Bioeffects Flashcards
Acoustic Variable
Pressure, Temperature, particle displacement, density
Attenuation
sound is absorbed by tissue (heat), non-linear frequency dependent absorption
Pulsed Wave Operation
Scanned vs non-scanned modality (M-mode vs B-mode)
Duty factor, frame rates
Transudcers
Focal point of beam, highest intensity,
System Operation
Output power, SNR
Doppler
Spectral Doppler and Continuous Doppler
Contributions to Bioeffects
Heating and Cavitation Mechanisms
Epidemiologic studies in human beings
Observation based research in cells, cultures, plants, and experimental animal studies.
Studies for bioeffects
Begin at the cellular level which provides information on interaction of mechanisms to guide
In animal studies…..
Information on thermal and non-thermal bio effects is provided
In epidemiology studies…..
Long term effects on patients with a history of previous ultrasound examinations are studied
Bioeffect
Refers to an undesired biological change to tissues as a result of interaction with the insonating beam
Sound beam is a
Transportation of energy
Energy is work with the possibility of work performed within the body can indicate a potential of adverse biological situations
Mechanisms of bioeffects
Thermal
Mechanical
What are the two principle mechanisms for bioeffects
Thermal effects
Mechanical bioeffects
Thermal bioeffects
Temperature rise related to: temporal average intensity duty factor scan time scanned vs. non-scanned modalities
mechanical bioeffects
Cavitation related to :
peak rarefactional pressure
What occurs with thermal bioeffects
Temperature in a region rises too high metabolic breakdown occurs and cellular damage can occur
What occurs with Mechanical bioeffects
Risk involves mechanical damage from interaction between the wave and the tissues within the body
This should not be surprising since sound is a mechanical wave
What are possible forms of mechanical bioeffects
Inertial / Transient Cavitation
Stable Cavitation
Radiation Force
Cavitation
Hollow space
Cavitation is when….
Bubbles are produced, vibrate or oscillate; bubbles burst or implode and a potential for biological effects is present
Stable Cavitation
- When oscillation of the microbubbles does not lead to collapse
- Bubbles oscillate with the varying acoustic pressure field in a stable manner
- Fluids surrounding the bubbles may begin to flow or stream
- Flow is the result of eddy currents which develop as energy is imparted to the fluid through the oscillating bubbles
- Referred to as micro-streaming
- Momentum of flow is potentially capable of inducing cellular wall stresses that can cause cellular harm
Inertial/Transient Cavitation
Unlike stable cavitation, this results in implosion of the microbubbles
The microbubbles may completely fragment or the ‘destruction’ may lead to a collection of smaller microbubbles
The likelihood of implosion is related to the peak rarefactional pressure
During this rarefaction phase, the pressure within the bubble relative to the lower decreasing pressure external to the bubble causes the bubble to expand
Clearly during the compression, the increasing external pressure causes the bubble to contract
With increasing ‘negative’ pressure of rarefaction, the bubble expands more and more
Inertial Transient Cavitation (Con’t)
Eventually the bubble expands so much that when compression begins, there is so much inertia from the surrounding fluid, that the bubble collapses inwardly (implodes)
This collapse can be extremely violent generating high amplitude shock waves and producing extremely high temperatures (as high as 10,000 Kelvin)
These shock waves and localized high temporature increases can cause damage to surrounding regions
Inertial Transient Cavitation Con’t 2
This can lead to cellular death
Fortunately the affected region is small, potentially only a few cells
The term transient is also used synonymously with the term inertial cavitation
The term transient is indicative of the fact that there is a change in “state” of the bubble
The word inertial is indicative of the fact that the dramatic change in state is the result of inertia
Radiation force
Is the force exerted by a sound beam on an absorber or a reflector
This force cab deform or disrupt structures
Can cause flow in an absorbing fluid
This flow can cause shear stresses that can deform or disrupt structures
Noncavitational mechanical interaction
Radiation force or direct force
is the interaction force of an ultrasound beam on an absorber or a reflector.
Basically when the ultrasound wave propagates through the medium it interacts between particles in the tissues
Target particles are pushed away from the transducer which causes acoustic streaming
This produces shear stresses that can disrupt or deform structures
Threshold Effect
limit or boundary
It implies that there is a limit above which a specific outcome is achieved, and below which a different outcome is occurs
Threshold effect-bioeffects
threshold bioeffects will force different behaviour and protocols than non-threshold effects
cavitation is stable up to…
A certain threshold, above which inertial cavitation occur
Long periods of time at lower refratctional pressures
will not produce inertial cavitation
There is no specific point…
at which a slight increase in power or scan duration will cause a dramatic increase in tissue temperature
Instead, the temperature change tends to be gradual over periods of time
This behavior is in stark contrast to the threshold behavior of cavitation
Mechanical bioeffects ehixibit
Threshold behaviour
What are mechanical bioeffects are related to ?
short-term event during the transmit burst and are not related to longer periods of time
Intensity
Power/Area
What is used to measure the risk of bioeffects
Common intensities
Common intensities: Concept 1
A peak is always greater than or equal to an average
Therefore an…..
SP is greater than an SA and
a TP is greater than a TA.
Common intensities: Concept 2
The first letter (S) refers to how the beam is distributed over physical space
Common Intensities: Concept 3
Pulse measurements
Common intensities: Concept 4
Temporal (PRP) measuremnts
Spatial Distribution
Refers to how the beam energy is distributed over physical space in the body.
The beam parameters principally determine the spatial distribution of the beam.
Temporal Distribution
Refers to how the energy is distributed over time.
