Power ○< (amplitude)^2

Intensity ○< power

Intensity ○< 1/beam area

Front 15

frequency correlation

Back 15

Pitxh

Front 16

Amplitude correlation (sound)

Back 16

Volume

Front 17

Units of period

Back 17

Time

Front 18

Units of wavelength

Back 18

Distance

Front 19

Distance/ range equation

Back 19

V=d/t

d=rt

13usec

Front 20

Half value layer

Back 20

Where the thickness of soft tissue @ which the intensity of the beam is reduced by 1 half

HVL= 6/ frequency (MHz)

Front 21

Sine val

Back 21

Sin 0=0

Sin 90=1

Sin 30=0.5

Sin 45=0.707

Sin60= 0.866

Front 22

Attenuation of soft tissue muscle n blood

Back 22

Soft tissue: 0.5 dB/cm MHz

Muscle: 1.0 dB/cn MHz

Blood: 0.125 db/cm MHz

Front 23

Attentuation

Back 23

Decrease in wave amp. (Or intensity) due to interaction w the medium

-implies decrease so neg sign not used

Front 24

Mechanisms of attenuation

Back 24

absorption- changing US energy into head

Specular reflection- reflectors larger than the WL

Scatterers- reflectors smaller than WL

Hint 24

Ass

Front 25

How does attenuation affect image

Back 25

Waves need to be able to go in and come back

Front 26

What can be done to mitigate attenuation

Back 26

Lower transducer frequency

Increase output

Increase gain

Adjust TGC

Change scanning path

Hint 26

LIICA

Front 27

Factors that affect absorption

Back 27

Dependant on the molecular interactions w/in the medium

Freq. Of moving (compressing) the medium

Front 28

Angle of incidence

Back 28

Angle formed btwn wavefront and and interface of reflection

Front 29

Normal incidence

Back 29

Wavefront is parallel to the reflecting structure

Front 30

Different types of reflection

Back 30

Specular reflection- mirror like reflection

Back scattering- occurs when reflecting surface is rough the reflection is redirected in many diff direction

Rayleigh scattering- when reflecting structures are v small w respect to WL (rayleigh scat. Increases w freq.)

Front 31

Acoustic impedance (define n formula)

Back 31

When theres a large mismatch theres a large reflection

Z=p x c

Z=kg/m^2 sec

Front 32

What is the critical angle?

Back 32

Angle at total internal reflection

Front 33

Matching layer

Back 33

Improves eff. Of sound transmissions in and out of the patient (helps mismatch)

Front 34

How the machine decides to place each echo in an image

Back 34

Itll record the time for the distance it has to travel and put it on image according to what comes back

Front 35

PW parameters

Back 35

PD

PRP

duty factor (time work is being done)

Spatial pulse length SPL

Range res./axial res - ability to see structures apart

Front 36

Scanned modalities

Back 36

Image is built up over time by transmitting and receiving in a specific location then moving over and repeat

Front 37

Nonscanned modalities

Back 37

Repeatedly transmitting and receiving from the same location over time

Front 38

Axial res equation

Back 38

AR= WL x # cycles/2

Front 39

PRF

Back 39

The reciprocal of PRP (prf=1/prp)

Front 40

Bandwidth

Back 40

Range of freq emitted by the transducer

Front 41

Centre freq.

Back 41

Abt middle of the bandwidth

Front 42

Fractional bandwidth

Back 42

Frac BW= BW(MHz)/ freq. (MHz)

Front 43

How dynamic freq. Works

Back 43

Produces a wide BW then filters the recieve freq. Based on imaging depth

-near high freq

Mid- intermediate freq

Far- low freq

Front 44

Relationship of PD and BW

Back 44

If signal rings for a long time there is a narrow bandwidth

Is the signal rings for a short time it has a broad bandwidth

Front 45

1/2and 1/4 BW

Back 45

1/2 power BW- 50%drop range of choices larger

1/4 power BW- 25% drop range even larger

Front 46

Distance / range equation

Back 46

V=d/t

d=rt (r=rate of avg speed)

Front 47

Power and amp

Back 47

Power o< amp^2

Front 48

Intensity

Back 48

I=power/beam area o<amp^2/beam area

I=1/beam area

I=P(W)/a

Front 49

Decibels

Back 49

dB=10log(Pf/Pi)

dB=10 log (I2/I1)

dB=20 log (A2/A1)

Front 50

Acoustic impedance

Back 50

Z= P x c

Front 51

Reflection & transmission

Back 51

Reflection % + transmission % =100%

1-reflection= transmission

Front 52

Intensity reflection coeff.

Back 52

IRC= ((Z2-Z1)/(Z2+Z1))^2

Front 53

Amp reflection coeff

Back 53

ARC= [Z2-Z1/ Z2+Z1]

Front 54

% reflection

Back 54

% reflection= [Z2-Z1/ Z2+Z1]^2

% x100

Front 55

Snells law

Back 55

Sin ○i/ sin ○t

Front 56

Total attenuation

Back 56

Atten coeff x path length (cm) x freq.

Front 57

HVL

Back 57

HVL= 6/f (MHz)

Front 58

PD

Back 58

PD= P • # cycles

Front 59

PRP

Back 59

PRP= 1/PRF or 13usec x depth

Front 60

Duty factor

Back 60

Duty factor= PD/PRP

Front 61

PRF

Back 61

PRF==1/PRP

Front 62

Frac BW

Back 62

Frac BW= BW (MHz)/ freq. (MHz)

Frac Bw= BW/Fc

Front 63

Q factor

Back 63

Q= freq. (MHz)/ BW (MHz)

Q= 1/ frac BW