US Physics Chapter 17-18 Flashcards
How are harmonics created with ultrasound
More powerful ultrasound creates harmonics as it passes through soft tissue interfaces. Weaker sound not so much so side and grating lobes can be taken out of the picture.
How does pulse inversion harmonics work
Pulse and inverted pulse sent out in quick succession. They cancel each other out at receiver, but harmonics are nonlinear and so don’t cancel each other out. Lets you separate out harmonics
downside to pulse harmonic inversion and pulse modulation harmonic
decrease in temporal resolution because twice as many impulses
How does power modulation harmonic imaging work
Two pulses sent - one is twice the strength of the second. Twice strength subtracted from doubled weaker one leaving harmonics behind. Harmonics are not created by the weaker pulse only the stronger one.
What in linearity of harmonics?
They are nonlinear.
How to contrast micro bubbles create harmonics?
When hit by high pressure wave of sound beam micro bubbles compress, when hit with low pressure wave of sound they expand to a greater degree than they compress and nonlinearity produces a harmonic.
What does mechanical index describe
Ability to create harmonics. Depends on the the ultrasound pulse and depends on MI = peak rarefaction pressure/√frequency
What do different mechanical indices do
<0.1 - no harmonics. 0.1-1 harmonics, >1 lots of harmonic production
What are two important components of micro bubbles
The gas - bigger molecules less likely to leak out. What the shell is made of - the tougher it less the less likely to break down.
What nonlinear behavior of sound produces harmonics
That sound travels faster in compression and slower in rarefaction.
What are two forms of laminar flow?
plug flow - all layers flow at same velocity
parabolic flow - velocity lowest on outer bit of vessel.
What are 3 types of blood flow and what are examples
pulsatile - arterial
phasic - accelerating and decelerating velocity - blood flowing in veins to lungs
steady - veins when breath held
What predicts turbulent flow? What is specific number predicting break point?
Reynolds number. If greater than 2000 predicts turbulent flow. Laminar flow < 1500
Blood flow equation
Pressure gradient = Flow x Resistance
What happens to venous flow during inspiration?
Diaphragm moves downwards leading to negative pressure in chest and increasing venous return to heart increasing flow from head/arms/vena cava. Flow decreases form the legs due to increased and pressure.
