Saturday, December 27, 2008

What Is A CT / CAT Scan (Computed Tomography)?

A CT or "Cat" scan has become a common term as the use of this modality has skyrocketed. Either through personal use or depictions on shows like "ER", the public has come to accept the technology without really understanding what it is. According to Wikipedia:

"Computed tomography (CT) is a medical imaging method employing tomography. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. The word "tomography" is derived from the Greek tomos (slice) and graphein (to write).

Computed tomography was originally known as the "EMI scan" as it was developed at a research branch of EMI, a company best known today for its music and recording business. It was later known as computed axial tomography (CAT or CT scan) and body section röntgenography.

CT produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to block the X-ray/Röntgen beam. Although historically (see below) the images generated were in the axial or transverse plane (orthogonal to the long axis of the body), modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.

In the early 1900s, the Italian radiologist Alessandro Vallebona proposed a method to represent a single slice of the body on the radiographic film. This method was known as tomography. The idea is based on simple principles of projective geometry: moving synchronously and in opposite directions the X-ray tube and the film, which are connected together by a rod whose pivot point is the focus; the image created by the points on the focal plane appears sharper, while the images of the other points annihilate as noise. This is only marginally effective, as blurring occurs only in the "x" plane. There are also more complex devices which can move in more than one plane and perform more effective blurring.

Tomography had been one of the pillars of radiologic diagnostics until the late 1970s, when the availability of minicomputers and of the transverse axial scanning method, this last due to the work of Godfrey Hounsfield and South African born Allan McLeod Cormack, gradually supplanted it as the modality of CT.

The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield in Hayes, United Kingdom at EMI Central Research Laboratories using X-rays. Hounsfield conceived his idea in 1967, and it was publicly announced in 1972. Allan McLeod Cormack of Tufts University in Massachusetts independently invented a similar process, and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine."

While this description is accurate, I'd like to put it in even simpler terms. A CT scan is simply a series of plain x-ray films that have been taken at precise intervals so that a computer can reconstruct an image of a 'slice' through that body. The computer uses geometry and other calculations to perform this feat. As an analogy, imagine you have a watermelon. A "w-ray" lets you see the shadow of the seeds inside the watermelon if you point the w-ray at it. If you shot one w-ray, you'd simply see a line on the other side, because the seed shadows would be compressed. However, if you shot another w-ray at an angle to the first, the shadows would shift but by differing amounts. Using geometry, you could use these shifts to triangulate the location of the seeds within the watermelon. If you shot enough w-rays, you would get a very precise picture of where exactly the seeds are located within the watermelon, the same as you would get if you had "sliced" the watermelon and looked at a flat section of it. This is the basic idea behind CT.

As CT scans have become more prevalent, the cost has fallen. However, they are still far more expensive and resource intensive than plain x-ray images. As CT scan have become more widely accepted by the medical community and requested by patients, insurance plans including Medigap insurance, have broadened the CT studies for which they would provide reimbursement. In most cases, especially in trauma situations, insurance providers will cover the majority of the cost of CT exams.

Recent innovations have helped expand the uses of CT into areas such as cardiac imaging. The original CT had a tough time imaging moving objects, such as the heart. However, new CT machines use multiple sources of radiation to speed up the acquisition time, allowing more precise imaging of moving structures. Such advances have helped the modality revolutionize the world of diagnostic imaging.

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