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

Vocabulary flashcards covering key terms related to X-ray production, interactions, image formation, and radiographic variables from the notes.

Primary X-ray beam

Beam emitted from the X-ray tube before passing through the patient; originates at the focal spot and diverges isotropically.

Remnant X-ray beam

X-ray radiation after passing through the patient; carries the image signal to the receptor; intensity < 1% of the primary beam.

Focal spot

The small area on the anode where electrons strike to produce X-rays; can be large or small.

Central ray

The straight beam through the center; other rays fan out around it.

Isotropic divergence

Primary X-rays radiate in multiple directions from the focal spot.

SID (Source-to-Image Receptor Distance)

Distance from the focal spot to the image receptor.

SOD (Source-to-Object Distance)

Distance from the focal spot to the upper surface of the object.

OID (Object-to-Image Receptor Distance)

Distance from the object to the image receptor.

Image receptor

Detector that captures remnant radiation; examples include DR panels, CR cassettes with phosphor, or traditional film.

CR (Computed Radiography)

CR uses photostimulable phosphor to store a latent image; readout converts it to a digital image.

Photostimulable phosphor

Phosphor material in CR plates that stores energy and is released as light during readout.

Digital radiography (DR) detector

Digital detector that directly captures X-ray signal as an electrical signal.

Six radiographic variables

Categories of factors affecting the image: technical, geometrical, patient status, IR systems, image processing, and viewing conditions.

Technical variables

Exposure-set variables at the console: mA, kVp, generator type, exposure time, filtration, and field size (collimation).

Geometrical variables

Factors affecting image geometry: SID, OID, focal spot size, beam angulation, alignment, motion, and beam-object-receptor relationships.

Patient status

Patient-related factors: body habitus, disease, age, gender, casts, contrast agents, breathing, prostheses, hardware.

IR systems

Image receptor systems and components (tabletop, cassettes, grids, phosphor types, CR/DR, film, CCDs).

Image processing

Digitization, pre-processing corrections, post-processing adjustments, and software-driven enhancements.

Viewing conditions

Ambient room lighting, background masking, back illumination, viewing angle, monitor settings, and possible artifacts.

Photoelectric interaction

Complete absorption of an X-ray photon by an inner-shell electron; ejects a photoelectron and increases subject contrast.

Compton interaction

Partial absorption with a scattered photon and recoil electron; reduces image contrast and adds noise.

Coherent scattering

Photon interacts with the atom as a whole and is re-emitted at the same energy in a different direction; no ionization.

Characteristic radiation

Photon emitted when an outer-shell electron fills an inner-shell vacancy; generally low energy in the body and often not reaching the image receptor.

Attenuation

Partial absorption and scattering of X-rays by tissue; decreases exponentially with thickness.

Subject contrast

Difference in remnant-beam intensity between different tissues; determined by tissue properties and penetration.

Differential absorption

Differences in attenuation between tissues due to thickness, density, and atomic number.

Transmission

Photons that pass through tissue without interaction reach the image receptor.

Backscatter

Scatter radiation directed back toward the source; degrades image quality by adding noise.

Four to five centimeter rule

For every 4–5 cm increase in part thickness, intensity halves; compensate by increasing technique (usually ~15% kVp or doubling mA).

kVp effect on photoelectric absorption

Higher kVp increases photon energy and penetration, reducing the proportion of photoelectric absorption and lowering contrast; lower kVp increases absorption and contrast.

Subject contrast vs image contrast

Subject contrast is the actual differences in attenuation within the patient; image contrast is what is visible on the radiograph.

Transmission vs absorption

Transmission: photons reach the receptor; absorption: photons are absorbed by tissue, contributing to attenuation.