X-ray physics Flashcards
The greatest # of x-rays emitted is approximately how much of the maximum energy?
1/3rd
What are 4 factors that influence the shape of the x-ray emission spectrum?
1) Projectile electrons accelerated from cathode to anode do not all have the peak kinetic energy. Depending on type of rectification/high voltage generator, some will have low energies when striking the target.
2) Projectile electrons have multiple interactions w/ target, each successive one @ lower E.
3) Low E x-rays are more likely to be absorbed by target
4) External filtration selectively removes low E x-rays.
What factors will affect the x-ray emission spectrum?
mAs kVp Added filtration Target material Voltage waveform/ripple
How does mAs affect the x-ray emission spectrum?
It increases the amplitude only.
How does kVp affect the x-ray emission spectrum?
It increases the amplitude and causes a right shift to the curve (mostly affects high energy x-rays).
What is the 15% rule?
A 15% increase in kVp, is equivalent to doubling the mAs.
How does filtration affect the x-ray emission spectrum?
Decrease in the amplitude (mostly at low E) and a right shift to the curve.
How does using a higher atomic number target affect the x-ray emission spectrum?
Increases the amplitude and causes a right shift to the curve.
How does increased voltage ripple affect the x-ray emission spectrum?
Causes a decrease in the amplitude (quantity) and a left shift of the curve (quality).
Using a 3-phase or high frequency equipment vs a single-phase is equivalent to what in terms of kVp/mAs?
A 12% increase in kVp or doubling of mAs
What is the relationship of kVp and x-ray quantity?
X-ray quantity is proportional to the square of ratio of kVp (eg. double kVp = increase x-ray intensity by x4)
What factors affect the x-ray QUANTITY?
a) mAs (proportionally)
b) kVp
c) distance (inverse square law)
d) filtration
What factors affect x-ray QUALITY?
a) kVp
b) Filtration
What produces more heat? A 3-phase system or a single phase?
3 phase
Coherent scattering occurs at what x-ray energy?
a) 10keV
b) 1.02MeV
c) 10.2MeV
d) can occur at any energy
10kev – very low energy x-rays
The probability of photoelectric interaction depends on:
1) photon E (inc. in photon E = dec. in probability of PE) = photon^3
2) atomic # (inc. atomic # = inc. probability of PE) = atomic number^3
An increase in atomic number does what to Compton’s effect?
No effect
What are 3 ways to reduce the anode heel effect?
1) Increase the SID
2) Decrease the field size
3) Increase the anode angle
At lower energies (kVp), most of the attenuation is due to what secondary interaction?
PE
At higher energies (kVp), most of the attenuation is due to what secondary interaction?
Compton’s
What is bucky factor?
Increased does to the patient d/t to the use of a grid.
What is stage 1 collimation?
Prevents off focus radiation – electrons that interact at positions on the anode other than the focal spot.
What is stage 2 collimation?
3mm thick lead shutter leaves
What is the function of a grid in terms of contrast?
Increases contrast.
Contrast improvement factor of 1 indicates what?
No improvement with the use of a grid.
At what kVp is contrast improvement factor measured at?
100kVp
What will cause a higher bucky factor?
1) Increased grid ratio
2) Increased kVp
Grid cut-off at the edges of the film most likely represents which grid abnormality?
Off-focus or upside-down
How does kVp affect the air gap technique and why?
Air gap technique doesn’t work with high kVp’s because, the scatter radiation is more forward directed towards the image.
What is the active ingredient of the emulsion?
Silver halide crystal (98% silver bromide; 2% silver iodide)
What holds the halide crystal in place in the emulsion?
gelatin
What is the sensitivity center/speck?
Imperfections in the crystals containing AgS (silver-gold sulfide) that attract photoelectrons & Ag ions to combine and form a latent image.
What is the supercoat?
Protects the emulsion from scratching, pressure and contamination.
What is the purpose of a double emulsion?
Faster speed
What is spectral matching?
Matching films to the spectrum of light emitted by the intensifying screens.
What are orthochromatic films?
rare earth screens and blue & green sensitive films
What are panchromatic films?
Films that are sensitive to the entire visible light spectrum.
What is the name of the production of the latent image?
Gerney-Mott theory
What are the reducing agents in a developer?
Hydroquinone & phenidone
What does the developer do?
Provides electrons to the sensitivity center of the crystal to change Ag+ to metallic Ag.
What controls the pH of the developer?
sodium carbonate & sodium hydroxide
What is the hardener in the developer and what does it do?
gluteraldehyde –> controls swelling & softening of emulsion
Which step is only used in manual processing?
Stop Bath – use acetic acid to stop development process
What is the clearing agent in the fixer?
Ammonium thiosulfate
What is the hardener used in the fixer?
potassium alum, aluminum chloride, chromium alum
What is the cause of deterioration/browning of the films after time?
Retention of the fixer in the emulsion combines with the silver –> forms silver sulfide –> oxidizes and causes image to discolor
What is the active layer in the cassette?
Phosphor layer (contains the phosphorous elements that glow)
What is the role of the reflective layer?
Intercepts light headed in other directions and redirects back to the film.
FYI, NOT present in the rare earth screens!
What is the relative speed of a par speed screen?
100
What is the fast speed screen?
High speed rare earth (up to 1200)
What is a disadvantage to the use of intensifying screens?
Image blur – decrease SPATIAL resolution (b/c light emitted activates a larger area of emulsion than direct x-rays)
What is intensification factor?
Ratio of exposures required to produce the same OD of a film w/ and w/out screens.
How is resolution quantitatively measured?
Line pair test = # line pairs/mm
Higher this number, smaller the object that can be imaged
What is the relationship of quantum mottle with intensifying screens and why?
Intensifying screens = inc. quantum mottle
Quantum mottle is decreased when larger # of x-rays are used. Intensifying screens are “faster” screens and hence fewer mAs (x-rays) are used, which means more quantum mottle.
What does an H&D curve plot? And what does it mean?
Density (y) to exposure (x)
Describes the change in optical density over each exposure interval.
What happens to density at very small and very large exposures, according to an H&D curve?
Large variances results in only small changes to OD.
What is the reciprocity law?
As long as the mAs is constant, it doesn’t matter if the exposure time is long or short. Therefore, the OD is only proportional to the total E imparted on the film.
What is latitude?
Range of exposures in which an x-ray film will respond with optical density in a radiographically useful range.
What is the relationship of latitude and contrast?
Inversely proportional –>
wide latitude = long grey scale
short latitude = short grey scale
The slope of a H&D curve is equivalent to:
The contrast of the image receptor.
What makes up radiographic contrast?
Image receptor contrast + subject (person) contrast
What does is the relative speed of image receptor A compared to image receptor B, if A is to the left side on an H&D curve?
Image receptor A is faster.
What increases penumbra?
1) large effective focal spots
2) short SID
3) large OID
4) geometry of object
5) greater on the cathode side
What is the minimum total filtration that must be used?
2.5mm Al
What is exposure linearity?
Ability of a unit to produce constant radiation output for multiple combinations of mA and exposure time. (Should be within +/-10%)
What does linear stochastic dose response mean?
That there is no threshold to which a biological response can be observed (i.e. response can be seen regardless of the size of the dose). Example: leukemia
What is linear energy transfer?
Measure of the rate @ which E is transferred from ionizing radiation to soft tissue.
What does an increased LET mean?
Radiation has a greater ability to produce a biologic response.
What are deterministic biological effects?
Non-stochastic (threshold response)
What is photostimulable luminescence?
Process of receiving x-rays and storing the energy as trapped electrons and releasing them as light when exposed to a different light source.
What do digital screens contain that allow them to store electrons and release them later?
photostimulable phosphors
What type of light is aimed at photostimulable phosphors to cause them to release the electrons and emit light?
Infrared light –> causes metastable electrons to return back to ground state
What color light is aimed at the photostimulable phosphor screens?
Red
What color light is emitted from the released electrons in a storage phosphor screen?
Blue (shorter wavelength)
What causes the latent image in CR?
Electrons being stored in a metastable state in the photostimulable phosphor screen.
How is the photostimulable phosphor erased?
Exposed to intense white light that returns any remaining metastable electrons back to ground state.
What is the active ingredient in a photostimulable phosphor screen?
Europium-activated barium fluorohalide
Grid cut-off on one side of image?
Grid off centered and off focus
Grid cut-off across entire image?
Either off centered or off level
List the different forms of energy along the electromagnetic spectrum, starting from smallest wavelength to greatest wavelength.
ia) gamma ray
ib) x-ray
ii) ultraviolet
iii) visible light
iv) infrared
v) microwave
vi) radiowave
List the imaging modalities from best spatial resolution to worst.
Radiography -- BEST Fluoroscopy CT MRI US Gamma camera --WORST
What is the maximum permissible dose for an adult occupational worker?
For an occupational worker under 18?
For a pregnant woman/fetus?
For the general public (not including medically required doses)?
All of these are annual: Adult occupational worker = 50mSv Child occupational worker = 1mSv Pregnant (and fetus) worker = 5mSv General public worker = 1mSv
