(2) Elliptical. The elliptical movement appears as an oblong circle. Most
polytomographic units will produce an elliptical movement longitudinally, crosswise, or
diagonally to the long axis of the table. It is usually used for lateral projections of the
lungs. It is also excellent for studies of the cervical and dorsal spines.
(3) Hypocycloidal. Hypocycloidal is the most complex of the
polytomographic movements. It makes a pretzel-like cut that produces the most
complete blurring. Virtually all overlying and underlying structures, bony or otherwise,
can be totally obscured by this movement. It can be used for skull detail and is
excellent for joint spaces.
4-17. PRINCIPLES OF TOMOGRAPHY
a. Apparatus. Basically, a rectilinear tomographic apparatus consists of a
standard radiographic table, a movable cassette tray, and a movable tube head.
(Polytomographic apparatus is designed specifically for polytomography and is much
more complex than rectilinear apparatus.) The tube head and the cassette tray are
attached to opposite ends of a vertical bar which is provided at a selected point between
the tube and the cassette tray.
b. Fulcrum Point. The point or level at which the vertical bar is pivoted is
known as the fulcrum point. At this point, there is controlled minimum motion. This is
the operating principle of tomography; above and below this point, there is motion when
the tube is moved. The fulcrum point also determines the focal plane.
c. Focal Plane. The level or plane of the body section to be examined is known
as the focal plane or datum plane. The level corresponds to the fulcrum point.
d. Operation. As the tube travels in one direction, the cassette tray travels in
the opposite direction. The exposure is made during this movement. Shadows of the
image at the focal plane will appear on the same area of the film throughout the
movement and exposure. This is true because at the fulcrum point or focal plane, there
is minimal movement. On the other hand, the shadows of all other planes below and
above the focal plane will be projected on different portions of the film during the
movement. The result will be a sharp and discernible image at the focal plane. All
other images will be blurred out and therefore, indistinct.
4-18. FACTORS OF SECTION THICKNESS
The thickness of the layer, commonly referred to as the cut, is measured in
millimeters (mm) and is controlled by two factors: the amplitude of tube travel and the