X-ray Physics

• Diagnostic X-Rays is Part 1 of the semester's medical physics course for University of Babylon's College of Medicine.
• The topics are X-ray production and types, methods of X-ray interaction with matter and radiographic image quality.

X-Rays

• Electromagnetic radiation (EMR) with short wavelengths (0.1-1 Å) and high penetrating power
• Used in diagnosis and radiotherapy
• Energy carried by each photon varies with radiation frequency

Electromagnetic Radiation

• X-rays are a type of radiation
• The transport of energy through space as a combination of electric and magnetic fields
• Includes radio waves, radiant heat, visible light, and gamma radiation
• Described by the equation E = hv = hc / λ
  • h = Planck's constant (6.6 * 10^-34 joule)
  • c = velocity of light (3 * 10^8 m/sec)
  • v = frequency of radiation.

X-Ray Production

• Produced when high-energy electrons interact with matter, converting kinetic energy into electromagnetic radiation.
• Main components of an X-ray tube:
  • A source of electrons (cathode/filament)
    • Number of electrons and produced X-ray photons controlled by tube current and time (mAs).
  • An evacuated space
    • The space in which electrons are speed up in (glass envelope).
  • High positive potential
    • This accelerates negative electrons, controlling electron and X-ray photon energy (kV)
  • A target
    • The target the electrons strike (anode)

X-Ray Tube Efficiency

• In X-ray tubes, up to 99% of accelerated electron energy converts to heat, approximately 1% to X-ray photons.
• Higher atomic number (Z) of the anode increases intensity of X-ray beam (Z of tungsten = 74).
• Increase current in the cathode circuit increases electrons
• Increase kV to increase electron speed, photon energy, and resolution

Target Properties inside an X-Ray Tube

• Anode material should have a high melting point (3400 °C for tungsten)
• X-ray tubes have two interchangeable filaments for large/small focal spots.
  • Focal spot is the area on the target struck by electrons
  • Small focal spots reduce image blurring but concentrate heat.
• Increasing area struck by electrons (focal spot area) without increasing image blurring achieved by angling anode 10-20° (line-focus principle).
• Rotating anode X-ray tubes rotate at 3600 rotations per minute.

Types of X-Ray Radiation

• Bremsstrahlung Spectrum (Continuous X-Ray)
  • Small fraction of accelerated electrons come near atomic nucleus within the target and are influenced by its positive electric field
  • Electrons decelerate and change direction
  • Results in a loss of kinetic energy that is emitted as an X-ray photon of equal energy (Bremsstrahlung radiation)
  • Amount depends on atomic target number and kilovolt (kV peak).
• Characteristic X-Ray
  • A fast electron strikes a K-electron in a target atom and knocks it out of its orbit, freeing the atom.
  • Vacancy in K-shell is immediately filled when an electron outer shell of the atom falls in to it, emitting a characteristic K X-ray photon
  • The difference in energy levels of the orbits in an atom specify that atom
  • Emitted radiation is characteristic

Diagnostic X-rays

• Usually have energies of 15-150 keV, while visible light photons have energies of 2-4 eV.
• The amount of electrons accelerated toward the anode depends on filament temperature
• Maximum energy of X-ray photons produced is determined by the accelerating potential (kVp)

Bremsstrahlung Spectrum

• Results in a broad, smooth curve and the spikes represent the characteristic X-rays.

X-Ray Energy

• One kilo electron-volt (keV) is the energy an electron gains or loses in going across a potential difference of 1000V, where 1keV=1.610-9 erg = 1.610-16 J
• The (kVp) used for an x-ray study depends on the thickness of the patient and the type of study.
  • Mammography: 25 to 50 kVp.
  • Chest: ≈ 350 kVp
• Electron current: 100-500 or 1000 mA
• X-ray energy produced is not monoenergetic, but a full spectrum.
• The power P (watt) = I (amp) * V (volt) = 1 A and 100 kV = 100 kW and 99% appears as heat: damaged anodes.

X-Ray Absorption

• Attenuation of an x-ray beam refers to its reduction due to absorption and scattering.
  • Intensity (I) decreases exponentially
  • Described by the equation I = Io * e^(-μx)
  • Io = initial beam intensity
  • I = unattenuated beam intensity
  • μ = linear attenuation coefficient,
  • e = 2.718
  • x = thickness of the attenuator
  • The following materials attenuate: brain tumor, bone, aluminum
  • The linear attenuation Coefficient (μ): measures measure probability that photon interact (absorbed or scattered) per unit length it travels in specified material.
    • Depends on energy of x-rays, atomic number (Z), density (p) of material
  • Half value thickness HVT (X1/2): thickness of material to reduce radiation intensity by half (50%).
    • Described by (HVT) X1/2 = 0.693 / μ

X- Ray Interaction Methods

• Photoelectric effect (P.E.)
  • Incoming X-ray photon transfers all energy to an electron, which uses it to overcome binding energy and get away from the nucleus.
  • Free photoelectron uses remainder of gained energy in ionizing surrounding atoms.
  • Common in high Z elements versus low Z, occurs more at low energies
  • At 30 keV, bone absorbs X-rays about 8x better than tissue because of the photoelectric effect.
• Compton Effect (C.E)
  • Occurs when X-ray photon collides with a loosely outer shell electron.
  • The electron receives part of the photon energy and the remainder energy is given to a Compton photon.
  • More likely to occur in material with low (Z) number.
  • Greatest in low Z material, e.g., water or soft tissue. More probable than P.E effect at energy ≥ 30 KeV. In bone C.E., P.E occurs at ≥ 100 KeV
• Pair Production
  • Rarely occurs at diagnostic energy range since the minimum energy needed for pair production is 1.02 Mev.
  • High energy photon enters electric field of nucleus and converts into two particles (electron and positron).
  • Particles vanish and mass-energy appears as two photons called annihilation radiation.

X-Ray Contrast Techniques

• Uses photoelectric effects to have Radiologists inject high Z material into parts of the body (contrasting media).
  • Barium and iodine compounds are commonly used contrast agents (Zbarium=56, Ziodine=53, Zsoft tissue=7.42).
    • Iodine injected into the bloodstream illuminates arteries
    • Iodine in an oily mist is sprayed into the lungs
    • Barium compound is swallowed to see gastrointestinal tract (upper GI)
    • Air replaces fluid ventricles of the brain during a pneumoecephalogram
    • Barium enemas are used to view the digestive system (lower GI)
    • In double contrasting studies, air and barium are used separately to show the same organ

X-Ray Image Production

• Requires X-ray source and an image receptor
  • Three types of image receptors or films:
    • Double-sided radiographic film: emulsion light-sensitive crystals coated on both sides of a transparent base material, and used in plain film imaging
    • Single-sided camera film: one emulsion layer in mammography
    • Non-screen film: x-ray photons directly expose film in dental x-rays

X-Ray Qualities

• Radiographic images present information in visual form, making it easy for trained observers to understand
• Different body parts absorb x-rays in different degrees
• Dense bone absorbs more radiation, while soft tissue (muscle, fat, organs) allow more radiation to pass through
  • Bones appear white on film, soft tissue is gray, and air is black

Enhancing Image Sharpness

• Sharpness is achieved by reducing blurring, positioning patients closer; increasing distance between x-ray source and film.
• Reducing scattered radiation reaching the film through grids (lead and plastic strips).
• Holding breath reduces blurring.

Radiographic Image Quality

• Blurring is the main problem in obtaining good X-ray images
  • Blurred edge on an X-ray image = penumbra
  • P (penumbra width) = D/L * l , where
    • D = focal spot size,
    • l = focal-object distance,
    • L = object-film distance
• A good x-ray image (without blurring) has: - A small focal spot (small D) to reduce penumbra, - The patient positioned close to the film (small l), - A large distance between tube and film (large L), - Reduced scattered radiation striking the film (grids), - No motion occurs.

Grids

• Reduces the amount of scattered radiation
• Should be in front of X-ray tube towards the body
• The grid = lead and plastic strips which absorbs the scattered radiation
• Disadvantage: increases radiation so patient may absorb some primary beam photons

X-Ray Filtration

• Low-energy x-rays will not penetrate the entire thickness of the body.
• Filters (thin aluminum, copper, or other materials) in front of the X-ray tube window get rid of low energy radiation

Measuring X-Rays

• Measure of X-ray ionizing ability = exposure.
• Measured in roentgen (R), where 1 R = 2.58 * 10^-4 C/kg of air.
• Exposure-area product (EAP): describes radiation relative to patient
  • EAP (rap) = exposure (roentgen) * area (cm²), where 1 rap = 100 R cm²
  • e.g., Exposing 33 cm² (typical dental exposure) to 0.6 R equals receiving 20 R cm² (or 0.2 rap)

Radiation Risk Factors

• Radiation risk refers to damage from ionizing radiation due to energy deposition in tissues
• Energy may result in ionization within the tissues
• Radiation interactions with tissue are either:
  • Direct: radiation energy is directly transferred to the DNA causing structural changes to its molecules.
  • Indirect: radiation energy is absorbed by water molecules, forming free radicals which cause damage to DNA molecules.
• Adverse health effects can be:
  • Deterministic: effects follow high radiation doses, can result in immediate damage (minutes to weeks).
  • Stochastic: effects follow low radiation doses may result in cancer development with a lag of at least 5 years and may reach 10 - 20 years.

Fluoroscopy

• Refers to the continuous acquisition of X-rays in movie-like format.
• Used to position catheters, visualize contrast, assist during surgery, and record motion in anatomic structures

Computed Tomography (CT) Scan

• Passing X-rays at a large number of angles by rotating the X-ray tube around the body.
• A detector array, opposite the x-ray source, collects the transmission projection data.
• Synthesized by computer into tomographic images.
• Tomography refers to picture of a slice (tomo).
• CT eliminates superimposition of anatomical structures through 3D slices.

Magnetic Resonance Imaging (MRI)

• Places the a patient magnetic field to utilize resonance properties of the proton in hydrogen nuclei of water molecules within the body.
• Coils generate a pulse of radio waves and the protons then absorb and re-emit
  • The radio waves emitted by the protons in the patient are detected by the coils

Comparing Medical Imaging Techniques

• X-Rays
  • Detects fractures and deformities, but not details in soft tissue
• CT scans
  • Cross-sectional images of bones and tissues
  • Help diagnose complex fractures
• MRIs
  • Images soft tissues such as tendons and ligaments
  • Does not show the bone, but helps diagnoses soft tissue problems

Questions covering X-ray interactions, attenuation, energy, and machine operation. These questions cover topics such as linear attenuation coefficient, energy calculation, types of X-ray interactions, transmitted intensity, and the concept of half-value thickness. Additional topics covered include kVp ranges in mammography, the need for high power in X-ray tubes, and the effects of tube current (mA) and voltage (kV) on X-ray production.