c. Positioning Instructions. In polytomography, the tube movement is usually
selected on the basis of how well it will demonstrate a particular body part. Many
tomographic units, however, only have a rectilinear movement in one direction,
longitudinally to the long axis of the table. With a rectilinear movement, it is necessary
at times to alter the patient's position to achieve the best results. For maximum blurring,
the superimposed structures should be at a 90 angle to the tube movement.
(1) If, for example, the ribs are under study, the patient should be positioned
across the table. This will place the spine (the major superimposing structure) at a 90
angle to the tube movement; consequently, it will be blurred. But the ribs, positioned so
that they are parallel to the tube movement, will be clearly demonstrated. When there
are two major superimposing structures at right angles to each other, the patient may be
positioned at an angle of 25 to 30 to the direction of tube travel. An example of this is
the sternum. If the patient is aligned parallel to the tube travel, the spine will be parallel
with the tube movement and will not be effectively blurred.
(2) If the patient is positioned across the table, the ribs will not be effectively
blurred because the ribs will be parallel to the tube movement. The solution, then, is to
position the patient so that both the ribs and spine, each about 25 to 30 to the
direction of the table movement, will be blurred. This will provide a good demonstration
of the sternum.
d. Patient Preparation. During an average tomographic examination, the
patient will receive about the same amount of radiation that he would receive in a
fluoroscopic and radiographic examination of the spinal canal (myelography). This is
true because the mAs is increased and because several "cuts" are usually required in
one examination. To minimize the amount of radiation the patient receives, you should
see that a minimum of 4 mm of total filtration is provided.
Section IV. ORTHORADIOGRAPHY FOR SKELETAL SURVEYS
a. Orthoradiography is a radiographic procedure whereby the factor geometrical
distortion is eliminated or is minimized to such an extent that it is possible to record the
exact form and size of structures inside the body. It is of particular value in obtaining
accurate measurements of the long bones of the skeletal system. It may be used, for
example, to determine the length of the femur prior to insertion of an intermedullary nail
or to maintain an accurate record of progress or regress in certain pediatric cases.
b. In orthoradiography, a particular portion of the x-ray beam is used in such a
way as to protect a specific dimension of an object in true size. The image produced,
when measured, corresponds in size to the actual dimension of the object under