Topic 16: Ultrasound biophysics and safety Flashcards
What different ways can ultrasound affect tissue?
- Thermal effects
- Bubble and cavitation effects - the negative pressure gets large enough
- Radiation effects (push stuff) - force produced by a change in energy density due to the absorption, scattering or reflection of ultrasound
Consider a plane wave propagating through homogeneous tissue. If there is no energy loss, the time averaged acoustic intensity in the direction of wave propagation will remain ____. However, in general, some acoustic energy will always be lost due to _____ _____, and thus the time averaged intensity will decay _____ in the direction of wave propagation. This loss in acoustic energy is converted to _____, where the rate of _____ is proportional to the spatial gradient of the intensity.
constant
acoustic absorption
exponentially
heat
heating
As discussed in Chapter 7, the pressure and velocity of a harmonic plane wave in an absorbing medium can be written in the form________
How can we obtain the volume rate of heat deposition Q.
Taking the spatial gradient of the intensity gives the volume rate of heat deposition Q.
How does frequency and nonlinearity affect heating?
As the absorption coefficient, a, increases as a function of frequency, using higher frequencies will also give increased heating.
Similarly, if the wave propagation is nonlinear, energy will be moved from the fundamental frequency to higher frequency harmonics where the absorption is higher, again leading to increased heating (often referred to as nonlinear enhanced heating).
how is ultrasound heating related to the acoustic pressure?
the square of the acoustic pressure
When you turn off the ultrasound the heat reduces through what mechanisms?
due to diffusion and perfusion
What are the two constituents of tissue?
water and extra-cellular matrix
ECM is largely constructed from _____
tri-helical collagen fibres.
The tissue is heated - what several changes occur?
- Above 40 degrees - cell proteins start to undergo conformational shape changes due to hydrogen bonds that break.
Above 65 degrees. The collagen fibres forming the ECM begin to shrink as the collagen’s tri-helical structure breaks apart.
Thermal dose is typically measured in ___ ___ ____
Thermal dose is typically measured in cumulative equivalent minutes
The actual thermal damage to the tissue depends on both the ____ and ______ .
temperature and the exposure time
The difference in the value of R below 43C is due to ?
The difference in the value of R below 43C is due to the development of thermotolerance during heating. This is acquired resistance to thermal toxicity regulated by heat-shock proteins. When cells are subject to thermal stress, these proteins are up-regulated and act as chaperones helping in protein-protein interactions, folding, preventing unwanted aggregation, and providing stability to partially unfolded proteins.
What is the most commonly used threshold for cell death?
t43 = 240 min
What is the thermal index?
Pdeg is the power required to raise the tissue by 1 K taken under reasonable worst-case conditions.
What is TI in words and what does it ignore? and what are the uncertainties?
TI is a measure of the predicted temperature rise. However, this ignores the structure of the acoustic beam (which will be non-uniform), and many other factors. There is also a large uncertainty in the values for Pdeg. Consequently, TI should be taken as a relative indicator of the likelihood of thermal e↵ects, rather than having any predictive value
The value for Pdeg will depend on
frequency and tissue type
Different thermal indices are used for different targets. If the tissue is soft tissue this index is called ___, for bone at the surface it’s called ___, and for bone below the surface, it’s called ___.
Different thermal indices are used for different targets. If the tissue is soft tissue this index is called TIS, for bone at the surface it’s called TIC (e.g., for imaging through the skull), and for bone below the surface, it’s called TIB
Some of the possible cavitation behaviour of a bubble subject to an oscillating ultrasound field.
What thermal index is too high for the scanning of an embryo or fetus according to the BMUS?
TI > 3.0
Acoustic cavitation definition
The formation and activity of bubbles of gas or vapour in a medium exposed to an acoustic field
Cavitation inception
small pre-existing bubbles or cavitation nuclei in the medium (fluids will always have some intrinsic nuclei)
Stable cavitation
acoustic pressure variations cause bubble oscillations which continue indefinitely
what is rectified diffusion as a form of acoustic cavitation?
active “pumping” of gas, initially dissolved in the surrounding fluid, into bubble.
Threshold process (depends on bubble size and acoustic pressure)
Inertial (transient) cavitation
bubble undergoes rapid expansion followed by violent collapse governed by the inertia of surrounding fluid
inertial cavitation ?
is a threshold event that depends on the peak negative acoustic pressure, and the presence and size of cavitation nuclei
Jetting
cavitation close to a boundary often takes the form of ‘jetting’ where the bubble collapses asymmetrically, smashing a ‘jet’ of fluid into the boundary
• Sonoluminescence
During the compressive cycle of a bubble oscillation, as the radius becomes very small, the temperature of the gas in the bubble can reach very high temperatures in excess of 5000 K (the temperature on the sun surface is roughly 6000 K) and emit picosecond pulses of light. Although the mechanism for the light emission is not completely understood, the most widely accepted theory is that an inward travelling shock wave creates a hot spot and the generation of a partly ionised plasma
Cavitation clouds
A gas-filled bubble has an acoustic impedance which is much smaller than the surrounding tissue. This causes strong acoustic reflections and the inversion of the incoming ultrasound wave such that a large peak positive pressure becomes a large peak negative pressure, leading to the cyclic growth of a bubble cloud.
Bulk heating
The oscillation and collapse of bubbles can heat tissue in several ways. First, there can be viscous heating due to the bubble motion and boundary layer e↵ects. Second, the nonlinear scattering of the ultrasound wave can generate higher frequency harmonics which are absorbed more rapidly. Finally, violent bubble collapse can cause the emission (and subsequent absorption) of broadband ultrasound and shock waves.
Mechanical action
The inertial collapse of a bubble generates large forces on surrounding tissue, and well as jetting and microstreaming. If the bubble is inside a blood vessel, this can cause vessel distension (outward push into tissue) and invagination (inward pull into lumen). Combined with jetting, this can cause distortion of the endothelial wall and vessel rupture. At a cellular level, cavitation can also cause cell lysis (where the cell membrane is ruptured and the cell is destroyed), and structural and functional changes to cells.
What mechanisms can cavitation affect tissue?
Bulk heating
Mechanical action
Chemical action
How does chemical action through cavitation damage tissue?
High temperatures during bubble collapse can form free radicals (H20 <-> H + OH ) and sonochemicals (e.g., hydrogen peroxide) which can cause DNA damage.
The mechanical index is what?
the on-screen measure of the potential for inertial cavitation during ultrasound scanning.
where f is the ultrasound centre frequency in MHz, and Pr is the peak rarefactional (negative) pressure in MPa (note the units!).
The peak negative pressure is measured in water, and then derated by 0.3 dB/(MHz cm) according to
where P(x) is the peak negative pressure measured in water as a function of distance x. In contrast to the thermal index, the mechanical index decreases with increasing frequency, as the cavitation threshold increases.
What levels of MI does the british medical ultrasound society say are acceptable under different scenarios?
MI > 0.3 There is a possibility of minor damage to neonatal lung or intestine. If such exposure is necessary, try to reduce the exposure time as much as possible.
MI > 0.7 There is a risk of cavitation if an ultrasound contrast agent containing gas microspheres is being used. There is a theoretical risk of cavitation without the presence of ultrasound contrast agents. The risk increases with MI values above this threshold.
How do you calculate the radiation force?
The transducer and target are placed in a small tank of deionized and degassed water, and the weight is measured before and after the transducer is turned on.
Equating the mass to force using F=mg the acoustic power can then be calculated.
The radiation force exerted by an ultrasound transducer is measured using this setup how?
What is passive cavitation detection?
Cavitation activity can be detected by passively listening for bubble signature
What is acoustic streaming?
the radiation force exerted by an ultrasound wave in an absorbing fluid medium can cause the fluid to move.
Streaming only occurs when the wave is being _____and ______
Streaming therefore only occurs when the wave is being attenuated and the medium can flow, which is not always true of soft tissue.
