ultrasound principle Flashcards
what does ultrasound use for detection
soundwaves
- high frequency (inaduible to human)
what frequency range are sound waves
about 20 kHz
ultrasound frequency begins at 20kHz
know that sound waves can be formed into a narrow bean and focussed
they can be reflected, refracted, differacted, scattered and absorbed
what type of wave is a sound wave and what structure/propergation of particle energy does it follow
mechnaical wave (not electromagnetic)
- longitudinal wave
what requirement is needed to be met for a sound wave to be able to travel
requires medium to travel through
what is the units US is measured in
Megahertz
are sound waves ionising
no
define a longitudinal wave
- particle motion parallel to the direction fo wave propagation
- particle motion is pushed back and further in direction of travel resulting in bands of high and low pressure
- pressure wave propagates forward
how do you describe the particle displacement in a longitudinal wave
- rarefaction (spaced)
- compressed (bunched)
why is a medium required for sound waves to be able to travel
needs a medium in order to compress and expand
areas of compression have higher pressure and areas of rarefracgion have lower pressure
know what even though wave propagates to the right, the individual particle is actually oscillating about a point
define frequency
- number of cycles per second
- Hz
what frequency sound wave is used in medical ultrasound
2 - 20Mhz or higher
at 2Mhz is where the ultrasound begins to be able to penetrate into the human body
define wavelength + units
- distance between consecutive wave crests
i.e between compression to compression
- meters
what is the wavelength used in medical imaging
0.1mm (for 15 Mhz)
0.5mm (for 3MHz)
what is the wave equation
c = f x wavelength what
is the speed of sound in tissue
similar to sound passing through sea water as tissue contains a lot of water
- 1540 meters per second
- higher in bone
what happens to the wavelength when you change from a 5MHz to 10Mhz probe
follow the wave equation so
- double frequency
- with same velocity
- requires you to half the wavelenght
what is acoustic pressure
the amount of acoustic energy per time unit.
-Acoustic power shows the amplitude of the pulse pressure of the ultrasound beam.
- sound wave pressure is the excess pressure, follows a sinusoidal curve and can be positive or negative
what machine control directly affects acoustic pressure
output
what effect is used to produce sound waves
- piezoelectric effect
what material is used in the piezoelectric effect
lead zirconate titanate
(type of ceramic)
explain how the piezoelectric effect works
- pressure is applied across crystal
- generating a voltage (released by crystal) proportional to pressure applied
- applying alternating voltage causes crystals to expand and contract with same frequency
- releasing sound waves of a certain frequency
the piezoelectric effect in US can act as a transducer, what is this
can transmit and recieve
the transducer will have a natural vibrational frequency called the resonance frequency, what is htsi
the natural frequency where a medium vibrates at the highest amplitude
what is the resonance frequency of a transducer dependant on
crystal thickness (PZT place)
speed of sound in crystals
in piezoelectric effect, when would resonance occur
when crystal thickness = half wavelength
- resulting in constructive interference between waves produced from each face
how does crystal thickness and frequency relate
the thinner the crystal, the higher the frequency
as the thinner the crystal, the smaller the wavelength which results in higher frequency
what is aimed for in order to get an efficient generation of US waves
- want to match the electrical driving frequency to the natural resonance of the crystal to get efficient generation of US waves
a resonating transducer will generate continuous ultrasound waves, to create short pulses of US, what must be done
damping
what is damping
Decrease or decay of ultrasonic wave amplitude with respect to time or distance.
what is the quality / Q factor of the transducer
- the more ‘damped’ a resonator is, the ‘sharpness’ of the resonant frequency decreases
how does Q factor, bandwidth and frequency relate
low Q factor (less damping) = broad broadband width = large range of frequencies
high Q factor (more damping) = short bandwidth, more specific frequency
know that high Q factor produces a pure note and the doppler response to only that note
why is low Q factor more preferred
low Q factor = low spatial pulse time = high bandwidth = high frequency = low wavelength overall better image quality
(shorter time is more preferred as well)
what are the 2 operational modes in ultrasound
- continuous
- pulsed
where is the continuous operational mode used in US
therapy deivces
non imaging doppler mode
where is the pulsed operational mode used in US
imaging
imaging dopple
what are the 3 types of broad bandwidth probes
- curvilinear
- linear
- sector
what is the frequency range for the 3 different types of probes
curvilinear = 1.5 - 6 MHz
linear = 4 - 11 MHz
sector probe = 1.8 - 5.2 MHz
know what the 3 different probes look like
find image of transducer and label their components and function
- PZT crystal
- front and rear electrodes to apply voltage
- housing and RF shielding
- backing material (damping)
- matching layer
- lens
what is the role of the front and rear electrode in a transducer
- apply voltage to PZT plate
role of backing material in transducer and how does it work
damping
- absorbs sound energy
role of matching layer in transducer
maximises transmission of US
role of lens in transducer
beam shaping and focus
in practice, the transducer is sub-divided into what 4 elements
- acoustic lens
- acoustic matching layer
- piezoelectric element
- backing material
how many leads are there connecting to the rear electrodes
128
describe the field of the beam from an unfocussed beam
- near to filed is parallel beam, far field it diverges
describe the field of the beam from a focussed beam
electronically controlled and uses focussing lend - divergence only occurs beyond the focal zone
what is another word used to describe the near and far field of beam
near = fresnel zone
far = Fraunhofer zone
divergent field is not useful for imaging
describe the pressure distribution in the near and far fild
near field = complex pressure distribution
far field = uniform pressure distribution
describe how the crystal elements of the PZT plate (in electronic array) can be fired to form a good beam shape or longer aperture
- each crystal element can be fired independently but
- fired in groups will achieve larger aperture/ good beam shape
know that the beam position can be moved along by width of single crystal so there is better line density
what is the number range of crystal elements in the electronic array
64-200
what is scan plane focussing
- appropriate delays to the pulses transmitted from each transducer array element
- so that the transmitted energy adds constructively a desired focal point to form a transmit beam.
what is elevation plane focus
The transmitted beams are focused both in elevation plane and in scan plane by energizing different apertures of the transducer at different focal depths
- fixed focus by lens
overall, what is electronic focusin
- adding time delay to firing of each of the piezoelectric elements
- pulses interact constructively to form a pulse which converges at the focal point
number of active crystals also altered to change beam characteristics
what is beam steering
altering the angle of the ultrasound beam with respect to the transducer without moving the probe.
define resolution
minimum spacing between 2 reflectors that allow them to be distinguished on display
define axial resolution (resolution along beam axis) of US and what would its value be
- ability to separate structures parallel to the ultrasound beam
-axial resolution is approx half the pulse elngth
e.g pulse is approx 3/4 wavelengths so axial resolution is around 1.5-2 wavelenghts
why is a long pulse bad
echoes overlap and cannot be resolved
what is lateral resolution
- ability to distingue structures along the beam long axis (perpendicular to the beam)
brightness profile of laterally spaced targets begins to merge when their spacing is less than the beamwidth
w
what is the value at which structures in the axial or longitudinal can be resolved
if beam pulsing/ pacing is about half the beam width
what is adpodisation
reducing side lobes in an ultrasound focusing system
(side lobes are an unwanted component of the US beam)
describe the difference in the beam shape of a larger and smaller diameter transducer
small =
- narrow beam in near field (short)
- widely diverting in far field
large =
- wider beam in near field (longer)
- less divergent in far field
describe the difference in beam shape of a higher and lower frequency transducer
low freq =
- shorter near field
- more divergence
high freq =
- longer near field
- less divergence
what is speed of sound in terms of US
how fast the pressure waves travel away from the sound source
what 2 components of the medium affect the speed of wave travel
stiffness
density
how does stiffness and density correlate to wave speed
increase stiffness = increased speed
increase density = decrease in speed
know that low masses can be accelerated quickly by stiff springs
a stiffer spring will transfer energy more efficient than a less stiff one
less dense masses will be more easily accelerated than heavier ones
define acoustic impedance
- It describes how much resistance an ultrasound beam encounters as it passes through a tissue
- gear that the medium operates in to transmit the ultrasound energy
what 2 things is acoustic impedance dependant on
stiffness and density
what 4 things does acoustic impedance determine what happens at interface between 2 mediums
reflection
refraction
scattering
absorption
how does acoustic impedance and reflection relate
the greater the mismatch in acoustic impedance, the greater the reflection
what is snells law of refraction
a relationship between the path taken by a ray of light in crossing the boundary or surface of separation between two contacting substances and the refractive index of each.
= n1 sin 1 = n2 sin 2
what is law of reflection
angle of incidence = angle of reflection
when does scattering occur in ultrasound
- when dimensions of the boundary are small compared with the wavelength
- strongly dependant on freq
what causes a speckle pattern to be seen on US
random interference pattern produced by scattering
how does absorption occur in US
- sound energy converted to heat energy by friction in medium
- motion of particles cant keep up with frequency of wave, go out of step and lose energy
how does sound frequency and absorption rate of particles in US relate
increasing sound frequency = increasing absorption
other than friction/frequency speed what other thing affects absorption
higher viscosity of tissue leads to higher absoprtion
what is the rule of thumb/ approx value of US beam penetration
100 wavelengths
define attenuation in US
loss of energy from ultrasound beam
what 3 things is attenuation dependent on
type of tissue
ultrasound freq
ultrasound beam shape
what is the attenuation coefficient in soft tissue
0.7dB /cm/MHz
how does attenuation and frequency link to penetration
In the low-frequency range, the attenuation coefficient increases approximately linearly with the increasing of frequency
- resulting in poorer penetration
how does scattering and frequency link to sensitivity
scattering increases with frequency which overall increases the sensitivity
what is the pulse-echo technique and how does it work
- uses a single transducer as both the transmitter and receiver
- at tissue interface, some pulse is transmitted and some is reflected (echo)
- echo signal detected by transducer
- echoes returning first are from structures close to probe vice versa
type of interface determines echo amplitudwe
what equation is used to calculate depth (which is also the speed taken for echo return)
d = c x (t/2)
describe the full process of forming an ultrasound image
- pulse sent out by transducer
- echo train detected at transcode = analogue signal
- Gain and TGC applied
- amplitude digitised and entered into memory location related to origin of echo
- interpolation required to fill in the gas
what is TGC
Time gain compensation (TGC)
- additional feature that reduces impact of wave attenuation by tissues through increased intensity of the received signal in proportion to the depth.
what is interpolation
a method of constructing new simulated data points to fill in the gaps.
what is spatial.frequency compound imaging
images obtained using multiple beam angles or multiple frequency (and summed up)
what is the benefit of compound imaging
- random pattern change with true echos remaining the same
- average echos from differenct directions smooth out speckle apperance
- improved identification of boundaries
- reduced effect of shadowing
what is THI
tissue harmonic imaging
- based on the nonlinear interaction of the emitted ultrasound with the tissues, resulting in higher frequencies that return to the probe.
- waveform distorts creating harmonic
how does distortion and depth relate
there increased distortion with increasing depth
what type of range is tissue harmonic image more effective in
mid to far range
what type of filtering is used in tissue harmonic imaging and why
- narrow bandwidth to decrease spatial resolution
what is the phase inversion technique used in THI
- 2 pulses of opposite phases sent
- returning signals are added
- linear response canceled out (removing original frequency)
- non linear response (harmonics) is left
know that THI has reduced side lobes
Describe the beam width in THI
narrower effective beam width
when phase inversion is used in THI, what remains maintained
axial resolution maintained if phase inversion used
describe the overall apperance of THI after all its factors accounted
images appear sharper, less noise, higher contrast resolution
what are some assumptions made in echo mapping
- US travels in straight line
- speed of sound is constant
- US beam is infinitely naoorw
- echo amplitude relates directly to a structure
- echoes received originate from most recently transmitted pulse
know that echoes that appear oj image that do not correspond in location or intensity to actual interfaces in patient are artefacts
what are the 2 categories of US artefacts
location (path or speed)
attenuation
what is a side lobe artefact/ what is seen on image
- high reflective object (black eclipse) adjacent to beam position
- can reflect secondary lobes, falsely displaying those echoes at beam position
- low energy off-axis beams immediately adjacent to primary beam
what is a reverberation artefact / what does it look like on image
generated when echo from strongly reflecting interface parallel to transducer is partially reflected from transducer face back to interface
(partial echo reflected back from transducer to interface)
- generates second echo, which is displayed at twice depth of interface
- often see lines or comet tail
how can we reduce the reverberation artefact
using tissue harmonic imaging
decreasing gain
changing AOI
using multiple windows
where is reverberation artefact most commonly found scannign
liquid filled areas e.g bladder
what are some structures that can be seen due to reverberation on image
- comet tail
- ring down (looks like a long extended version of comet tail)
what is a mirror artefact
occur when the transmitted pulse and returning echo reflect off of a highly reflective interface (an acoustic mirror) and change direction before returning to the transducer
what is the affect of a mirror artefact on an image / what do u see
structure images to be deeper than it actually is
- see a curved bright line
what will happen to an image if there is refraction
if beam is refracted at angled surface, the display places object where it its expected not at the true location
why is it important to localise an AOI in more than one plane
to aid in location of objects true position as refraction can change where object is found on imaging
what can be seen on image as a result of refraction and what is this
ghosting
- object may appear in duplicate or triplicate due to refraction
compared to the true object, what does the duplicated structures look like in ghosting artefact
looks grey
refraction can also cause edge shadowing, what is this
edge shadowing (defocusing) at edge of structure with different speed of sound than surrounding medium
- refraction greatest at edges, resulting in shadowing due to decreased beam intensity to deeper tissue
what does edge shadowing artefact look like on image
- extend shadow from edge of object
how does shadowing relate to frequency
shadowing increases with increased frequency
(and THI)
what can be done to reduce shadowing
reduce the echo strength distal to highly attenuating or reflective object
what is a boundary distortion / what causes it
- speed of sound of object is less than assumed during calculation of depth
- longer time for transmitted and returning echoes = mis-registration of target
boundary distortion artefact occurring deep to a structure with a lower speed of sound than surround tissue
what is beam width artefacrt
- US beam assumed to be linear but actual beam narrows at focal point then widens distal to focal one
what do you usually see on beam width artefact
echos within structure
- small grey echo within black structure
what is the doppler effect
- difference in frequency of the transmitted and received wave
- moving blood causes change in US frequency and can detect and calculate velocity
(racing car sound analogy)
change in frequency = doppler effect
how does Doppler effect relate to velocity between transmitter and recieveer
Doppler effect is PROPORTIONAL to relative velocity between transmitter and receiver
what 3 ways is doppler information displayed graphically
- using spectral doppler
- colour doppler (directional) (flow)
- power doppler (non directional) (volume)
what is a duplex scan
image + doppler
- allows angle of incidence to be measure and velocity to be estimated
what is power doppler
displays amplitude of doppler signal rather than the detected frequency
- relates to blood volume and tissue perfusion
what are some adv and dis of power doppler
adv
- not angle dependant
- no aliasing
- better vessel wall definition
dis
- no velocity info
- non directional
- low temporal resolution
longer pulse gives more accurate measurement
