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 (current module)
Radiation safety in X-ray imaging
Fluoroscopy
Mammography
Contrast resolution (sometimes called contrast detectability) is the ability of an imaging system to represent differences in X-ray attenuation or signal intensity as visible differences in image brightness.
In other words, it reflects how finely the system can divide the full range of exposures into distinct grey levels.
It determines whether two tissues with similar densities (for example, liver and spleen) can be visualised as distinct regions in the image.
High contrast resolution is particularly important in soft-tissue imaging (e.g. abdominal radiography and mammography).
The Physics of Image Contrast
In radiography we can think of contrast in two distinct ways, subject contrast and image contrast.
Subject contrast arises from differential attenuation of X-rays as they pass through different tissues.
Image contrast is the ability for an imaging system to detect and display subject contrast. It depends on detector performance (how well signal is detected, stored and processed) and display processing (how brightness and contrast are mapped to grey levels and displayed).
Factors affecting subject contrast
| Parameter | Effect on Subject Contrast |
|---|---|
| kVp | ↑kVp → ↓ contrast (beam more penetrating, attenuation differences reduced) |
| Tissue composition | ↑ difference in Z or density → ↑ contrast |
| Thickness difference | Greater difference → ↑ contrast |
| Beam filtration | Removes low-energy photons → ↓ contrast |
| Scatter | Adds uniform background → ↓ contrast |
Factors affecting image contrast
1. Bit depth
The bit depth determines how many distinct grey levels can be used to represent the detected signal.
Number of grey levels = 2n
| Bit Depth (n) | Number of Grey Levels |
|---|---|
| 8-bit | 256 |
| 10-bit | 1024 |
| 12-bit | 4096 |
| 16-bit | 65,536 |
A higher bit depth provides finer gradations between pixel values. This enables detection of smaller differences in signal intensity (higher contrast resolution).
However, the full bit depth may not be visible on display monitors (typically 8–10 bits), so windowing is used to optimise visible contrast.
2. Detector performance (DQE and noise)
The ability to distinguish subtle differences is noise-limited. Even if two tissues produce slightly different signals, noise can obscure that difference if it’s larger than the signal difference itself.
Minimum detectable contrast ∝ 1/SNR
Thus:
- Higher SNR → smaller detectable contrast difference.
- High DQE detectors (efficient at preserving SNR) → better contrast resolution.
3. Windowing (width and level)
- Window width (WW): determines the range of pixel values mapped to grey scale.
- Narrow WW → fewer values displayed → high display contrast.
- Wide WW → more values displayed → low display contrast.
- Window level (WL): sets midpoint of displayed range → adjusts brightness.
Digital systems can therefore manipulate image contrast post-acquisition which is a major advantage over film-screen systems.
Key Takeaways and Exam Tips:
- Contrast resolution = ability to distinguish small differences in attenuation.
- Determined by SNR, bit depth, DQE, and beam energy (kVp).
- Lower kVp increases subject contrast but raises dose.
- High DQE and wide bit depth improve low-contrast detectability.
- Windowing allows post-acquisition contrast adjustment..
- Common exam question: “Define contrast resolution and list the factors that affect it in digital radiography.”
Up Next
Next, we’ll move on to Image Sharpness and Unsharpness, covering geometric, motion, and system unsharpness.