X-ray Attenuation & Contrast
An X-ray image is formed because different paths leave different intensities at the detector. A visible image needs a difference between those detected intensities.
Thickness and material both affect transmission
X-rays obey the same exponential attenuation form used for ultrasound. A larger attenuation coefficient or a greater thickness leaves less intensity at the detector.
For several layers, add the separate attenuation products in the exponent.
An X-ray beam passes through 1.5 cm of material with μ = 0.80 cm−1. Find the transmitted fraction.
Show worked answer
About 30% of the incident intensity is transmitted.
Contrast is a detector difference
Definition
1 markWhat is meant by contrast in an X-ray image?
Model answer: The difference in the degrees of blackening of different parts of the image.
- If two paths leave very different detected intensities, the image has good contrast.
- If the detected intensities are similar, the difference in blackening is small and the contrast is poor (9702/42/O/N/25 Q10(b)(iii)).
- Contrast depends on the complete path: both the attenuation coefficient and the thickness of every material matter.
State what is meant by contrast in an X-ray image. [1]
Show worked answer
The difference in the degrees of blackening in the image.