Ultrasound Flashcards
what are the 3 categories of sound
- audible sound: 20Hz-20kHz
- ultrasound: >20kHz
- diagnostic ultrasound: 1MHz - 20MHz
what are the 2 categories that waves are classified
- mechanical waves
- electromagnetic waves
what are the 2 types of mechanical waves
- transverse wave
- longitudinal wave
what type of wave is ultrasound
longitudinal
what is the concept of transverse wave
- particles of a medium moves perpendicular to the direction of wave propogation
what is the concept of longitudinal wave
- 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
what is the formula of wave speed
c = f x wavelength
what is the avg speed of sound in soft tissue
1540m/s
what is speed of sound dependent on
- density
- compressibility of media
how does density affect speed of sound
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
how does compressibility affect speed of sound
compressibility = how squashy is the material
the more compressible, the lower the velocity
stiffer medium = faster wave propagation
so how does both compressibility and density of the same medium affect the speed of sound
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
what is frequency
- number of cycles per sec
- units: Hertz
- inverse of frequency is period (T)
what is wavelength
- length of space which one completes wave cycle occurs (dist)
interaction of sound and matters
- attenuation
- reflection
- scattering
- refraction
what is attenuation
attenuation = absorption + scattering
what is absorption of beam
- 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
factors affecting attenuation
- type of tissue (fluid attenuate less than tissues)
- frequency
- depth
what is acoustic impedance
- defined as the resistance of a medium to the transmission of sound
- if two media have different impedances, they are acoustically different
what is impedance mismatch
- 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
explain reflection of beam
- reflection occurs at the boundary/interface between 2 adjacent tissues
- difference in acoustic impedance cause reflection of sound wave
what are the 2 types of reflections
- specular reflection
- non-specular reflection (scattering)
explain specular reflection
- US strikes smooth boundary
- interface large and relatively smooth
- maximum reflection at 90deg
explain non specular reflection
- US strikes boundary with irregular surfaces with multiple individual interfaces
- sound deflects in multiple directions
what is refraction
- 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
what is critical angle
- angle of incidence for which the angle of refraction is 90deg
what are some commonly encountered artefacts
- reverberations
- dirty vs clean acoustic shadowing
- acoustic enhancement
- comet tail artifacts
when does reverberation occur
when beam encounters two highly reflective interfaces
what is posterior acoustic shadowing
- reduction in echo strength distal to a highly attenuating or reflective object
clean vs dirty shadowing
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
what is posterior acoustic enhancement
- increased intensity echoes relative to surrounding tissues occurring distal to low-attenuating structure
what is comet tail artifact
- multiple internal reflections within a small reflective object producing a series of short tapered echoes
what is piezo-electric effect
- 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
what is reverse piezo-electric effect
- voltage applied across the surface of crystal, crystal undergoes deformation
- deformation depends on polarity of voltage
relationship between wavelength and frequency
- inverse relationship
- wavelength decrease, f increase
what kind of crystals produce higher resonant frequencies
thinner
0.2-1mm
2MHz: 1mm
7.5MHz: 0.3mm
formula for distance measurement in US
s=ct
how do transducers receive signals
- 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
what makes up a simple transducer
- backing material
- connector
- matching layer
- crystal
- casing and acoustic insulator
types of transducer
- curvi-linear
- linear
- phased array
- transvaginal, transrectal
describe the curvi-linear trasnducer
- low frequency
- deep tissue: abdo, pelvis, kidney, liver
describe the linear transducer
- superficial tissue - thyroid, testis, breast, vascular
- higher frequency
- lower penetration
- lower resolution
use of crystal in the transducer
- acts as both sound source and sound receiver
- thickness of crystal according to desired frequency of output
what is the electrode for in the transducer
- electrodes plated onto 2 faces
- act as electrical connections to carry electric impulses to the crystal and from it during echo reception
what is damping
- 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
what is absorption
- 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
what are the possible artefacts caused by transducer
- dropped/ handed roughy- “dead” elements
parameters for testing
- sensitivity
- calibration
- dead space
- axial resolution
- lateral resolution
- cyst resolution
- contrast resolution
