MAPPING ANATOMY - Radiation Oncology: A Physicist's-Eye View - page 28

of the tissues is measured, resulting in an array of values of that
property. From such an array one can generate an image, the intensity
of which at any point is proportional to, or related to, the value of the
property at that point. The terms series and study are used here
interchangeably to describe a sequence of 2D sections which, together,
comprise a 3D data set.
Although the patient extends in three-dimensions, patient anatomy
can only be displayed on a screen or on paper as two-dimensional
(2D) images. These can be of two basic types:
Sectional images
A sectional image is a two-dimensional representation of a thin
“section” or “slice” taken through a 3D data set. Usually the slice is a
plane, but it can also be curved. A conventional single-slice CT scan-
ner, for example, produces a sequence of parallel sectional images -
usually, but not necessarily, transverse sections. A B-mode ultra-sound
scan also provides a sectional image. In sectional images the anatomic
information comes from a very thin slice of the patient and, therefore,
there is hardly any superimposition of anatomic information; the inten-
sity at a point in the image corresponds to a property of the tissue
at that point.
Projection images
The prime example of a projection image is an X-ray radiograph. In
such a radiograph, the image intensity at any point in the radiograph
is related to the attenuation of the X-rays by all the tissues which lie
between the radiation source and the point in question. In this case,
the projection is what is termed a perspective projection as the lines
appear to diverge from a well-defined point in space. DRRs (see
below) are another example of projection images.
An under-valued form of projection image is the photograph. A
photograph of the patient's skin surface taken from the presumed
source of radiation can be a valuable guide in planning and verify-
ing radiation therapy. A photograph differs from the usual projection
image in that it is not so much a superposition of information as a
map of the closest visible points on the surface of the patient.
C OMPUTED T OMOGRAPHY (CT)
X-radiographs are two-dimensional, formed by superimposing infor-
mation about the tissues lying between the source and a point in