X-ray Production and Interactions

• X-rays travel at the speed of light; electrons moving from cathode to anode travel at about half the speed of light
• Electrons interact with the top 0.5mm surface of the anode target

X-Ray Production Interactions

• Two interaction types are responsible for x-ray production: Bremstrahlung and Characteristic interactions

Bremstrahlung Interactions

• Bremstrahlung means “braking” or “slowing down radiation”
• Electrons avoid orbital electrons, traveling close to the nucleus of the tungsten atom
• The closer the electron is to the nucleus, the higher the attraction, leading to loss of energy and change in direction
• The loss of energy results in an x-ray photon; more energy lost by the electron results in a stronger x-ray photon
• Conversely, if the incoming electron travels farther from the nucleus, the attraction will be weak, there is less loss of energy, and the resulting x-ray photon will be weaker with lower energy
• X-ray energy is measured in kiloelectron volts (keV); 1 keV equals 1000 electron volts
• To determine the energy of a bremsstrahlung x-ray photon subtract the initial energy from the final energy after atom exit
• As an example, an electron enters with 100keV and exits with 40keV, which creates a photon with about 60keV of energy

Characteristic Interactions

• Resulting x-rays have energies characteristic of the tungsten element and its binding energy values
• The projectile electron interacts with an inner shell (K-shell) electron of a tungsten atom
• The projectile electron's energy must be equal to or higher than the binding energy of the electron shell

Process during Characteristic Interactions

• When the electron is ejected from the K shell, a vacancy is created and electrons from outer shells (L or M) fill the vacancy
• Electron transition creates an energy difference that results in the x-ray photon
• To calculate the energy of a characteristic x-ray photon subtract shell energies; a projectile electron removes a K-shell electron, and an L-shell electron fills the vacancy
• Binding energy of K shell: 69.5keV and L-Shell: 12.1keV; to calculate resulting photon subtract energies, 69.5 – 12.1 = 57.4keV

Diagnostic X-Ray Range

• The diagnostic energy range is 30keV to 150keV
• In the diagnostic range most x-ray interactions are bremsstrahlung
• Results of kVp when below 70: The entire x-ray beam (100%) results from bremsstrahlung interactions
• Results of kVp when at 70 or higher: About 85% of the beam results from bremsstrahlung, and about 15% from characteristic interactions
• The lowest energies in the x-ray emission spectrum range from 15 to 20 keV, while the highest energy cannot exceed the kVp selected

X-Ray Switches

• Deadman switches are used to make an x-ray exposure
• Older systems had two switches: a rotor or prep switch, and the exposure switch; newer systems combine the two
• These switches require positive pressure during the entire exposure. If pressure is removed, the exposure terminates
• Pushing prep button induces an electrical current across the filament
• The filament current is about 3-5 Å, and 10 V
• Selection at the panel shows the mA, determining the amount of current at the filament
• Space charge occurs when a cloud of electrons around the filament is a result from thermionic emission

Space Charge Effect

• The cloud of electrons around the filament prevents more electrons from being boiled off the filament
• Similar to a crowded room, "crowding" makes it harder for new people (charges) to enter or move around freely
• Tube current is the flow of electrons from cathode to anode and it is measure in milliamperes (mA)
• Quality and quantity of x-rays are altered by controlling the kVp, mA, and exposure time
• Quantity is how many x-ray photons are in the x-ray beam; quality is their penetrating power
• Kilovoltage set by the operator determines the speed of the electrons
• The kV is applied across the tube from cathode to anode and determines the electron speed, kV is directly related, but not proportional
• When Kv increases, the energy increases and vice versa, but when the speed doubles, the energy level increases by its square
• A kVp meter measures the actual kilovoltage, can vary by +/- 5%, and when this is too high x-ray quality will be affected

Generators

• A generator is required to convert low voltage to high voltage to provide a sufficient potential difference (kVp) for x-ray production, and there 3 generator types:
• Single-phase generators: voltage ripple 100%
• Three-phase generators: 13%
• High Frequency (HF) generators: 1%
• Waveform production for x-ray tubes happens in each system
• In single phase systems, voltage drops from 100 to 0 and goes up again, so the voltage fluctuation is 100%
• In three phase systems, there are three waveforms offset 120 degrees, so voltage never drops to zero, creating a ripple of 13%
• In High frequency generators use high frequencies (kHz to MHz)

Electrons

• The console selection of mA controls the number of electrons in the beam.
• Number of electrons are in proportion to an increase in mA
• mA does not impact the energy or quality of the electrons
• Exposure time determines the length of time over which the x-ray tube produces x-rays
• A higher exposure time leads to a greater flow of electrons, meaning there can be more x-rays
• Exposure time is expressed in seconds (s) or milliseconds (ms); 1 second is 1000 milliseconds
• mAs = mA x seconds
• 200mA X 0.25s= 50 mAs as an example
• Divide milliseconds by 1000 to convert milliseconds to seconds; Multiply seconds by 100 to convert seconds to milliseconds
• A digital timer device measures the actual exposure time with a variability of +/-5% for times >10ms and +/-10% for times <10ms
• Radiation output is tested with a dosimeter; Three tests are conducted

Radiation Tests

• Reproducibility: measures radiation must stay consistent for a repetition of set exposure factors with an acceptable variability of +/-5%
• mAs reciprocity: measures radiation output when mA and exposure times are altered; radiation output is consistent whether mA is high and exposure time is low or vice versa, and variability should be +/-10%
• mA and exposure time linearity: measures radiation output consistency when mA/exposure time are adjusted; Increasing either raises the output and vice versa, variability acceptable is +/-10%
• The actual focal spot is the size of the area on the anode target exposed to electrons
• The effective focal spot is the projected focal spot size as measured underneath the anode target. When an angle is small the effective focal spot size will be smaller, which leads to better image quality due to the higher x-ray concentration

Line Focus Principle

• Explains the relationship between the actual and effective focal spot where the smaller the angle of the target, the smaller the effective focal spot

Anode Heel Effect

• It occurs because of the target angle, and says that x-rays on the cathode side of the tube will be more intense (greater in quantity) because they travelled through a shorter path through the anode material
• Because X-rays on the anode side travel farther (due to the angle), energy loss or absorption can occur by the heel of the anode, decreasing intensity (less quantity)
• The anode heel may be intentionally used to image body parts that have varying densities, such as the thoracic spine, where the smaller vertebrae has the head towards the anode for less intensity
• Primary beams are the photons that exit or leave the x-ray tube
• Remnant/exit beams are the photons that exit the patient after interaction to contribute to image formation
• Beam filtration removes low energy photons from the primary beam because they do not penetrate the patient, contributing to patient dose without contributing to image quality
• Inherent filtration and added filtration are the types of filtration

Inherent Filtration

• Inherent filtration is permanently in the path of the x-ray beam
• Components of inherent beam filtration:
  • The envelope
  • The dialectric oil
  • The window in the tube housing

Added Filtration

• Any filtration added below the port of the x-ray tube
• Consists of a thin sheet of aluminum (Al) sheet placed right below the port, and the mirror below the collimator
• Total filtration (inherent + added) is set by the U.S. government to ensure low radiation doses
• 2.5mm of aluminum is the minimum filtration for tubes operating at/above 70kVp

Half-Value Layer (HVL)

• The amount of added filtration that reduces the beam intensity to half of its original intensity
• Polyenergetic beams make it hard to express beam quality with a value, necessitating this
• By adding filtration of different thicknesses (aluminum) and a detector, a physicist measures beam's quality and determine aluminum needed to reach 50% of its original intensity
• New tubes have better output, with decreasing outputs over time. This test helps determine when the output reaches an unacceptable level based on its HVL
• Compensating filters, placed between tube and patient, helps create more uniform image by compensating for differences in tissue thickness

Filter Types

• Wedge filters have thick sides (blocks more x-rays) aligned over the thin body part, and the thin blocks less rays
• Wedge filters are placed over thick parts because thinner parts require less radiation than thicker parts
• Trough filters have double wedges, can be good for imaging the spine (because type of filter is good for imaging the spine)
• Measured amount of heat units with any given exposure from produced any given exposure (HU); Depends on exposure factors selected and type of generator

Heat Unit Calculations

• Calculate heat units (HU): HU= mA x Time x kVp X generator factor
• For example: 600 mA x 0.05s x 75 kVp x 1.35(factor for three phase) = 3037.5 HU
• Heat units are calculated in an X-ray tube to ensure that the tube can safely handle the heat generated during the exposure process
• Earlier techniques relied on techs evaluating exposure selections to avoid excessive heat loads, but modern systems prevent tube damaging exposures
• Overheating can cause pitting (indentations or craters) in the anode target or can cause melting of the focal track
• The tube life is extended by warming up the tube after 2+ hours of non-use
  • Avoid consecutive high exposures
  • Don't hold down the rotor without making an exposure
  • Stop using the rotor after noticing noises until serviced
  • Don't move the energized tube because it can damage the anode
  • use current/exposure for minimal filament wear

Explore X-ray production principles including Bremsstrahlung and characteristic radiation. Understand how kVp and mA settings, the space charge effect, and safety mechanisms like deadman switches affect X-ray generation and diagnostic energy ranges. This covers electron interactions and energy transfer in X-ray tubes.