Bioeffects & Safety

Question 1

Which exposure parameters are measured?

Answer 1

Energy
Power
Intensity

Question 2

What is the main cause of energy transfer to tissue?

Answer 2

Attenuation

Question 3

Which factors determine the amount of heating caused by ultrasound

Answer 3

Frequency
Intensity
Degree of focussing
Scanning pattern

Question 4

Describe cavitation

Answer 4

Gas bubbles in the body vibrate in response to the pressure variations caused by ultrasound
As pressure increases and decreases, the bubbles expand and contract
Over time, these bubbles either stabilise (non-inertial cavitation) or collapse (inertial cavitation)
The collapse of these bubbles results in extreme energy exposure

Question 5

Which factors determine whether cavitation occur?

Answer 5

Whether gas bubbles are present
The ultrasound frequency
Maximum negative pressure created in the tissues (peak rarefaction)

Question 6

What is the thermal index

Answer 6

Estimate of the maximum likely increase in temperature of the tissues

Question 7

How is the thermal index calculated

Answer 7

Machine calculates:
Wdeg - Transmitted ultrasound power that would be likely to raise the tissue by 1 degree
W - Transmitted power
TI = W/Wdeg
TI = 1.0 = maximum likely 1.0 degree

Question 8

Below what thermal index can scanning continue indefinitely

Answer 8

<0.7

Question 9

What is the mechanical index

Answer 9

Indicates whether inertial cavitation is likely to occur
Also a good estimate of other non-thermal bioeffects

Question 10

How is mechanical index calculated

Answer 10

Peak rarefaction pressure / square root of the frequency

Question 11

What is the generally accepted maximum MI

Answer 11

MI <0.7