Creation of the Projected Image and Radiographic Variables Study of Radiographic Variables

The X-Ray Beam and Radiographic Variables

• The Primary Beam:
  • Defined as primary radiation ($PR$) that has not yet passed through any object.
  • It remains measurably unaffected by the air through which it travels.
• The Remnant Beam:
  • This beam carries the organized signal that serves as the basis for the projected image.
  • On average, the total intensity of the remnant beam constitutes less than $1\%$ of the intensity of the primary beam.
  • Remnant radiation ($RR$) includes some primary rays as well as secondary and scattered radiation.
• The Focal Spot ($FS$):
  • The specific point of origin of the x-ray beam located on the anode.
  • Primary x-rays diverge isotropically (evenly in all directions) from the focal spot.
  • X-rays that penetrate the object continue moving in straight lines toward the image receptor ($IR$).
• The Central Ray ($CR$):
  • Identified as the only "straight" ray within the x-ray beam.
  • It is oriented exactly perpendicular to the long axis of the x-ray tube.
  • Because the beams diverge, all anatomy projected by the diverging portions of the beam is distorted in shape to some degree in the final image.

Six Types of Radiographic Variables

• 1. Technical Variables (Electrical Factors):
  • Milliamperage ($mA$).
  • Peak kilovoltage ($kVp$).
  • Generator type.
  • Exposure time.
  • Filtration.
  • Field size limitation (collimation).
• 2. Geometrical Variables:
  • Source-to-Image Receptor Distance ($SID$).
  • Source-to-Object Distance ($SOD$).
  • Object-to-Image Receptor Distance ($OID$).
  • Focal spot size.
  • Angulation of the beam, part, and $IR$.
  • Alignment of the beam, part, and $IR$.
  • Positioning: Includes flexion-extension, rotation, and tilt.
  • Motion of the beam, part, or $IR$.
• 3. Patient Status:
  • Body habitus and general condition.
  • Diseases and age.
  • Gender.
  • Presence of casts or contrast agents.
  • Stage of breathing.
  • Interventional changes: Including prostheses, hardware, or surgical removals.
• 4. Image Receptor ($IR$) Systems:
  • Materials: Tabletop, cassette, and detector front materials.
  • Grids.
  • Types of phosphors used in:
    • Computed Radiography ($CR$).
    • Indirect-capture Digital Radiography ($DR$).
    • Film/screen systems.
  • Direct-capture electronic $DR$ detectors.
  • Fluoroscopic image intensifiers.
  • Charge-Coupled Devices ($CCDs$) and TV cameras.
• 5. Image Processing:
  • Digitization of image data.
  • Conversion of image data into electronic, magnetic, or optical patterns for storage.
  • Pre-processing corrections applied by software.
  • Default post-processing refinements applied by software.
  • Post-processing changes made by the operator (radiographer or radiologist), such as windowing, formatting, and special features.
• 6. Viewing Conditions:
  • Ambient (room) lighting and peripheral lighting (application of black-masking).
  • Adequacy of back-illumination.
  • Viewing artifacts: Such as monitor smudges.
  • Electronic noise (display monitors).
  • Limited viewing angles (inherent to $LCDs$).
  • Monitor settings: Brightness, contrast, and resolution.
  • Monitor surface reflectance.

X-Ray Interactions Within the Patient

• Three Primary Interactions of Interest:
  • The Photoelectric Interaction.
  • The Compton Interaction: Accounts for approximately $97\%$ of all scatter. Also known as Incoherent Scatter or Modified Scatter.
  • Coherent Scattering: Accounts for the remaining $3\%$ of scatter. Also known as Thompson interaction, Raleigh interaction, Classical scatter, or Unmodified scatter.

Coherent Scattering (Thompson/Raleigh/Classical/Unmodified)

• Mechanism:
  • Occurs when the energy of the incoming x-ray photon is less than the binding energy of a strongly-bound orbital electron.
  • The x-ray photon's energy is momentarily absorbed by the whole atom or an orbital electron.
  • The orbital electron is momentarily raised to a state of excitation.
  • To release the extra absorbed energy, the atom emits a secondary x-ray photon.
• Characteristics:
  • The atom is NOT ionized.
  • The emitted photon possesses identical energy to the original photon.
  • Because it is scattered in a different direction (straight-line travel), it is considered a different x-ray from the original.
  • The scattered photon may reach the $IR$ and degrade image contrast by producing noise ($3\%$ contributions).

The Compton Effect (Incoherent/Modified)

• Mechanism:
  • An incident x-ray photon interacts with an outer-shell electron.
  • The outer-shell electron is ejected from the atom, becoming a "recoil electron."
  • Secondary radiation is emitted as a scattered x-ray photon.
  • The energy of the incident photon is divided between the ejected recoil electron and the secondary scattered radiation.
• Characteristics:
  • The atom is ionized.
  • Recoil electrons cannot reach the $IR$; however, the scattered x-ray photon can.
  • Compton scattering provides no useful information on the image; it produces a uniform optical density that reduces image contrast.
• Probabilities and Variables:
  • Probability is a function of the energy of the incident x-ray.
  • Probability is inversely proportional to x-ray energy ($1/E$).
  • Probability is independent of the atom's atomic number ($Z$); it does not depend on tissue atomic number because interactions occur in loosely-bound outer shells.
• Safety and Occupational Exposure:
  • Compton scattering is the source of most occupational exposure, particularly in fluoroscopy.
  • Protective shielding in x-ray rooms is necessary primarily because of Compton scattered x-rays.
• Backscatter:
  • X-rays scattered back in the direction of the incident x-ray beam.
  • In radiography, this can cause a "cassette-hinge image" on the radiograph, even when the hinge is on the back of the cassette. This occurs when x-rays backscatter from the wall or table rather than the patient.
  • Lead aprons placed behind a plate can eliminate evident backscatter.

The Photoelectric Effect

• Mechanism:
  • Complete absorption of the x-ray photon's energy by an inner-orbital electron (typically the K-shell).
  • The absorbed energy increases the electron's kinetic energy, causing it to be ejected from the atom.
  • The ejected electron is called a photoelectron.
  • No secondary or scatter x-rays are left over from the initial interaction.
• Characteristics:
  • The atom is ionized.
  • The photoelectron cannot reach the $IR$.
  • This effect only occurs when the energy of the incoming x-ray photon is equal to or slightly higher than the binding energy of the inner-shell electron.
• Characteristic Radiation:
  • May follow any ionizing event in the body.
  • When an electron falls from a higher orbit to fill a vacancy, potential energy is lost and emitted as electromagnetic radiation.
  • These photons typically have too low an energy to exit the patient or reach the $IR$.
• Probability and Mathematical Relationships:
  • Probability is a function of both x-ray energy ($E$) and tissue atomic number ($Z$).
  • Inversely proportional to the cube of the x-ray energy: $\frac{1}{E^3}$.
  • Directly proportional to the cube of the atomic number of the absorbing material: $Z^3$.
  • Probability decreases rapidly as x-ray energy increases; most interactions occur when the photon energy is just slightly higher than the electron binding energy.
• Significance:
  • The photoelectric effect is primarily responsible for subject contrast in the latent image.
  • X-ray imaging would be impossible without it (it produces the "whites").

Comparison of interactions: Compton vs. Photoelectric

• Mnemonic for Radiographers:
  • Compton: Related to the worker ("Workers Comp"). The worker stays outside the room during exams (Outer shell).
  • Photoelectric: Related to the patient ("taking a photo"). The patient is inside the room (Inner shell).
• Effect on Contrast:
  • Photoelectric interactions produce subject contrast (Signal).
  • Compton interactions destroy contrast (Noise/Fog).

High-Energy Interactions (Non-Diagnostic)

• Pair Production:
  • Occurs when an incident x-ray has energy of at least $1.02\,MeV$.
  • The x-ray bypasses electron interactions and is influenced by the nuclear field.
  • The x-ray disappears and two particles appear: one positron (positive) and one electron (negative).
  • Does not occur in diagnostic radiography; used in Positron Emission Tomography ($PET$) in Nuclear Medicine.
• Photodisintegration:
  • Occurs with energies above approximately $10\,MeV$.
  • X-rays are absorbed directly by the nucleus.
  • The nucleus enters an excited state and emits a nucleon or nuclear fragment.
  • The atom is changed into a different element/isotope.
  • Does not occur in diagnostic radiography.

Attenuation and Subject Contrast

• Attenuation: Defined as the partial absorption of the x-ray beam.
  • Primary radiation reaching the $IR$ unattenuated produces "pitch black" background density.
  • Muscle/soft tissue provides slight attenuation, producing dark grays.
  • Organs with contrast media provide more attenuation, producing light grays.
  • Bone attenuates the most radiation, resulting in the lightest densities (whites).
• Exponential Reduction:
  • X-rays are reduced by a certain percentage for each incremental thickness of tissue.
  • Generally, every $4-5\,cm$ (range of $3-6\,cm$) of tissue thickness reduces x-ray intensity to about one-half ($50\%$).
  • Technical Adjustment: For every $4-5\,cm$ increase in part thickness, increase technique by a factor of $2$ (either double the $mAs$ or increase $kVp$ by $15\%$).
• Differential Absorption:
  • The subtle differences in attenuation between tissues due to Thickness, Physical (Mass) Density, and Molecular (Average) Atomic Number ($Z$).
  • Atomic Number ($Z$): High $Z$ means high electron density. Compton effect is independent of $Z$. Photoelectric effect is proportional to $Z^3$.
  • Mass Density: Interaction chance is directly proportional to mass density. If density doubles, the chance for both Compton and Photoelectric interactions doubles (twice as many electrons available).
  • X-ray Energy ($kVp$): As energy increases, transmission increases. Photoelectric effect plummets ($1/E^3$), while Compton decreases only gradually.

Subject Contrast Details

• Definition: The difference in radiation intensity between various portions of the projected image carried by the remnant beam.
  • All tissue composition information is contained within the subject contrast of the remnant beam.
  • Ideal Subject Contrast: A level where every anatomical detail is depicted as a shade of gray, avoiding "blank white" or "pitch black" areas where information is missing.
• Contrast Agents:
  • Positive Contrast Agents: Barium ($Z=56$) and Iodine ($Z=53$). These are radiopaque (attenuate x-rays), resulting in low-density (clear/white) areas.
  • Negative Contrast Agents: Air, Nitrous Oxide, and Carbon Dioxide. These are radiolucent (allow penetration), resulting in darker/denser radiographic images.
  • Double-contrast: Use of both positive and negative agents (e.g., Barium Enema).
• Minimum kVp for Sufficient Penetration:
  • Iodine Contrast Agent: $76\,kVp$.
  • Air Contrast / Barium studies: $92\,kVp$.
  • Barium Esophagrams: $92\,kVp$.
  • Solid-Column Barium studies (Abdomen): $110\,kVp$.

Questions & Discussion

• Q: Of the three interactions discussed, which two interactions result in ionizing the atom?
  • A: The Compton interaction and the Photoelectric effect.
• Q: As $kVp$ increases, the probability of photoelectric interactions:
  • A: Decreases (Inversely proportional to the cube of the energy).
• Q: Which type of interaction is responsible for making a radiographic image possible?
  • A: Photoelectric effect.
• Q: The Compton effect is:
  • A: Independent of atomic number ($Z$).