Bioeffects Flashcards
Particle displacement/acoustic pressure
Amplitude
What determines the pulse’s amplitude?
The pulser
What are the 5 mechanisms of attenuation?
Absorption, wavefront divergence, reflection, refraction, scatter
Power =
Intensity x area
This is a measure of the total energy transmitted summed over the cross sectional area of the beam per unit time
Power
The absolute unit of power
Watt (joule/s)
What determines the power?
The pulser
Intensity =
Power/Area
This is a measure of the strength of a sound wave and is equal to the power per unit area
Intensity
The absolute unit of intensity
mW/cm2 or W/cm2
If power increases, what happens to intensity?
It also increases
If area decreases, what happens to intensity?
It increases
What is the relationship between intensity and amplitude?
A small change in amplitude will result in a large change to intensity
BUR =
SP/SA
A high BUR does what for focus? What is the BUR for a perfectly uniform beam?
High BUR = high focus.
BUR would be 1
DF =
TA/PA
Where does the information on bioeffects come from?
Epidemiology, in vitro, animal studies
What are the set intensity thresholds for unfocused and focused ultrasound?
Unfocused: 100mW/cm2
Focused: 1000mW/cm2 (1 W/cm2)
2 main categories for bioeffects
- thermal
- non-thermal (mechanical)
Simple oscillation of bubbles that can result in the streaming of liquid resulting in shear stresses
Stable cavitation
When the bubble actually collapses and produces a shock wave that can result in localized high temperatures and light emission.
Transient cavitation
What is the thermal index (TI)
The amount of acoustic power ended to raise the temperature in tissue 1 degree.
Example, a TI of 2 could possibly raise the temp by 2 degrees
TIS, TIB, TIC
TIS: soft tissue
TIB: bone near the focus and obstetrics
TIC: bone near the surface (cranial studies)
What is the mechanical index (MI)
The likelihood that the energy used will lead to cavitation
How is MI related to frequency?
Inversely proportional to the square root of the frequency.
The frequency would have to quadruple for the MI to halve
What are the TI and MI maximums?
TI- 6
MI- 1.9
The balance between benefit and risk
ALARA
What tissue is most susceptible to TI?
Fetal tissue
