A-Level Physics May/June 2024 Q8(d): Explain why X-rays can be used to produce images of internal body structures that have…

1 answer

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  Internal body structures are composed of different materials, primarily bone and soft tissue.

  These different materials have significantly different linear attenuation coefficients ($μ$). Bone, being denser and containing elements with higher proton numbers (like calcium), has a much higher attenuation coefficient than soft tissue.

  Due to this large difference in attenuation, bone absorbs a much greater fraction of the incident X-rays compared to soft tissue. This results in a large difference in the intensity of the X-rays transmitted through the bone and the soft tissue. This significant variation in transmitted intensity is detected and creates an image with high contrast.

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  How the marks are awarded

  • B1 — The answer identifies the key internal body structures being compared, specifically 'bone and soft tissue'.
  • B1 — The answer explains the underlying physical difference, stating that these tissues have 'different linear attenuation coefficients' or that their absorption/transmission properties differ.
  • B1 — The answer links the difference in attenuation to the final image by explaining that it causes a 'large difference in the intensity of the X-rays transmitted', which is the definition of good contrast.

  Common mistakes

  • Simply stating that 'different parts of the body absorb X-rays' without specifying bone and tissue, and without mentioning the concept of attenuation.
  • Confusing contrast with resolution. An answer might discuss the short wavelength of X-rays allowing fine detail, which is not what the question is asking.
  • Failing to complete the explanation by linking differential absorption to the difference in transmitted intensity that forms the image.
  • Describing the process for only one tissue type (e.g., 'bone absorbs X-rays well') without the comparison to soft tissue, which is essential for explaining contrast.

  Examiner tip: For 'explain' questions, structure your answer as a logical chain of cause and effect, starting from the fundamental physical property and ending with the observed phenomenon.

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