X-Rays

Overview

This lesson develops X-rays as a medical imaging method. You will explain how X-rays are produced, calculate the minimum wavelength from accelerating voltage, then connect attenuation and contrast to plain X-ray images and CT scans.

What You Need to Know

• X-rays are produced when high-speed electrons are stopped or deflected by a metal target.
• The maximum photon energy comes from an electron losing all of its kinetic energy in one event, which sets the minimum X-ray wavelength.
• X-ray images depend on different tissues attenuating the beam by different amounts.
• Contrast is the difference in intensity or brightness between regions of an image.
• X-ray intensity in matter follows an exponential attenuation model.
• CT scanning combines multiple X-ray images from different angles to build 2D slices, then combines slices to form a 3D image.

How to Work Through It

1. Start by linking electron acceleration through a p.d. to kinetic energy.
2. Use energy conservation to calculate the minimum X-ray wavelength.
3. Compare how different tissues affect transmitted intensity and image contrast.
4. Trace the CT process from projections to slices to a reconstructed 3D image.

Check Your Understanding

• Why does increasing the accelerating p.d. decrease the minimum X-ray wavelength?
• What makes bone appear with stronger contrast than soft tissue in a plain X-ray image?
• What does the attenuation coefficient describe?
• Why does CT need many X-ray images from different angles?

Common Mistakes

• Thinking all X-ray photons have the minimum wavelength rather than a spectrum with a lower limit.
• Using wavelength equations without converting electronvolts and joules correctly.
• Describing contrast as sharpness rather than difference between regions.
• Saying CT is just a stronger X-ray rather than a reconstruction from many projections.

Next Steps

• Practise one minimum-wavelength calculation and one attenuation calculation.
• Bring nuclear decay ideas into the next lesson on PET scanning.