In many ways, CT scanning works very much like other x-ray examinations. Very small, controlled amounts of x-ray radiation are passed through the body, and different tissues absorb radiation at different rates. With plain x-rays, when special film is exposed to the absorbed x-rays, an image of the inside of the body is captured. With CT, the film is replaced by an array of detectors, which measure the x-ray profile.
Inside the CT scanner is a rotating gantry that has an x-ray tube mounted on one side and an arc-shaped detector mounted on the opposite side. An x-ray beam is emitted in a fan shape as the rotating frame spins the x-ray tube and detector around the patient. Each time the x-ray tube and detector make a 360 degree rotation and the x-ray passes through the patient's body, the image of a thin section is acquired. During each rotation, the detector records about 1,000 images (profiles) of the expanded x-ray beam. Each profile is then reconstructed by a dedicated computer into a two-dimensional image of the section that was scanned. Multiple computers are typically used to control the entire CT system.
You might think of it as looking into a loaf of bread by cutting the bread into thin slices. When the image slices are reassembled by computer, the result is a very detailed, multidimensional view of the body's interior.
A relatively new technique, spiral (helical) CT has improved the accuracy of CT for many diseases. A new vascular imaging technique-spiral CT angiography-is noninvasive and less expensive than conventional angiography, and allows doctors to see blood vessels without the need for more invasive procedures.
The term "spiral CT" comes from the shape of the path taken by the x-ray beam during scanning. The examination table advances at a constant rate through the scanner gantry while the x-ray tube rotates continuously around the patient, tracing a spiral path through the patient. This spiral path gathers continuous data with no gaps between images.
With spiral-or helical-CT, refinements in detector technology support faster, higher-quality image acquisition with less radiation exposure. The current spiral CT scans are called multidetector CT and are most commonly four- or 16-slice systems. CT scanners with 32, 40 and 64 detectors now becoming available. These instruments provide either faster scanning or higher resolution images. Using 16-slice scanner systems the radiologist can acquire 32 image slices per second. A spiral scan can usually be obtained during a single breath hold (less than 30 seconds). Such speed is beneficial in all patients, but especially in populations in which the length of scanning was often problematic, such as elderly, paediatric or critically-ill patients. The multidetector CT also allows applications like CT angiography to be more successful.
With conventional CT, small lesions may frequently go undetected when a patient breathes differently on consecutive scans, as a lesion may be missed by unequal spacing between scans. The speed of spiral scanning and a single breath hold increase the rate of lesion detection.