20.21. X-rays Flashcards
What is the voltage difference used in diagnostic X-rays?
100eV-120keV
Typical energy range of X-ray photon
100eV - 120keV
Typical wavelength range of X-ray
10 picometers to 10 nanometers
How is the electron beam generated?
thermionic emission
What is the energy of the emitted X-ray photon proportional to?
to the distance of electron with the nucleus : the closer it gets, the more it is deflected and the more of its energy goes to the photon
Which type of contrast generation is photoeffect a main contributor of?
for tissues with similar density but different atomic number
Duane-hunt law
min wavelength = (hc/e) * (1/U) h : plank's constant c : speed of light in vacuum e : charge of electron U : accelerating voltage
What is the spectrum of Bremsstrahlung?
Power/lamda vs lamda
- for different accelerating voltages
- power is area under the curve
What does the minimum wavelength correspond to?
the process of converting the total energy of 1 electron into 1 photon with h*fmax energy
What is an Auger electron?
Energy of an electron transition to a lower level can be used to eject an outer shell electron: the auger electron
What is the characteristic X-ray spectrum?
line spectrum
What is the total power of the X-ray tube?
Px = Cx * Uanode² * Ianode * Z Cx : X-ray constant Uanode : accelerating voltage Ianode : anode current = number of electrons hitting the anode Z : atomic number of anode atoms
What is the efficiency of the X-ray tube?
ratio of total power / power invested. Typically low because much of the energy is lost as heat
Energy range of rayleigh scattering in soft tissue
1-30keV
Energy range of photoeffect absorbtion in soft tissue
10-100keV the limit is higher when the atomic number is higher
Energy range of compton absorbtion in soft tissue
0.5 - 5 MeV
Which substance are used for positive contrast?
High Z substance, for example Barium
What can be used for negative contrast?
gases with low specific gravity : O2, CO2
What is a summation image?
sum of all the elementary densities of all the layers the x-ray photon goes through
What does the X-ray amplifier do?
Produces an amplified, digital image
What are the advantages of using an X-ray amplifier?
can use lower E photons since it will be amplified + digital so no need for a light behind the scan etc (useful for surgery)
How does the X-ray amplifier work?
- Input phosphor : produces a large number of light photons (lower E)
- Photocathode releases electrons (pptional to number of photons)
- electrons are accelerated to anode
- electromagnetic grids focus and minify electron distribution, which augments inensity
What is DSA?
digital substraction angiography
What is a CT scan?
combines a series of X-ray images taken from different angles around the body and uses computer processing to make cross-sections
