Attenuation
Medical Imaging - OCR A-Level Physics
Key Definition
Attenuation
The reduction in intensity of X-rays as they pass through a material, due to absorption and scattering.
The reduction in intensity of X-rays as they pass through a material, due to absorption and scattering.
$$I = I_0 e^{-\mu x}$$
- The attenuationThe reduction in intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. of radiation (such as X-rays) as it passes through a material, due to absorption and scattering. coefficient μ depends on the material (densityMass per unit volume of a material. Measured in kg m⁻³. and atomic numberThe number of protons in the nucleus of an atom (also called proton number, Z).) and the photon energyThe capacity to do work. Measured in joules (J)..
- Bone (high densityMass per unit volume of a material. Measured in kg m⁻³., high Z) has a much larger μ than soft tissue - this creates the contrast in an X-ray image.
- The half-value thicknessThe thickness of material that reduces the intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. of X-rays to half its original value. (HVT) $x_{1/2}$ is the thickness that reduces intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. to half: $x_{1/2}$ = $\frac{\ln 2}{\mu}$.
- Contrast mediaSubstances (e.g. barium, iodine) injected or ingested to increase X-ray attenuation differences between tissues. (e.g. barium meal for the gut, iodine for blood vessels) are high-Z substances that absorb X-rays more than surrounding tissue, making soft-tissue structures visible.
Common Mistake
MEDIUM
Wrong: Halving the thickness halves the intensity.
Right: X-ray attenuationThe reduction in intensity of radiation (such as X-rays) as it passes through a material, due to absorption and scattering. is exponential, not linear. Halving the thickness does NOT halve the intensity. Use $I = I_{0} e^(-\mu x)$ for the correct calculation.
Right: X-ray attenuationThe reduction in intensity of radiation (such as X-rays) as it passes through a material, due to absorption and scattering. is exponential, not linear. Halving the thickness does NOT halve the intensity. Use $I = I_{0} e^(-\mu x)$ for the correct calculation.