Physics of Radiology Flashcards
Atomic structure
Atomic number
Atomic Mass
- Nucleus consists
- Z atomic number
- number of protons
- Determines identity of element
- A Atomic Mass
- Number of protons and neutrons
- Determines stability of element
- Z atomic number
Relation of size of atom, number of protons and neutrons and stability
- Neutrons =Protons Stable
- More neutrons larger atom, less protons= Unstable
- Larger atom= more neutrons= unstable
Ionization
- A neutral atom loses an electron
- Ion pair is formed
- Positive atom and negative free electron
Electron Binding energy
- Energy that an atom holds onto an electron
- Larger atoms have higher binding energies
- Closer orbital shells have higher binding energies
Cathode and anode charges
- Cathode Negative
- Anode Positive
- High voltage between cathode and anode
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Where are electrons generated from and where do they travel from there
Electrons stream from filament to cathode target
The negative cathode send electrons from cathode to anode (+)
Cathode
Filament
Focusing Cup
- Filament
- Tungsten wire
- Filament is heated by flow of current from low voltage source and emits electrons proportional to tempurature
- Electron cloud forms around filament
- Focusing Cup
- Molybdenum
- Negatively charged reflector
- Focuses electrons from filament into narrow beam directed to anode focal spot
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Anode
- Tungsten target
- Focal spot
- Heat vs sharpness
- Smaller= sharper image
- Smaller= hotter
- Copper stem- heat sink
The electrons smash onto tungsten target
- Tungsten target is wrapped in copper to absorb heat
What happens at tungsten target
99% of energy converted to HEAT
Radiation is produced
Tungsten Properties
- High atomic number Z
- Efficient X ray production
- High melting point
- Withstand heat generated
- High thermal conductivity
- Helps dissipate heat
- Low vapor pressure
- Helps maintain vacuum in tube
Radiation definition
Transmission of energy though space and matter
Particulate Radiation
- Large atoms= unstable
- Atoms break up releasing
- Alpha particles
- Beta particles
- Gamma rays
- Radioactivity: The emission of ionizing radiation or particles by spontaneous disintegration of nucleus
Electromagnetic radiation definition
Movement of energy through space as a combination of electric and magnetic fields
Small discrete bundles of energy that are photons
An electron volt is the unit of energy of a photon
Electromagnetic radiation is generated when
- EM radiation is generated when the velocity of an electrically charged particle is altered (accelerated or decelerated)
- Velocity has magnitude and direction
- If electron changes direction it releases energy in the form of radiation (X-ray)
Bremsstrahlung Radiation
- The sudden stopping or slowing of electrons produces bremsstrahlung photons
- Main mechanism to produce radiation produced in X ray beam
- Direct hit
- Electron can smash into the nucleus of the tungsten nucleus
- Then all its mass and energy is converted into a photon (X-ray)
- Strongest bc all of the energy of the electron is converted into a photon
- Indirect hit
- Negatively charged electron nearly misses the nucleus but is attracted to the nucleus
- Changes direction, and velocity is a vector
- Energy is released as a photon (xray) deacceleration
- This is happening at the tungsten plate
Charecteristic Radiation
- Moving electron hits and ejects inner electron
- Outer orbital electron attracted to void
- Must lose energy to go to lower energy state
- Photon is emitted
- Higher level/ outer shell electron will fall to a lower level shell
- Called charactereistic bc every atom has its unique electron energy shell, characteristic to the element that it is involving
Voltage
Definition
Deteremined by
Greater voltage potential leads to
- Voltage is electromotive force (amplitude)
- Movement of electrons across wire
- Amplitude of the force is deteremined by the potential differnce between 2 points
- Greater the voltage potential the greater
- Greater push btw 2 plates
- Higher energy of x ray
- Electrons move faster
Tube voltage
- Between cathode and anode
- Voltage across the tube
- Want HIGH voltage kilovolts
- Alternating current has spectrum of voltages, the highest is the peak, kVp usually 120
Filament Voltage
- Voltage across the filament
- Want LOW voltage ~10 volts
- Filament is very thin and can burn out easily
- Thin so electrons can evaporate to make electron cloud
Current
- Amperage (Amps)
- Measure of flow of the electrical charge
- How many electrons are passing though a particular spot
- Charge cant flow without voltage
- Increase of Amps (current) more electrons more photons
Filament Current
- Number of electrons through filament
- Increasing current through filament will increase tube current
Tube Current
- Number of electrons from cathode to anode
- Filament (step down) transformer (low for filament voltage)
- Reduces voltage of incoming AC current to 10 v
- Voltage regulated by mA selector
- System regulates filament temperature and number of electrons emitted by cathode to anode
- Tube current=Number of electrons=Quantity of beam=Controlled by mA
What are the 6 factors that control the X ray beam
- Exposure time
- Tube current
- Tube voltage peak kVp
- Filtration
- Collimation
- Inverse Square Law
Exposure time
Measured in seconds
Number photons (quantity) proportional to number of seconds
Quality styas the same
