Optimising image quality and dose

Optimisation is the adjustment of imaging parameters and system design to ensure that:

• The image contains sufficient information for diagnosis (adequate quality), and
• The radiation dose to the patient is as low as reasonably achievable (ALARA), consistent with that aim.

It lies at the heart of radiation protection principles and is a central learning objective of the radiology physics curriculum.

The goal is not to maximise image quality, but to achieve the best diagnostic outcome for the least dose.

The SNR–Dose Relationship

• Image quality improves as radiation dose increases because more photons → higher SNR → lower noise.
• However, the improvement follows a square-root relationship: SNR ∝ √Dose
• Doubling dose only improves SNR by ~41%.
• Beyond a certain point, further dose increases bring little diagnostic benefit but greater patient risk.

This defines the point of optimisation. It’s the region where image quality is diagnostically sufficient, not necessarily perfect.

Parameters Affecting Image Quality and Dose

Let’s review and summarise some of the key parameters affecting image quality and dose.

Parameter	Effect on Image Quality	Effect on Dose	Optimisation Consideration
kVp	↑ kVp → ↓ contrast, ↑ penetration	↓ dose (per photon)	Use highest kVp consistent with adequate contrast (“high-kVp technique”)
mAs	↑ mAs → ↑ SNR, ↓ noise	↑ dose	Adjust to maintain adequate detector exposure
Filtration	Removes low-energy photons	↓ skin dose	Added filtration improves beam quality
Collimation	Reduces scatter → ↑ contrast	↓ dose	Always collimate to anatomy of interest
Grids	↓ scatter → ↑ contrast	↑ dose (Bucky factor)	Use only when necessary (thick body parts)
SID / OID	Affects magnification and unsharpness	Minor	Use appropriate geometry
AEC (Automatic Exposure Control)	Ensures consistent detector exposure	Optimises dose automatically	Regular calibration required

The Role of Exposure Index (EI) and Deviation Index (DI)

Digital systems monitor the amount of radiation reaching the detector using exposure indices.

• EI (Exposure Index): proportional to detector air kerma. This is measured at the detector.
  • Ensures exposures are neither too low (noisy) nor too high (unnecessary dose).
• DI (Deviation Index):

DI = 10log⁡₁₀(EI/EI_T)

where EI_T = target EI.

• DI = 0 → correct exposure
• DI = +1 → twice the target exposure (≈ +26% dose)
• DI = –1 → half the target exposure (≈ –26% dose)

Deviation index gives a measurement for how closely the measured exposure at the detector and target exposure match. A wide discrepancy will prompt the operator to adjust scanning parameters.

Purpose: Feedback tool to prevent dose creep. The gradual increase in exposure settings over time due to digital post-processing hiding underexposure noise.

What strategies can we use to optimise dose and image quality?

A. Technique Optimisation

1. Use the highest kVp compatible with diagnostic contrast.
2. Use AEC or standard exposure charts for consistent detector exposure.
3. Collimate tightly to the region of interest.
4. Use added filtration (e.g. 2.5 mm Al) to remove low-energy, non-contributory photons.
5. Apply pulsed fluoroscopy and last image hold to minimise dynamic exposure.
6. Avoid grids for thin anatomy (<10 cm); use appropriate grid ratio for thicker parts.

B. System and Detector Optimisation

• High DQE detectors require fewer photons for the same SNR → lower dose.
• Maintain flat-field calibration to prevent noise amplification.
• Ensure monitor calibration and ambient light control for consistent perceived image quality.

C. Operator and Clinical Protocol Optimisation

• Tailor exposure parameters to patient size and region (paediatric vs adult, chest vs abdomen). See table below.
• Use standardised protocols to ensure reproducibility and auditability.
• Regular training and feedback to maintain awareness of dose indices and optimisation principles.

Increase in Parameter	Effect on Contrast	Effect on Noise (SNR)	Effect on Dose
↑ kVp	↓	↑ (better penetration)	↓
↑ mAs	—	↓ (improves SNR)	↑
↑ Filtration	↓ (harder beam)	↓ (removes low-E scatter)	↓
↑ Collimation	↑	↓ (less scatter)	↓
Use of Grid	↑	↓	↑

Key Takeaways and Exam Tips:

• Optimisation = adequate image quality at minimal dose (ALARA).
• SNR ∝ √Dose, there are diminishing returns beyond diagnostic threshold.
• EI / DI provide essential dose feedback for digital systems.
• Optimise using high kVp, low mAs, tight collimation, and AEC.
• High DQE detectors improve dose efficiency.
• Avoid “dose creep” by regular auditing of exposure indices.
• Common exam question: “Describe how image quality and radiation dose are optimised in digital radiography.”

Up Next

You’ve now completed the Image Quality and Optimisation section, an essential bridge between image physics and radiation protection.

The next major section in the curriculum naturally follows as Radiation Safety in X-ray Imaging.