The GATE Grind

GATE 2024 BM – Question 27

Medical Imaging Systems · X-Ray and Computed Tomography · 1 mark · Multiple select

Monochromatic gamma photons enter homogeneous tissue. Which relationships hold for half-value-layer thickness?

  1. First HVL thicker than second
  2. Second HVL thicker than first
  3. All HVLs equal
  4. First/second HVL ratio changes with incident intensity

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Correct answer: (C) All HVLs equal

Explanation

For fixed photon energy and homogeneous tissue, attenuation is I=I0e^(−μx) with constant μ. Every factor-of-two reduction requires the same distance ln2/μ, independent of depth or initial intensity. Hence C only; beam-hardening differences of polychromatic beams do not apply.

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