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)
The image quality requirements in mammography are more demanding than in any other diagnostic X-ray modality.
Breast tissue offers minimal inherent contrast, and clinically significant lesions (such as microcalcifications) are small and often subtle.
Mammography therefore relies on optimised system design and precise exposure control to achieve high spatial resolution, excellent contrast, and minimal noise, all at the lowest possible dose.
1. Spatial Resolution
Spatial resolution defines the ability to visualise fine detail and separate closely spaced structures.
In mammography, this determines the visibility of microcalcifications (≈ 100–200 µm) and fine spiculations in soft-tissue masses.
| Determinant | Optimisation in Mammography |
|---|---|
| Focal spot size | 0.3 mm (standard), 0.1 mm (magnification) for reduced geometric unsharpness |
| Detector pixel size | 50–100 µm depending on detector type |
| System geometry | Short SID (≈ 65 cm) and fixed alignment reduce magnification variability |
| Motion control | Compression immobilises tissue, enabling short exposure times |
Typical limiting resolution:
- Digital systems: 5–10 line pairs/mm
- Film–screen systems: up to 15 lp/mm (though now obsolete)
2. Contrast Resolution
Mammography operates in the low-energy range (17–23 keV), where photoelectric interactions dominate, producing strong attenuation differences between glandular and adipose tissue.
However, those differences are still small (≈ 1–2% attenuation variation), so maintaining contrast is a key design objective.
The following factors serve to optimise contrast.
| Factor | Effect on Contrast |
|---|---|
| Low tube potential (25–32 kVp) | Enhances photoelectric contrast |
| Target–filter selection (Mo/Mo, Rh/Rh, W/Rh) | Shapes spectrum to match breast composition |
| Compression | Reduces scatter, improves contrast-to-noise ratio (CNR) |
| Anti-scatter grid | Further improves contrast; Bucky factor ≈2 |
| Digital image processing | Allows fine contrast enhancement and adaptive windowing |
Proper selection of target/filter combination and adequate compression are the most effective methods for improving contrast without increasing dose.
3. Noise and Signal-to-Noise Ratio (SNR)
We’ve discussed this in detail before. But to revise.
At mammographic photon energies, image noise is primarily quantum noise, arising from the statistical variation in photon detection.
SNR ∝ √N
where N is the number of detected photons.
Since dose is proportional to photon fluence, increasing dose improves SNR — but only with diminishing returns.
Optimisation
- Use detectors with high Detective Quantum Efficiency (DQE) (typically 0.6–0.8).
- Maintain appropriate exposure levels via Automatic Exposure Control (AEC).
- Minimise system and electronic noise (especially in direct a-Se detectors).
- Apply temporal and spatial noise reduction algorithms in digital systems.
High SNR ensures the visibility of low-contrast lesions, especially in dense breast tissue.
Key takeaways and exam tips:
- Mammography requires exceptional spatial and contrast resolution due to subtle tissue differences and small lesion size.
- Operates in the 17–23 keV range, optimising photoelectric contrast.
- Compression is essential for dose efficiency, uniform exposure, and motion control.
- High DQE detectors maintain adequate SNR at low doses.
- Common exam question: “Describe the image quality requirements of mammography and the factors influencing contrast and resolution.”
Up next
Next, we will move on to Tomosynthesis and Advanced Mammography Techniques, covering the physics principles of digital breast tomosynthesis (DBT), its advantages and limitations, and emerging technologies such as contrast-enhanced spectral mammography.