RadiologyLesson 4 of 4 in the course Computed tomography

Hounsfield Units: CT Attenuation, Calibration, and Interpretation

5 flashcards · 4 quiz questions · 5 match pairs · 4 fill-in sentences · English

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Summary

The Hounsfield scale is used in computed tomography (CT) to describe how strongly materials attenuate X-rays. Its values, called Hounsfield units (HU) or CT numbers, help distinguish air, water, soft tissues, and bone in CT images.

The Hounsfield scale transforms a material’s linear attenuation coefficient, represented by μ\mu, into a relative value. By convention, distilled water at standard pressure and temperature is assigned 0 HU, while air is assigned −1000 HU.

All flashcards

What is the Hounsfield scale?
A CT scale for expressing radiodensity relative to reference materials; its values are called Hounsfield units (HU) or CT numbers.
How are water and air represented on the HU scale?
Distilled water at standard pressure and temperature is assigned 0 HU, while air is assigned approximately −1000 HU.
What does 1 HU represent approximately?
A change of approximately 0.1% of water’s attenuation coefficient.
Why are HU calibration checks useful in radiotherapy planning?
HU values may be converted into electron-density estimates, so reference-material checks help standardize scanner response.
Why are HU values not perfectly fixed across examinations?
Readings can vary with acquisition and reconstruction settings, contrast agents, and imaging modality; CBCT values are unreliable for HU-based material differentiation.

Quiz questions

  1. Why is calibration with reference materials especially important when CT is used for radiotherapy treatment planning?Answer: HU values may be converted into electron-density estimates used in planning.
  2. Which factor can cause HU readings to vary between CT examinations?Answer: Tube voltage, filtration, or reconstruction settings.
  3. Which CT modality supports HU-based material differentiation, unlike cone-beam CT?Answer: Medical-grade dual-energy CT.
  4. What does an adrenal lesion measuring less than 10 HU generally suggest?Answer: A relatively fatty lesion that is almost certainly a benign adenoma.

Based on the Wikipedia article Hounsfield scale, licensed CC BY-SA 4.0. Adapted into a summary, flashcards and quizzes.

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