Radiation Measurement and Interactions

• Radiation measurement units include Gray (Gya) for air kerma, Gray (Gyt) for tissue absorbed dose, Sievert (Sv) for effective/equivalent dose, and Becquerel (Bq) for radioactivity.

Ionizing Radiation

• Removes electrons from atoms, which is called ionization.
• Ionization can lead to unstable atoms, free electrons, or harmful new molecules.
• Cell damage can be somatic (to the cell itself) or genetic (to the cell's DNA).
• Natural background radiation is always present, with radon being the largest source.
• Human-produced radiation comes from activities or inventions like medical imaging.
• CT scans have significantly increased the total and medical radiation dose to the population.
• The total radiation dose to the U.S. population has doubled since the 1980s.

Photon-Tissue Interactions

• Attenuation refers to changes in the x-ray beam's intensity as it passes through the patient.
• Photoelectric interaction involves complete x-ray photon absorption, creating image contrast.
• Compton interaction results in photon scattering, which must be removed before reaching the image receptor.
• Compton scatter is the primary exposure source for radiographers/radiologists during fluoroscopy.

Annual Dose Limits

• Annual dose limits are set by the NCRP.
• The effective dose limit is the maximum dose allowed annually with negligible risk of damage.
• ALARA (as low as reasonably achievable) guides radiographers to minimize doses.
• The linear-nonthreshold relationship assumes no radiation level is completely safe and response is proportional to dose.
• Stochastic effects are random radiation effects with probability proportional to dose, not severity.
• Tissue reactions (deterministic effects) increase in severity above a threshold dose.
• NCRP Report #116 provides annual dose limit recommendations.
• Occupational annual effective dose limit: 50 mSv.
• Cumulative effective dose limit: age (years) × 10 mSv.
• General public annual effective dose limit: 1 mSv (frequent) or 5 mSv (infrequent).
• Embryo/fetus effective dose limit for gestation: 5.0 mSv.

Cell

• Main parts: cell membrane, cytoplasm, and nucleus.
• The nucleus contains DNA.
• The cytoplasm contains organelles and water.
• Interphase is the period before mitosis in the cellular life cycle.
• Mitosis is somatic cell division, including prophase, metaphase, anaphase, and telophase.
• After mitosis, each new cell has 46 chromosomes.
• Meiosis is germ cell division that halves the chromosome number, so that the union of two germ cells produces a new cell with 46 chromosomes.
• Linear Energy Transfer (LET) is the energy deposited per unit length of radiation travel.
• Increased LET increases the potential for biological damage.
• Relative Biological Effectiveness (RBE) is the radiation's ability to cause biological damage, varying with LET.
• Direct effect occurs when radiation directly impacts DNA or RNA.
• Mutation is the passing of incorrect information to future generations through cell division.
• Indirect effect occurs when radiation energizes water in the cytoplasm, causing radiolysis and producing free radicals or H2O2.
• Law of Bergonié and Tribondeau states that immature, undifferentiated, rapidly dividing cells are most radiosensitive.
• Oxygen Enhancement Ratio (OER) says that more oxygenated cells are more susceptible to radiation damage.
• As cells mature and specialize, they become less radiation-sensitive.
• Lymphocytes are the most radiosensitive blood cells.
• Bone marrow stem cells are especially radiosensitive.
• Epithelial tissue is highly radiosensitive.
• Muscle is relatively insensitive to radiation.
• Adult nerve tissue is relatively insensitive, requiring very high doses for damage.
• Immature sperm cells are very radiosensitive.
• Ova in female fetuses/children are very radiosensitive.
• Ova radiosensitivity decreases until middle age, then increases.
• Somatic effects occur in the exposed individual.
• Early tissue effects include hematopoietic, GI, and central nervous system syndromes, erythema, epilation, and decreased blood count.
• Late tissue effects include cataractogenesis and embryologic effects.
• Stochastic effects include carcinogenesis and genetic effects.
• Genetic effects manifest in future generations due to DNA damage, following a linear-nonthreshold curve.
• Doubling dose is the radiation amount that doubles the mutation number in a population and is estimated at 1.56 Sv for humans.

Patient Exposure and Protection

• Always adhere to ALARA.
• Beam limiters include collimators, cylinder cones, and aperture diaphragms.
• Positive Beam Limitation (PBL) is automatic collimation.
• Beam filtration removes long-wavelength rays; total filtration must be ≥ 2.5-mm aluminum equivalent.
• Exposure factors: use optimum kVp and lowest practical mAs.
• Grids remove scatter radiation, increasing patient dose due to increased mAs.
• Maintain a minimum 12-inch source-to-skin distance for portable radiography.
• Use intermittent fluoroscopy.
• Maintain a minimum 15-inch source-to-tabletop distance for fixed fluoroscopes.
• Maintain a minimum 12-inch source-to-tabletop distance for portable fluoroscopes (15 inches preferred).
• Fluoroscopy timer must alarm after 5 minutes (300 seconds) of beam-on time.
• Fluoroscopy foot switch must be the dead-man type.
• Fluoroscopy dose at the tabletop should be ≤ 100 mGya/minute.
• MMD is the average dose to active bone marrow.
• Cardinal principles: time, distance, shielding; distance provides the best protection.
• Dose is governed by the inverse square law.
• Lead aprons must be ≥ 0.25-mm lead equivalent (preferably ≥ 0.5-mm).
• The radiographer must never be exposed to the primary beam.
• Scatter radiation from Compton interactions in patients during fluoroscopy, portable radiography, and surgical radiography is the main source of exposure to the radiographer.
• Scattered beam intensity is about 1/1000 the primary beam's intensity at a 90-degree angle, 1 m from the patient.
• Beam collimation reduces Compton interactions, decreasing scatter from the patient.
• Primary protective barriers must be at least 1/16-inch lead equivalent and extend from the floor to 7 feet high.
• Secondary protective barriers must be at least 1/32-inch lead equivalent, extending from the primary barrier to the ceiling, overlapping by ½ inch.
• Barrier thickness depends on distance, occupancy, workload, and use.
• Uncontrolled area: general public areas.
• Controlled area: for trained personnel wearing monitoring devices.
• X-ray tube leakage: ≤ 1 mGya/hour at 1 m from the housing.
• Fluoroscopic protective curtain: ≥ 0.25-mm lead equivalent.
• Bucky slot shield: ≥ 0.25-mm lead equivalent.
• Portable x-ray machine exposure switch must have a cord ≥ 6 feet long.
• OSL dosimeters: aluminum oxide layer stores energy, released by laser, correlating to dose; reads as low as 10 μGya; insensitive to environmental factors.
• TLDs: lithium fluoride crystals store energy, released when heated, correlating to dose; reads as low as 50 μGya; insensitive to environmental factors.
• Digital ionization dosimeters measure exposures as low as 50 μGya.
• Handheld ionization chambers measure area radiation, measuring doses of 10 μGya/hour.
• Geiger-Mueller detectors detect radioactive particles, with readings in counts per minute.

Overview of radiation measurement units including Gray, Sievert and Becquerel. Discussion of ionizing radiation effects, natural and human-produced radiation sources. Photon-tissue interaction and attenuation are also explained.