X-rays in Medical Diagnosis – Absorption, X-ray Spectrum and CT
PhysicsQuantum & Nuclear PhysicsAges 17–18Virtual lab
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An X-ray beam passes through different tissue layers: soft tissue absorbs little, bone absorbs much more, giving the image on the film its contrast. Adjust the tube voltage, tissue thickness and tissue type to see how the absorption law I = I₀·e^(−µx) determines image quality, and why the patient's radiation dose must be weighed carefully. The X-ray spectrum screen shows the continuous spectrum with its shortest wavelength λmin = hc/(eV) and the anode's characteristic Kα and Kβ lines; record λmin against V to find Planck's constant. The CT scan screen takes many projections of an abdomen slice and rebuilds it by filtered back-projection, with CT numbers (HU), contrast medium and dose.
Lesson: Physics applications in medical diagnosis
What it shows
X-rays are attenuated exponentially in matter, I = I₀·e^(−µx); bone has a larger µ than soft tissue, which gives a radiograph its contrast, and harder X-rays (higher kV) penetrate better but give less contrast. The X-ray spectrum screen shows how a tube produces them: a continuous bremsstrahlung spectrum that ends at λmin = hc/(eV), plus the anode's characteristic Kα and Kβ lines once eV exceeds the K-shell binding energy. The CT scan screen takes projections of an abdomen slice from many angles and rebuilds it by filtered back-projection, showing CT numbers, the effect of a contrast medium and the trade-off between dose and image noise.
How to use
On Absorption, move Thickness, Tube voltage and Bone fraction and compare contrast and relative dose. On X-ray spectrum, change Tube voltage V, Anode (target), Tube current and Aluminium filter, press Record at several voltages and use the graph of λmin against 1/V to find h. On CT scan, press Scan, lower the Number of projections, untick Filter before back-projection, tick Contrast medium injected and tap the image to read CT numbers.
Parameters you can change
- Tube voltage 40–140 kV
- Body part thickness 2–30 cm
- Bone fraction along the path 0–60 %
- Screen Absorption, X-ray spectrum, CT scan
- Tube voltage (spectrum) 10–150 kV
- Anode material Tungsten, Molybdenum, Copper
- Tube current 1–10 mA
- Aluminium filter thickness 0–5 mm
- Spectrum horizontal axis Wavelength, Photon energy
- CT projections 1–180
- CT dose Low, Standard, High
- Filter in CT reconstruction
- Contrast medium
Questions to explore
- Why does bone look lighter than soft tissue on the X-ray image?
- Why does raising the tube voltage shorten λmin but leave the Kα line at the same wavelength?
- Why does a CT image made from only a few projections show streaks, and what does the filter change?
