6.5MedicalStretch

X-ray Imaging

A broken bone shows up sharply on an X-ray image while the surrounding muscle barely appears at all — not because bone and muscle look different to X-rays, but because they absorb them by very different amounts. Understanding that difference in absorption is the whole basis of X-ray imaging.

45 min Video by DrPhysicsA Module 6.5 Medical imaging

X Rays — A Level PhysicsWatch the full walkthrough before the notes below.

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What you'll be able to do

Describe how X-rays are produced in an X-ray tube
Explain the exponential attenuation of X-rays passing through matter
Use the attenuation equation $I = I_{0} e^{- μx}$
Explain how differences in attenuation produce image contrast

Producing X-rays

In an X-ray tube, a heated filament releases electrons by thermionic emission; these are accelerated through a large potential difference toward a metal target (often tungsten). Rapid deceleration of the electrons on impact produces a continuous spectrum of X-ray photons — this is $Bremsstrahlung$ ("braking radiation"), radiation emitted because a charged particle is decelerating.

Tip — A small fraction of the electron’s kinetic energy converts to X-ray photon energy at each deceleration event, which is why a continuous range of photon energies is produced, up to a maximum set by the tube’s accelerating voltage.

Attenuation

As X-rays pass through matter, their intensity decreases exponentially — $attenuation$ — due to absorption and scattering within the material.

I = I_{0} e^{- μx}

μ = attenuation (absorption) coefficient (m⁻¹), x = thickness of material.

1Use

I = I_{0} e^{- μx}

\frac{I}{I _{0}} = e^{- 0.30 \times 3.0} = e^{- 0.90}

Answer

\frac{I}{I _{0}} \approx 0.407

(about 41% transmitted)

Tip — The attenuation coefficient depends on both the material AND the X-ray photon energy — denser materials (like bone) and lower-energy X-rays are both attenuated more strongly.

Image contrast

X-ray imaging relies entirely on differences in attenuation coefficient between different tissues: bone (dense, high atomic number calcium content) attenuates X-rays far more strongly than soft tissue, so more X-rays pass through soft tissue to reach the detector, creating strong image contrast. Where two tissues have very similar attenuation coefficients (such as different soft tissues), a $contrast medium$ — for example, a barium meal, which strongly absorbs X-rays — can be introduced to artificially increase the contrast.

Tip — Good image contrast fundamentally requires a big DIFFERENCE in attenuation coefficient between adjacent structures — not just any absorption at all.

Equation recap

I = I_{0} e^{- μx}

X-ray attenuation.

Common mistakes to avoid

Thinking X-ray intensity decreases linearly with material thickness.

Attenuation is EXPONENTIAL, I=I₀e^(−μx), not a simple linear decrease.

Assuming the attenuation coefficient is a fixed property of a material alone.

μ depends on both the material AND the energy of the X-ray photons used — it is not a single fixed constant for a given tissue.

Believing image contrast just requires strong absorption everywhere.

Contrast requires a DIFFERENCE in attenuation between adjacent tissues — uniformly strong (or weak) absorption everywhere gives no contrast at all.

Key takeaways

X-rays are produced by rapid deceleration of electrons at a metal target (Bremsstrahlung), giving a continuous photon energy spectrum.
X-ray intensity attenuates exponentially through matter: I=I₀e^(−μx).
Image contrast arises from differences in attenuation coefficient between tissues; contrast media enhance this where natural differences are small.

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X-ray Imaging

45 min Video by DrPhysicsA Module 6.5 Medical imaging