Presentation for Prof. Carpenter

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What is Linear Tomography ?

Tomography is imaging by section or sectioning. The word was derived from the Greek word "tomos", which means "a section", "a slice", or "a cutting" and "graphien", which means to write or document.
Linear Tomography is a radiographic technique that uses motion to demonstrate anatomy lying in a plane of tissue, while blurring or eliminating structures above and below the plane of interest. This technology led to the development of CT (Computerized Tomography), which is more widely used today.

Purpose for Linear Tomography

In Tomography, radiologic staff make a sectional image through a body by moving an x-ray source and the film in opposite directions during the exposure. As a result, structures in the focal plane appear sharper, while structures in other planes appear blurred. By adjusting the direction and range of the movement, operators can select different focal planes which contain the structures of interest. This technique began in the 1930s by the radiologist Alessandro Vallebona, it proved useful in reducing the problem of superimposition of structures in projectional (shadow) radiography. By modifying the direction and extent of the movement, operators can select different focal planes which contain the structures of interest.

In figure A to the left above you see a PA chest x-ray, with the arrow pointing to an inflammatory lesion.

In figure B to the right above you see a left lung apex tomography, where the inflammatory lesion is seen unequivocally, along with its size as well as any caviations.

Principles of Linear Tomography

The Tomographic Principle is based on the synchronous movement of two of the three elements in a tomographic system : the tube, the object, and the image receptor.

The tube and the image receptor move during the exposure in opposite directions around a stationary fulcrum, the pivot point.

The tube and the image receptor are attached by a rod as described in the equipment section of this blog.

The objecct to be imaged is placed at the level of the fulcrum or pivot point. The area placed at the pivot point will not be blurred, because it has not moved in relationship to the tube and image receptor. Thus tomography enhances visualization of superimposed structures.

Equipment used in Linear Tomography

Most features of a tomographic x-ray imaging system appear similar to those of a conventional radiographic imaging system. Note the rod that connects to the x-ray tube above the table with the image receptor below the patient to enable movement in the opposite direction of the tube.

Applications for Linear Tomography

Tomography is commonly used when improved radiographic contrast is essential in your diagnostic exam. Through blurring of overlying and underlying tissues, the subject contrast of tissue in the tomographic section is enhanced.

Anatomy at the target level remains sharp, while structures at different levels are blurred. By varying the extent and path of motion, a variety of effects can be obtained.
Although largely obsolete, conventional tomography is still used in specific situations such as dental imaging (orthopantomography) or in intravenous urography.