Biophysics & Safety Flashcards
What can ultrasound induce?
Thermal effects: absorption of ultrasound heats tissue
Bubble and cavitation effects: the formation, oscillation, and
collapse of bubbles of gas and vapour due to ultrasound
Radiation effects: force produced by a change in energy density due to the absorption, scattering or reflection of ultrasound
What can the effects of ultrasound be used for?
They can be exploited for ultrasound therapy
What happens due to absorption to the beam?
There is a loss in acoustic energy
The time average intensity will decay exponentially along the beam axis
What is the rate of heating proportional to?
Spatial gradient of intensity
What is P_A (x)?
The time average pressure which is spatial varying
What does the local pressure amplitude indicate about the heating by ultrasound?
Heating by ultrasound is proportional to the absorption coefficient, and the square of the local pressure amplitude and inversely proportional to acoustic impedance
What happens after the ultrasound is switched off?
The heat will gradually decrease due to diffusion (conduction) and perfusion
What is the temperature in the absence of diffusion and perfusion (if heat is deposited) quickly?
The change in temperature is δT/δt = (1 / ρ_0 C_0) Q
What is the total temperature rise?
T = T_0 + ΔT
ΔT = temperature rise (no diffusion)
Why can the temperature rise equations be used in a focused transducer?
Strictly only valid for a harmonic plane wave, in the focal region of a focused transducer, but the wave-field is approximately plane, so they can still be applied
What do nonlinear effects move?
Energy to higher frequencies (leads to enhanced heating)
What affects heating rate?
Human tissue has a range of absorption values
What are Q and ΔT proportional to?
α
What happens as frequency is increased?
Absorption increases and higher frequencies will give increased heating
What does the area under the curve of attenuation vs frequency graph give?
The thermal dose delivered at different temperatures
What happens to tissue at temperature < 40 degrees?
No irreversible damage (renaturation can occur: regains activity)
What happens to tissue at temperature > 40 degrees?
Cell proteins (e.g., enzymes) start to undergo conformational (shape) changes due to violent vibrations of the molecule and they begin to denature (loss of biological activity) and lose function
What happens above 65 degrees?
Collagen fibres shrink, tri-helical structure breaks, tissue coagulates
What does the actual thermal damage to tissue depend on?
Both the temperature and exposure time
What do heated cells look like?
Rounded shape
Lack of organised cytoskeleton
What is thermal dose measured in?
Cumulative equivalent minutes (CEM 43 degrees or t_43)
t_43 = time in minutes
How is the thermal dose found for different temperatures (equation)?
t_43 = ∫ R^43-T dt
Gives the number of minutes to achieve the same effect as at 43 degrees C
What is the difference in slope between 43 degree C due to?
The development of thermo-tolerance during heating
What is thermo-tolerance?
an acquired resistance to thermal toxicity regulated by heat-shock proteins, which are up-regulated when cells are exposed to thermal stress
Also a memory effect
What is the most commonly used threshold for cell death?
t_43 = 240 min
How long does it take to reach a given thermal dose above 43 degrees C?
For every degree increase above 43°C, time to effect is halved
(in most cases, heating isn’t uniform)
What does the exact thermal dose required to achieve cell death depend on?
Depends strongly on the tissue and cell type
Not binary – some tissue damage may occur before thresholds shown
What improves image quality?
Using higher acoustic pressures can improve the signal-to-noise ratio (SNR)
Similarly, using higher frequencies, because they have shorter wavelengths, can improve the spatial resolution
What does the use of both higher frequencies and higher pressures lead to?
Increased heating in the tissue
What is the thermal index?
On-screen measure of the potential for tissue heating (doesn’t have predictive value)
TI = P_0/ P_deg
What is P_deg dependent on?
on frequency and tissue type and taken under “reasonable worst-case conditions”
What are the different thermal indices used for different targets?
TIS: soft tissue
TIC: bone at the surface (e.g. for imaging through the skull)
TIB: bone below the surface (e.g. fetal imaging)
Bone is more absorbing, so changes for TIB and TIC
What are the published guidelines for scanning times for different scenarios?
Exposure limit is time dependent as thermal dose is cumulative
TI < 0.7 : no risk of tissue damage
TI > 0.7: The overall exposure time (including pauses) of an embryo or fetus should be restricted in accordance (0.7 TI = 60 mins)
TI > 3: Scanning of an embryo or fetus is not recommended, however briefly
What is the order of modes of diagnostic ultrasound least to most affected by increase of power and heating?
B-mode Imaging
->
Harmonic Imaging
->
M-mode Imaging
->
Colour Doppler
->
Spectral Doppler
What is acoustic cavitation?
The formation and activity of bubbles of gas or vapour in a medium exposed to an acoustic field
What is cavitation inception?
Small pre-existing bubbles or cavitation nuclei in the medium (fluids will always have some intrinsic nuclei)
There will always be a quantity of tiny bubbles stabilised by some mechanism against dissolution
What is stable cavitation?
Acoustic pressure variations cause bubble oscillations which continue indefinitely (low amplitude)
Radius continually increasing and decreasing in response to the acoustic field
low pressure = bubble expands
high pressure = bubble contracts
What is the resonant frequency of a gas bubble of radius R_0 in water at atmospheric pressure?
f_0 = 3/R_0
(For coated bubbles, the resonance frequency will be higher)
What is rectified diffusion?
Active “pumping” of gas, initially dissolved in surrounding fluid, into bubble due the difference in pressure and surface area when the bubble is compressed and expanded
What happens at different pressures in rectified diffusion?
High pressure in bubble: small surface area and diffusion out of bubble is difficult
Low pressure in bubble:
large surface area and diffusion into bubble is easier
What is inertial (transient) cavitation?
Bubble undergoes rapid expansion followed by violent collapse governed by the inertia of surrounding fluid (bubble may rebound, fracture into smaller bubbles, or disappear)
When does inertial cavitation occur?
When the total fluid pressure is negative (fluid is under tension)
p_T = p_0 + p < 0
Tensile strength of tissue can range from 1-100 MPa
What does the threshold pressure for cavitation depend on?
Tissue type, existing cavitation nuclei, radius of any bubbles, etc
What decreases the likelihood of cavitation?
Increasing frequency (contrasting with absorption)
At higher frequencies, the period is decreased, and thus there is less time available for bubble growth before the next compression phase of the acoustic wave
What are the several mechanisms which by which cavitation can affect tissue?
Bulk heating: viscous heating due to bubble motion and boundary layer effects
Mechanical action: Violent bubble collapse generates large forces on surrounding tissue, as well as jetting and microstreaming
Chemical action: high temperatures during bubble collapse can form free radicals and sonochemicals
What can a collapsing bubble produce?
An outward travelling shock wave
What is sonoluminescence?
Collapsing oscillating bubble can also produce temperatures in excess of 5000 K and emit picosecond pulses of light
What is jetting?
Bubble oscillation near boundary causes asymmetric collapse, smashing a ‘jet’ of fluid into the boundary
What can bubble oscillation and collapse cause?
vessel distension (outward push into tissue) and invagination (inward pull into lumen)
Combined with jetting, this can cause distortion of endothelial wall and vessel rupture
What can cavitation cause at cellular level?
Cell lysis where cell membrane is ruptured and cell is destroyed
Structural and functional changes to cells, or effects on DNA
How can cavitation activity be detected?
By passively listening for bubble “signature”
What is the Mechanical Index (MI)?
On-screen measure of the potential for inertial cavitation
How is the peak rare fractional pressure measured?
Measured in water and then derated (figure out equivalence in tissue) by 0.3 dB cm-1 MHz-1
When is damage more likely?
At lower frequencies due to lower cavitation threshold
(mechanical index decreases with increasing frequency)
There is no time dependence as a threshold effect
What are the effects of different mechanical index under different scenarios?
MI < 0.3: no damage
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 micro- spheres is being used. The risk increases with MI values above this threshold
What happens if an ultrasound wave is absorbed or reflected?
Wave absorbed: there is a change in momentum and the wave must exert a force on wall
Wave reflected by wall: must reverse direction of wave so twice the force
(can measure output of an acoustic probe)
How are acoustic radiation forces caused?
Radiation force is due to mean rate-of-change in momentum
the time-averaged momentum will not necessarily be zero, and this gives rise to an additional force
What is the volume radiation force, F^V for a plane wave in an absorbing medium?
F^V = Q/c_0
What is the force for a plane wave incident on a completely absorbing or reflecting target?
F_absorb = P/c_0
F_reflect = 2P/c_0
How is the power of an ultrasound transducer measured?
the radiation force exerted on an absorbing or reflecting target
How is the radiation force measured?
Radiation force measured using sensitive weighing scale (F =mg)
The transducer and target are placed in deionised and degassed water and the weight is measured before and after the transducer is turned on.
(includes transducer, water tank, absorber, balance)
What is the mass change per per Watt of acoustic power in an transducer?
∼69 mg mass change per Watt
What can the radiation force exerted by an ultrasound wave in an absorbing fluid medium cause?
The fluid to move (acoustically-induced flow is called acoustic streaming)
Streaming up to 10 cm/s can be observed using clinical US equipment
What can bubble cavitation cause?
Mechanical (non-thermal) damage to tissue
When does acoustic streaming occur?
Streaming therefore only occurs when the wave is being attenuated and the medium can flow, which is not always true of soft tissue
