medical physics Flashcards
What is the piezo-electric effect?
certain crystals produce an emf when compressed or stretched, they also become stretched or compressed when there is a p.d across it
What is a piezoelectric crystal?
crystals that exhibit the piezoelectric effect
What is the intensity reflection coefficient?
intensity of relected ray/ intensity of inscidant ray
What is acoustice impedence?
product of density of material and the speed of ultrasound in that substance Z=pc
What is impedence matching? What is it also called?
when two substances have similar acoustice impedences to minimise the ultrasound reflected at the boundary of the 2 substances.
What is the use of coupling gel?
A get that has similar acoustic impedence to skin, it is used between the transducer and the skin of the patient to minimise the reflection of the ultrasound.
What is Thermionic Emission?
When electrons are released from a material by heating the material
What is Attenuation?
The decrease in intensity of em radiation when it travels through some matter
What is the Attenuation Coefficient?
a measure of the absorption of x-ray photons by a substance. It is proportional to the z number cubed
What is simple scattering?
x-rays interact with electron in an atom but doesnt have sufficient energy to remove the electron. The xrays then scattre withought any change in energy
When does simple scattering occur?
primarily for x-rays with energy of 20keV
What is compton scattering?
x-rays remove electron from atom and gets scattered and gets reduced in energy.
When does compton scattering usually occur?
x-ray photons with energy between 0.5-5.0 MeV
What is pair production?
when an x-ray interacts with an atoms nucleus it disapears and creates an electron and positron
When does pair production usually occur?
when X-ray photons have energy equal to or greater than 1.02 MeV
