Radiation Physics and Radiobiology Review Notes

Radiation Physics and Radiobiology

A. Principle of Radiation Physics

1. X-ray Production

• Source of Free Electrons

  • X-rays are produced in the x-ray tube at the cathode filament through a process called thermionic emission.

  • Thermionic Emission: The release of electrons in response to heat.

• Formation of Electron Cloud

  • When the filament gets extremely hot, it generates a cloud of electrons.

• Role of Kilovoltage Peak (kVp)

  • Creates a strong negative charge at the cathode and accelerates electrons toward the positively charged anode.

• Energy Release at Anode

  • Upon hitting the anode, electrons decelerate and release energy both as heat and x-rays.

• Factors Affecting X-ray Production

  • mA and Exposure Time: Directly related to the number of x-ray photons produced.

  • Increased kVp: Results in increased x-ray energy and an increased number of x-rays generated.

2. Target Interactions

• Requirements for X-ray Creation

  • Source of Electrons: Electrons generated at the cathode.

  • Acceleration of Electrons: By kVp towards the anode.

  • Deceleration of Electrons: When electrons hit the anode, they lose energy by producing x-rays.

• Bremsstrahlung

  • A German term meaning “braking radiation.”

  • Interaction: Fast-moving electrons approach a positive nucleus (usually tungsten).

  • Deceleration: Electrostatic attraction slows down the electron and bends its path.

  • Emission: Sudden loss of kinetic energy emitted as an x-ray photon.

  • The maximum energy of emitted photons corresponds to the set kVp, although actual energy varies based on proximity to the nucleus.

• Characteristic Interaction

  • An inner shell electron is ejected following a collision with another electron.

  • This process creates a vacancy in the inner shell, which is filled by an outer shell electron, emitting a photon in the process.

  • This is described as a cascade effect.

  • To calculate the energy of emitted characteristic photons, observe the difference in binding energies of involved shells:

  • K-shell = 69.5 kEv

  • L-shell = 12 kEv

  • M-shell = 3 kEv

  • N-shell = 0.5 kEv

  • Increased kVp leads to more characteristic x-rays but does not increase their energy.

  • No K-shell characteristic photons will be generated if kVp is below 69.5 since it won’t have the energy to eject an electron.

3. X-ray Beam Characteristics

• Wavelength: Distance measurement between peaks of waves; measured in nanometers (nm).

• Frequency: Number of wavelengths per second; measured in hertz (Hz).

• Relation between Wavelength and Energy: Decreased wavelength corresponds to increased frequency and energy.

Beam Characteristics Defined

• Quantity: Number of x-rays in the beam, also referred to as beam intensity.

  • Measured in units of coulombs/kg (C/kg) or air kerma in gray (Gy).

  • Determined by kVp, mAs, and exposure time.

• Quality: Average energy of the x-ray beam, dependent on kVp and filtration.

  • High kVp means more penetrability and information but also increases scatter and dose.

• Isotropically: Means x-rays diverge equally in all directions, traveling in straight lines unless obstructed.

• Electromagnetic Spectrum: Range of photon energies including radiowaves, microwaves, visible light, x-rays, and gamma rays.

• Photon Speed: Photons travel at the speed of light.

• Attenuation: Reduction in x-ray intensity due to absorption and scatter in matter.

• Primary Beam: Original intensity before attenuation.

• Exit/Remnant Beam: Intensity after passing through the patient.

4. Inverse Square Law

• Formula: I_1/I_2 = D_2^2/D_1^2

  • Used to compare intensity (quantity) of x-rays relative to distance from the source.

B. Photon Interactions with Matter

1. Photoelectric Absorption

• An x-ray photon is fully absorbed by an inner shell electron resulting in ionization and producing a photoelectron.

• The ejected electron leads to a cascade effect where an outer shell electron fills the vacancy, emitting a characteristic photon.

• Consequences: Increases patient dose but enhances image contrast since scattered photons don’t contribute to the image.

2. Compton Scatter

• An x-ray photon is partially absorbed by an outer shell electron, leading to the ejection of that electron and the emission of a lower-energy scattered photon.

• Results in biological damage and contributes to increased patient dose due to the scattered photon also being absorbed in tissues.

• Compton scatter is a significant source of occupational dose, as the produced scatter reduces image contrast.

3. Coherent Scattering

• An x-ray photon temporarily absorbed by the entire atom is immediately re-emitted in a different direction as a scatter photon with the same energy.

• Known as Thomson scattering, classical scattering, elastic scattering, and Rayleigh scattering.

• Effect on Dose: Coherent scattering does not affect patient or occupational dose, though it does reduce image quality due to increased noise.

4. Attenuation by Various Tissues

• An x-ray beam can either be absorbed, fully transmitted, or scattered when passing through matter.

• Factors Influencing Attenuation:

  • Increased part thickness leads to increased attenuation due to more atomic interactions.

  • Higher density body parts lead to greater attenuation.

  • Higher atomic number materials (e.g., bone) result in greater attenuation due to increased electron interactions.

  • Lower kVp increases attenuation since the x-ray photons will have insufficient energy to penetrate the matter.

Biological Effects of Radiation

1. SI Units of Measurement

• Absorbed Dose (Gy): Measures energy absorbed per unit mass; calculated from photoelectric absorption and Compton scattering.

  • 1 Gray = 1 Joule/Kilogram (J/kg).

  • Short-term injuries (e.g., skin erythema, epilation) can be predicted based on absorbed dose.

• Dose Equivalent (Sv): Measures biological damage across different radiation types, using the formula: D imes W_r (where $D$ is absorbed dose and $W_r$ is the weighting factor).

• Exposure (C/kg): Measures ionizations in air, correlating to the number of x-ray photons in a beam.

• Effective Dose (Sv): Evaluates risk for long-term radiation effects, using: D imes W_r imes W_t (where $W_t$ is the tissue weighting factor).

• Air Kerma (Gy): Represents energy of ionizations in air; KERMA means Kinetic Energy Released per unit of Mass.

2. Radiosensitivity

• Definition: The susceptibility of a cell, tissue, organ, or organism to radiation damage.

• Factors Affecting Radiosensitivity:

  • Age: Children are more radiosensitive due to immature cells and inability to repair damage.

  • Children are approximately 10 times more radiosensitive than adults.

  • Tissue Sensitivity: Tissues likely to develop cancer include lungs, breast, gonads, and bone marrow.

  • Tissue Weighting Factor ($W_t$): Used to calculate effective dose based on tissue sensitivity.

  • Women have a higher susceptibility to radiation-induced lethality than men due to higher volume of reproductive tissues.

  • Radiation Types: Alpha radiation is significantly more damaging than X-rays.

  • Dose Rates: A large single dose is generally more harmful than the same dose spread out over time.

• Types of Ionizing Radiation: X-ray photons, beta particles, protons, neutrons, alpha particles — with varied sensitivity.

• Linear Energy Transfer (LET): Average energy deposit per unit distance; expressed in keV/μm.

  • X-rays have a low LET leading to widespread, less concentrated biological damage.

• Relative Biological Effectiveness (RBE): Effectiveness in causing biological damage is greater at high LET.

• Radiation Weighting Factor ($W_r$): Indicates potential biological harm from radiation exposure.

  • e.g., Protons = 5.

• Oxygen Enhancement Ratio (OER): Indicates increased biological harm of radiation in oxygenated cells compared to hypoxic cells.

  • OER for X-rays = 3.

• Law of Bergonie and Tribondeau: Most radiosensitive cells are immature, unspecialized, and multiply rapidly.

• Lethal Dose (LD50/60): Average dose causing death in 50% of a population within 60 days is 3-4 Gy for humans.

3. Somatic and Genetic Effects

• Somatic Effects: Effects on the irradiated body (soma means body).

  • Genetic Effects: Effects that manifest in future generations due to germ cell irradiation.

• Classification of Somatic Effects:

  • Early (deterministic): Symptoms manifest soon after exposure (e.g., skin burns, hair loss, diminished sperm count).

  • Late (stochastic): Cancer and cataracts manifest years after exposure; latency period varies based on dose.

• Deterministic Effects: Occur after surpassing a threshold dose; examples include:

  • Skin erythema begins at doses of 2 Gy or more.

  • May also involve acute radiation syndrome (ARS) under extreme exposure conditions.

• Stochastic Effects: Observed at any dose, with probabilities of occurrence increasing with dose (e.g., cancer).

• Target Theory: Adverse effects observed when sensitive target molecules (specifically DNA) are affected by radiation.

• Radiolysis: Ionization of cellular water leading to production of free radicals which can damage DNA.

• Direct Action vs. Indirect Action:

  • Direct action occurs when radiation directly interacts with DNA (rare), while indirect involves water ionization causing chemical changes resulting in DNA damage.

• Biological Effects from Radiation:

  • Base pair lesions are the most repairable, whereas double-strand breaks can cause carcinogenesis due to unrepaired genetic material.

• Teratogenic Effects: Negative effects from radiation exposure during pregnancy, including miscarriages and malformations.

  • Pre-implantation Stage: High susceptibility leading to lethal effects or normal development (all or nothing effect).

  • Organogenesis Stage: Risk of physical malformations and growth retardation at thresholds of about 100 mGy.

  • Fetal Period: Cerebral effects most likely manifest, potentially leading to developmental disorders.

1. Acute Radiation Syndrome (ARS):

• Significant and deterministic effects resulting from whole-body exposure to high doses of radiation.

  • Phases:

  • Prodromal Phase: Early symptoms, such as nausea and vomiting, appear soon after exposure.

  • Latent Phase: Initial reactions subside and may appear normal before full illness manifests.

  • Manifest Illness: Full-scale illness occurs, with severity related to the dose.

  • Syndromes:

  • Hematopoietic Syndrome: Bone marrow destruction leads to reduced blood cell production.

  • Gastrointestinal Syndrome: Impacts digestive tract function leading to dehydration and electrolyte imbalance.

  • Cerebrovascular Syndrome: Damage to the brain's blood vessels leading to rapid deterioration and death.

• Carcinogenic Effects: Most likely adverse effect to occur in medical imaging, though unlikely.

  • Three outcomes of cellular radiation exposure: repair, cell death, or DNA damage leading to mutation/cancer.

• Linear Non-threshold Model (LNT): Predicts risk of radiation-induced cancer is linear, with any dose above zero increasing cancer risk.

Radiation Protection

A. Minimizing Patient Exposure

1. Exposure Factors

• kVp (Kilovoltage Peak):

  • Maximum voltage that accelerates electrons in the x-ray tube; increasing kVp enhances x-ray energy and quantity.

  • 15% Rule: Increasing kVp by 15% doubles receptor exposure; decreasing it by 15% halves receptor exposure.

  • Changes in kVp: Impact beam quality, quantity, patient dose, and receptor exposure.

• mA (Milliamperage):

  • Represents electron flow in the x-ray tube; increased mA results in more x-rays produced.

  • Relation to exposure time: mAs = mA imes s.

• Automatic Exposure Control (AEC): Regulates exposure time based on radiation reaching the receptor, reducing the risk of over-exposure.

  • The system includes an ionization chamber between the patient and detector, shutting off exposure after reaching maximum radiation levels.

  • Backup timer of 5 seconds prevents overexposure in case of a positioning issue.

2. Beam Restriction

• Purpose: To protect patients by limiting the radiation area.

• Types of Beam Restrictors:

  • Collimators: Variable aperture devices that include lead shutters to control beam size and minimize off-focus radiation.

  • Cylinder Cones: Metal cylinders that restrict the beam to a small circle; extended cones may be used for specific imaging applications.

  • Aperture Diaphragm: A flat plate of lead with an opening placed as close to the tube window as possible.

3. Patient Considerations

• Positioning: Utilize appropriate kVp and the lowest possible mAs, avoiding unnecessary radiation to sensitive regions.

• Communication: Clearly explain procedures to patients, tailoring communication for pediatric and geriatric populations to provide reassurance and understanding.

4. Filtration

• Purpose: To reduce patient dose by removing low-energy photons from the x-ray beam.

• Types of Filtration:

  • Inherent Filtration: Built into the x-ray tube (glass envelope, insulating oil).

  • Added Filtration: Additional aluminum plates.

• Total Filtration Requirements: At least 2.5 mm equivalent aluminum for x-ray tubes operating above 70 kVp.

5. Radiographic Dose Documentation

• Whole-body yearly exposure limit for professionals = 50 mSv.

• Public exposure annual limit = 5 mSv.

• Lens of the eye threshold = 150 mSv.

• Student radiographer exposure limit = 1 mSv.

• Skin & Extremities exposure limit = 500 mSv.

• Total fetal exposure limit = 5 mSv;

• Monthly fetal exposure limit = 0.5 mSv (10-month pregnancy).

6. Image Receptors

• Digital Radiography (DR) reduces patient dose compared to Computed Radiography (CR) or film, allowing lower mAs values.

7. Grids

• Function: To prevent scatter radiation from reaching the image receptor.

• Grid Construction: Thin lead strips; photons can be absorbed, transmitted, or scattered.

• Grid Ratio: Height of lead strips divided by the distance between them.

• Use grids for body parts exceeding 10 cm (2.5 inches).

• Grid Types: Parallel vs. Focused (angling of strips), and Linear vs. Crossed.

• Grid Errors: Classified by off-level, off-center, off-focus errors, leading to reduced image quality or exposure.

8. Fluoroscopy

• Pulsed Mode: Reduces patient dose and motion.

• Factors Influencing Quality: Include contrast, distortion, resolution, and quantum mottle.

• Timers: An audible alarm indicates 5 minutes of fluoroscopy usage.

• Personnel Safety: Minimum source-to-skin distance guidelines to minimize dose during procedures, with adjustments made for optimal positioning to reduce exposure.

B. Personnel Protection (ALARA)

1. Sources of Radiation Exposure

• Primary X-ray Beam: Largest amount of scatter at x-ray entry; keep the source underneath the patient during procedures.

• Secondary Radiation: Minimal scatter observed at a 90-degree angle to the patient.

2. Basic Protection Methods

• Time: Minimize time in the fluoroscopy suite; consider pulsed/intermittent fluoroscopy.

• Distance: Maximize distance from the source; employ remote controls where feasible.

• Shielding: Utilize lead equivalent materials for protection.

3. Protective Devices

• Types:

  • Lead equivalent aprons, lead curtains, lead barriers, and bucky slot covers.

  • Minimum lead equivalent standards (NCRP): Primary barriers = 1/16 inch Pb; secondary barriers = 1/32 inch Pb.

4. Special Considerations for Mobile Units and Fluoroscopy

• Guidelines: Ensure regulatory compliance for shielding and protective measures during mobile and fluoroscopy setups.

5. Radiation Exposure and Monitoring

• Monitoring Devices: Use dosimeters to track occupational exposure levels; personal monitoring systems should be employed for applicable staff.

• Types of Dosimeters: Include thermoluminescent ring dosimeters (TLDs) and optically stimulated luminescence devices (OSLs).

6. Handling and Disposal of Radioactive Material

• Safety Measures: Enforce PPE usage, proper waste segregation, labeling, and documentation protocols.

• Disposal Methods: Vary based on the radioactive properties and half-lives, with protocols for decay-in-storage and authorized personnel management.

Key Considerations for Handling Radioactive Materials

• Documentation of all activities and disposal must align with established protocols to ensure compliance and safety in handling radioactive waste.