Chapter 1 Notes: Essentials of Radiologic Science (Radiologic Physics and X-Ray Discovery)

Sources of Ionizing Radiation

  • Two main categories of annual radiation dose for the US population:
    • Natural environmental radiation: approximately 3 mSv3\ \text{mSv} per year
    • Man-made radiation: approximately 3.1 mSv3.1\ \text{mSv} per year
  • Historical context for total population dose:
    • 1990: about 3.6 mSv3.6\ \text{mSv} per year
    • 2006: about 6.2 mSv6.2\ \text{mSv} per year
  • Medical imaging as a major man-made source:
    • Diagnostic x-rays constitute the largest man-made source of ionizing radiation, about 3.0 mSv/year3.0\ \text{mSv/year} (as of 2006)
    • Earlier NCRP estimate (1990) for medical exposure was about 0.5 mSv/year0.5\ \text{mSv/year}; the rise is mainly due to CT use and higher-level fluoroscopy
  • Composition of man-made exposures outside medical imaging:
    • Nuclear power, research applications, industrial sources contribute very little to the population dose
    • Consumer products (watch dials, exit signs, smoke detectors, camping lantern mantles, airport surveillance systems) contribute about 0.1 mSv/year0.1\ \text{mSv/year}
  • Regional variation in natural background radiation:
    • Gulf Coast and Florida: ≈ 0.2 mSv/year0.2\ \text{mSv/year}
    • Rocky Mountains region: ≥1.0 mSv/year\geq 1.0\ \text{mSv/year} or higher
  • Rationale for radiation safety in medicine:
    • The average annual medical radiation dose is comparable to natural background; thus, prudent use and dose reduction are essential
    • Responsibility lies with radiologic technologists who control x-ray imaging systems during examinations
  • Calculation of the share of annual dose from medical imaging:
    • Percentage from medical imaging = medical dosetotal dose×100%=3.0 mSv6.2 mSv≈0.484≈48%\frac{\text{medical dose}}{\text{total dose}} \times 100\% = \frac{3.0\ \text{mSv}}{6.2\ \text{mSv}} \approx 0.484 \approx 48\%
  • Additional context on radiation sources:
    • Nuclear power and other industrial applications contribute very little to the population dose
    • Consumer items contribute approximately 0.1 mSv/year0.1\ \text{mSv/year} as noted above
  • Live connection to the figure in the text:
    • FIGURE 1.3 shows the contribution of various sources to the average US population radiation dose for 1990 and 2006
    • NOTE: This pie chart is referred to again in Chapter 39

Ionizing Radiation: Concepts and Definitions

  • Mass–Energy equivalence:
    • E=mc2E = mc^2 where
    • EE = energy,
    • mm = mass,
    • cc = speed of electromagnetic radiation in vacuum
  • Radiation and exposure definitions:
    • Radiation: energy emitted and transferred through space
    • Radiation type can be classified as electromagnetic (e.g., visible light, ultraviolet, x-rays) or particle-based
    • Exposure vs irradiation:
    • A person exposed to radiation has the radiation intersecting the body (irradiated)
  • Ionizing radiation:
    • Any type of radiation capable of removing an orbital electron from an atom
    • Ionization occurs when an x-ray (or other radiation) transfers sufficient energy to eject an electron from an atom
    • Ion pairs are formed: the ejected electron is a negative ion and the remaining atom is a positive ion
    • Most common ionizing radiations include x-rays and gamma rays (electromagnetic), ultraviolet light with sufficient energy, and certain fast-moving particles
    • Some fast-moving particles (alpha, beta) are also ionizing; they are particles, not rays, and are discussed further in Chapter 3
  • Ionization terminology:
    • Ionization = removal of an electron from an atom
    • Ion pair = ejected electron + resulting positively charged ion
  • Units and dose concepts:
    • The dose to populations is expressed using the sievert (mSv for convenience for populations)
    • The mSv is used to express effective dose and associated risk in populations

Natural Environmental Radiation: Components and Dose

  • Four components of natural environmental radiation:
    • Cosmic rays: particulate and electromagnetic radiation from the sun and stars; intensity increases with altitude and latitude
    • Terrestrial radiation: from uranium, thorium, and other radionuclides in the Earth; intensity dependent on local geology
    • Internally deposited radionuclides: mainly potassium-40 (40K); natural metabolites that have always been with us and contribute an equal dose to each person
    • Radon: a radioactive gas produced by natural decay of uranium; present in all earth-based materials (concrete, bricks, gypsum wallboard); radon emits alpha particles which are not highly penetrating and contribute an inhalation dose primarily to the lungs
  • Dose from natural environmental radiation:
    • Collectively, these sources result in approximately
    • 3 mSv3\ \text{mSv} per year or more broadly within the natural background range
  • Largest natural source:
    • Radon accounts for the largest share of natural environmental radiation dose
  • Typical waist-level exposure in the United States:
    • Approximately 300 to 1000 μSv/h300\text{ to }1000\ \mu\text{Sv/h} (0.3 to 1.0 mSv/h) at waist level
  • Figure references:
    • FIGURE 1.4: Radiation exposure at waist level throughout the United States
  • Context:
    • Natural background has persisted through human evolution; there is ongoing discussion about the role of ionizing radiation in evolution and whether exposure control is warranted in modern medicine

Discovery of X-Rays and Early History

  • Crookes tube and precursor devices:
    • The Crookes tube was a partially evacuated glass tube used to study cathode rays and produced x-rays in certain configurations
  • Discoverer and date:
    • Sir Wilhelm Conrad Roentgen (Roentgen) discovered x-rays on November 8, 1895, at Würzburg University, Germany
  • Experimental observations leading to discovery:
    • Roentgen darkened his lab; enclosed his Crookes tube with black photographic paper; a barium platinocyanide screen glowed (fluorescence) when x-rays were emitted
    • The glow increased as the fluorescent plate was brought closer to the tube, indicating the presence of an unseen radiation
    • He named the radiation with the initial tag "X" for unknown
  • Early investigations and reporting:
    • Roentgen interposed various materials (wood, aluminum, his hand) between the tube and screen to study interaction
    • Reported his experimental results to the scientific community by the end of 1895
    • Nobel Prize in Physics awarded in 1901 for this work
  • First medical x-ray image:
    • Early 1896: Roentgen published the first medical x-ray image of his wife's hand (Fig. 1.6)
  • Early US exposure:
    • The first medical x-ray examination in the United States occurred in early February 1896 at Dartmouth College (Fig. 1.7, Eddie McCarthy's wrist)
  • The Crookes tube family:
    • The tube Roentgen used had existed for years; it remained largely unchanged into the early 20th century with some modifications
  • The table of Roentgen's original properties (1.1):
    1. X-rays are highly penetrating, invisible rays that are a form of electromagnetic radiation.
    2. X-rays are electrically neutral and therefore not affected by electric or magnetic fields.
    3. X-rays can be produced over a wide variety of energies and wavelengths.
    4. X-rays release very small amounts of heat upon passing through matter.
    5. X-rays travel in straight lines.
    6. X-rays travel at the speed of light, c=3×108 m/sc = 3 \times 10^8\ \text{m/s} in a vacuum.
    7. X-rays can ionize matter.
    8. X-rays cause fluorescence of certain crystals.
    9. X-rays cannot be focused by a lens.
    10. X-rays affect photographic film.
    11. X-rays produce chemical and biological changes in matter through ionization and excitation.
    12. X-rays produce secondary and scatter radiation.
  • Other notable developments:
    • The discovery and feeding frenzy of x-rays involved serious safety concerns early on, including x-ray burns; the first US x-ray fatality was Clarence Dally in 1904

Development of Medical Imaging: Modalities and Imaging Techniques

  • Three general types of x-ray examinations:
    • Radiography: uses a solid-state image receptor (IR) with a ceiling-mounted x-ray tube capable of moving; provides fixed radiographic images
    • Fluoroscopy: usually uses an x-ray tube under the table; dynamic moving images on a digital display
    • Computed Tomography (CT): uses a rotating x-ray source and detector array to acquire data for 3D image reconstruction in coronal, sagittal, transverse, or oblique planes
  • Imaging equipment is broadly similar across modalities, with variations in geometry and detectors
  • Radiation production principles:
    • X-ray beam requires high voltage (kVp) and electric current (mA)

X-Ray Measurements: Voltages, Currents, and Distances

  • Key units and definitions:
    • X-ray voltages are measured in kilovolt peak (kVp)
    • 1 kilovolt (kV) = 1000 V
    • X-ray currents are measured in milliampere (mA); 1 A = 1 C/s
    • The prefix milli means 1/1000, i.e., 1 mA=10−3 A1\text{ mA} = 10^{-3}\ \text{A}
  • Source-to-image receptor distance (SID):
    • Typical SID during radiography is 1 extm1\ ext{m}
    • Conversion to millimeters: 1 m = 1000 mm; thus, 1000 mm equals 1 m, i.e.
    • 1 m=1000 mm1\ \text{m} = 1000\ \text{mm}
  • Historical context of x-ray generation:
    • Roentgen's era relied on static generators; currents of only a few mA and voltages up to about 50 kVp50\ \text{kVp} were available
    • Modern practice commonly uses around 1000 mA1000\ \text{mA} and 150 kVp150\ \text{kVp} for many diagnostic procedures
  • Exposure times and image quality:
    • Early radiographic procedures often required exposure times of up to 30 minutes30\ \text{minutes}; long exposures caused image blur
    • The introduction of fluorescent intensifying screens in combination with glass photographic plates significantly reduced exposure times
  • Intensifying screens and film evolution:
    • Pupin demonstrated the use of radiographic intensifying screens around 1896; many years later gained adequate recognition
    • Double-emulsion radiography (two glass x-ray plates with emulsion surfaces together) reduced exposure time by about half; demonstrated by Charles L. Leonard in 1904; commercially available by 1918
    • WWI disruptions limited high-quality glass plates; radiologists shifted to film
  • Substitutes for glass plates:
    • Cellulose nitrate became a substitute for glass plates as demand rose (army use)
  • Fluoroscopy and Edison:
    • Fluoroscope developed by Thomas A. Edison in 1898
    • Original fluorescent material: barium platinocyanide; Edison tested >1800 materials, including zinc cadmium sulfide and calcium tungstate; these latter materials remain in use in some forms
  • Safety innovations at the turn of the century:
    • William Rollins (Boston dentist) introduced early dose-reduction techniques: collimation with a lead diaphragm having a central hole and the use of filtration to reduce patient exposure while preserving diagnostic quality
  • High-voltage power generation advances:
    • 1907: H. C. Snook introduced an interrupterless transformer, a high-voltage power supply that significantly improved radiographic capability
    • The diffusion of these devices was accelerated by the introduction of the Coolidge tube (hot-cathode vacuum tube), which could sustain higher voltages and currents safely
  • The Crookes tube legacy:
    • The Crookes tube Roentgen used in 1895 existed for years and remained essentially unchanged into the early 20th century, albeit with various refinements

Roentgen's Properties of X-Rays (Table 1.1) — Key Concepts

  • Overview of the properties Roentgen observed about X-rays (as originally listed):
    1) X-rays are highly penetrating, invisible rays that are a form of electromagnetic radiation.
    2) X-rays are electrically neutral and therefore not affected by either electric or magnetic fields.
    3) X-rays can be produced over a wide variety of energies and wavelengths.
    4) X-rays release very small amounts of heat upon passing through matter.
    5) X-rays travel in straight lines.
    6) X-rays travel at the speed of light, c=3×108 m/sc = 3 \times 10^8\ \text{m/s} in a vacuum.
    7) X-rays can ionize matter.
    8) X-rays cause fluorescence of certain crystals.
    9) X-rays cannot be focused by a lens.
    10) X-rays affect photographic film.
    11) X-rays produce chemical and biological changes in matter through ionization and excitation.
    12) X-rays produce secondary and scatter radiation.
  • Significance:
    • These properties laid the foundation for understanding imaging physics, safety, and the behavior of radiation in diagnostic radiology

Safety, Ethics, and Practical Implications

  • Benefits of medical imaging are clear and substantial, but prudent use is essential to minimize unnecessary exposure for patients and staff
  • The radiologic technologist plays a central role in controlling exposure through technique, equipment settings, shielding, collimation, filtration, and optimization of imaging protocols
  • Early safety challenges (e.g., x-ray burns, Dally’s case) underscored the need for protective measures and dose optimization
  • Practical implication:
    • As imaging use grows, the medical community must balance diagnostic benefit with minimizing dose, adhering to ALARA principles (as low as reasonably achievable)
  • Real-world relevance:
    • The discussion of natural background vs medical exposure informs public health policy, radiologic safety training, and diagnostic imaging guidelines

Connections to Foundational Principles and Applications

  • Foundational physics:
    • Mass–energy equivalence links to energy considerations in radiation production and interactions with matter
  • Epidemiological and risk considerations:
    • Understanding dose in mSv and its components helps quantify population risk and informs regulatory standards (NCRP, etc.)
  • Practical radiology implications:
    • Knowledge of dose sources, imaging modalities, and dose-reduction strategies underpins safe and effective diagnostic imaging practice