Ultrasound

Question 1

what are the 3 categories of sound

Answer 1

• audible sound: 20Hz-20kHz
• ultrasound: >20kHz
• diagnostic ultrasound: 1MHz - 20MHz

Question 2

what are the 2 categories that waves are classified

Answer 2

• mechanical waves
• electromagnetic waves

Question 3

what are the 2 types of mechanical waves

Answer 3

• transverse wave
• longitudinal wave

Question 4

what type of wave is ultrasound

Answer 4

longitudinal

Question 5

what is the concept of transverse wave

Answer 5

• particles of a medium moves perpendicular to the direction of wave propogation

Question 6

what is the concept of longitudinal wave

Answer 6

• particles of medium moves parallel or antiparallel to the motion of the wave
• particles oscillate back and fourth about their resting positions, in line with the direction of the wave travel

Question 7

what is the formula of wave speed

Answer 7

c = f x wavelength

Question 8

what is the avg speed of sound in soft tissue

Answer 8

1540m/s

Question 9

what is speed of sound dependent on

Answer 9

• density
• compressibility of media

Question 10

how does density affect speed of sound

Answer 10

density = number of particles per unit vol
as density increases, there are more particles to move
so they become harder to move

denser medium = slower sound wave propagation

Question 11

how does compressibility affect speed of sound

Answer 11

compressibility = how squashy is the material
the more compressible, the lower the velocity

stiffer medium = faster wave propagation

Question 12

so how does both compressibility and density of the same medium affect the speed of sound

Answer 12

when there is a higher density medium = harder to compress
but higher density = slower wave
harder to compress = faster wave

when think about velocity, look in terms of compressibility

Question 13

what is frequency

Answer 13

• number of cycles per sec
• units: Hertz
• inverse of frequency is period (T)

Question 14

what is wavelength

Answer 14

• length of space which one completes wave cycle occurs (dist)

Question 15

interaction of sound and matters

Answer 15

• attenuation
• reflection
• scattering
• refraction

Question 16

what is attenuation

Answer 16

attenuation = absorption + scattering

Question 17

what is absorption of beam

Answer 17

• process whereby the US beam gives up energy to the tissue it is travelling in
• the tissue energy content increases while beam energy decreases
• 2 effects occur: tissue molecules vibrate with greater amplitude and tissue heats up

Question 18

factors affecting attenuation

Answer 18

• type of tissue (fluid attenuate less than tissues)
• frequency
• depth

Question 19

what is acoustic impedance

Answer 19

• defined as the resistance of a medium to the transmission of sound
• if two media have different impedances, they are acoustically different

Question 20

what is impedance mismatch

Answer 20

• if acoustic impedance is same in 2 medium, sound will readily travel from one to the other
• if there is a difference in acoustic impedance, causes a portion of the sound to be reflected at the interface

Question 21

explain reflection of beam

Answer 21

• reflection occurs at the boundary/interface between 2 adjacent tissues
• difference in acoustic impedance cause reflection of sound wave

Question 22

what are the 2 types of reflections

Answer 22

• specular reflection
• non-specular reflection (scattering)

Question 23

explain specular reflection

Answer 23

• US strikes smooth boundary
• interface large and relatively smooth
• maximum reflection at 90deg

Question 24

explain non specular reflection

Answer 24

• US strikes boundary with irregular surfaces with multiple individual interfaces
• sound deflects in multiple directions

Question 25

what is refraction

Answer 25

- US crosses a boundary where there is a velocity difference, it will be refracted - change in the direction of the wave, where is passes obliquely through another medium with different velocity - increase difference of velocity = increase refraction

Question 26

what is critical angle

Answer 26

- angle of incidence for which the angle of refraction is 90deg

Question 27

what are some commonly encountered artefacts

Answer 27

- reverberations - dirty vs clean acoustic shadowing - acoustic enhancement - comet tail artifacts

Question 28

when does reverberation occur

Answer 28

when beam encounters two highly reflective interfaces

Question 29

what is posterior acoustic shadowing

Answer 29

- reduction in echo strength distal to a highly attenuating or reflective object

Question 30

clean vs dirty shadowing

Answer 30

clean shadowing commonly occurs distal to larger calculi and bon, appears as dark anechoic band dirty shadowing is commonly seen distal to a highly reflective surface such as gas

Question 31

what is posterior acoustic enhancement

Answer 31

- increased intensity echoes relative to surrounding tissues occurring distal to low-attenuating structure

Question 32

what is comet tail artifact

Answer 32

- multiple internal reflections within a small reflective object producing a series of short tapered echoes

Question 33

what is piezo-electric effect

Answer 33

- properties of some crystalline material - when force is applied, these materials change shape - and develop an electric charge on their surface (electrically polarised - generates voltages) - when short electrical burst is applied - material vibrates

Question 34

what is reverse piezo-electric effect

Answer 34

- voltage applied across the surface of crystal, crystal undergoes deformation - deformation depends on polarity of voltage

Question 35

relationship between wavelength and frequency

Answer 35

- inverse relationship - wavelength decrease, f increase

Question 36

what kind of crystals produce higher resonant frequencies

Answer 36

thinner 0.2-1mm 2MHz: 1mm 7.5MHz: 0.3mm

Question 37

formula for distance measurement in US

Answer 37

s=ct

Question 38

how do transducers receive signals

Answer 38

- produce US pulse in response to electric stimulus - convert electrical energy to mechanical energy in the form of vibration - convert sound energy (echoes) into electrical signals to generate sonogram

Question 39

what makes up a simple transducer

Answer 39

- backing material - connector - matching layer - crystal - casing and acoustic insulator

Question 40

types of transducer

Answer 40

- curvi-linear - linear - phased array - transvaginal, transrectal

Question 41

describe the curvi-linear trasnducer

Answer 41

- low frequency - deep tissue: abdo, pelvis, kidney, liver

Question 42

describe the linear transducer

Answer 42

- superficial tissue - thyroid, testis, breast, vascular - higher frequency - lower penetration - lower resolution

Question 43

use of crystal in the transducer

Answer 43

- acts as both sound source and sound receiver - thickness of crystal according to desired frequency of output

Question 44

what is the electrode for in the transducer

Answer 44

- electrodes plated onto 2 faces - act as electrical connections to carry electric impulses to the crystal and from it during echo reception

Question 45

what is damping

Answer 45

- when a crystal is pulsed, it will ring - but if the crystal is still producing sound then it cannot react to returning sound - therefore need to damp the crystal to produce short pulse - short pulses allow better better resolution

Question 46

what is absorption

Answer 46

- absorbing sound produced by the back face of the crystal - if sound is allowed to reach the front of the crystal, it would destructively interfere with the sound produced by the front face and reduce intensity of the US beam - backing material is selected to be as absorbing as possible

Question 47

what are the possible artefacts caused by transducer

Answer 47

- dropped/ handed roughy- "dead" elements

Question 48

parameters for testing

Answer 48

- sensitivity - calibration - dead space - axial resolution - lateral resolution - cyst resolution - contrast resolution