X-Ray Interactions with Matter

Overview of X-Ray Interactions with Matter

  • Purpose of Studying X-Ray Interactions:

    • Understanding x-ray photon interaction mechanisms minimizes radiation harm to patients.

    • Facilitates the selection of appropriate technical exposure factors to produce higher quality diagnostic images.

  • Five Primary Mechanisms of X-Ray Interactions with Matter:

    • Classical interactions (also known as coherent or Thomson scattering)

    • Compton scattering

    • Photoelectric effect

    • Pair production

    • Photodisintegration

  • Diagnostic Energy Range:

    • Only the first three interaction processes—classical scattering, Compton scattering, and the photoelectric effect—occur within the energy range utilized in diagnostic radiography.

Classical Interactions


Classical scattering schematic

  • Nomenclature:

    • Also referred to as coherent scattering or Thomson scattering.

  • Interaction Process:

    • The incident x-ray photon interacts with an orbital electron of a target tissue atom.

    • The interaction causes the incident x-ray photon to change direction (scatter).

  • Energy and Ionization Characteristics:

    • No energy is transferred to the atom or electron, and no energy is lost by the photon.

    • The target atom is not ionized during this interaction.

Compton Interactions


Compton scattering schematic showing primary and secondary Compton interactions

  • Interaction Mechanism:

    • An incident x-ray photon enters a tissue atom and interacts with a loosely bound outer-shell orbital electron.

    • The incident photon removes the electron from its shell.

    • During the interaction, the incident photon loses up to 13\frac{1}{3} of its initial energy and is deflected in a new direction.

  • Three Consequences of Compton Interaction:

    • Atom Ionization: Ejection of the outer-shell electron ionizes the atom, leaving it chemically unstable.

    • Ejection of Compton Electron: The ejected electron, called a Compton electron or secondary electron, leaves the atom with sufficient kinetic energy to undergo further ionizing interactions in adjacent atoms.

    • Creation of Compton Scatter Photon: The deflected incident photon becomes a Compton scatter photon. It retains enough energy to undergo additional interactions within surrounding body tissues or to exit the patient entirely.

  • Impact on Image Quality:

    • Compton scatter photons that travel toward the image receptor strike it in unintended places, producing unwanted background exposure known as image fog.

    • Image fog reduces radiographic contrast and image clarity.

    • Compton scatter should be minimized as much as possible through beam restriction and scatter reduction techniques.

  • Prevalence and Physical Probability:

    • Compton scattering is one of the most prevalent interactions between x-ray photons and the human body in general diagnostic imaging.

    • Atomic Number Independence: The probability of a Compton interaction occurring does not depend on the atomic number (ZZ) of the target tissue atom.

    • Photon Energy Dependence: The probability of Compton scattering is primarily related to the energy level of the incident x-ray photon.

  • Occupational Exposure and Radiation Safety:

    • Compton scatter photons retain approximately 23\frac{2}{3} of their original energy and frequently exit the patient's body.

    • Scatter photons exiting the patient represent the major source of occupational radiation exposure to radiographers and other healthcare personnel.

    • Radiation shielding (such as lead aprons, shields, and protective barriers) is mandatory during any clinical procedure where healthcare workers must remain near the patient and x-ray tube during exposure.

Photoelectric Interactions


Photoelectric interaction schematic illustrating inner-shell electron ejection and characteristic cascade

  • Interaction Mechanism:

    • An incident x-ray photon interacts directly with an inner-shell orbital electron of a target tissue atom.

    • The photon ejects the inner-shell electron from its orbit.

    • In this process, the incident x-ray photon gives up all of its energy and is totally absorbed by the tissue atom.

  • Photoelectron Dynamics and Energy Equation:

    • The ejected orbital electron is designated as a photoelectron.

    • The energy transferred to the photoelectron is equal to the incident photon energy minus the binding energy of the inner-shell electron:     Ephotoelectron=E0−EbE_{\text{photoelectron}} = E_0 - E_b

    • The photoelectron acquires sufficient kinetic energy to undergo subsequent interactions in nearby matter before eventually filling an electron vacancy in another atom.

  • Impact on Patient Dose:

    • Photoelectric absorption is the primary interaction responsible for radiation dose absorbed by the patient during diagnostic imaging.

    • Although photoelectric absorption is essential for creating diagnostic tissue contrast on radiographs, the radiographer is responsible for selecting technical parameters that achieve an optimal balance between high image quality and minimal patient dose.

  • Atomic Instability and Secondary Radiation Cascade:

    • Ejection of the inner-shell electron leaves an inner-shell vacancy, causing tissue atom ionization and instability.

    • To regain atomic stability, outer-shell electrons cascade down to fill inner-shell vacancies (characteristic cascade).

    • This characteristic cascade releases secondary x-ray photons.

    • These secondary photons are of low energy, are rapidly absorbed locally within the patient's body through secondary photoelectric events, and contribute further to total patient dose.

  • Probability Determinants:

    • Incident Energy Threshold: The energy of the incident x-ray photon must be greater than or equal to the binding energy of the targeted inner-shell orbital electron.

    • Atomic Number Dependence: The probability of photoelectric interaction is directly proportional to the third power (cube) of the atomic number of the absorbing atom (Z3Z^3).

    • Energy Inverse Dependence: As incident photon energy increases, the probability of photoelectric interaction decreases dramatically (inversely proportional to the cube of the energy).

Pair Production


Pair production schematic showing nuclear interaction, positron and electron creation, and annihilation event

  • Interaction Mechanism:

    • Occurs when an incident x-ray photon possesses sufficient energy to escape interaction with orbital electrons and directly interacts with the powerful electric field of the atomic nucleus.

    • The incident photon is transformed into two distinct particles: an electron (negatron) and a positron.

  • Energy Requirements:

    • For these particles to be created, each particle must have a rest mass energy equivalent of 0.51 MeV0.51\,\text{MeV}.

    • Consequently, pair production requires a threshold incident photon energy of at least 1.02 MeV1.02\,\text{MeV} (2×0.51 MeV2 \times 0.51\,\text{MeV}).

  • Particle Trajectory and Annihilation Event:

    • Both generated particles travel outward from the target atom.

    • The electron undergoes multiple ionizing collisions until coming to rest in another atom.

    • The positron travels through matter until it collides with a free electron, resulting in a mutual annihilation event.

    • The annihilation event completely converts particle mass into two 0.51 MeV0.51\,\text{MeV} photons emitted in opposite directions (180∘180^\circ apart).

  • Diagnostic Relevance:

    • Pair production requires photon energies far above diagnostic radiographic ranges (1.02 MeV1.02\,\text{MeV} minimum). Therefore, pair production does not occur in diagnostic radiography.

Photodisintegration


Photodisintegration schematic showing direct nuclear absorption of a high-energy x-ray photon and ejection of a nuclear particle

  • Interaction Mechanism:

    • Occurs when photons with extremely high energy (typically greater than 10 MeV10\,\text{MeV}) directly strike the nucleus of an atom, causing it to absorb the photon energy and become extremely unstable.

  • Nuclear Stabilization Process:

    • The excited target nucleus regains stability by ejecting a nuclear particle (such as a neutron, proton, alpha particle, or deuteron).

  • Diagnostic Relevance:

    • Because diagnostic radiology utilizes photon energies far below this threshold, photodisintegration does not occur in diagnostic radiography.

Differential Absorption and Image Formation


Primary beam interaction with torso showing transmission, absorption, and scatter creation


Differential absorption mechanism showing absorbed photons, scattered photons, and transmitted photons reaching the image receptor

  • Definition of Differential Absorption:

    • Differential absorption is the difference between x-ray photons that undergo total photoelectric absorption within anatomical tissues and those that penetrate the body without interacting.

    • Different human tissues absorb x-ray photons to varying degrees, creating subject contrast in diagnostic images.

  • Transmission versus Absorption:

    • Transmission: X-ray photons that pass directly through the patient's body without interacting and reach the image receptor, forming dark areas on the image.

    • Absorption: X-ray photons that undergo attenuation and absorption within anatomical structures, failing to reach the image receptor and leaving light areas on the image.

    • Photon absorption depends heavily on the biological tissue density and atomic composition of the traversed body part.

  • Tissue Classifications:

    • Radiopaque Structures: High-density structures or tissues with high atomic numbers (e.g., bone or barium contrast media) that readily absorb x-rays, creating light areas on the radiograph.

    • Radiolucent Structures: Less dense anatomical structures (e.g., air, lung, or soft tissue) that have a significantly lower probability of x-ray absorption, allowing photon transmission to produce darker image areas.

Radiation Effects on Biological Macromolecules

  • Macromolecular Structure:

    • Macromolecules are large, complex biological molecules containing thousands of atoms (e.g., proteins, nucleic acids).

    • When an x-ray photon interacts with atoms inside a macromolecule, energy transfer can alter the structural integrity of the molecule.

  • Three Common Types of Radiation Damage to Macromolecules:

    • Main-Chain Scission:

    • Breakdown of the long-chain macromolecule backbone.

    • Reduces a long single molecule into several smaller molecular fragments, decreasing solution viscosity.


    Main-chain scission in a macromolecule caused by x-ray interaction

    • Cross-Linking:

    • Production of small, spur-like side structures on the macromolecule post-irradiation.

    • These sticky ends attach to adjacent macromolecules or to another segment of the same molecule, increasing solution viscosity.


    Cross-linking of macromolecules resulting from x-ray interaction

    • Point Lesions:

    • Disruption of individual chemical bonds within the macromolecule.

    • Causes localized molecular lesions that can impair cell function or induce minor structural modifications without initially altering macromolecular size.


    Point lesions within a macromolecule following x-ray interaction

  • Impact on Deoxyribonucleic Acid (DNA):

    • Analogous radiation damage—including main-chain scission, cross-linking, and point lesions—can occur within DNA\text{DNA} molecules as a result of direct or indirect radiation exposure, potentially resulting in cell dysfunction, genetic mutation, or cell death.