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X-ray image formation: tube, beam and image quality
In the tube X-rays arise mainly from braking of electrons (bremsstrahlung), kV sets beam energy and contrast, mAs sets the number of photons and the noise — and the filter, grid and geometry set dose, contrast and sharpness.
Why it matters in the exam
X-ray physics questions repeat the same components. In the tube, electrons leave the heated cathode and strike the anode (usually tungsten). Most radiation is bremsstrahlung with a continuous spectrum; characteristic radiation appears at fixed energies set by the anode material — this is the answer when asked "what determines characteristic radiation". The heel effect: beam intensity is lower on the anode side, so the thicker body part is placed on the cathode side. Mammography uses a molybdenum, rhodium or tungsten anode and filter at 25–32 kV to obtain low energies that increase soft-tissue contrast.
The two important interaction processes: the photoelectric effect — the photon is fully absorbed and releases an inner-shell electron; its probability rises steeply with atomic number (about Z³) and falls with energy (about 1/E³), and it is the source of contrast between bone, iodine and tissue. Compton scatter — the photon loses part of its energy and changes direction; it is the commonest interaction in soft tissue at radiography and CT energies, lowers contrast and is the main source of staff dose. The (aluminium) filter removes low-energy photons that the skin would absorb without adding anything to the image — it reduces skin dose and "hardens" the beam.
The grid absorbs scattered radiation and improves contrast, but requires a higher dose; it can be omitted for thin body parts (limbs, small children). A focused grid whose distance does not match the focal distance causes grid cut-off. Raising kV lowers contrast and skin dose; raising mAs lowers noise but does not improve contrast. Increasing the object–detector distance increases magnification and blur (except for the air-gap technique, which reduces scatter); good sharpness comes from a small focal spot, a short object–detector distance and a long focus–detector distance. In CR the latent image is stored in a phosphor plate and read by a laser; in DR a flat-panel detector reads directly, or through a scintillator layer that converts X-rays into light. With automatic exposure control (photo-timer), too low a kV lengthens the exposure and gives the patient a higher dose.
Key points
- Characteristic radiation — set by the anode material; most radiation — bremsstrahlung.
- Photoelectric — depends on Z³, the source of contrast.
- Compton — scatter, lowers contrast, staff dose.
- Filter lowers skin dose; grid improves contrast and raises dose.
- Sharpness: small focal spot, object close to the detector.
Summary table
| Parameter or component | What it changes | Direction of change | Typical question |
|---|---|---|---|
| kV | Beam energy and penetration | High kV → low contrast, low skin dose | Why mammography uses low kV |
| mAs | Number of photons | High mAs → less noise, higher dose | Does mAs improve contrast? No |
| Filter | Removes soft photons | Low skin dose, "hard" beam | Role of the filter in the tube |
| Grid | Absorbs scattered radiation | Good contrast, higher dose | When it is omitted |
| Focal spot and distances | Sharpness and magnification | Small focus and short object–detector distance → sharp | What happens when object–detector distance increases |
Free exam questions
מה תפקיד הסריג( GRID ) במערכת DR ? (the question as examined, in Hebrew)
- א. מקצר את זמן הצילום ומונע תזוזה
- ב. גורם לקבלת תמונה בעלת ניגודיות טובה
- ג. מקטין את חשיפת החולה לקרינה
- ד. מעלה את כשר ההפרדה המרחבי של התמונה
מי מהשיטות הבאות לא משפרת קונטרסט ברקמה? (the question as examined, in Hebrew)
- א. הורדת kV
- ב. העלאת mA
- ג. שימוש בסורג( grid .)
- ד. מתן חומר ניגוד בוריד
In the full version for subscribers
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Sources
- Bushberg et al., The Essential Physics of Medical Imaging, 4th edition (2020) — X-ray production, X-ray tubes and generators; Radiography
- IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students, 2014 — Interactions of radiation with matter; Scattered radiation and grids
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.