Chapter 18 Computed tomography
The earliest types of computed tomography (CT) scanner were known as sequential scanners and acquired images a single slice at a time. This was because the gantry contained large high-tension power cables, and in order to prevent these cables becoming tangled after every 360° rotation, the X-ray tube had to reverse rotate back to the start point before the next slice could be acquired. These were known as first and second generation scanners.
The 1980s saw a significant advancement in technology with the introduction of slip rings, which eliminated the need for a high-tensioncable supply to the X-ray tube and detectors and allowed the X-ray tube to rotate continuously in one direction around the patient. This is known as spiral or helical CT. Slip rings are ‘electromechanical devices consisting of circular electrical conductive rings and brushes that transmit electrical energy across a rotating interface’.1 Slip ring technology was first incorporated into the third generation scanners. The most common type of CT scanner in use today is the fourth generation scanner, incorporating a stationary circular ring of detectors and a large fan beam of X-rays.
The advantage of slip ring technology is continuous rotation of the X-ray tube, meaning it spirals around the patient as the table moves through the gantry, allowing a volume of data to be collected rather than individual slices. It also means that large volumes can be scanned in a single breath hold as a result of faster scan times.
The patient couch adjusts to a range of heights to allow patients to mount and dismount with ease. This also enables safe transfer of patients from beds or trolleys of varying heights. To enable coverage during scanning the patient couch also moves horizontally through the gantry, allowing the patient to be scanned from head to mid thigh without the need to reposition.
The gantry is square with a circular opening. Staff are able to access the patient from both the front and the back of the gantry. The circular opening through which the patient couch travels is known as the ‘aperture’; this is also the opening at which the patient is positioned for scanning. Most scanner apertures measure 70 cm in diameter, enabling access to the patient should it be needed. The gantry also has laser lights to aid patient positioning. An important feature of the CT gantry is the tilting range to aid positioning of patients and the variety of clinical examinations. The degree of tilt varies but on most scanners +/− 30 ° is usually standard. For spiral scanning the tilt often has to be at 0 ° to enable reconstruction of images. Housed within the gantry are important system components, such as the high-tension generator and X-ray tube on a rotating scan frame, a ring of detectors, slip rings and collimators.
The high-tension generator and X-ray tube are connected via a short cable, allowing the generator to sit on a rotating frame with the X-ray tube and eliminating the need for long high-tension cables. Situated in front of the X-ray tube is a collimator (similar to those used on a conventional X-ray tube), which determines the slice thickness of the scan being performed and can be adjusted. Another collimator is also situated directly opposite the X-ray tube on the opposite side of the rotating frame. This is known as a post-scan collimator and its purpose is to eliminate the low energy scattered radiation emitted from the patient during the scan, enabling the scanner to produce sharper, more detailed images.
The detectors are situated on a stationary ring inside the gantry. They are designed to measure the attenuation of the transmitted X-rays from the patient and convert them into an electrical signal. The electrical signal is then amplified and analysed by the DAS (data acquisition system). The DAS is situated between the detector ring and the computer system.
The most common types of CT scanner in use today are multi-slice scanners, i.e. 16 and 64-slice scanners (Fig. 18.1). The number of slices refers to the number of rows of detectors contained within a gantry; for example a 64-slice scanner will contain 64 rows of detectors.
Slip rings are situated at the diameter of the gantry and contain wire brushes made from conductive material which carry the electrical supply required for the scanner to operate. Slip rings provide continuous rotation of the gantry through eliminating the need for long high-tension cables to the X-ray tube.
The CT gantry must be kept cool in order for the system components to operate efficiently. This can be either air or water-cooled. The scanning rooms are also air-conditioned to ensure the system does not overheat.