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Radiography Lecture Notes: X-ray Interactions, Image Formation, and Radiographic Variables

Remnant X-ray Beam and Primary Radiation

  • Primary radiation: produced at the X-ray tube; has not yet passed through any object (e.g., patient).
  • Remnant X-ray beam: the portion of the primary beam after it has passed through the object/patient; carries the organized signal used to form the image.
  • On average, the remnant beam intensity is less than 1% of the primary beam. rac{I{ ext{remnant}}}{I{ ext{primary}}} < 0.01.
  • Remnant radiation includes some primary rays plus secondary and scattered radiation.
  • Focal spot: located at the anode; there can be a large or small focal spot; it is the origin of the X-ray beam.
  • Primary X-rays diverge isotropically from the focal spot, radiating in multiple directions.
  • Central ray: the only beam that travels in a straight line; other rays fan out.
  • Anatomical projection: all anatomy is distorted in its final image due to the diverging beams; changing tube angle changes the image.

Geometry and Distances

  • SID: Source-to-Image Receptor Distance; measured from the focal spot to the image receptor.
  • SOD: Source-to-Object Distance; measured from the focal spot to the upper surface of the object.
  • OID: Object-to-Image Receptor Distance.
  • These distances are used in geometric calculations and impact magnification and distortion.
  • Magnification is determined by geometry; a useful relation (for intuition) is M = \frac{SID}{SOD} = \frac{SID}{SID - OID}.

Image Receptors and Detector Types

  • Image receptors can be:
    • Digital radiography detectors (DR).
    • CR (computed radiography) cassette with photostimulable phosphor; requires a reader to release the latent image.
    • Traditional photographic film (largely obsolete).
  • In CR: remnant signal is stored as energy in the phosphor; later read out to form an image.
  • In DR: direct or indirect capture electronic detectors exist; direct capture converts X-rays to electric signal; indirect capture first converts X-ray to light then to an electronic signal.
  • Indirect conversion vs direct conversion:
    • Indirect: X-ray → light (via phosphor) → electronic signal.
    • Direct: X-ray → electronic signal directly (e.g., using certain detector materials or scintillators).
  • Image processing and viewing rely on these detectors and their materials to convert remnant radiation into a viewable image.

Six Types of Radiographic Variables Affecting the Projected Image

  • Technical variables (machine settings): MA, kVp, generator type, exposure time, filtration, and field size (collimation).
  • Geometrical variables: SOD, SID, OID, focal spot size, beam angulation, alignment of the heart and image receptor, and any motion of the beam, heart, or receptor.
  • Patient status: body habitus, general condition, diseases, age, gender, casts, contrast agents, stage of breathing, prostheses or hardware; these influence how X-rays interact with tissues.
  • Image receptor (IR) systems: tabletop/cassette detectors, detector materials, grids, phosphor types, CR vs DR, and the specifics of conversion to an electronic image.
  • Image processing: digitization, pre-processing corrections, default post-processing, post-processing adjustments by the operator (contrast/brightness), formatting, and special features.
  • Viewing conditions: ambient room lighting, peripheral/back illumination, masking, back illumination of the image, viewing artifacts, electronic/noise issues on displays, limited viewing angles (LCDs), monitor brightness/contrast, resolution, and surface reflectance.

Interactions of X-rays with Atoms

  • Three main types of interactions that determine latent image formation:
    1) Photoelectric interaction (absorption): complete absorption of the X-ray photon by an inner-shell electron; ionizes the atom; ejects a photoelectron; leaves a vacancy that is filled by an outer-shell electron, often emitting a characteristic X-ray photon.
    2) Compton interaction (scattering): partial absorption; a recoil electron is ejected and a scattered photon emerges with reduced energy; the scattered photon may reach the image receptor and contributes to noise/contrast reduction.
    3) Coherent scattering (Thomson/Rayleigh): momentary excitation of the atom without ionization; the photon is scattered with nearly the same energy in a different direction; energy of scattered photon is nearly identical to the incident photon.
  • Coherent scattering specifics:
    • Also called Thompson, Rayleigh, unmodified, elastic scattering.
    • Occurs primarily at very low photon energies.
    • Does not affect patient dose (photon does not ionize).
    • Causes a small decrease in image contrast due to added scatter (noise).
  • Photoelectric interaction details:
    • Energy of incoming photon is absorbed by an inner-shell electron; results in ionization and ejection of a photoelectron.
    • Outer-shell electrons fill the vacancy, emitting characteristic X-rays with energies equal to the difference in binding energies between shells.
    • This process is a major contributor to subject contrast in the latent image.
    • Occurs when Ep > Ebinding (inner shell). The energy left for the photoelectron is $E{ ext{KE}} = Ep - E_b$.
    • The likelihood of photoelectric interaction is strongly dependent on tissue properties and beam energy; it increases at lower kVp and higher atomic number materials.
    • Photoelectric absorption is not good for the patient dose (ionization increases patient dose) but it is essential for image contrast and overall image formation.
    • The amount of photoelectric absorption increases with tissue thickness, density, atomic number, and decreases with higher beam energy (kVp).
  • Compton interaction details:
    • Partial absorption with recoil electron ejected and a scattered photon that may reach the image receptor.
    • Scatter reduces image contrast and acts as noise; it contributes to occupational and patient dose depending on geometry.
    • The energy of the scattered photon depends on the scattering angle; as the deflection angle θ increases from 0 to 180 degrees, the scattered photon energy decreases from near the incident energy toward lower values. A standard relation (Compton formula) is:
      E' = \frac{E}{1 + \frac{E}{me c^2}(1 - \cos\theta)} where $E$ is the incident photon energy, $me c^2 \approx 511\ \text{keV}$, and $E'$ is the scattered photon energy. The energy transferred to the recoil electron is $E - E'$.
    • Energy retention in scatter: backscatter can retain a substantial fraction of the original energy; in diagnostic radiography, backscatter photons can be 68% or more of the original energy depending on angle.
  • Characteristic radiation (within the body):
    • When a vacancy in an inner shell is filled by an outer-shell electron, a characteristic photon is emitted.
    • In body atoms (which are smaller than tungsten in X-ray tubes), these characteristic photons are often in the UV range (energy is typically low) and may be absorbed locally rather than escape to the image receptor.
  • Attenuation and contrast basics:
    • Attenuation is partial absorption of the X-ray beam by tissues; both absorption (photoelectric) and scattering (Compton, coherent) contribute to attenuation.
    • X-ray intensity diminishes exponentially with tissue thickness: I = I_0 e^{-\mu x}. Here (\mu) is the linear attenuation coefficient.
    • For every ~4–5 cm of tissue thickness, the incident intensity can drop by about a factor of 2 (roughly a half-value concept): to compensate, technique (kVp or mAs) may be adjusted.
    • Two routes to compensate: increase kVp by about 15% or double the patient mass; the latter increases patient dose more than increasing kVp.
    • Differential absorption (subject contrast) arises from differences in thickness, density, and atomic number among tissues; higher atomic number materials (e.g., iodine, barium) show greater absorption and appear brighter on radiographs.
  • Subject contrast vs image contrast:
    • Subject contrast: differences in absorption within the patient that would exist if you could directly observe the latent energy differences in the body.
    • Image contrast: what is actually visible on the radiograph; influenced by how the remnant beam is recorded by the detector, processing, and viewing conditions.
    • Adequate penetration must match tissue thickness, density, and atomic number so that some X-rays penetrate through all tissues to produce an image with good subject contrast.
    • With underexposure, information can be lost (cannot be recovered via processing); overexposure can be compensated to some extent by processing, but too much exposure can still degrade image quality if dynamic range is exceeded.
  • Attenuation in the image receptor chain:
    • The same physics apply to the image receptor as in tissues; the detector aims to absorb as much energy as possible in a thin layer, with materials having higher atomic numbers increasing photoelectric interactions and improving signal.
  • Practical notes from the transcript:
    • Four specific factors influence photoelectric absorption: tissue thickness, tissue density, average atomic number, and beam energy (kVp).
    • Lower kVp increases the proportion of photoelectric absorption (higher contrast) but increases patient dose; higher kVp increases penetration but also increases scatter and reduces contrast.
    • Coherent scattering, while not contributing to dose, reduces image contrast slightly and is more likely at very low photon energies.
    • For the exam-style example: a scenario where a 40 keV incident photon interacts via photoelectric effect with an L-shell electron in calcium, with L-shell binding energy ~0.5 keV and recoil electron KE = 5 keV; the energy of any Compton-scattered photon in that context would be approximately E' \approx Ep - Eb - E_{ ext{KE}} = 40 - 0.5 - 5 = 34.5\ \text{keV}. (From the transcript, this setup is used to illustrate that the scattered photon can carry most of the energy, though note that in reality a single interaction can involve photoelectric emission, characteristic photons, or Compton scattering depending on the event.)
  • Backscatter considerations:
    • Backscatter X-rays can retain at least about 68% of the original photon energy and travel at backward oblique angles toward the patient’s rear; they still contribute to image noise and can degrade image quality.

Image Processing and Viewing Conditions (Overview)

  • Image processing steps include digitization (for CR), pre-processing corrections, and post-processing adjustments (contrast/brightness) by the operator.
  • Viewing conditions impact how the radiologist perceives the image (ambient light, monitor settings, resolution, and surface reflectance). Proper settings help maximize the visibility of anatomic details and reduce misinterpretation due to display artifacts.

Connections to Practice and Real-World Relevance

  • Understanding primary vs remnant beams helps explain how radiographs are formed and why exposures must be calibrated to maximize image quality while minimizing patient dose.
  • Geometry (SID, SOD, OID) directly affects magnification and geometric unsharpness; tube angulation and object alignment can distort anatomy on the radiograph.
  • Detector choice (CR vs DR, indirect vs direct) influences how X-ray energy is converted to a usable image and impacts dose efficiency and image quality.
  • Balancing kVp and mA is critical: lower kVp improves contrast via photoelectric absorption but raises patient dose, whereas higher kVp improves penetration and reduces dose but can lower contrast due to increased scatter.
  • Understanding attenuation and the different interactions aids in selecting appropriate contrast agents (iodine, barium) and recognizing artifacts related to scattering and imperfect alignment.

Quick Reference Formulas and Key Concepts

  • Intensity relation after patient: rac{I{ ext{remnant}}}{I{ ext{primary}}} < 0.01.
  • Attenuation: I = I_0 e^{-\mu x}.
  • Magnification: M = \frac{SID}{SOD} = \frac{SID}{SID - OID}.
  • Photoelectric interaction: energy balance for inner-shell absorption, E{ ext{KE}} = Ep - Eb, where Ep is the incident photon energy and E_b is the inner-shell binding energy.
  • Compton scattering (energy of scattered photon):
    E' = \frac{E}{1 + \frac{E}{m_e c^2} (1 - \cos\theta)},
    with transferred energy to recoil electron E - E'.
  • Coherent scattering: photon energy remains essentially the same; scattering directed forward; minimal dose effect but slight image contrast reduction.

Note: The transcript includes some exam-style examples and instructor commentary. In practice, always cross-check with standard references for specific numerical values and terminology, but use this as a detailed study scaffold that mirrors the concepts emphasized in the provided content.