On a non-contrast CT head, streaks are seen between the petrous temporal bones at the skull base, and soft tissue values are falsely elevated toward the centre of the scan compared with the periphery. Which physical process explains this appearance?
- A Preferential absorption of low-energy photons as the beam passes through dense bone, shifting the effective energy of the beam upward ✓
- B Scattered photons from adjacent slices entering the detector array
- C Averaging of bone and soft tissue attenuation within a single voxel
- D Overshooting of the reconstruction filter kernel at sharp density transitions
Correct answer: A. Preferential absorption of low-energy photons as the beam passes through dense bone, shifting the effective energy of the beam upward
Explanation
The polychromatic X-ray beam becomes harder as lower-energy photons are preferentially absorbed by dense structures such as bone. The detector then records attenuations inconsistent with the assumed monoenergetic beam, producing streaks between dense bones and cupping, where central values appear higher than peripheral ones. Voxel averaging describes partial volume effect, scatter relates to noise and shading, and filter overshoot produces edge enhancement, none of which cause cupping.
Reference: Bushberg The Physical Principles of Medical Imaging, 4th ed.
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Written and medically reviewed by the StethoPrep medical team.