Reconstruction of the Human Airways - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 47

Fig. 3.2 Anatomical planes and position definition. The axial plane also called transverse plane

expressed in terms of Hounsfield units (HU), defined as

μ P − μ W

HU = 1000 ·

μ W

where μ P is the mean x-ray attenuation coefficient of the pixel, and μ W is the mean
x-ray attenuation coefficient of water. Bone structures attenuate the beam the greatest
while air has minimal attenuation properties.
The scanned raw images are a series of cross-sectional images containing different
greyscale information with respect to different organs or tissues. A typical CT image
has a possible 4,096 grey levels (12-bit image) in a 512

512 pixel resolution. The
greyscale represents a pixel map of the linear x-ray attenuation cogefficient or a
measure of the radio-density. These greyscale pixel values can be represented in
the Hounsfield scale (name after Sir Godfrey Hounsfield) and its typical values are
shown in Fig. 3.3 . Since each cross-sectional slice is separated by a known thickness
the pixel data in the 2D-slice changes into a volume element known as a voxel.
The risk of any x-ray scan is the exposure to radiation which is relatively higher
than other diagnostic x-ray procedures. Measurements of exposure are dependent

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