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 employs a dedicated geometric design and controlled breast compression to optimise spatial resolution, reduce scatter, and minimise patient dose.
Precise alignment of the X-ray tube, compression paddle, and detector ensures consistent image quality and reproducible positioning across examinations.
System Geometry
Mammography systems use a dedicated, fixed geometry optimised for small field sizes and soft-tissue contrast.
| Parameter | Typical Value | Purpose / Effect |
|---|---|---|
| Source-to-image distance (SID) | 60–70 cm | Balances magnification and geometric unsharpness |
| Anode angle | 6–12° | Provides uniform coverage and small effective focal spot |
| Focal spot size | 0.3 mm (standard), 0.1 mm (magnification) | Determines limiting spatial resolution |
| Field size | ~24 × 30 cm | Matches average breast dimensions |
| Tube orientation | Cathode near chest wall | Compensates for heel effect and tissue thickness |
Line focus principle
The apparent focal spot is reduced by angling the anode.
This provides high spatial resolution while maintaining sufficient heat capacity in the actual focal track.
Heel Effect in Mammography
Due to the steep anode angle, the heel effect (variation in X-ray intensity across the field) is more pronounced in mammography.
Intensity decreases from the cathode side (chest wall) toward the anode side (nipple).
To ensure uniform image brightness:
- The tube is positioned so that the cathode faces the chest wall, compensating for greater tissue thickness.
- The anode side faces the thinner peripheral breast region.
This alignment balances beam intensity across the image receptor.
Compression: Purpose and Mechanism
The breast is compressed between a clear plastic paddle and the image receptor during every mammography exposure.
Compression is an essential design feature, not simply for positioning, but for image optimisation.
Benefits of compression include:
| Benefit | Physical Principle / Outcome |
|---|---|
| Reduced tissue thickness | Less attenuation → lower dose and scatter |
| Reduced motion blur | Due to immobilisation |
| Improved image contrast | Reduced scatter fraction → higher CNR |
| Uniform tissue thickness | Reduces dynamic range requirements on detector |
| Better detail visibility | Brings anatomy closer to detector, reducing geometric unsharpness |
Compression therefore directly improves image quality while simultaneously lowering radiation dose.
Typical Compression Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Force applied | ≈100–200 N | Adjusted automatically for comfort and breast size |
| Compressed thickness | 2–8 cm typical | Determines kVp and filter selection via AEC |
| Exposure time | 0.5–1.5 s | Shorter due to reduced motion and tissue attenuation |
Compression should be firm enough to achieve tissue uniformity and dose efficiency, but not so high as to cause excessive discomfort or risk tissue injury.
Magnification Mammography
Magnification views are used to evaluate microcalcifications or small lesions requiring fine detail.
Principle
Magnification is achieved by:
- Increasing object-to-image distance (OID) by raising the breast above the detector.
- Using a smaller focal spot (0.1 mm) to limit geometric unsharpness.
- No anti-scatter grid (increased OID reduces scatter naturally).
Magnification improves spatial resolution at the expense of increased exposure and small loss of sharpness due to focal spot blur.
Scatter Reduction
In addition to compression, mammography employs focused anti-scatter grids to further improve contrast.
| Feature | Typical Specification |
|---|---|
| Grid ratio | 4:1–5:1 |
| Grid frequency | 30–50 lines/cm |
| Bucky factor | ≈2 (increases dose slightly) |
| Moving grid mechanism | Reduces grid-line artefacts |
Some digital systems use slot scanning or software-based scatter correction, eliminating the need for a physical grid.
Key Points and Exam Tips:
- Mammography systems use fixed geometry (SID ≈ 65 cm) with a small focal spot (0.1–0.3 mm).
- The cathode side of the tube faces the chest wall to compensate for the heel effect.
- Compression is essential for image quality and dose reduction.
- Magnification views use small focal spots and increased OID for improved detail.
- Scatter reduction is achieved through compression, grids, or digital correction.
- Common exam question: “Explain the purpose of breast compression in mammography and describe its effects on image quality and radiation dose.”
Up next
Next, we’ll briefly discuss Image quality considerations in mammography. We’ve discussed image quality before, I just want to highlight key considerations in mammography.