Quiz 1 - Weeks 1-3 Flashcards
Are most X-rays made by man or nature?
Nature (about 85%)
The basic energy unit is
the electron volt – eV
Cathode or filament
Cathode or filament
Very similar to a light bulb
The more electric current you pass through the filament (wire) the more electrons boil off
An electron (e-) cloud is formed around the filament
Electric current is measured in milliamperes – mA and is the mA setting on the x-ray machine (importance to follow)
Filament is usually made of tungsten
Anode or target
X-rays are produced at the anode
A voltage potential (Vp) is applied across the anode and cathode
The potential is electrically positive at the anode and negative at the cathode
The Vp accelerates the e- cloud toward the anode
kVp
kVp = quality of the x-ray
Increasing the kVp increases the energy of the x-ray photon the number of x-rays generated
The number of x-rays increases due to increased interactions in the anode (i.e. more braking and collision radiation)
mA and s
mA and s = quantity of the x- ray
The more current that passes through the filament the more electrons boil off
Often mA is discussed as mAs or milli- amp-second
The product of mA and s(econds) is mAs
mA and s = quantity of the x- ray
The more current that passes through the filament the more electrons boil off
Often mA is discussed as mAs or milli- amp-second
The product of mA and s(econds) is mAs
Photoelectric effect
The x-ray photon (energy) is totally absorbed
Scatter radiation is not produced
Ejects an electron from the orbit of an atom and produces a characteristic x- ray
electron results in subsequent ionizations within the body
the characteristic x-ray has low energy and absorbed locally
Primarily responsible for the patients is absorbed dose
What is the principle responsible for image generation?
Photoelectric effect
Different opacities:
Fat – mostly carbon (C-12)
Bone – mostly calcium (Ca-20)
123 = 1728 203 = 8000
Better chance of having absorption of a
x-ray with Ca than C
If the energy is to high, photoelectric effect is reduced, resulting in contrast loss
Compton Scattering
Responsible for nearly all scattered radiation in diagnostic radiology
Similar to photoelectric but the incoming photon energy is not completely absorbed
An orbital electron is ejected and an energy reduced photon continues on…
What principle causes film fogging?
Compton Scattering
Probability of Compton effect is independent of Z, but proportional to physical and electron density
Effect does not contribute to formation of image
But does contribute to film fogging (later) and exposure to patient and surrounding participants
How could we make the film more black
Doubling the mAs will double the film blackness.
Increasing the kVp by ~15% will double film
blackness
How can we make a film less dark?
Halving the mAs with halve film blackness
Decreasing kVp by ~15% will halve film blackness
Heel Effect
Is due to a portion of the x-ray beam being absorbed by the anode
Results in an x-ray beam that is less intense on the anode side and more intense on the cathode side
Clinical correlation:
Since the x-ray beam will be more powerful on the cathode side, always place thicker body parts under the cathode side to give a uniform exposure across the radiograph
Factors affecting image detail
Motion
Film speed
Focal spot size
Focal spot –film distance Object film distance
Intensifying screens
Grids
Motion
Results in reduced image sharpness
Correct by:
Decrease exposure
time
Sedate or GA
Use a regular (non- detail) screen/film combination
Reduce grid ratio
Reduce FS to FD
Focal Spot Size
Most X-ray generators have two filaments
A small and large one
Large mAs values would melt small
filaments
Large filaments able to produce a larger number of x-rays:
make larger effective focal spots decrease sharpness of the image
Focal Spot to Film Distance
Mentioned previously
A long FS to FD increases detail
But… higher mAs’s are required
Typically a standard setting that is used routinely
Object Film Distance
Keep patient as close as possible to film
As the OFD increases:
Magnification occurs
Decreases image detail
Occasionally useful… magnification is sometimes desirable
Intensifying Screens
Film emulsion is insensitive to x-rays compared to visible light
Screens convert x- rays into visible light through phosphorescence
Various types of screens based on composition, thickness and size of the crystal
Screen vs. Non Screen Technique
Non-screen has superior detail but greater exposure (i.e. greater kVp’s and mAs)
X-rays alone are used to expose silver halide crystals in the emulsion
Information from a screen technique is spread out over a larger area
Other Devices that Limit Scatter
Collimator
Grids
Shielding
Grids
Grids
Reduces scatter radiation
Plates that are placed between the patient and the cassette (film and screens)
Contain rows of lead strips Characterized by a ratio:
The higher the more effective at reducing scatter
But must increase mAs to compensate (2-3 x)
Contrast
Contrast is the opacity difference between adjacent areas on radiograph
The confusing part is coming =
The scale of contrast refers to the number of density gradations between the lightest radiograph shadow and the darkest radiographic shadow.
An image with a lot of contrast is said to have a
An image with a lot of contrast is said to have a short scale of contrast.
A high contrast image
A high contrast image only has a few shades of gray between the lightest and darkest areas of the radiograph
How can we get short scale contrast?
High mAs and low kVp exposure factors are one method of achieving short scale contrast.
What kind of rads do we take with short scale contrast?
Abdominal radiographs are taken with a short scale contrast.
How do we get long scale contrast?
Low mAs and high kVp exposure factors are one method of achieving a long scale contrast
What kind of rads do we take with long scale contrast?
Thoracic radiographs are taken with a long scale contrast
Factors Affecting Contrast
Subject contrast
Thickness
Density
Atomic number differences
X-ray beam energy
Film contrast
Inherent property of the film
Fog and scatter
Scatter radiation use a grid
Avoid high temperature, visible light or pressure
Watch expiration dates… film fogs spontaneously
