Biomedical Engineering Reference
In-Depth Information
FIGURE 15.10
Hounsfield scale for absorption coefficients.
The scale developed by Hounsfield that is shown in Figure 15.10 demonstrates the values
of absorption coefficients that range from air (-1,000) at the bottom of the scale to bone at
the top. This original scale covers some 1,000 levels of absorption on either side of water,
which is indicated as zero at the center. This is chosen for convenience, since the absorption
of water is close to that of tissue due to the high percentage of water that is found in tissue.
In using this scale, the number 1,000 is added to the absorption coefficient of each picture
element. In the process, the absorption value of air becomes zero, while that of water
becomes 1,000. Having assigned each pixel to a particular value in this scale, the system
is ready to display each element and create the reconstructed image.
EXAMPLE PROBLEM 15.9
Illustrate the process of back propagation.
Solution
The process can be illustrated by means of a simple analog. In the figure below, a parallel beam
of x-rays is directed past and through a cylinder-absorbing substance. As a result, a shadow of the
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