1 to 1- Exposure and dose - manipulation and management Flashcards
What is ESD?
ESD - is the measure of the radiation dose that is absorbed by the skin as it reaches the patient. Entrance skin dose is a directly measurable quantity
Measured in grays (Gy)
What is effective dose?
Measure in sieverts, usually mSv and is linked to grays but it is dependant on the body part receiving the dose.
What is DAP?
Dose area product
A multiplication of the absorbed air dose (Gy) and the area exposed (m2)
Gy.m2
Difficult to estimate skin or effective dose from DAP
Convenient
What is mAs?
mAs is the current we put into the cathode tube, kind of like a filament in a light bulb.
High mA will boil off electrons.
Each electron could become a x-ray photon. More mA more electrons more x-ray photons.
Time is how long the x-ray time is on, boiling off the electrons.
What is kVp?
kVp is the energy given to the photons.
Higher kVp, the faster the electrons, the more penetrating, more likely to go through the patient.
Filtration is used to get the low energy photons that would get absorbed by the patient.
How does mAs and kVp affect dose?
mAs has a liner realtionship to dose, as it makes more photons that could be absorbed.
However, kVp is complicated as it will increase the penetrating power of the electrons they will go straight through the patient and not get absorbed which means lower absorbed dose. However, it still might increase DAP.
When using a grid do we increase/or decrease mAs?
For grid we have to increase mAs because we need more photons as some of the useful image absorbing photons are blocked by the grid.
Why do we use a grid for Chest PA (kV 120, mAs 1) and not chest AP (kV 80, mAs 3.2)?
and why different exposure factors.
We dont use a grid for chest AP because of grid cutoff, its very difficult to get the tube parallel to the grid with AP.
This means we use different exposure factors, higher kVp means more scatter, over 100 you get alot of scatter. Too low mAs causes quantum mottle.
What other factors affect dose?
High kV, low mAs tends to be lower dose
Dependant on other factors
Collimation
SID
ODD (object detector distance)
What is exposure index and what is deviation index?
Exposure index
Derived from analysis of image pixel values
Deviation index
An indication of how the exposure index deviates from the target exposure index
Arguably easier to apply in practice
How does exposure index work?
Selecting the algorithm effectively selects an optimal lookup table (LUT) histogram of expected pixel exposure
After exposure the data from the pixels is analysed
The LUT histogram is applied to the pixels
This automatically rescales to optimise image brightness and contrast
The EI is determined from the midpoint of the histogram
Problems with EI and thus DI
Not only affected by exposure factors
Patient positioning
SID
Unexpected material in FOV
Collimation margins not detected
Extreme under- or over-exposure
Extraneous exposure information, including scatter
Rule of 10 number 1
If the volume being x-rayed doubles (or halves)
We need to double (or half) the exposure used
To double exposure:
Add 10kVp
Double mAs
Thus minus 10kVp and half mAs to half exposure
If we want to manage absorbed dose we reduce mAs we have to increase kV.
The standard exposure for an AP CXR is:
90kVp and 1.5mAs
We are imaging this patient (large/high bmi patient)
What exposure do we give?
The standard exposure for an AP CXR is:
90kVp and 1.5mAs
We are imaging this patient (large patient)
Double the volume of tissue to “average”
Or thereabouts
Double the exposure
= 100kVp and 3mAs
Rule of 10 number 2
To reduce absorbed dose but maintain image quality we need to reduce mAs and increase kVp
Half the mAs
Add 10kVp
This will decrease image contrast but lead to the same overall image brightness (and probably EI/DI)
