X-rays Flashcards
Discovery of x-rays
Wilhelm Conrad Roentgen Nov 8, 1895
won nobel prize in 1901
electormagnetic radiation relevant to DX
x-rays and gamma rays
Are gamma or x-rays more powerful?
trick question - depends on energy of electron interacting with atom
Velocity =
frequency x wavelength
= speed of light
Energy =
Planck’s constant x speed of light/wavelength
What is the changing factor with electromagnetic radiation energy?
wavelength
What level of energy required to produce x-ray?
15eV causes ionization of atoms
Equivalencies of roentgen rads gray sievert and rem
roentgen = 2.58X10^-4 C/kg
1Gy = 1 joule/kg or 100rads
1rem = 10mSv
1 rad = 100ergs/g
1 rem = dose x weighting factor
1 sievert = Gy x weighting factor
Does an equivalent dose of Gy cause the same biologic damage between alpha particles and x-rays?
No, higher ionization density of alpha particles causes more biologic damage
Gy to rads
1 Gy = 100 rads
Sv to rem
1Sv = 100rem
What is determinstic?
threshold of dose to severity
What is stochastic?
random and not dose dependent
Is cataract formation deterministic?
yes
Is the International Commision on Radiological Protection a government organization?
No, they are independent internationa organization that provide guidance and recommendations
Maximum mSv per year
20 mSv averaged over 5 years with max of 50 mSv in any one year
Allowable lifetime accumulation of mSV
10mSv x age in years
What is the nuclear regulatory commison
federal exposure standards
<50mSv/year which is 5 rem
<1mSv for public
Relation of distance to x-ray reduction of dose
logarithmic
Thickness of lead gowning should be?
0.5mm Pb equivalent
Film badge vs modern dosimetry badge
film with exposure vs. now radiation-sensitive aluminum oxide or lithium flouride
What does increasing mA do?
Increases electric current passing through filament from cathode to anode by increasing the amount of free electrons at the cathode
What does changing kVp do?
Changes voltage difference between filament so increases energy via velocity differential
What is binding energy?
Energy required to eject electron - specific to atom and orbit level
Binding energy is related to characteristic x rays via radiative interaction?
False, via collisional interaction
What is radiative interaction? And what is another name for it?
Brehmsstrahlung or braking radiation Energy given off by “braking” of electrons near the positively changed Tungsten nucleus
Most x-rays arise from collisional or radiative interactions?
Radiative, and electrons can “brake” multiple times
kVp vs kEv?
kVp is the energy across the tube from cathode to anode at absolute peak in a revolving wave from -kVp to +kVp at maximum and minimum. Now this is not actually true because the tube never goes negative and “high frequency generators” keep the actual kVp very high but not always maximum kEv is the ACTUAL energy of the electron as it crosses the tube which is dependent on the relation of release from the cathode and wave of kVp.
Why is there a range of energy of electrons leaving anode?
Variance in kVp AND electrons must “brake” all at once to give off all their energy which is rare
Why is Tungsten used?
High melting point of 3422°C or 6192°F High elecrron number increasing efficiency by harder braking
Properties of anodes?
Small focal spot and rotating
When do you use a small filament?
Exotics or very small - decreased focal spot but limited mA
What is full-wave rectification?
Keeping kVp in a high positive energy High frequency generators
What is coherent scattering?
Photon interacting with object with no loss of energy or absorption Scatter 5% of x-rays produced
What is photoelectric effect?
Electron emitted after x-ray knocks one out of orbit
Is characteristic x-ray production in tissues the same energy as produced in the x-ray tube?
No because Tungsten has high energy binding and tissues have low energy binding of orbits
What is Compton scattering?
X-rays that hit outer shell electrons in tissues but still have energy to continue on at different angle and energy
Is Compton scatter dependent on atomic number?
No, only tissue density
How does the tube filter out low energy x-rays produced?
A filter before the collimator
What emulsion is present in film?
Silver halide
What factors change film blackness
kVp mAs Distance to plate
What mAs would be needed in this scenario. 10mAs at distance of 40inches X mAs at distance of 30inches
7.5mAs
Why lower mAs with smaller focal size?
Filament will overheat
What is the edge unhappiness with large focal spots called?
Penumbra
How can we make film more efficient?
Using an intensifying screen
What is an intensifying screen?
Flourescent material that converts x-rays to visble light which is more efficient at sensitivity the film emulsion
What is the relationship of intensifying screens to speed and detail?
Thick flourescent layer allows more x-ray absorption and conversion but also more diffusion before hitting film which decreases detail Speed and detail inversely related
How many intensifying screens are used?
Two, one on each side of film, slight decrease in image quality for large increase in efficiency
What is a way to decrease diffused light in a thick flourescent screen?
Light-absorbing dye
What are rare earth phosphors
Atomic numbers 57-71 These allow lower mAs settings as they produce more visible light in relation to the original phosphor used which was calcium tungstate
How to prevent fog?
Grids
What three things increase photon scattering?
Density if patient Amount of tissues Beam energy (kVp)
What is a grid composed of?
Alternate layers of absorbing (lead) and transparent (aluminum) material Aligns x-rays withpatient to prevent image degradation
How do you compensate for decreased x-rays getting to the patient with grids?
2-3x increase mAs
What thickness should you use grids
>10cm
What is the name of an oscillating grid?
Bucky
Pros and cons of Bucky grid
Decreases chance of grid lines Loud when moves
What are the two things that can be changed in a grid?
Strips per linear inch - decreases grid lines Grid ratio - height to width ratio of strips (usually 5mm x 0.4mm)
T or F Higher grid ratio = less effective absorption of scatter radiation
False
How are grid strips aligned?
Diverging to match beam
What is a focused grid?
Peripheral divergence angulation to match beam Defined range of FFD’s
How are grid strips aligned with x-ray table?
Parallel with long axis and centered
What is lateral decentering?
Loss of primary beam from central malalignment
When do you get distortion?
Closer to x-ray tube than plate
Biggest effects on contrast?
Relationship of kVp and mAs Fogging
What is the most important component on over or underexposure
kVp as it dictates energy
What is long scale vs short scale
Long scale is low contrast (more shades of gray) Short scale is high contrast (less shades of gray)
How to produce short scale?
Low kVp high mAs X-rays are absorbed or not and less range
What scale does high kVp low mAs produce?
Long scale Higher energy allows for variance in absorption
What radiographic technique allows predominance of photoelectric absorption?
High mAs low kVp More dependent on atomic number
What radiographic technique results in more Compton process?
High kVp low mAs Less absorption Independent of atomic numbers Less difference in soft tissue and bone
What can fog a film by environmental exposure?
Light Pressure High temperatures
What photoconductor are direct flat panel detectors made of?
Selenium
Is light intermediary necessary for charged couple device?
Yes Similar to camcorders and digital cameras
What is the difference between indirect and charged coupled?
Fiberoptics to focus light from intensifying screen
What is the difference between the relationship of intensity to distance and mAs to distance?
Intensity to distance is inversely proportional mAs to distance is directly proportional
Rayleigh scattering and coherent scattering are the same? T or F
True
What is a thin film transistor?
The screen that turns pixels on or off
Pixel size is related to resolution how?
Smaller pixel size = improved spatial resolution
Pixel depth is related to resolution how?
More bit depth = better contrast resolution
How many bits in a byte?
8 bits in a byte
Storage data for image dependent on?
Bit depth x matrix size
