Biomedical Engineering Reference
In-Depth Information
cylinder is cast on the x-ray film. The density of the exposed and developed film along the line AA
can be regarded as a projection of the object. If a series of such radiographs are taken at equally
spaced angles around the cylinder, these radiographs then constitute the set of projections from
which the cross section has to be reconstructed.
An approximate reconstruction can be reproduced by directing parallel beams of light through
all the radiographs in turn from the position in which they were taken, as shown below.
The correct cross section can be reconstructed by back-projecting the original shadow, as
shown above, and subtracting the result of back-projecting two beams placed on either side of
the original shadow as shown in the following. Mathematically, this is the equivalent of taking
each transmission value in the projection and subtracting from it a quantity proportional to
adjacent values. This process is called convolution and is actually used to modify projections.
In the display provided by any medical imaging device, factors affecting the resolution
or accuracy of the image must be recognized and modified. Since the information provided
by a CT scanner in essence reflects the distribution of x-ray absorption in a cross section of
a patient, the smallest change in x-ray absorption that can be detected and projected in
the image is defined as the density resolution. In addition, the smallest distance apart
that two objects can be placed and still be seen as separate entities is defined as the spatial
