Biomedical Engineering Reference
In-Depth Information
Positional information, such as how far the scanning frame was along its scan
Reference information from the reference detector, which monitored the x-ray output
Calibration information, which was obtained at the end of each scan
The bulk of the readings—the actual absorption information obtained from the detectors
Electronic interfaces are now available that convert this information to digital form
and systematically store it in the memory of the resident computer. The next step is the
actual process of image reconstruction (Figure 15.14).
It will be recalled that the first stage of this computation process is to analyze all of the
raw data and convert them into a set of profiles, normally 180 or more, and then convert
these profiles into information that can be displayed as a picture and used for diagnosis.
This is the heart of the operation of a CT scanner—that element that makes CT totally
different from conventional x-ray techniques and most other imaging techniques. The algo-
rithms or computer programs for reconstructing tissue x-ray absorption properties from a
series of x-ray absorption profiles fall into the categories of simple back-projection (some-
times called summation), Fourier transforms, integral equations, or series expansions. The
choice of an algorithm for a CT scanner depends on the algorithm's speed and accuracy.
Mathematical algorithms for taking the attenuation projection data and reconstructing an
image can be classified into two categories: iterative and analytic. The iterative techniques
(also known as the algebraic reconstruction technique [ART]), such as the one used by
Hounsfield in the first-generation scanner, require an initial guess of the two-dimensional
pattern of x-ray absorption. The attenuation projection data predicted by this guess are then
calculated and the results compared with the measured data. The difference between the
measured data and predicted values is used in an iterative manner so the initial guess is
modified and that difference goes to zero. In general, a large number of iterations are
required for convergence, with the process usually halted when the difference between the
FIGURE 15.14
A modern display terminal for a CT scanner.
