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Bohr’s model
- not most up to date; but visually most satisfying
- Miniature of solar system: nucleus (protons and neutrons) in the center (the sun); Orbited by an electron cloud (the planets).
- All electrons are alike, as are all protons, and all neutrons.
- H - 1e, 1P
- He - 2e, 2P, 2N
- Li - 3e, 3P, 3N
- Electrons circle the nucleus at high speeds in orbits/shells - K, L, M, N, O, P, principle quantum numbers.
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Modern view
electrons can be anywhere in the cloud
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Atomic Shells
- No atom is known to have more than seven shells.
- Only two electrons may occupy the K shell.
- Increasingly larger numbers of electrons occupy the outer shells.
- Formula: 2n². L shell = 2(2)² = 8 electrons in shell.
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Atomic Structure
- Subatomic Particles:
- - Electrical Charges: e: -1; Proton: +1; Neutron: 0
- - Particle Mass: proton and neutron weigh ~2kX of e; Almost the entire mass of the atom is in the nucleus. Total number of protons and neutrons in the nucleus of the atom is the atomic mass (A).
- Number of protons determines the positive charge and the identity of the element (atomic number, Z).
- All atoms of the same element have the same atomic number, but isotopes have different atomic masses (different number of neutrons)
- Each element has its own atomic number, corresponding number of electrons, and unique chemical and physical properties.
- atom is electrically neutral, number of protons = number electrons
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The Nature of Radiation
- Defined as the transmission of energy through space and matter.
- Can be visualized in two forms: Particulate; Electromagnetic.
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Particulate Radiation
- Subatomic particles and atomic nuclei moving at high speed.
- Include - α particles, β particles, Cathode rays (mainly high speed e).
- Significant ability to ionize other atoms - dependent on mass, velocity, and charge.
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Subatomic Particles
- More than 100 subatomic particles have been described.
- Fundamental subatomic particles – electrons, protons and neutrons.
- These are of most interest because generation, emission, and absorption of radiation occurs at the subatomic level.
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α Particles
- Helium nuclei (2 protons, 2 neutrons, 0 electrons).
- Formed from radioactive decay.
- Highly ionizing.
- Shallow penetration (a few microns).
- Stop, acquire 2 e-, and become helium atoms.
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β Particles
- Electrons emitted by radioactive nuclei.
- Better penetration (up to 1.5 cm).
- Less ionizing.
- Used in radiation therapy for skin lesions.
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Cathode Rays
- High speed electrons.
- Man-made
- - X-ray and Cathode ray tubes.
- - Older Television sets and Computer screens (not flat panel).
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Linear Energy Transfer (LET)
- The loss of energy from a particle as it travels through the tissue by causing ionizations.
- The more interactions, the greater the energy loss.
- Capacity to ionize atoms depends on mass, velocity, and charge of the particle.
- Rate of energy loss is the Linear Energy Transfer.
- Greater physical size and charge and lower velocity, the greater the LET.
- - α particles transfer more energy in a given path than β particles and are therefore more damaging per unit dose.
- - Both penetrate a relatively small distance into tissue.
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Electromagnetic Radiation
- Movement of energy through space as a combination of perpendicular electric and magnetic fields.
- Generated when the velocity of an electrically charged particle is changed.
- Examples of Electromagnetic Radiation (with decreasing energy levels): Gamma Rays, X-rays, UV, Visible, IR, Microwaves, Radio waves
- All electromagnetic radiation travels at the speed of light
- X-rays are produced outside the nucleus, generally by interactions between electrons and nuclei within an x-ray machine (overlaps w/ gamma; considered man-made, gamma considered nature-made).
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Terminology
- X-ray - an invisible beam/photon of energy.
- X-ray film, plate, or sensor - the image receptor.
- Radiograph - the resultant image.
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Components of the X-ray Machine
- Head – contains the x-ray tube (aka the tube head).
- Power Supply.
- Control panel.
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Tube head consists of
support arm, tube head, and aiming cone/device
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The X-ray tube is positioned within the tube head along with some components of the power supply. The tube may be recessed within the tube head to improve the radiographic image quality.
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Components of x-ray tube head
- x-ray tube
- power supply
- oil (to cool and maintain a constant temperature because a lot of heat is generated; can leak out)
- aluminum filter
- collimator
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x-ray tube
- aka Coolidge tube
- Basic design introduced in 1913.
- Composed of: Anode, Cathode (source of electrons), Evacuated glass tube.
- Electrons from cathode strike anode, producing x-ray photons.
- Cathode - tungsten filament coiled and sitting in the focusing cup, generates the electron cloud when warmed up
- Anode - copper stem and tungsten target
- Emitted x-ray goes through porte filter and diaphragm
- Power supply functions:
- - Heat the filament to generate electrons.
- - Establish high-voltage potential between the anode and the cathode to accelerate the electrons.
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Cathode
- Filament (source of electrons)
- - Coiled tungsten wire, 1cm long, 0.2cm in diameter.
- - Mounted on 2 stiff wires for support and carrying electric current.
- - Wires connect to high and low voltage electric sources.
- - Incandescence of the wire causes the release of electrons (boiling off of electrons).
- Focusing cup
- – negatively charged concave reflector (charge repels electrons).
- - Nickel or molybdenum
- - directs electrons toward the focal spot on anode (positively charged).
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Anode
- Tungsten target embedded in copper stem (heat transmitting).
- Converts the kinetic energy of electrons into photon energy (x-rays).
- Inefficient process – 99% of energy is lost as heat.
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Tungsten (W)
- Tungsten - "heavy stone" in Swedish.
- W - wolfram, medieval German smelters who found that tin ores containing tungsten had a much lower yield. It was said that the tungsten devoured the tin "like a wolf".
- First isolated by two Spanish chemists, the de Elhujar brothers in 1783.
- Greyish white lustrous metal
- Solid at room temperature
- Has the highest melting point (3370º C) and lowest vapor pressure of all metals - Helps withstand the high temperatures and maintain the vacuum
- High atomic number (74) - Most efficient in producing x-rays.
- High thermal conductivity - Dissipates heat into the copper stem.
- 2-8-18-32-12-2
- At temperatures over 1650°C has the highest tensile strength.
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Tungsten target
- Embedded in a large block of copper (a good thermal conductor).
- - Dissipates heat from the tungsten – reduces
- risk of target melting.
- Insulating oil surrounds the x-ray tube.
- Stationary anode.
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Rotating anode
- higher energy levels, longer exposure time, such as in medicine, heat is more of a problem
- Target rotates so a different portion is exposed each time - lasts longer
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Focal spot
- Area on target where focusing cup directs electrons
- Image sharpness increases as focal spot size decreases.
- - larger focal spot wanted by engineers, but for good images smaller one is needed
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Electric circuits
- Rate of flow (electrons/second) is measured in amperes and depends on the voltage and resistance
- Ohm’s law: V = IR
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Power supply
- Tube current (mA) - low-voltage current to heat the filament and generate electrons.
- Tube voltage (kVp, peak lvl) - high voltage potential between the anode and the cathode to accelerate the electrons.
- Together, mA and kVp determine the intensity of the x ray beam, along with the target material and the filtration used.
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Transformers
- Two coils wrapped around a closed core
- - Primary circuit - first coil; current creates a magnetic field within the core
- - Secondary circuit - 2nd coil; current induced by magnetic field
- Two types: Step-up and step-down
- Step-down transformer: for the filament; AC 110V -> 10V
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Autotransformer
- Regulated by kVp control
- Controls voltage between cathode and anode (tube voltage, peak level 60-100kV, at least 70kVp for tungsten target to produce the K-characteristic radiation)
- The greater the tube voltage, the greater the kVp will be.
- For AC current, only positive cycle generates x rays and generates the strongest x-ray at peak voltage level
- kVp determines the (maximum) energy of the electrons that generate, hence the quality of the x ray.
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The higher the mA, the more electrons generated, the more x rays produced
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Timer
- Determines the duration when the high voltage is on.
- Together with mA setting determines the amount of photons produced, the quantity of x ray (mAs, milliamp sec)
- Older machine use impulses (1/60 sec) in stead of seconds as the time unit.
- Units need to be recorded in the chart.
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Half-wave rectification
- aka self-rectification, eliminates negative portion of the cycle.
- Vs. Full-wave rectified - negative portion reversed
- Almost used by all AC dental x-ray units
- DC - continual, shorter period needed
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Tube rating
The maximum exposure time that an machine may be energized for a single exposure.
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Duty cycle
Frequency with which exposures can be made. Limited by the heat build-up on the anode, heat dissipating time.
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X-rays are produced by two processes
- Bremsstrahlung
- Characteristic radiation
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Bremsstrahlung
- "Braking" radiation
- Most x-rays in a dental x-ray unit are generated in this way.
- Produced by sudden stopping or slowing of electron at the target, during which 99% of the energy is converted to heat, and 1% to x-ray photons, when the electron hits target nucleus (photon w/ maximal energy, same energy level and direction as the incoming electron) or passes the nucleus closely (with a portion of the kinetic energy of the incoming electron).
- Positively charged nucleus attracts the high speed electron -> deceleration -> kinetic energy converted to photon
- The closer the electron passes to the nucleus, the greater the change in direction of the electron, the greater the intensity of the energy (x-ray photon) given off, and the less altered path of the new photon from the incoming electron.
- Electrons participate in many Bremsstrahlung reactions, losing energy with each reaction, until they pass from the field or lose all their energy.
- Broad spectrum, no more than kVp keV
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Characteristic radiation
- Incoming electron displaces electron from inner shell, causing ionization.
- Outer shell electron drops in to fill the void, emitting photon with energy equivalent to the difference in the orbital binding energies.
- Atomic number of the target determines the energy - Tungsten (74) – 57 to 69 keV.
- - Tin (50) – 25 to 29 keV.
- - Lead (81) – 72 to 88 keV.
- Fingerprint-like spectrum
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Thermionic Emission
- Current heats the filament of the cathode, releasing electrons
- Higher temperatures produce more free electrons (electron cloud).
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Focal spot
- 1mm X 3mm actual focal spot size
- 1mm X 1mm effective focal spot size
- Angled (90 degrees) target to decrease effective focal spot size while maximizing distribution of electrons over a large target
- The smaller the focal spot, the smaller the penumbra, the sharper the image and higher resolution, the less dissipated heat
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Polychromatic radiation
X-ray beam w/ varying energies and wavelengths.
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Filtration
- Needed to remove long wavelength, low energy photons, which do not contribute to the useful image.
- External filtration - aluminum (discs) filter over the exit port of the tube head; reduces the total energy, by blocking of the low energy photons, but increases the mean energy, high energy ones go through intact.
- Inherent filtration - materials in path between the focal spot to the point of exiting the x-ray head, including glass tube wall, insulating oil, barrier surrounding oil; 0.5-2.0mm aluminum equivalent (AE)
- Total filtration - required to be 1.5mm AE for beams up to 70kVp and 2.5mm for over 70kVp.
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Half-value layer
- The thickness (mm or cm) of any given material after which 50% of incident energy has been attenuated.
- Like the attenuation coefficient, it is photon energy dependant. Increasing the penetrating energy of a stream of photons will result in an increase in a material's HVL.
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Primary radiation
- Main beam from the tungsten target.
- Used to record the image.
- Central portion - central ray or central beam.
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Collimation
- Collimated to limit the shape and size of the resultant beam to the shape and size of the image receptor, to remove the useless portion of the beam, improving image quality by reducing scatter radiation (less scatter -> better contrast), and protect the patient from unnecessary irradiation.
- - longer collimator -> longer beam, more central parallel rays (reduces size of focal spot even further, less divergence); more difficult to maneuver.
- - Short collimator -> more magnification, more area of the person gets irradiated
- Rectangular collimator used to limit the beam for intraoral radiographs (periapical and bitewings)
- Round vs rectangular - rectangular best matches, image quality, and safety.
- Can also use external positioning metal instrument/shield to introduce collimation near the patient; not part of the machine; not as easy to use as the collimator
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Secondary radiation
- Primary radiation that reflects from the patient and other objects (walls, ceiling, floor, dental unit, etc.).
- AKA scatter radiation.
- Detracts from image quality, overall darkening w/o adding to the quality, reducing the contrast.
- One way to improve contrast is to reduce secondary radiation.
- Patient cannot be protected from it by wearing the lead apron.
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Atoms
Cannot be subdivided by chemical methods, but can be broken down into subatomic particles by high-energy techniques.
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Atomic shells
- K, L, M, N, O, P, Q
- No known atom has more than 7 shells
- Two electrons for K shell
- N=2n^2
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Forces
- Electrostatic forces - maintain electrons in the shells (electrons and protons).
- Centrifugal forces - balance electrostatic forces on revolving (orbiting) electrons.
- Electron binding energy (ionization energy) - the amount of energy required to remove an e- from its shell; shell-specific; greater than the electrostatic force; the greatest for K shell electrons (greatest electrostatic force) and decreases in successive shells.
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Ionization
- An electrically neutral atom loses an electron, usually from outer shell, and becomes positively charged ion.
- Electron becomes a free, negatively charged ion.
- Occurs by heating, collisions with high energy x-rays or particles.
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High LET radiation is more likely to interact w/ DNA than low LET radiation.
Greater physical size and charge, and lower velocity.
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Electromagnetic (EM) Energy
- Generated when the velocity (speed) of an electrically charged particle is altered, ionizing or non-ionizing.
- Movement of energy through space or matter as a combination of electric and magnetic fields.
- Includes radio waves, radiant heat, visible light, and γ radiation.
- Enough energy will knock an orbital electron out of its shell – ionizing radiation.
- Increases with frequency.
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Inverse Square Law
- The intensity of an x-ray beam is inversely proportional to the square of the distance between the source and the point of measure.
- I1/I2 = (D2)^2/(D1)^2
- Used to calculate exposure factors.
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Attenuation
The reduction in intensity of an X-ray beam as it travels through matter, outside the tube/collimator, inside the patient.
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Attenuation Depends on:
- Energy of the initial beam, determined by kVp; lower energy x-rays are attenuated to a greater extent than the high energy ones; thus as the beam is attenuated, the mean energy increases while the number of photons decreases, aka filtration, and results in beam hardening .
- Thickness of absorber, exponential [different w/ Inverse Square Law].
- Density of absorber.
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Types of interaction between x-rays and matter:
- Compton scatter, 49% for dental radiographs.
- Photoelectric absorption, 23%.
- Coherent scatter, 7%.
- Remaining has no interaction.
- Scattered photons may have further interaction.
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Photoelectric Absorption
- Essential in diagnostic imaging.
- Photon interacts with inner shell electron.
- Electron is ejected and becomes a recoil electron. Incident photon no longer exists.
- Energy of recoil electron is same as the incident photon minus the binding energy of the electron.
- Atom is ionized.
- Outer shell electron falls in to fill vacancy and characteristic radiation is released as a photon.
- Resultant photons are of low energy and usually remain in absorber.
- This effect varies with the material, greater in bone than soft tissue.
- Resultant image with differences in densities.
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Compton Scattering
- Incident photon collides with outer shell electron.
- Incident photon is deflected.
- Energy of photon is that of the original minus energy given up to electron (minor).
- Absorber is ionized.
- Another free electron comes in to fill the void, since the binding energy is low, no additional photon is emitted.
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Coherent scattering
- Low energy incident photon passes near an outer electron of an atom.
- Electron becomes excited and photon ceases to exist.
- Electron returns to its former energy level as it emits a new photon.
- New photon exits at angle to incident photon.
- Does not contribute to the image quality.
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Differential Absorption
- The number of Photoelectric and Compton interactions are greater in hard (calcified) tissues than in soft tissue.
- More photons pass through soft tissues and exit the patient as part of the remnant beam.
- This heterogeneous remnant beam contains data.
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Dosimetry
- Dose: Absorption of energy per unit of mass at site of interest.
- Exposure: Measure of radiation based on its ability to produce ionization in air at Standard Temperature and pressure (STP).
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Units of radiation measurement:
- 1 Gray = 1 Roentgen (exposure/dose)
- 1 Gray = 100 Rad (absorbed dose)
- 1 Sievert = 100 Rem (equivalent dose and effective dose)
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Exposure
- SI unit is air KERMA (Kinetic Energy Released in Matter)
- Measures kinetic energy transferred from x-ray photons to electrons in matter
- Expressed in units of dose Gray (Gy).
- 1 Gy = 1 Joule/kilogram
- Old unit = Roentgen
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Absorbed Dose
- Also measured in grays (Gy). 1 Gy = 1 Joule/kg; old unit (rad = 0.01 Gy)
- Can be in any type of matter
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Equivalent dose
- Measured in sieverts (Sv); old unit: rem = 0.01 Sv
- Used to compare biological effects of different types of radiation on a given tissue or organ (thyroid is more susceptible to radiation damage).
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Effective Dose
- Used to estimate the risk to humans
- Combines equivalent dose to each tissue or organ and tissue weighting factors
- Unit of measure is the sievert (Sv)
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Fluorescence
- Luminescence - emission of light upon application of a stimulus.
- Fluorescence - visible light ...; When stimulus is removed, fluorescence ceases immediately.
- Used in extraoral radiology.
- X-ray photons can cause fluorescence in certain substances.
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