Exposure Factors Flashcards
How are x-rays produced?
(3)
Electrons are emitted by thermionic emission
They’re accelerated with a high voltage towards a target
They slow down, and this deceleration emits x-rays
What does mAs mean?
Current (the no. of electrons going into the x-ray tube)
What does the mAs do?
It controls the quantity (amount) of radiation in the x-ray beam, which correlates with the number of photons in the beam, or its intensity
What does kV mean?
Voltage
What does the kV do?
It controls the penetrating power of the x-ray beam
What happens if we increase the quality and intensity of the x-ray beam?
Why?
More x-rays get through the target, and more x-rays reach the detector
Because less of the x-rays are attenuated
What is contrast?
The difference in density or grayness between areas of the radiographic image (the ability to tell the difference between different things)
What does it mean if we have high contrast?
It’s easy to tell the difference between adjacent structures
What does it mean if we have low contrast?
It’s hard to tell the difference between adjacent structures
How many shades of grey can humans distinguish?
30 shades
Is this high, medium or low contrast?
High contrast
Is this high, medium or low contrast?
Medium contrast
Is this high, medium or low contrast?
Low contrast
What is the normal kV and mAs of a hand x-ray?
60 kV
2 mAs
If there’s a high kV, is there more or less contrast?
Less contrast
If there’s a low kV, is there more or less contrast?
More contrast
What are x-rays used to detect?
Water- they detect water where there shouldn’t be any, which reveals abnormalities
Are chest x-rays low or high contrast images?
Why?
Low contrast images
Because if they were high, we wouldn’t be able to see the lung linings
Is a hand or chest x-ray higher contrast?
Hand x-ray is higher contrast
What is contrast dependent on?
(4)
X-ray beam quality/kV
Thickness
Density
Atomic number
Is a higher or lower kV more penetrating?
Higher kV
Does a thicker or thinner material attenuate more x-rays?
Thicker
Does a more or less dense material attenuate more x-rays?
More dense
Does a higher or lower atomic number attenuate more x-rays?
Higher
What is the normal kV and mAs of a chest x-ray?
100 kV
1/2 mAs
What does AEC stand for?
Automatic
Exposure
Control
Why does a high kV cause a lower contrast image?
(4)
Increase in the kV increases the photon energy
This is because the increase in the kV increases the speed of the electrons in the x-ray tube, which increases the photon energy
This increases the penetrating power of the x-ray beam
This will reduce contrast
Why does a lower kV cause a higher contrast image?
(3)
Lower kV reduces the penetrating power of the x-ray beam
This results in less absorption and less transmission into the anatomy
So a wider variety of of x-ray photons are coming out of the other side, causing a higher contrast
What does density of the image mean?
How many photons get through the patient
What is the density of the image a measure of?
Image darkening (how many photons get through the patient)
More photons = more dense
What is the density of the image proportional to?
The number of photons that are received by the detector
What is the density of the image controlled by?
(3)
The current in the x-ray tube (mAs)
Source image distance (SID)
Th material being imaged
What do changes in the mAs have a direct effect on?
Density
When the mAs decreases by half, what happens to the density?
The density decreases
When the mAs is doubled, what happens to the density?
There’s an increase in density
Does the image have a low or high density?
Why?
Low density
Because its been under-exposed
What causes an image to have a low density?
(4)
Too few photons are reaching the imaging receptor (lower mAs)
This results in quantum noise
This gives a grainy, mottled appearance
The image lacks density
In practice, when will a low density image only be seen?
If there’s a drastic under-exposure error made
What causes a high density image?
(4)
Too many photons reach the imaging receptor, causing the receptor to become saturated
This increases the density
This causes a poor definition between the soft tissue and the other tissues
The dose to the patient is therefore too high
How do we know if an image has good contrast?
If we can make out the details in the image
What distance should the chest be from the detector?
Why?
180cm
Because it reduces magnification- as the photons travel through the air, the distance is higher so more photons are attenuated
What causes the heart to appear larger if we do an AP chest x-ray?
The heart is more anterior, so it’ll be closer to the detector
What does kV affect?
Contrast
What does mAs affect?
Density
What is the normal kV and mAs of an AP pelvis?
85 kV
16 mAs
What is the normal kV and mAs of the knee?
63 kV
2.5 mAs
What is the normal kV and mAs of a PA chest?
0 kV
1.6 mAs
