X-ray Radiography
Seeing inside with transmitted photons.
How does an invisible beam become an image of the body? Follow the path from X-ray production to detection, and discover the physics that shapes a radiograph.
Follow the photons. Hover over a component, or select its label below.
Select a component to explore its role in forming an X-ray image.
Understand how X-rays are produced, why materials attenuate them differently, and how geometry and detection shape image quality.
X-ray Production
Where do the photons come from?
Electrons emitted by a heated cathode accelerate toward a metal anode. Their interactions produce a continuous bremsstrahlung spectrum and characteristic X-rays. Much of the electron energy becomes heat. Tube voltage and filtration shape the spectrum; tube current and exposure time influence the number of photons produced.
From Electrons to Photons
Explore the parts of an X-ray tube and follow representative interactions. Particle counts, speeds, glow, and rotation are illustrative; they do not predict radiation output.
Inside the tube
2D cutaway · Not to scaleCathode filament. Heating the filament releases electrons by thermionic emission. Filament heating current and the tube current (mA) are different quantities.
Playback clock: 0.0 / 8.0 s at 1×
Exposure time represented: 0.000 / 0.20 s
The full exposure is stretched to 8 seconds of illustration at 1×. Playback speed changes viewing time only. Changing a tube setting restarts the illustration. Reset restores settings and restarts the current view. Switching views or mechanisms resets playback. Switching to another browser tab pauses it.
Reduced motion: particle positions, glow, and disc orientation stay still while the progress clock advances.
X-ray spectrum
SpekPy 2.5.4 · casimCurrent spectrum
80 kVp · 200 mA · 0.20 s · 40 mAs
Relative spectral fluence (keV⁻¹)
- Current total
Axes rescale to the visible curves; compared curves always share one scale.
No saved spectrum. Save a baseline, then change a tube setting.
kVp changes the energy endpoint and shape. At fixed kVp, this constant-potential model scales fluence linearly with mA × exposure time. The representative animation particles are unchanged.
One baseline is kept in memory for this lab; it survives view switching and clears on page reload.
One common reference: integrated fluence of the 80 kVp, 1 mAs spectrum equals 1. Every curve uses that same divisor, so mA and exposure changes remain visible. Curves are never independently normalized.
Inspect spectrum values
- Total
- 7.4676e-1 keV⁻¹
- Bremsstrahlung
- 7.4676e-1 keV⁻¹
- Characteristic
- 0.0000e+0 keV⁻¹
Current spectrum bin-averaged relative fluence density. Displayed digits aid inspection; they are not a statement of model accuracy.
Source, units, and assumptions
Idealized constant-potential tungsten reflection target, 12° anode, 2.5 mm Al external filtration, central axis at 100 cm. No additional air attenuation, patient, scatter, or detector response. The tube drawing is schematic and does not define these quantitative dimensions. No dose or machine-calibrated absolute output is displayed.
Generated offline for every integer kVp from 40–150 with 0.5 keV bins, from 1 keV up to the tube-voltage endpoint. No interpolation or smoothing. Peak heights depend on bin width. Low-energy lines may be strongly suppressed by filtration; K lines require sufficient tube potential.
Published model verification: three separate reference-setup HVL checks reproduced Table V within 1%, and its experimental references within 2%. This validates the pipeline against those benchmarks, not a particular X-ray machine.
SpekPy model and validation paper · Version and software license
200 mA × 0.20 s. Assumes constant tube current. The spectrum uses model-based relative fluence; no dose or temperature is calculated.
Advanced mode
These topics explain the fixed spectrum setup and future extensions. Additional numerical controls are not implemented.
Filtration
Preferentially removes lower-energy photons. The spectrum currently uses a fixed 2.5 mm Al external filter; adjustable materials and thicknesses are not implemented.
Target material
Affects characteristic energies and bremsstrahlung production. The spectrum uses a fixed tungsten reflection target; material selection is not implemented.
Anode angle
Affects projected focal spot and the usable field. The spectrum assumes 12 degrees on the central axis. The tube sketch is illustrative; off-axis heel-effect exploration is not implemented.
Additional components
The spectrum assumes constant potential. Tube-window materials, added layers, generator ripple, and detector response require separately documented extensions.
Heat loading
Requires waveform, tube ratings, thermal properties, and a heat-transfer model. No heat units, temperature, or safe operating limit is calculated.
Connect what you see to the physics
Thermionic emission
A heated filament releases electrons. Its temperature helps control the electron supply; the glow here is an explanatory cue, not a temperature reading.
Electron acceleration
kVp specifies peak tube voltage. A larger potential difference can give electrons more kinetic energy before they strike the target; the animation does not calculate their speed.
Bremsstrahlung
Electrons deflected and slowed in the target can emit X-rays with a range of energies. This is the origin of the continuous part of a tube spectrum.
Characteristic emission
An incident electron can create an inner-shell vacancy. When another electron fills it, the atom may emit an X-ray with an energy determined by its shell structure.
Heat at the anode
Most incident electron energy becomes heat rather than useful X-rays. Rotation distributes this energy across a focal track; it does not remove the need for cooling.
kVp, mA, and mAs
kVp describes peak voltage; mA describes tube current. For constant current, mAs = mA × exposure time in seconds. mAs is an electrical exposure setting, not a photon count or dose.
Assumptions, simplifications, and references
The tube is a conceptual local cutaway, not a mechanical drawing. The target rim section and separate face-on disc inset are complementary views. The inset’s stationary amber point represents the impact location; its moving spokes represent target material passing through that location.
The focal spot is treated as a point. Electron motion ends there; X-ray symbols originate there. Real emission occurs over a range of directions and energies. Only representative useful directions are drawn. Brown dots and teal rings are distinct illustrative samples, not a one-to-one conversion or measured particle counts. X-ray symbols have equal radius everywhere.
Most deposited electron energy becomes heat; amber cues have no numerical scale. Rotation is not a cooling calculation. Bearings, drive system, oil, shielding, filtration, space-charge dynamics, spectral lines, heel effect, and thermal limits are not modeled. Neither particle velocity nor anode RPM responds to the settings. There is no anatomy in this tube-only view.
- IAEA — Diagnostic Radiology Physics (2014), chapter 5 — Tube components, line-focus geometry, X-ray production, and rotating anodes.
- Bushberg (1997) — X-ray tubes, RadioGraphics — Tube design, focal spots, and thermal constraints.
Photon Attenuation
Why do tissues look different?
As X-rays travel through matter, absorption and scattering remove photons from the primary beam. Attenuation depends on photon energy, material composition, density, and path length. Differences in transmitted intensity make internal structures visible. A clinical beam contains many photon energies, so attenuation cannot generally be described by one energy-independent coefficient.
Scattering
What happens when a photon changes direction?
Scattered photons may reach the detector from directions unrelated to their original paths, reducing image contrast. Compton scattering is an important source of scatter in diagnostic radiography. Collimation restricts the irradiated field, while an anti-scatter grid can reject some scattered radiation before it reaches the detector.
Imaging Geometry
How does position change the projection?
Radiography maps a three-dimensional object onto a two-dimensional detector. Diverging rays cause magnification, and a finite focal spot produces geometric unsharpness. Source, object, and detector distances affect both. Structures along the same ray overlap in the image.
Detector
How are X-rays turned into a signal?
Digital detectors convert deposited X-ray energy into an electrical signal. Indirect detectors first produce light in a scintillator; direct detectors generate charge in a photoconductor. Sampling, signal conversion, and detector noise all influence the recorded image.
Image Formation
How does a measurement become an image?
Detector signals are digitized and processed into a displayed image. Calibration and image processing help make the signal useful for interpretation. Display brightness is a chosen mapping of the measured data, rather than a direct measurement of tissue density.
Image Quality
What makes a useful radiograph?
Image quality depends on the imaging task. Contrast, spatial resolution, noise, and artifacts affect what can be seen. Exposure choices involve a balance between image quality and radiation dose; increasing photon counts can reduce relative quantum noise but does not solve every limitation.
One system. Many interacting choices.
A future Full X-ray Lab will connect the chapter’s concepts in a single exploration.
View the planned Full X-ray Lab ↗References
Starting points for this introductory overview and future development.
- IAEA — Diagnostic Radiology Physics: A Handbook for Teachers and Students ↗
Foundational reading on production, interactions, detectors, and image quality.
- NIST — XCOM: Photon Cross Sections Database ↗
A candidate source for future energy-dependent attenuation data. No dataset is imported in Phase 1.