6.1 X-ray Production & Interactions with Matter

6.1 X-ray Production & Interactions with Matter

Quick Answer: X-rays are produced in a vacuum tube when high-speed electrons from the cathode strike the anode. Most useful diagnostic photons come from bremsstrahlung; characteristic x-rays appear only above the target’s K-edge. mAs mainly sets how many photons are made; kVp sets both how many and how penetrating they are. In tissue, photoelectric absorption builds contrast and patient dose, while Compton scatter blurs contrast and drives technologist exposure. Subject contrast is differential attenuation among tissues before the detector ever records an image.

RTR.2 (Manage imaging systems, about 5–10% of the CAMRT Radiological Technology exam, roughly 9–19 items) expects you to operate imaging systems safely and knowledgeably. That starts with radiation physics: how photons are made, how they interact with matter, and why technique choices change image quality and dose. Entry-level RTR candidates must apply these ideas under pressure—not just recite definitions.

X-ray Tube Components

A diagnostic x-ray tube is a diode (two electrodes) enclosed in a glass or metal envelope under high vacuum.

Thermionic emission frees electrons when filament current heats the filament. Tube current (mA) is the flow of those electrons across the tube. kVp is the high voltage applied between cathode and anode; it accelerates electrons and sets their kinetic energy (in keV, roughly equal to the kVp setting for peak photons).

Focal spot and heat management

The actual focal spot is the area of electron bombardment on the angled target. The effective (projected) focal spot is smaller because of the line-focus principle (anode angle). Smaller effective focal spots improve geometric sharpness; larger spots tolerate higher heat loading. Rotating anodes spread heat over a circular track so higher technique factors are possible without melting the target.

Exam application: Overheating, cracked anode disks, or filament failure are equipment problems you recognize as performance issues (linked to RTR.2.8), but the physics of heat loading also explains why high-mA short-time techniques and dual-focus tubes exist.

How X-rays Are Produced

When accelerated electrons hit the target, over 99% of their kinetic energy becomes heat. Less than 1% becomes x-rays by two processes.

Bremsstrahlung (braking) radiation

An electron is deflected by the nuclear field of a target atom and loses kinetic energy. That energy loss appears as an x-ray photon. Bremsstrahlung forms a continuous spectrum from near zero up to a maximum photon energy equal to the peak tube voltage (kVp). Most diagnostic image-forming photons are bremsstrahlung.

• Raising kVp shifts the spectrum to higher energies and increases the total number of x-rays produced (quantity rises roughly with kVp squared, often approximated as ∝ kVp² to kVp³ depending on filtration and the energy range considered).
• mAs scales the height of the spectrum (more electrons → more photons) without changing the maximum photon energy.

Characteristic radiation

If an incident electron ejects an inner-shell (usually K-shell) electron from a target atom, an outer-shell electron fills the vacancy. The energy difference is emitted as a characteristic x-ray with discrete energies unique to the target element.

For tungsten, K-shell binding energy is about 69–70 keV, so useful K-characteristic x-rays appear only when tube potential is above ~70 kVp. Below that threshold, the beam is essentially pure bremsstrahlung (plus filtration effects).

Quantity vs Quality of the Beam

Filtration (inherent glass/oil/window plus added aluminum or equivalent) removes low-energy photons that would only increase skin dose. Canadian and international practice expects minimum total filtration appropriate to tube kVp (commonly expressed as mm Al equivalent).

Interactions of X-rays with Matter

Once the beam enters the patient, individual photons may pass through, be absorbed, or scatter. Three interactions matter for diagnostic radiography.

1. Photoelectric absorption

The photon transfers all its energy to an inner-shell electron (photoelectron). The atom is ionized; characteristic radiation or Auger electrons may follow at atomic scale, but the diagnostic photon is removed from the beam.

• Probability rises sharply with atomic number (roughly ∝ Z³) and falls steeply with increasing photon energy (roughly ∝ 1/E³).
• Dominant in bone vs soft tissue contrast and at lower diagnostic energies.
• All energy is deposited locally → major contributor to patient absorbed dose and to subject contrast.

2. Compton scatter

The photon collides with a loosely bound outer electron, ejects it (recoil electron), and continues as a lower-energy photon in a new direction.

• Probability depends mainly on electron density (similar for soft tissues of similar density) and decreases slowly with energy in the diagnostic range.
• Dominates at higher kVp and in thick body parts.
• Scatter reaching the detector reduces image contrast (fog).
• Scatter leaving the patient is the primary source of occupational exposure in radiography and fluoroscopy.

3. Coherent (classical / Rayleigh) scatter

The photon interacts with the atom as a whole and is redirected with no ionization and essentially no energy loss. It is more likely at low energies and contributes little to diagnostic imaging or dose, but is still part of the interaction inventory on exam outlines.

Pair production requires photon energy ≥ 1.02 MeV and is not relevant in general diagnostic radiography energies—do not invoke it for standard RTR items.

Attenuation and Subject Contrast

Attenuation is the reduction of beam intensity as photons are removed by absorption and scatter. Transmitted intensity follows an exponential relationship with thickness and the material’s linear attenuation coefficient, but for the exam you need the applied idea: thicker, denser, or higher-Z structures attenuate more.

Subject contrast is the difference in x-ray intensity transmitted through adjacent tissues before detector processing. It is created by differential attenuation:

• Bone (high Z, high density) vs soft tissue → strong photoelectric contrast, especially at moderate kVp.
• Soft tissue vs air (lungs, bowel gas) → large density difference → high contrast even when photoelectric probability is lower.
• Soft tissue vs soft tissue (e.g., kidney vs liver) → low subject contrast; often needs contrast media or optimized technique/windowing later in the imaging chain.

Raising kVp increases penetration and relatively increases the Compton share, which reduces subject contrast while lowering patient dose for a given receptor exposure (within reasonable technique). Lowering kVp can increase contrast (and dose) when differential photoelectric absorption is desired—classic example: extremity work and some contrast studies—always balanced against ALARA and diagnostic need.

CAMRT Application Focus

On RTR.2.1-style stems, map the clinical choice to the physics:

• Why does scatter increase with field size and patient thickness? More tissue volume → more Compton events → more scatter toward the IR.
• Why does bone “white out” relative to soft tissue at lower kVp? Photoelectric absorption rises steeply with Z and falls with energy.
• Why does increasing mAs darken a film-screen or raise exposure index without changing contrast much? Quantity changes; beam energy spectrum is essentially unchanged.
• Why is the technologist’s dose mostly scatter-driven? Compton photons leave the patient toward the room.

Master these links and the later RTR.2 topics—grids, collimation, AEC, and QC—become consequences of the same physics rather than isolated equipment trivia.