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X-ray Production
Conversion of projectile-electron kinetic energy into heat, infrared radiation, and x-rays at the anode target.
Projectile Electron
Electron accelerated from the cathode that strikes the anode target and produces heat and x-rays.
Electron-Target Interaction
Encounters between projectile electrons and the orbital electrons or nuclear field of target atoms that generate x-ray photons and heat.
Thermionic Emission
Release of electrons from a heated filament on the cathode side of the x-ray tube.
Space Charge Effect
Self-limiting build-up of electrons around the filament that restricts further thermionic emission.
Focusing Cup
Negatively charged cathode component that confines the electron cloud into a narrow beam toward the anode.
Tube Current
Stream of electrons traveling from cathode to anode during exposure; measured in milliamperes (mA).
Anode Heat
Thermal energy generated when 99 % of projectile-electron kinetic energy interacts with outer-shell electrons of the target.
Characteristic Radiation
X-rays emitted when an outer-shell electron fills an inner-shell vacancy created by a projectile electron; energy is discrete and element-specific.
K-Characteristic X-Ray
Characteristic photon produced from a K-shell vacancy; requires ≥ 69 kVp in tungsten targets.
Bremsstrahlung Radiation
‘Braking’ x-rays produced when projectile electrons are decelerated by the nuclear field of target atoms; continuous energy spectrum.
Emission Spectrum
Graphic display of x-ray beam energies showing discrete characteristic peaks superimposed on a continuous Bremsstrahlung curve.
X-Ray Quantity (Intensity)
Number of x-ray photons in the useful beam; measured in milligray in air (mGya) or exposure.
Milliampere-Seconds (mAs)
Product of tube current and exposure time; x-ray quantity is directly proportional to mAs.
Kilovolt Peak (kVp)
Peak tube potential that controls beam energy and affects quantity as the square of the kVp ratio.
Inverse Square Law
X-ray intensity is inversely proportional to the square of the distance from the source (I ∝ 1/d²).
Square Law
mAs must be changed by the factor (SID₂)²/(SID₁)² to maintain beam intensity when source-to-image distance changes.
Filtration
Use of metal (usually 1–5 mm Al) to remove low-energy photons, reducing patient dose and beam quantity while hardening the beam.
Inherent Filtration
Permanent filtration from the glass envelope, insulating oil, and mirror inside the collimator.
Added Filtration
Filtration that is added to the part of the x-ray tube. Sheets of aluminum placed at the tube port to absorb low-energy x-rays.
Total Filtration
Sum of inherent and added filtration; must be ≥ 2.5 mm Al for tubes operating above 70 kVp.
Compensating Filter
Accessory filter (e.g., wedge, bow-tie) that evens out non-uniform anatomy by differentially attenuating the beam.
Wedge Filter
Most common type of compensating filter; thick edge is aligned with thinner anatomy to equalize exposure.
Bow-Tie Filter
Curved compensating filter used in CT to match the rounded shape of the head or body.
Bilateral Wedge (Trough) Filter
Double-wedge filter commonly used for AP thorax to balance exposure between lungs and mediastinum. Performs similar function to the wedge filter.
X-Ray Quality (Penetrability)
Ability of x-rays to pass through tissue; increases with higher kVp and added filtration.
Coherent (Classical) Scattering
Low-energy (< 10 keV) interaction in which x-ray excites an atom and is re-emitted with equal energy but different direction.
Diffraction X-ray
<10 kVp; used in research for structural and molecular analysis
Grenz rays
10–20 kVp; used in dermatology for medical purposes.
Superficial X-ray
50–100 kVp; used in therapy of superficial tissues.
Diagnostic X-ray
30–150 kVp; used in medical imaging of anatomical structures and tissues.
Orthovoltage X-ray
200–300 kVp; used in therapy of deep-lying tissues.
Supervoltage X-ray
300–1000 kVp; used in therapy of deep-lying tissues.
Megavoltage X-ray
>1000 kVp (1 MV); used in therapy of deep-lying tissues and in industry for checking integrity of welded metals.
Compton Scattering
Interaction of diagnostic-range x-rays with outer-shell electrons, producing a scattered photon of lower energy and a Compton (recoil) electron.
Compton Electron
Ejected outer-shell electron resulting from Compton scattering. Most of the energy is divided between scattered x-ray and Compton electron.
Recoil electron
The ejected electron; Other term for Compton electron
Photoelectric Effect
Total absorption of an x-ray by an inner-shell electron, which is ejected as a photoelectron; dominant at low kVp/high-Z materials.
Photoelectron
Inner-shell electron ejected during a photoelectric interaction.
Pair Production
High-energy (≥ 1.02 MeV) interaction near the nucleus where the x-ray converts to a positron–electron pair.
Annihilation Radiation
Process where a positron combines with an electron, converting their mass to two 0.511 MeV photons at 180° apart.
Photodisintegration
Absorption of an x-ray (> 10 MeV) by the nucleus, followed by emission of a nuclear fragment.
Compton Scattering and Photoelectric Effect
The only important X-ray Interaction to Radiology
Effective Atomic Number
Composite atomic number that expresses overall electron density of a compound or mixture (e.g., bone 13.8, soft tissue 7.4).
Emission Button (Exposure Switch)
Control that applies high voltage across the tube, driving electrons to the anode and producing x-rays and heat.
Rotor/Prep Button
Switch that heats the filament for thermionic emission and accelerates the rotating anode to operational speed.
Electron Cloud (Space Charge)
Collection of electrons around the filament prior to exposure.
Penetrability
Depth to which x-ray photons can travel in tissue; synonymous with beam quality.
Outer-Shell Interaction
Collision of projectile electrons with outer-shell electrons in the target that produces heat, not useful x-rays.
Bremsstrahlung Continuous Spectrum
Range of photon energies from zero up to the maximum kVp, characteristic of Bremsstrahlung production.
Characteristic Peak
Discrete spike in the x-ray spectrum corresponding to energy differences between specific electron shells (e.g., Kα, Kβ).
Aluminum (Z = 13)
Material chosen for filtration because it is inexpensive, readily available, and easily shaped while effectively absorbing low-energy photons.
99 % Heat / 1 % X-Rays
Approximate distribution of projectile-electron kinetic energy at the anode
kVp–Quantity Relationship
X-ray quantity changes proportional to (kVp₂/kVp₁)²; doubling kVp increases quantity fourfold (factor of 4).
mAs–Quantity Relationship
X-ray quantity is directly proportional to mAs; doubling mAs doubles quantity.
Distance–Quantity Relationship
X-ray intensity decreases with the square of the distance per the inverse square law.
Filters and Beam Quality
Adding filtration removes low-energy photons, increasing average beam energy (quality) while decreasing quantity.
Diagnostic Range
X-ray energy spectrum of approximately 30–150 kVp used for medical imaging.
Positron Emission Tomography (PET)
Nuclear medicine technique relying on pair production and annihilation radiation to detect metabolic activity.