X-ray physics notes curriculum
Fundamentals of radiation
The X-ray machine
Production of X-rays
Interaction of radiation with matter
X-ray detection and image formation
Image quality
Radiation safety in X-ray imaging
Fluoroscopy
Mammography (current module)
Mammography systems use X-ray spectra specifically tailored to image soft tissues of the breast.
This requires low photon energies (≈17–23 keV) to maximise photoelectric contrast while keeping the mean glandular dose as low as possible.
To achieve this, mammography X-ray tubes use special target materials, anode designs, and beam filtration distinct from those used in general radiography.
Target Materials and Characteristic Radiation
Purpose
The goal is to produce an X-ray spectrum concentrated around photon energies that enhance contrast between glandular and adipose tissue.
Key principle
Characteristic radiation is emitted when inner-shell (K-shell) electrons are ejected and outer electrons fill the vacancy.
The energy of this radiation is specific to the target element and defines the spectral peak.
| Target Material | K-shell binding energy (keV) | Characteristic peaks (keV) | Clinical use |
|---|---|---|---|
| Molybdenum (Mo) | 20.0 | 17.4 and 19.6 | Standard for fatty or average breasts |
| Rhodium (Rh) | 23.2 | 20.2 and 22.7 | Dense breasts (requires slightly higher energy) |
| Tungsten (W) | 69.5 | 59 and 67 (filtered for lower energies) | Digital systems; wide spectral range |
By selecting the appropriate target material, the X-ray spectrum is matched to the thickness and composition of the breast.
Beam Energy and Tube Potential
Typical tube potentials for mammography are 25–32 kVp, far lower than those in general radiography (50–120 kVp).
At these voltages, both Bremsstrahlung and characteristic radiation contribute to the output spectrum, but characteristic peaks dominate the useful range.
- Lower kVp → improved subject contrast (photoelectric interactions dominate).
- Higher kVp → reduced contrast but improved penetration for dense or thick breasts.
- Tube voltage is automatically selected by Automatic Exposure Control (AEC) based on breast thickness and density.
Target–Filter Combinations
Filtration in mammography fine-tunes the X-ray spectrum by removing both:
- Low-energy photons (which increase dose but contribute no image information), and
- High-energy photons (which reduce image contrast).
Filters are made of thin metallic foils chosen so that their K-edge matches the characteristic radiation of the target.
| Target–Filter Pair | Filter thickness (mm) | Application |
|---|---|---|
| Mo/Mo | 0.03–0.04 | Standard; thin or fatty breasts |
| Mo/Rh | 0.03–0.04 | Slightly higher effective energy; thicker or denser breasts |
| Rh/Rh | 0.05 | Dense breasts or thicker tissue |
| W/Rh or W/Ag | 0.05 | Digital systems with tungsten targets; broad spectral output |
K-edge filtration allows the beam to be tightly centred around 17–23 keV — the optimal range for soft-tissue imaging.
Anode Design and Focal Spot
Anode angle
- Mammography systems use steep anode angles (≈6–12°) to maintain coverage of the breast while achieving a small effective focal spot (0.1–0.3 mm).
- A smaller focal spot improves spatial resolution but limits tube loading.
Line focus principle
The effective focal spot is reduced by using an angled anode while maintaining a larger actual focal area to dissipate heat.
Heel effect
- More pronounced due to steep anode angle and large field coverage.
- Tube and detector are aligned so that the cathode side (more intense beam) is positioned toward the chest wall, compensating for greater breast thickness.
Key Points and Exam Tips:
- Mammography spectra are centred around 17–23 keV for optimal soft-tissue contrast.
- Target material determines the characteristic photon energies: Mo (17–19 keV), Rh (20–23 keV).
- K-edge filtration (Mo, Rh, Ag) selectively shapes the spectrum.
- Steep anode angle (6–12°) improves spatial resolution and field coverage.
- Heel effect is compensated by orienting the cathode toward the chest wall.
- kVp selection balances contrast and penetration; controlled automatically by AEC.
- Common exam question: “Explain how target and filter selection in mammography produces an X-ray spectrum suitable for breast imaging.”
Up next
Next, we will move on to Filtration and Beam Quality, where we will examine how K-edge filtration modifies the X-ray spectrum, defines the half-value layer (HVL), and optimises image contrast and patient dose in mammography.