XR production 1 Flashcards
What are some elements of an XR tube?
-Anode
-cathode
-electrons
-vacuum
-target
-source of electrons
why is a vacuum needed?
So there’s nothing else the electrons will interact with between the cathode and the anode.
Why are electrons attracted to the anode?
Electrons are negatively charged, and a positively charged anode which creates a force of attraction called a coulomb force.
what happens when electrons interact with the anode material?
Convert potential energy to kinetic energy, but when they interact, they give up that energy in the
form of radiation (x-ray photons).
what is the definition of an electron volt?
defined as the amount of energy gathered by a single electron when moved across a potential difference of one volt.
what are these radiographic factors?
-kV/kVp?
-mA
-ms
-mAs
kV- potential difference between anode and cathode
mA- current flowing between anode and cathode
ms- time during which current flows in the tube
-mAs- product of the tube current in milliamps and the time in seconds (how many electrons are flowing during our exposure).
x rays are produced by which 2 processes?
-General radiation (bremsstrahlung)- e- interacting with the nucleus of target atoms.
-Characteristic Radiation e- interacting with electrons in the shells of target material.
Outline bremsstrahlung
-tungsten atom in anode has many electrons.
-accelerated electrons are attracted to the nucleus because of coulomb force.
-As the electron gets closer to the nucleus its diverted from its path and gives up some of its kinetic energy.
-This produces either heat or photon energy.
-the closer it gets to the nucleus the more energy is released because there is a higher breaking force.
What does bremsstrahlung produce?
-Wide range of photon energy
-mostly heat
-Energy depends on proximity of electron to the nucleus and charge associated with the nucleus.
What is filtration?
matter that gets in the way of the x ray photons as its trying to get out of the x ray tube, e.g. glass.
What does the minimum and maximum energy depend on regarding bremsstrahlung?
Minimum- filtration of XR beam
Maximum- Applied voltage
What is photon energy?
Measure of the ability of the XR photon to penetrate matter.
-higher the energy of XR photon, more likely it is to penetrate matter compared to a lower energy photon.
Outline the concept of XR beam intensity and quantum noise.
-XR beams distributed randomly, XR beam intensity won’t be the same across the whole image field.
-there will be some variation, this called quantum noise.
-If the noise is too big it can obscure things in the image.
-Reduce the noise by increasing no. of XR photons per mm²
In simple terms, what is characteristic radiation?
outer-shell electrons fill a vacancy in the inner shell of an atom, releasing X-rays in a pattern that is “characteristic” to each element.
Explain the process of characteristic radiation
- Accelerated electrons interact with the k shell electrons.
- If the electron has sufficient kinetic energy to overcome the binding energy of this k shell electron, it can eject it out of its orbit.
- That will leave a vacancy which is then filled from the higher shells above it.
- Electron drops down from L shell to K shell, in doing so it gives off characteristic radiation.
The energy given off is equal to the difference in the binding energies between the two shells.
what happens to any excess energy during characteristic radiation?
it is shared between the electrons
what is the XR beam spectrum?
At a high level the x-ray spectrum is a plot of the Number of X-ray Photons (y-axis) as a function of the energy level in keV (x-axis).
What does the XR beam spectrum show us?
Needs enough energy to overcome the binding energy.
Max kinetic energy depends on the voltage
If voltage isn’t high enough, we don’t see characteristic x ray peaks.
XR beam quality
-indicates ability of XR beam to penetrate an object or indicate the energy of the beam.
-beam quality is referred to as being hard or soft, meaning it is more of less penetrative.
XR beam spectrum: Effect of altering exposure factors. What 3 factors can you alter?
-Tube potential (kV)
-XR tube current (mA)
-Increasing filtration thickness
XR beam spectrum: What are the effects of changing tube potential (kV)
increases:
-intensity- more X-rays per mm^2
-average energy
-maximum energy
-Peaks on the spectrum are characteristic of the anode material.
-if kV is high enough, characteristic XRs will be produced.
XR beam spectrum: What are the effects of increasing XR tube current (mA)
increases: intensity
no change:
-average energy
-minimum energy
-maximum energy
Don’t get any difference in the distribution of energies. The average beam energy stays the same.
XR beam spectrum: What are the effects of increasing filtration thickness
increases: average energy
decrease: intensity
lower energy photons are preferentially absorbed compared to higher energy.
More filtration leads to more photons being absorbed. So you get a spectrum that is reduced.
Low energy XR photons can be easily absorbed by the patient. So we need to get rid of these as these are dangerous and can produce skin burns. To do this we add some extra filtration, e.g. 2mm of aluminium.
Air Kerma
-quantity used to express radiation the concentration delivered to a point, such as entrance surface of a patients body
-Measure of the amount of radiation energy in Joules (J), deposited in or absorbed in a unit mass (kg) of air
-Also referred to entrance surface dose and has units of Gray
-Not actual dose in an individual patient as this is impossible to calculate accurately.
Divergent XR beam
-Photons diverge, covering a wider area,
Same no. of photons but wider area, means XR beam intensity has decreased.
What is inverse square law
-When you double the distance, you quarter the XR beam intensity.
This is called inverse square law. As the intensity is inversely proportional to the square of the distance.
-As distance increases the XR be a intensity and exposure decreases.
