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IMG-24

Hounsfield unit

HU = 1000 (μ − μw)/μw. Water is 0, air is −1000.

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CTHounsfield

Governing equation

HU=1000(μ−μw)/μw\mathrm{HU}=1000(\mu-\mu_w)/\mu_w

where

\mu
Attenuation μ (1/cm)
\mu_w
Water μ (1/cm)
HU
Hounsfield unit (HU)

Lecture brief

Historical brief

Larmor precession, CT Beer projections, SNR and Nyquist sampling are why MRI and CT images exist as numbers. The lab computes frequency, dose and resolution limits. This sheet (IMG-24 — Hounsfield unit) is the form associated with Hounsfield. Working symbols: μ\mu, μw\mu_w →\rightarrow HUHU. Bone ~ +400 to +1000, fat ~ −50 to −100. kV and reconstruction kernel shift HU slightly.

Purpose

Purpose: compute HUHU from μ\mu, μw\mu_w in Medical imaging via HU=1000(μ−μw)/μw\mathrm{HU}=1000(\mu-\mu_w)/\mu_w HU = 1000 (μ − μw)/μw. Water is 0, air is −1000. Use it when a real medical imaging question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given μ=0.220 1/cm\mu = 0.220\,\mathrm{1/cm}, μw=0.200 1/cm\mu_w = 0.200\,\mathrm{1/cm}, the governing relation HU=1000(μ−μw)/μw\mathrm{HU}=1000(\mu-\mu_w)/\mu_w yields HU=100 HUHU = 100\,\mathrm{HU}. A grey scale anchored at water and air. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Outputs

  • Hounsfield unit HU100 HU
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IMG-24 · spectrum
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Narration of this film

A grey scale anchored at water and air.

Bone ~ +400 to +1000, fat ~ −50 to −100. kV and reconstruction kernel shift HU slightly.

Reading speed

Watch on YouTube