Linear Tomography

For Linear Tomography, X-ray images are collected while the X-ray tube moves through a range of positions, generating images with a range of exposure angles.

(fig. 1)
Siemens Iconos R 200 X-ray machine with Linear Tomography capability. (From:
http://www.siemens.com)


PURPOSE OF LINEAR TOMOGRAPHY

Tomography can be defined as the radiographic technique that employs motion to show anatomical structures lying in a plane of tissue while blurring or eliminating the detail in images of structures above and below the plane of interest.


PRINCIPLES OF LINEAR TOMOGRAPHY

Tomographic principle: you must have 2 of the 3 elements in synchronous movement during your exposure (for tomography, this would be the tube and image receptor, while the patient lies still as in fig.1).

The motion of X-ray tube and film in linear tomography.
(fig.2)








The angle of movement is called the tomographic angle. You must have an adjustable fulcrum (pivot point) that allows you to change the area of beam focus. This is the "cm" adjustment you make between tomo slices, and should be adjusted to the height of the anatomy of interest. object Plane: region in which the image exhibits satisfactory recorded detail, and is controlled by the level of the fulcrum. This region will show the least amount of radiographic motion, thus will appear clearly defined on the radiograph. Anything above or below this plane will be blurred due to tube/receptor motion. (fig.3)

(fig.3)















Details from other planes in the object which would otherwise contribute confounding detail to the image, are blurred and effectively removed from visual consideration in the image. A variety of tomography techniques have been developed, which differ primarily in the manner in which the X-ray source and film move.

Linear tomography is one of the most basic techniques As the tube and film move from the first position to the second, all points in the object plane project to the same position on X-ray film.


(fig.4)













To complete the tomographic calculation, the relative positions of the X-ray tube, imager and subject must be precisely known. In the typical case the patient and imager are stationary and only the X-ray tube moves (as shown in fig.1)
Section thickness: Consider the diagrams below in which the the width of the object plane is controlled by the exposure (or tomographic) angle as seen in the diagram. The exposure angle is inversely proportional to section thickness. As exposure angle increases, section thickness decreases (fig.5). If you want the whole anatomy to appear more focused, you would have a narrow exposure angle (10 degrees for example), giving you a "thicker" cut (fig.6) The thinner your slice, the more detail for small parts will be visible.

(fig.5)


(fig.6)
















A tomogram showing the kidneys














Equipment used