US Tutorial/Revision Lecture Flashcards
What is the speed of sound in tissue assumed to be in most US scanners? How does the speed of sound in fat and muscle compare to this? (3 marks)
1540 m/s, fat is slower and muscle is faster.
What is the equation for the speed of sound in tissue? (1 mark)
c = (kappa/rho)^0.5
kappa = bulk mod (Pa) rho = density
What is the speed of sound in the liver and what implications does this have for images of the liver? (2 marks)
Liver c = 1590 m/s
If we assume this then normal images become stretched by 3% as the sound travels further in the same time, therefore the image we see is actually squashed slightly due to the slower assumed speed.
What is the equation for the RIL and RPL? Why are these scales useful rather than pressure/intensity? (3 marks)
RIL = 10 log(I_2/I_1) RPL = 20 log(p_2/p_1)
These logarithmic scales are more useful as they enable us to see much weaker echoes from deeper depths. It gives a more usable scale.
What is the equation for acoustic impedance? (1 mark)
Z = rho*c
What does the thermal index mean? (6 marks)
TI = W_0 / W_deg (W_deg is power output to get a 1 degree temp rise)
TI is the worst case estimate of the maximal temperature rise. It is not the actual temperature rise in the tissue.
There are different values:
- TIS = soft tissue and first trimester
- TIB = bone and second/third trimester
- TIC = cranial
What limits are there on acoustic outputs and where do they come from? (4 marks)
- Come from the FDA
- TI is limited to 6
- MI is limited to 1.9
- I_spta (mW/cm^2) limited to 720
(spta = spatial peak time average)
How can ALARA be implemented? (5 marks)
ALARA:
- Use freeze controls
- Change defaults if they are giving higher outputs than required
- Take care when using power doppler modes
- Keep power as low as possible so long as adequate image quality is achieved
- Gain does not affect output but frequency does
