CT Scanning

The technique of CT scanning was developed in 1973 by Hounsfield. A thin fan beam of X-rays generated by a conventional X-ray tube passes through a single 'slice' of a patient through to a bank of X-ray detectors. The detectors can be manufactured as a 'solid state' component or may be composed of the inert gas xenon. The detectors produce a voltage proportional to the intensity of incident X-rays. The voltage is read and digitized for subsequent processing in a computer. By taking thousands of readings from multiple angles around the patient, a two dimensional image can be reconstructed. 'CT' stands for Computerized Tomography, tomography referring to a cross sectional image in a single plane. Modern CT scanners acquire images rapidly with each slice taking as little as a fraction of a second to obtain. Multiple slices are obtained in sequence. By stacking the data from multiple slices together, a three dimensional image can be reconstructed. The processed images are transferred onto special laser film with between 6 to 24 images per film.
The radiographers responsible for the examination undertake special training in CT scanning. The images are interpreted by a Radiologist who issues a diagnostic report. CT scanning is now commonly performed particularly for imaging the head, sinuses, spine, chest, abdomen, pelvis and limbs. The radiographers and radiologist are happy to answer any questions you may have.

An injection of intravenous contrast may be given. Contrast increases the density of blood vessels and vascular structures, improving their visibility. A pressure injector may be used to inject the contrast automatically. Patient movement creates severe artifacts on the images and therefore it is important to keep still. Examinations take between 10 minutes to 30 minutes to complete.

Spiral CT
In the 1990's spiral CT scanners were developed. A Spiral CT has a slip ring which enables the tube to spin continuously at one revolution or more per second. Data is obtained in a continuous spiral or helix rather than in individual slices. The advantages of spiral CT include faster scanning with reduced examination time, greater accuracy especially for small lesions in the liver or lungs, better intravenous contrast usage and the ability to obtain seamless 3D images. Reduced examination times are important for seriously ill patients, children, restlessness or claustrophobia.

Multi-slice CT
A major development has been the introduction of multi-slice CT. The latest multi-slice scanners have 16 detector rows allowing the acquisition of 16 slices of information each tube rotation. The advantage is improved volume coverage and longitudinal spatial resolution. New applications have evolved including cardiac scanning, virtual endoscopy and CT angiography.