Reconstruction of the Human Airways - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 46

Fig. 3.1 Three different types of computed tomography scanners and how they work

2004). During a typical CT procedure, the patient is placed on a table. The table then
moves the patient in a supine position through a rotating gantry (a donut-shaped
device), which houses the x-ray tube and electronic x-ray detectors. Multiple x-rays
are taken in thin cross-sections in the region of interest along the person's body form-
ing slices (like slicing a loaf of bread). The detectors collect the x-ray information
from each cross-section and send them to a computer that combines them into a
single image. In helical (spiral) CT continuous rotation can occur which averts the
need to stop and start (Fig. 3.1 ).
These helical CT scanners provide quick scans which are significant for entire
anatomic regions such as the lung which have to be scanned with the patient in
one position and within one breath hold. The general procedure may include the
following steps:

• A large drink or intravenous injection containing a type of dye may be given to
the patient if a contrast agent is needed to enhance pictures.
• All metal objects need to be removed.
• The patient lies on the scanner in a supine position and is moved through the
rotating gantry.
• The gantry moves in a circle around the patient as it takes the x-ray images, taking
less than a second per revolution.
• Depending on the medical investigation, the CT scan typically take anywhere
from 5-30 min.
The difference between a CT scan and a regular conventional x-ray, is that a CT
study comprises of a set of 2D-cross-sectional slices instead of a single 2D image that
represents a 3D body structure volume that have tissues and structures superimposed
onto a 2D image plane. Therefore CT scans can be read into various planes of view
(sagittal, coronal, or axial) or even as a 3D object (see Fig. 3.2 for all anatomical
position definitions).
Each individual CT image is typically acquired with a matrix of 512 512 pixels

and a resolution of 16-bit. During the scan the x-ray beam consisting of photons
pass through, absorbed or redirected (i.e. scattered) by a structure which reduces the
strength of the x-ray beam. The degree to which an x-ray beam is reduced is called
attenuation. Each pixel is assigned a numerical value (gray scale), based onthe degree
to which the structure corresponding to that pixel attenuates the x-ray beam. This is

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