Computed tomography (CT) merges x-ray technology with the computer to provide detailed digital cross-sectional images of the body relatively free from superimposition of the different tissues.
CT, which revolutionized the ability to visualize soft tissues, was invented in 1972 by Godfrey Hounsfield, a British engineer at EMI Laboratories. Hounsfield was awarded the Nobel Prize in medicine in 1979 for his invention. His work was based on Alan Cormack's mathematical formulas for reconstruction of images from digital signals.
The first scanners were dedicated to scanning the head, but larger scanners, accommodating the whole body, were introduced in 1976 and became widely available during the 1980s. In the early years, acquiring the CT image data took hours and reconstruction of the image took days, but advancements in computer technology have decreased image acquisition time to where it is possible to collect the information needed for imaging the entire chest in seconds. The amount of information collected, and with it detail and clarity, has also increased dramatically. The first scanners had an image matrix of only 80 by 80 (6,400) data points, whereas modern scanners may have a matrix of 1,024 by 1,024 pixels, or 1,048,576 data points.
CT is based on some of the same imaging principles as conventional radiography. CT employs x-rays that are attenuated by body tissues. The radiodensities of the body tissues are represented in the image as shades of gray. The main difference between the two modalities is that CT creates images based on cross-sectional (axial) slices, created by up to 1,000 projections from different angles.
The following description refers to spiral/helical scanners, which are now most commonly used (Fig. 4-1). A CT scanner has three components:
Spiral scanning. The patient's table moves continually during the examination while the x-ray tube moves in continuous circles around the patient, projecting to stationary detectors.
The gantry contains the x-ray tube, its high-voltage generator, a collimator assembly, a detector array, and a data acquisition system.
CT employs a high-intensity x-ray tube in order to provide uniform penetration of the tissues and reduce attenuation by bone relative to soft tissue.
The fan-shaped x-ray beam must be tightly collimated. The collimators are apertures through which the x-rays pass on the way to the patient and serve the following functions: