In CT imaging, which interaction of X-rays with tissue is MOST responsible for image contrast at the energies used in diagnostic CT (80–140 kVp)?

• A Compton scatter ✓
• B Photoelectric effect
• C Pair production
• D Coherent (Rayleigh) scatter

Explanation

At diagnostic CT energies (80–140 kVp), Compton scatter is the dominant interaction of X-ray photons with soft tissue, accounting for most of the X-ray attenuation. Compton scatter is energy-dependent and proportional to electron density. The photoelectric effect predominates at lower energies and is important for contrast between bone and soft tissue (high Z difference), and for contrast media. Pair production requires >1.022 MeV (therapeutic radiotherapy range). Coherent scatter occurs at very low energies and contributes minimally.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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