Lec 1: Radiobiology and Radiation Safety Notes

History of Protection

  • Early observations and warnings:

    • "X-ray Light Kills" — William Rollins; 1901. Demonstrated that guinea pigs died from overexposure to x-rays.

    • Lower doses caused fetal death in a guinea pig; later experiments noted cataracts as a consequence of exposure.

  • Notable figures:

    • Thomas Edison and Clarence Dally (historical context of early radiology practices).

    • The early era included experimentation and evolving understanding of radiation risks and protection.

  • Documentation and devices from the era (e.g., protection equipment and medical instruments) highlighted the need for shielding and safety culture in medical imaging.

    • X-ray machine operators, not patients, are at a higher risk of the

      adverse affects of radiation exposure

      • Metals such as gold, lead, and aluminum blocked out the softer more harmful rays.

Radiation Protection Units of Measure

  • Units used in diagnostic radiology:

    • R (roentgen): amount of radiation in dry air.

    • Rad (radiation absorbed dose): quantity of radiation absorbed by the patient.

    • Rem (Rad equivalent man): unit expressing the dose received by radiation workers.

  • For diagnostic radiology purposes:

    • R = rad = rem (these units are treated equivalently in this context).

Cells and Radiosensitivity

  • Radiosensitive cells:

    • Reproductive cells (immature, rapidly dividing).

    • White blood cells (especially lymphocytes).

  • Less radiosensitive cells:

    • Muscle, nerve, and cortical bone (highly specialized cells).

Biological Effects

  • Genetic effects (molecular):

    • Damage to DNA molecules can cause mutations in offspring.

  • Somatic effects (cellular):

    • Biological damage to the individual (e.g., burns, hair loss, cancer) that is not passed to offspring.

Embryologic Effects

  • Embryo/fetus during development:

    • First trimester is the most radiosensitive period; most severe effects occur during the first six weeks.

    • Sensitivity decreases during the second and third trimesters.

Pregnant Patients: Safety Message

  • If pregnant or possibly pregnant:

    • Tell your doctor before getting an x-ray or prescription imaging.

  • Caprile Vertex presentation (head out canal first) and Breech (legs out first, not safe)

    • relevant to discussing imaging considerations in pregnancy.

How We Protect from Ionizing Radiation

  • Four primary strategies to reduce exposure (ALARA framework is overarching):
    1) Reduce the time of radiation exposure.
    2) Increase the distance from the radiation source.
    3) Provide radiation shielding between the individual and the radiation source.
    4) Collimate (focus) the radiation beam.

  • Note: Medical exposure to the patient for diagnostic purposes is not counted toward occupational protection dose limits.

  • Dose monitoring and maximum permissible doses have been calculated for radiation workers; dose meters exist to monitor exposure.

Inverse Square Law (Distance as a Protection Method)

  • Distance is the most effective protection method.

  • Inverse square law: I ∝ 1d2I \,\propto \,\frac{1}{d^2}

    • The intensity of the beam is inversely proportional to the square of the

      distance from the source. Therefore, as the distance from the radiation source increases, the intensity of radiation exposure decreases significantly, illustrating the importance of maintaining adequate distance as a means of minimizing exposure in radiation safety protocols.

Shielding

  • Lead is the metal most commonly used for shielding in diagnostic radiology.

  • Shield ALL patients of childbearing age, and ALL pediatric patients if possible.

  • Beam restriction (collimation) is another key shielding method; the collimator is the best device for restricting the primary beam.

Collimation and Beam Restriction

  • Collimation confines the x-ray beam to the area of interest, reducing patient dose and scatter.

  • Visual aid references include diagrams of the X-ray tube, shielding, and beam path.

X-ray Tube and Beam Components

  • Main components and pathway:

    • X-ray tube with anode (+) and cathode (−).

    • X-ray tube window/port.

    • Focal spot location at the anode/cathode interface.

    • Aluminum filter to absorb low-energy photons.

    • Collimator and light source with shutters (upper and lower shutters).

    • Off-focus radiation (arises from outside the focal spot).

    • Film area (projection) on the radiographic image receptor.

    • Film carrier and cassette.

    • Front and rear fluorescent coatings in the cassette for screen-film radiography.

Occupational Exposure and ALARA

  • ALARA concept: keep exposures As Low As Reasonably Achievable.

  • Occupational dose limits are set by the National Council on Radiation Protection (NCRP).

  • Practical emphasis on minimizing dose while achieving diagnostic quality.

Dose Limits and Monitoring

  • Occupational exposure limits:

    • 5 rems per year is the total dose limit for radiation workers

      • 0.5 rems per year is the total dose limit for the rest of the population

    • Rem = convention method of radiation absorbed dose measurement

      (used in the US)

      • Sievert (Sv)= Standard International (SI) unit of measure

      1 Sv = 100 rem or 1 rem = 0.01 Sv

    • 5 rems (0.05 Sv) per year is the total dose limit for radiation workers

      • 0.5 rems (0.005 Sv) per year is the total dose limit for the rest of the

      population

  • Monitoring includes two main types:

    • Film badges.

      • Film badges consist of a piece of film similar to that of dental film within a plastic holder

        • Should be worn at chest level (on the collar)

        • Changed every 4 weeks

    • Thermoluminescent dosimeters (TLD).

      • TLD contains small chips of lithium fluoride

        • Should be worn at chest level

        • Changed every three months

Image Production: Film and Screens

  • Key elements:

    • Film, latent image, screens, cassettes.

    • Density, contrast, and factors affecting image quality.

Film Characteristics

  • Roll film introduction:

    • Based on cellulose nitrate coated with silver halide emulsion (Kodak).

  • Base materials:

    • Cellulose nitrate: highly flammable.

    • Cellulose acetate: tended to wrinkle and mold.

Latent Image vs Manifest Image

  • Latent image:

    • The invisible image on radiographic film before development.

  • Manifest image:

    • The visible image after film development.

Creating a Radiograph: Basic Assembly

  • Rear fluorescent coating – front fluorescent coating – radiographic film in between – cassette cross-section:

    • Front phosphor-containing coating,

    • Radiographic film,

    • Back fluorescent coating.

  • Components and arrangement:

    • X-ray tube -> primary beam -> patient -> scatter -> cassette with screens -> film -> processed image.

Screens and Speed

  • Screen-film systems:

    • Screen film is sensitive to fluorescent light from crystals in the intensifying screens

      • Slow speed screens have smaller crystals: higher image detail but increased patient exposure.

      • High speed screens have larger crystals: reduced patient exposure but less detail.

Density and Image Quality

  • Density factors:

    • mAs

    • TFD

    • kVp

    • Film-screen combination

    • Filtration

    • Tissue thickness

    • Processing

    • Fog and artifacts

Latent vs Manifest: Summary Visuals

  • Latent image forms during exposure; becomes manifest after development process.

  • Key steps in film processing:

    • development, fixation, washing, drying

      • Development: Converts latent image to visible image through chemical reactions.

      • Fixation: Stops the development process and makes the image permanent by removing unexposed silver halide.

      • Washing: Removes residual chemicals from the film to ensure longevity and quality of the image.

      • Drying: Prepares the film for handling and eliminates any moisture that could damage the image.

Summary of Key Concepts

  • Patient history is essential when evaluating an image.

  • The discoverer of x-rays is Wilhelm Konrad Roentgen (1895).

  • A high kVp, low mAs technique is generally better for patient exposure than a low kVp, high mAs technique because higher kVp reduces the number of soft-energy photons in tissues while maintaining penetration; soft-tissue evaluation may require lower kVp.

  • Most radiosensitive cells include reproductive cells and WBC/lymphocytes; genetic effects affect offspring, while somatic effects affect the individual and are not inherited.

  • In pregnancy, first trimester is the most radiosensitive period; distance is the most effective protection method.

  • ALARA principle guides all radiation safety practices.

  • Occupational exposure limits, 5rems; general public 0.5rems

  • Rem and Sv units: 1 Sv = 100 rem; 1 rem = 0.01 Sv.

  • Monitoring devices (film badges and TLDs) are used to track exposure, with regular replacement intervals.

  • Image quality depends on exposure factors (mAs, kVp), geometry (distance, alignment, filtration), and processing; screen speed affects dose and detail.

  • Historical and practical notes emphasize the evolution of protection standards and ongoing emphasis on patient and worker safety.