Safety in Ultrasound Flashcards
(17 cards)
What are US thermal effects?
Absorbed energy turned into heat producing a temperature rise, with temperature rise varying with the varying intensity of the field. Rises quicker in focal regions before an equilibrium is reached, conducting heat away.
Which tissue absorbs US more?
Bone
Which tissue is most at risk of thermal effects due to US?
Bone
What causes transducer heating?
Impedance mismatch between the transducer crystal and tissue causes a lot of energy being absorbed by the face of the probe, heating it up (especially in air) which can transfer to the patient
What are the WFUMB guidelines for US temperature rises?
A diagnostic procedure should not cause a rise in temperature of more than 1.5 degrees
What is TI in US?
Thermal index is an estimate of the increase in temperature that the US exposure is causing (e.g. TI of 2 equates to an approximate increase of 2 degrees)
TI = w/wdeg
How should heating and cavitation effects be minimised in US?
Minimise scan times and use lower powers whilst still getting a diagnostic result
What is US?
Mechanical wave identical to sound above human hearing (>20 kHz)
What frequency is used for diagnostic US?
1020 MHz (1.5 mm to 77 microm)
What is US intensity>
Energy flux - energy flow through unit area per unit time
I=P^2/Z
What is US attenuation and what is it caused by?
Exponential loss of pressure amplitude with depth, caused by scatter and absorption (absorption dominates in diagnostic)
What does the rate of US attenuation depend on?
Frequency - higher frequency = more attenuation = less penetration
Why is US focusing used?
To improve lateral resolution by increasing intensity
What is acoustic power?
Measure of the rate at which energy is emitted by the transducer (W), for diagnostic: ~1 mW - 100s mW
What is cavitation?
At low pressure amplitides, bubbles oscillate in size but reamin stable
At high pressure amplitudes, bubbles will grow by a process of rectified diffusion - once they get too big, they implode violently causing damage to nearby cells
What is MI in US?
Way to quantify the likelihood of interial cavitation
MI = P-/root(fc)
What is the MI threshold in US?
0.7 0 physical conditions cannot exist to support bubble growth
