Topic 5: quality assurance Flashcards
Define the quality of radiation
- Measures beam’s overall energy
- How readily beam penetrates material
- HVT ∝ quality of beam
Define the intensity
- Measure of number of x-ray photons
- Quantity of radiation
- Bremsstrahlung curve ∝ intensity of X-ray
What is half value thickness/HVT/HVL?
- Amount of given material which will attenuate 50% intensity of X-ray bean
- Guide to penetrating power
What is the linear attenuation coefficient?
- Fraction of photons interacting per unit thickness of attenuator
What is the formula for beam attenuation?
𝑁 = 𝑁0𝑒^−𝜇𝑥
- N = number of particles after traveling through material
- N0 = initial number of particles
- 𝜇 = linear attenuation coefficient
- x = thickness
- e = Euler’s number = 2.718
Define E-max
- Max beam energy
- Diagnostic X-ray = E-max equal to generating voltage of X-ray beam = kVp
What is mA?
- Measure of current flowing across X-ray tube = tube current
- Current = flow of e-
- Value in mA determined by = number of e- flowing per unit time
Describe the change in mA
- Each e- = same potential difference = same chance at creating X-ray photon
- Therefore doubling current = doubling e- = doubling mA = doubling number of X-rays produced
Describe the relationship between mA and beam intensity + quality
- mA doesn’t affect energy range = quality unaffected by changes in mA
- Intensity ∝ mA
What is keV?
- X-ray tube voltage
- Determines potential difference between cathode + anode of X-ray tube
- Determines KE in each e- as current flows
What is kVp?
- Max cathode-anode voltage measured
- E.g. kVp = 150 kV = e- KE 150 keV before hitting anode
Describe the change in keV
- keV increase = speed e- impact on anode increases = more opportunity for energy conversion to X-ray
- More energy =
> Max photon energy achievable increases
> Increase in average energy of each photon
> Total number of photons increases + intensity of X-ray beam increases
Describe the relationship between keV and bam quality + intensity
- If changes reversed = reduction in tue voltage
- Beam quality ∝ (keV)^2
- Beam intensity ∝ keV
What is the relationship between contrast and kvP?
- More kvP = more penetration
- More kvP = low contrast
What is the relationship between contrast + mA?
- More mA = More black background = more contrast
What does the inherent filtration consist of?
1) Glass/ceramic envelope = maintaining vacuum
2) Cooling oil
3) Window of X-ray tube
How is inherent filtration measured?
- In aluminum equivalents = mm Al
- This is the thickness of Al = produce same degree of attenuation as thickness of material being used
Why do X-ray tubes have additional filteration?
- To remove lower energy/softer X-rays from beam
Describe the filters
- Made of aluminium = use with diagnostic range of X-rays + has low atomic number = more likely absorb low energy photons
- Copper better = high energy radiation
- Size depends on max generating voltage = 1.5-2.5mm
Describe the effects of filtration
- Doesn’t affect E-max
- Intensity decreased
- Average energy of beam increases
- Therefore beam quality ∝ filtration
- Beam intensity inversely proportional to filtration
Define quality assurance as per council directive
- Planned + systematic actions necessary = provide structure/system/component/procedure
- Perform satisfactorily in compliance with agreed standards
- Quality control part of it
Define quality control as per council directive
- Set of operations = maintain/improve quality
- Monitoring + evaluation + maintenance
- Performance of equipment defined + measure + controlled
What is the difference between QC/QA?
- QC = verify quality + output
- QA = process of managing quality
What is the QA protocol for ionizing equipment?
- IPEM report 91
- Specifies set of test = comply with EU + international legislation
