Image contrast

Image contrast is the difference in signal intensity between two regions of the image.

It reflects how differently X-rays are attenuated by different tissues, a property known as differential attenuation.

Relationship Between Attenuation and Contrast

Contrast arises because different tissues absorb X-rays to different degrees:

• Bone and contrast media (high Z) → strong photoelectric absorption → appear white.
• Soft tissue (low Z) → moderate attenuation → grey.
• Air → minimal attenuation → black.

Contrast ∝ Difference in attenuation coefficients between tissues.

As we’ve seen when looking at the linear attenuation coefficient, the degree of differential attenuation (and therefore contrast) depends on photon energy, atomic number, and tissue thickness.

The Role of Scatter Radiation

Scatter radiation refers to photons that have changed direction after interacting with the patient (mainly via Compton scatter). Some reach the detector, contributing to a uniform background exposure that reduces image contrast.

If the primary intensity reaching the detector is I_p​ and scattered intensity is I_s​, then:

Image contrast ∝ Iₚ / (Iₚ + Iₛ)

As I_s​ (scatter) increases, the ratio decreases and contrast falls. The goal is to maximise primary radiation and minimise scatter at the detector.

Consequences of Scatter

• Reduces contrast (adds unwanted fog).
• Decreases image sharpness and detail visibility.
• Increases exposure to staff.

Factors Increasing Scatter Production

• High kVp (more Compton interactions).
• Large field size (more irradiated volume).
• Greater patient thickness (more opportunities for scatter).

How can we reduce scatter?

1. Beam Collimation

• Limits field size → fewer photons interact with tissue → less scatter produced.
• Most effective and dose-efficient method.

2. Anti-Scatter Grids

• Lead strips absorb scattered photons before they reach the detector.
• Improves contrast but increases dose (requires higher mAs to maintain exposure).
• Grid ratio = height / spacing of lead strips.
• High-ratio grids remove more scatter but require precise alignment.

3. Air Gap Technique

• Increasing distance between patient and detector allows scattered photons to diverge and miss the detector.
• Improves contrast at the cost of slight magnification.

4. Tissue Compression

• Reduces tissue thickness and scatter volume.
• Used effectively in mammography.

5. Lowering kVp (where appropriate)

• Reduces Compton scatter fraction.
• Improves contrast but increases dose.

Summary

Method / Parameter	Effect on Scatter	Effect on Contrast	Effect on Dose
Collimation (↓ field size)	↓	↑	↓
Grid use	↓	↑	↑
Air gap	↓	↑	Slight ↑ (magnification)*
Compression	↓	↑	↓
High kVp	↑	↓	↓
Low kVp	↓	↑	↑
Contrast agents	N/A	↑↑	↑

Key Takeaways and Exam Tips

• Image contrast arises from differential attenuation between tissues.
• Scatter (mainly Compton) reduces contrast by adding uniform background exposure.
• Contrast ∝ Iₚ / (Iₚ + Iₛ). Minimise scatter to maximise contrast.
• Best ways to reduce scatter: collimation, grids, compression, air gaps.
• kVp selection: trade-off between dose and contrast.
• Contrast agents enhance photoelectric absorption in soft-tissue imaging.
• Common exam question: “Explain how scattered radiation affects image contrast and describe methods used to reduce it.”

Up next:

We’ve reached the end of Section 4 – X-ray Interaction with Matter

You’ve now covered:

1. Photoelectric absorption – the physics behind image contrast.
2. Compton scatter – the main source of image degradation.
3. Attenuation and HVL – how beam penetration and quality are quantified.
4. Scatter control – practical strategies for maintaining image quality and optimising dose.

Our next section will be X-ray detection and image formation.

*Why dose can slightly increase with the air gap technique

There are two main reasons:

1. Increased geometric magnification → larger apparent field

• The detector is further away from the patient, but the X-ray beam still diverges from the same focal spot.
• To cover the same anatomy on the detector, you must open the collimator slightly or use a larger beam area.
• A larger irradiated field means more tissue is exposed, so total patient dose increases slightly.

2. Reduced intensity at the detector → compensation with higher exposure

• Because of the inverse square law, moving the detector further away reduces the X-ray intensity at the detector (intensity ∝ 1/d²).
• To maintain the same image brightness or receptor exposure, radiographers often increase mAs.
• That increase in tube output results in a slightly higher patient entrance dose, even though less scatter reaches the detector.