Radiation Protection: Principles, lonizing Radiation, Biological Effects, Measurement, and Units
Radiation Protection Principles and Practice
Benefit vs. Risk in Medical Applications: In medical settings, exposure to ionizing radiation presents both advantages (diagnostic information for patient management from X-ray examinations) and disadvantages (a small but definite risk). The diagnostic benefit often significantly outweighs the associated X-ray exposure risk, providing essential information for modern medical practice.
Radiographer's Responsibility: Radiographers are responsible for ensuring that each patient receives the minimal radiation dose necessary to produce a diagnostic image, consistent with examination requirements. They must also protect all individuals from unnecessary radiation exposure.
The Nature of lonizing Radiation
Definition: Ionizing radiation creates positively and negatively charged particles upon interaction with matter.
Sources: It originates from both natural and human-made sources.
Mechanism of Harm: Its energy can displace atomic electron bonds and break molecular electron bonds, leading to chemical changes. These chemical changes can cause metabolic alterations in the body, resulting in harmful biological effects.
Classification: Ionizing radiation is categorized into two main groups:
Particulate Radiation: High-energy particles that produce ionization via direct atomic collisions.
Types: Includes high-energy electrons, neutrons, and protons.
Alpha Particles:
Associated with radioactive decay.
Composed of $2$ protons and $2$ neutrons, making them equivalent to a helium nucleus.
Emitted from the nuclei of very heavy elements undergoing radioactive decay.
Characterized by great mass and a positive charge.
Transfer energy over a very short range; for instance, they can travel approximately $5 \text{ cm}$ in air.
From external sources, alpha particles are essentially harmless due to their short range.
Beta Particles:
Associated with radioactive decay.
Identical to electrons but originate from the nuclei of radioactive material (whereas electrons orbit the nucleus).
Are negatively charged and very light.
Travel farther in matter than alpha particles, approximately $10 \text{-} 100 \text{ cm}$ in air.
Electromagnetic Radiation: Consists of x-rays and gamma rays, which produce ionization through different interaction types.
X-rays and Gamma Rays: Are fundamentally the same but differ in origin.
Gamma rays are emitted by the nuclei of radioactive materials.
X-rays are human-made in an x-ray tube.
Energy Transfer in Matter (for medically useful x-ray energy range <200 \text{ keV}):
Involves processes like photoelectric absorption and Compton scattering.
This is a two-step process:
An incident photon interacts with an atom, setting an electron in motion and releasing kinetic energy in the material.
The released kinetic energy from the electron is then absorbed through subsequent excitation and ionization events.
Biological Damage:
Excitations and ionizations occurring along the tracks of these charged particles cause biological damage in tissue.
Direct Interaction: Cellular macromolecules are directly excited or ionized by the charged particles set in motion.
Indirect Interaction: Radiation is absorbed by water molecules, producing highly reactive species such as free radicals. These free radicals diffuse from their origin site and subsequently cause biological damage.
For x-rays and gamma rays, roughly two-thirds of the biological effects on tissue result from indirect actions.
Biological Effects of lonizing Radiation Exposure
Consequences: Exposure to ionizing radiation affects various organs and tissues, carrying a finite probability of radiation-induced disease in exposed individuals and their descendants.
Influencing Factors: Health effects are influenced by radiation characteristics and biological factors.
Health Effects Include: Cancer induction, genetically determined ill health, nonspecific life shortening, developmental abnormalities, and degenerative diseases.
Classification of Effects:
Somatic Effects:
Become evident in the irradiated individual.
Are generally not expected in individuals exposed occupationally in the medical environment.
Require a quite high dose to demonstrate a human radiation response within a few days to weeks.
Examples: Skin erythema, cataracts, and radiation-induced malignancies.
Genetic Effects:
Do not produce a significantly observable effect in the exposed individual.
May appear in the descendants of the exposed individual.
Can lie dormant for several generations and may even be eliminated completely from the genetic pool.
Result from alterations in reproductive cells, potentially leading to defects in offspring.
Detectable radiation-induced mutations can occur if reproductive cells are exposed to an appreciable amount of radiation.
It is improbable for a medical worker to be exposed to a level high enough to cause appreciable genetic effects transmission.
Factors Influencing the Effect of Exposure: Total dose received, the rate at which the dose was received, age at exposure, the type of radiation, the sensitivity of the irradiated cells, and the portion of the body that was irradiated.
History of Radiation Protection
Early Discoveries and Injuries: Early scientists were unaware of the harmful effects of x-rays, leading to injuries among initial radiation workers. Injuries were reported in Europe within the first year of x-radiation investigation. The first radiation fatality in the United States, a radiation-induced cancer, was reported in $1904$.
Increasing Awareness: Over the next decade, blood disorders (leukemia, anemia), cancerous skin lesions, and cancer deaths were reported among radiation workers and unequivocally linked to x-ray exposure.
Formation of Protection Organizations:
In $1921$, the British X-Ray and Radium Protection Committee was formed to study methods for reducing radiation exposure to patients and medical personnel.
The International Commission on Radiological Protection (ICRP) was established in $1928$.
In $1929$, the Advisory Committee on X-Ray and Radium Protection evolved into the National Council on Radiation Protection and Measurements (NCRP) in the United States.
Goal of Radiation Protection: To limit human exposure to ionizing radiation to a degree that is reasonable and acceptable in relation to the benefits gained from activities involving exposure, thereby reducing the likelihood of somatic and genetic effects.
Sources and Magnitude of lonizing Radiation Exposure
Universal Exposure: Everyone is exposed to sources of ionizing radiation, with varying degrees of exposure to different sources.
Source Categories:
Natural Origin: Sources undisturbed by human activities or enhanced by them.
Human-Made Sources.
NCRP Data Collection: The NCRP regularly collects data to assess ionizing radiation exposure in the U.S. population.
NCRP Report No. $160$ (released in $2009$) superseded NCRP Report No. $93$ (released in $1987$).
Report No. $93$ detailed radiation exposure in the early $1980\text{s}$, while Report No. $160$ provided data from $2006$.
Five Major Areas of Radiation Exposure (NCRP Report No. $160$):
Exposure from ubiquitous background radiation, including radon in the home.
Exposure to patients from medical procedures.
Exposure from consumer products or activities involving radiation sources.
Exposure from industrial, security, medical, educational, and research radiation sources.
Exposure to workers resulting from their occupations.
Ubiquitous Background Radiation:
External Sources: Space (solar particles, cosmic radiation) and terrestrial radiation (naturally occurring radioactive sources in the ground).
Internal Sources: Radionuclides naturally present in the body and inhaled radionuclides of natural origin.
Examples of Inhaled Radionuclides: Radon () and Thoron ().
Human exposure varies based on locality and other circumstances.
Enhanced Natural Sources: Occur when human actions, deliberate or otherwise, increase exposure to natural radiation sources.
Human-Made Sources of Radiation:
Result from various human-made materials and devices.
Examples: X-rays and radiopharmaceuticals (for medical procedures); building materials, commercial air travel, cigarette smoking, and combustion of fossil fuels (consumer products/activities); industrial, security, medical, educational, and research activities; and occupational exposures.
Medical Procedures Exposure (Report No. $160$ further details five sources):
Computed Tomography (CT).
Conventional Radiography and Fluoroscopy.
Interventional Fluoroscopy.
Nuclear Medicine.
External-Beam Radiotherapy.
Statistical Data (2006 vs. Early 1980s):
Collective Effective Dose ($S$): The product of the mean effective dose for a population and the number of persons in the population (e.g., U.S. population was $300$ million in $2006$).
Total Effective Dose per Individual ($E_U$): Exhibited a significant increase from the early $1980\text{s}$ to $2006$, primarily due to increased use of ionizing radiation in medical procedures.
Early $1980\text{s}$: Total $E_U = 3.6 \text{ mSv}$.
$2006$: Total $E_U = 6.2 \text{ mSv}$.
This increase is almost exclusively attributed to medical procedure exposures, which rose from $0.53 \text{ mSv}$ in the early $1980\text{s}$ to $3.00 \text{ mSv}$ in $2006$.
This represents a percentage change from $15\%$ of the total in the early $1980\text{s}$ to $48\%$ of the total in $2006$.
Despite tremendous benefits and lives saved from increased medical use of ionizing radiation, there is heightened awareness and a focus on reducing this dose, particularly in computed tomography.
Quantities and Units Relevant To Radiation Protection
Purpose: Various quantities, units, and radiation dosimetry concepts are developed to quantify radiation received by individuals.
International System of Units (SI Units):
Developed in $1948$ by the International Committee for Weights and Measures.
Adopted by the NCRP in $1985$ for ionizing radiation. While traditional (conventional) units are still widely used, SI units are the international standard.
Exposure:
Conventional Unit: Roentgen ($R$).
Represents a unit of exposure in air.
Defined as the quantity of x-rays or gamma rays required to produce a given amount of ionization (charge) in a unit mass of air.
One roentgen creates $2.08 \times 10^9$ ion pairs per cubic centimeter of air, resulting in a total ion charge of $2.58 \times 10^{\text{-}4} \text{ coulomb (C)}$ per kilogram ($ ext{kg}$).
Limited to measuring exposure in air only and not applicable to photons with energy above $3 \text{ MeV}$ or to particulate radiations.
The roentgen has no direct SI equivalent and is no longer used.
SI Unit Equivalent: Expressed directly in $\text{C/kg}$ or its equivalent $\text{Ckg}^{\text{-}1}$.
Absorbed Dose:
Conventional Unit: Rad (radiation absorbed dose).
Developed as a unit of absorbed energy or dose, applicable to any material.
Defined as $100$ ergs of energy absorbed in $1$ gram of absorbing material.
SI Unit: Gray ($ ext{Gy}$).
Replaced the rad in the SI system.
Defined as $1$ joule ($ ext{J}$) of energy absorbed in each kilogram ($ ext{kg}$) of absorbing material.
Conversions: $1 \text{ Gy} = 100 \text{ rads}$ and $1 \text{ rad} = 10 \text{ mGy}$.
Not restricted to air and can be measured in other absorbing materials.
Kerma/Air Kerma:
Kerma: Acronym for Kinetic Energy Released in MAtter.
As radiation passes through matter, the energy carried by photons is transformed into kinetic energy of charged particles (e.g., electrons in photoelectric and Compton interactions).
Kerma is the energy imparted directly to these electrons, per unit mass.
If some of the kerma is radiated away as bremsstrahlung (e.g., if electrons interact with atomic nuclei), kerma and absorbed dose would not be identical. However, at diagnostic energies, minimal bremsstrahlung is produced in tissues, so kerma typically equals absorbed dose.
SI Unit: Gray ($ ext{Gy}$).
Air Kerma: The kinetic energy released per unit mass of air.
X-ray tube outputs and inputs to image receptors are sometimes described in air kerma.
An air kerma of $1 \text{ cGy}$ ($1 \text{ rad}$) corresponds to an exposure of approximately $1 \text{ R}$.
Integral Dose:
Describes the total amount of energy imparted to matter.
It is the product of the dose and the mass over which the energy is imparted.
Example: For a CT scan of the abdomen, if the dose per section (irradiated volume) is $1 \text{ rad}$ ($10 \text{ mGy}$), and the patient undergoes $20$ scan sections, the integral dose would be approximately $20 \text{ rad}$ ($200 \text{ mGy}$). The dose to any single irradiated volume remains $1 \text{ rad}$ ($10 \text{ mGy}$), but the total energy imparted over multiple sections accumulates.
Equivalent Dose ($H_T$):
Accounts for the fact that different types of radiation (e.g., alpha, beta particles, neutrons versus gamma or x-radiation) produce varying degrees of biological damage for the same absorbed dose.
Conventional Unit: Rem (radiation equivalent in man).
NCRP Report No. $116$: The term equivalent dose ($H_T$) replaced the previously used dose equivalent ($H$) in defining dose limits.
Dose equivalent ($H$) was based on absorbed dose at a "point" in tissue.
Equivalent dose ($H_T$) is based on the average absorbed dose in the entire tissue or organ.
Formula: $HT = DT \times W_R$
$D_T$: Average absorbed dose in a specific tissue ($T$) due to a radiation type ($R$).
$W_R$: Radiation weighting factor (formerly known as the quality factor $Q$). This factor is particular to specific types of radiation and accounts for their biological effectiveness.
For gamma or x-radiation, the radiation weighting factor ($W_R$) is $1$. This means $1 \text{ rad} = 1 \text{ rem}$.
For alpha particles, the radiation weighting factor ($W_R$) is $20$. This means $1 \text{ rad} = 20 \text{ rem}$ for alpha particle absorption.
SI Unit: Sievert ($ ext{Sv}$).
Replaced the rem in the SI system.
Defined as the product of the absorbed dose in gray and the radiation weighting factor.
Conversions: $1 \text{ Sv} = 100 \text{ rem}$ and $1 \text{ rem} = 10 \text{ mSv}$.
Effective Dose ($E$):
Represents the sum of the weighted equivalent doses for all irradiated tissues and organs.
It considers that not all tissues are equally sensitive to the effects of ionizing radiation.
NCRP Report No. $116$: The term effective dose ($E$) replaced the effective dose equivalent ($H_E$) in defining dose limits.
Purpose: Used to compare the detriment from irradiating a limited portion of the body with the detriment from irradiating the entire body.
Employs weighting factors for the relative risks associated with the irradiation of various body tissues.
Definition: The sum, over specified tissues, of the products of the equivalent dose in a tissue ($T$) and the weighting factor for that tissue.
Activity ($A$):
Describes the quantity of radioactive material.
Expressed as the number of radioactive atoms that undergo decay per unit time (disintegrations per second).
Conventional Unit: Curie ($ ext{Ci}$).
Defined as $3.7 \times 10^{10}$ disintegrations per second ($ ext{dps}$).
One curie is a very large amount of radioactive material. Typical nuclear medicine procedures use activities ranging from $0.1$ to $30 \text{ mCi}$.
SI Unit: Becquerel ($ ext{Bq}$).
Defined as $1 \text{ dps}$.
For further information on SI units in radiation protection, refer to NCRP Report No. $82$.
Detection and Measurement of lonizing Radiation
Dosimeters: Devices employed for detecting and measuring radiation exposure from x-rays.
Classification: Dosimeters are classified as either field survey instruments or personnel monitoring devices.
Field Survey Instruments
Portable instruments used for radiation detection and measurement.
Should be calibrated at least annually.
Types:
Geiger-Mueller (GM) Survey Instruments/Counters:
Primarily used to detect the presence of radiation rather than providing exact measurements.
A gas-filled detector consisting of a gas volume between two electrodes. Ionizing radiation produces ion pairs in the gas that are collected and measured.
Very efficient at detecting charged particles (e.g., beta particles) but relatively inefficient at detecting x- and gamma radiations.
Most commonly used in nuclear medicine as radioactive contamination survey instruments.
Scintillation Detection Devices:
Combine a scintillator (material that emits visible or ultraviolet light when exposed to ionizing radiation) with a device that converts light into an electric signal.
This electric signal is then measured.
Most common applications are in gamma cameras used in nuclear medicine.
Ionization Chamber Instruments:
Commonly used for measuring the primary and secondary radiation beams, evaluating equipment performance, assessing environmental scatter and leakage radiation, and measuring patient exposure.
Principle: When radiation interacts with air electrons, positive ions are produced, generating an electrical charge that can be measured.
Components: A typical ionization chamber dosimeter comprises a chamber with a known volume of air and an electrode. A small voltage is applied between the electrode (positive) and the chamber wall (negative).
Mechanism: As x-ray photons pass through the chamber, they ionize the air. The free electrons are attracted to the positive electrode and measured by an electrometer. The intensity of the signal is proportional to the radiation exposure in the air volume.
Readings can be displayed in $R$ or $\text{C/kg}$ units.
Can be designed to measure specific ranges of radiation intensity.
Personnel Monitoring Devices
Purpose: Supplied to individuals regularly exposed to ionizing radiation to estimate their received exposure. They are not radiation protection devices.
Recommendation: Recommended when there is a possibility an individual could receive more than $1/10$ of the recommended dose limit due to occupational activities.
Wearing Location:
If only one device is issued, it should be worn on the anterior surface of the body, between the chest and waist level.
When a lead apron is worn and only one device is issued, it should be worn outside the apron at the collar level.
Consult the local radiation safety officer for specific guidance due to historical controversy regarding placement with lead aprons.
Consistency in dosimeter wearing location among all personnel is crucial.
An additional "whole-body" dosimeter (usually positioned near the waist, under the lead apron) may be worn along with a collar dosimeter if there's significant potential for thyroid or eye exposure. This combination can help determine the effective dose. These two dosimeters must never be interchanged.
Most Common Types:
Optically Stimulated Luminescence (OSL) Dosimeter:
The most common type of radiation monitoring device.
Measures radiation passing through a thin strip of aluminum oxide ().
A laser light stimulates the aluminum oxide, causing it to luminesce proportionally to the received radiation exposure.
Distinct Advantages (over film badge and TLD):
Can report doses over a wide range, from as low as $1 \text{ mrem}$ with a precision of $\pm 1 \text{ mrem}$.
Allows for complete reanalysis to confirm exposure without information loss.
Offers excellent long-term and environmental (temperature, humidity) stability.
Luxel® OSL Dosimeter (Landauer, Inc.): A self-contained packet containing an aluminum oxide strip, positioned between three filters and an open window.
The open window and the copper and tin filters are used to determine the energy levels of the exposure.
An imaging filter helps determine if the exposure was static (dosimeter not moving, potentially implying it wasn't worn or an accidental exposure occurred) or dynamic (blurred appearance, indicating it was worn during exposure).
Film Badge Dosimeter:
Usually issued monthly.
Consists of two pieces of film with different sensitivities to x-rays, enclosed in a light-tight envelope.
This film packet is placed in a holder containing various filter elements (typically copper, cadmium, and aluminum) fixed relative to the film.
Information on the film packet includes the individual's name, issue date, and other coded data.
Exposure to ionizing radiation darkens the film emulsion proportionally to the dose.
The resultant optical density is measured with a densitometer and calibrated to the radiation exposure.
The filters in the holder allow for determining the energy of the incident radiation, as absorption by a material varies with radiation energy.
Capable of measuring exposures from approximately $10 \text{ mrem}$ ($0.1 \text{ mSv}$) to $2,000 \text{ rem}$ ($20 \text{ Sv}$).
Most commonly used for measuring total body exposure. Readings less than $10 \text{ mrem}$ ($0.1 \text{ mSv}$) are generally undetectable and reported as "minimal" (M).
Thermoluminescent Dosimeter (TLD):
Uses small chips of a thermoluminescent material, typically lithium fluoride ($\text{LiF}$).
When exposed to radiation, a portion of the absorbed energy is stored in the crystal structure of the $\text{LiF}$ chips in metastable states, where it can remain for long periods.
Heating the $\text{LiF}$ chips causes the stored energy to be released as visible light.
This heating and measurement are performed in a device called a reader, and the amount of light measured is proportional to the absorbed radiation dose.
TLDs offer a similar measurement range to film badges and can be used as whole-body or collar badges.
Due to their small size, TLDs are frequently used for monitoring extremity exposure (e.g., as ring badges).
Pocket Dosimeter:
Includes older "direct reading conductive fiber electroscopes" and newer "electronic personal dosimeters."
Both types resemble large pens or flash drives.
Provide immediate personal dosimetry, which is particularly useful in environments like interventional radiology laboratories where delayed readings are impractical.
Older Electroscopes: When irradiated, air ionization in a small chamber partially neutralizes a positively charged electrode (a quartz fiber on a wire frame), causing the fiber to move on an exposure scale. The ionization amount and fiber movement are proportional to the radiation exposure. These required a separate charging unit for reading.
Newer Electronic Personal Dosimeters: Utilize a similar air ionization system but provide digital readouts on the side of the dosimeter.
Care of Personnel Monitoring Devices:
Proper care and handling are essential for accurate results.
Must only be worn by the assigned individual, in the correct location, for the prescribed period, and returned for processing on time.
OSL dosimeters are less susceptible to adverse conditions due to their stability, precision, and wide dose range.
Film badge and TLD dosimeters can be adversely affected by heat, humidity, mechanical pressure, inadvertent light exposure, and prolonged delays between exposure and processing.
Exposure from the rear or oblique angles can also lead to inaccurate results.
A control dosimeter (unexposed) is typically provided with a group of personnel dosimeters. It must be stored in an unexposed area at the facility and returned with the group for processing to provide a baseline for evaluation.
Personnel monitoring devices should never be worn during medical or dental examinations, nor should they be intentionally exposed to the primary radiation beam.
Individuals should regularly review their exposure readings on facility-posted reports and consult the facility radiation safety officer if any aspect of the report is unclear or causes concern.
Dosimetry Reporting
Processing: After each wear period (which can range from weekly to quarterly), dosimeters are returned to a laboratory for analysis.
Report Contents (example for Luxel® OSL dosimeters from Landauer, Inc.):
Participant identification information.
Type of dosimeter used.
Radiation quality that exposed the dosimeter (e.g., x or gamma photons, beta, neutrons).
Radiation exposures, expressed as dose equivalents in millirem, for:
The current wear period.
A quarterly accumulated dose equivalent.
A year-to-date dose equivalent.
A lifetime dose equivalent for each participant.
For each exposure column, specific dose equivalents are provided:
Deep-dose equivalent: Applies to external whole-body exposure at a tissue depth of $1 \text{ cm}$.
Eye dose equivalent: Applies to the external lens of the eye at a tissue depth of $0.3 \text{ cm}$.
Shallow-dose equivalent: Applies to the external exposure of the skin at a tissue depth of $0.007 \text{ cm}$.
Participants should diligently check their dosimetry report for each wear period.