Introduction

• Computed tomography produces cross-sectional images using diagnostic x-rays as the energy source, and a powerful computer to create the images.
• The major advantages of this technique over conventional x-ray techniques are its superior soft tissue differentiation and separation of overlying structures.
• In 1972, coincident with the explosion in computer technology, the principle of tomography was used in the earliest CAT (computed axial tomography) scanner. This followed the work of Hounsfield in the early 70s, Oldendorf and Cormack in the early 60s and, believe it or not, the work of mathematician Radon (appropriate name!) in 1917 who proved that an image of a three-dimensional object could be constructed from its mathematical projections. A TV picture is composed from a large number of tiny dots (pixels) and if we consider a black and white image, each dot is a shade of gray from black to white. The shades can be assigned numbers from 09 (dark to bright). These numbers can then be displayed as a large and unintelligible matrix of numbers on the screen and it is obvious that if we ask the computer to add 1 or subtract say 3 from all the numbers we would be able to make the whole picture lighter or darker. What the computer can do is obviously much more complex than this and as time goes by more and more complicated and "outrageous" adjustments can be made. If a slice picture is made in one plane the computer can reconstruct a picture at right angles to the original plane the image is a little less sharp but acceptable. Recent improvements allow the computer to reconstruct a 3dimensional image, which can be rotated on the screen, which is especially useful for surgical planning especially in tissue reconstruction.
• The development of the CT (computed tomography, now preferred to the term CAT) scanner has gone through four generations, each generation has brought about improvements in the way the radiographic image is made in addition to computer technology advances. To a large extent the speed of data collection and image construction has improved exponentially with the evolution of the equipment.

Helical (spiral) CT

Veterinary use of CT

Further Reading