INTRODUCTION

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.

SYSTEM COMPONENTS

System components for CT generally fall into four categories:

IMAGING SYSTEM

Patient couch

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.

Gantry

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.

Functions of a data acquisition system

• Measures the transmitted radiation from the patient.

• Assigns numerical (binary) values to the transmitted data.

• Transmits the binary data to the computer.

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.

Figure 18.1 16 slice CT scanner.

Advantages of multi-slice scanning

• Greater amount of data acquired per rotation of the X-ray tube.

• Shorter scan times.

• Finer slices acquire more detail per rotation of the X-ray tube.

• Use of thinner slices reduces the partial volume artefact.

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.

IMAGE RECONSTRUCTION

Once the scan has been performed the data collected by the detectors must be processed and displayed as an image. This process occurs in the image reconstruction system (IRS). There are three main stages to this process:

The IRS is driven by high specification computer systems, which allow multiple reconstructions with different slice thickness, algorithms and preset viewing windows to be carried out for each set of acquired data. If the IRS is not functioning then, as a fail-safe the scanner will not function.

With the fast computing systems now available we are no longer limited to basic 2D axial images. Although many post-processing functions may be undertaken on the operator console a separate workstation now comes as standard with all scanners. Radiographers and radiologists can manipulate and view the data using multi-planar reformats (MPR), optimised filters and volume rendering techniques (VRT).

ARTEFACTS

Artefacts affect the quality of the CT images that we produce. A scan degraded by artefact can be much more difficult for the radiologist to interpret.

METAL ARTEFACT

The blocking of radiation from the detectors by the metal present causes metal artefacts. Metal artefacts will typically be seen as a star burst effect emanating from the metal (Fig. 18.2).

Figure 18.2 Metal artefact.

Courtesy of Siemens Medical.

Steps should be taken to remove all external metal objects before scanning. For internal metal, selection of an appropriate algorithm can reduce the effect. Most scanners will also have a specific postprocessing VRT for minimising this even further (Fig. 18.3). This can prove useful when imaging patients after spinal fusion surgery to check the position of the pedicular screws.

Figure 18.3 Post-processing volume rendering technique (VRT) to reduce metal artefact.

Courtesy of Siemens Medical.

PATIENT CARE

The vast majority of patients encountered will be mobile, self-caring outpatients.

PRE-SCAN PREPARATION

Oral contrast

Depending on what type of scan a patient is having they made require oral contrast prior to the scan. Some departments will use positive contrast agents such as barium or gastrografin and others may use negative contrast agents such as water. Oral contrast agents can be administered at any time between 4 hours and 15 min prior to a scan commencing. This information is all dependent on individual departmental scanning protocols.

Pregnancy status

It is essential under the IR(ME)R guidelines that when scanning patients of childbearing age (commonly between the ages 12 and 55, but may vary), you must establish a pregnancy status (Fig. 18.4). This should apply to all scans in the region between the diaphragm and the knees.

Figure 18.4 ‘Last menstrual period’ (LMP) check procedure prior to examination.

Courtesy of Charing Cross Hospital Radiological Services Unit.

Siting of a cannula

Some patients require a cannula for the introduction of contrast medium. The following points should be remembered:

• Ensure you are able to prepare an injection tray with all the required components.

• Be sensitive to needle phobic patients who may require extra care whilst the needle is being sited.

• Be aware if a patient is diabetic and has been fasting. This is because the blood sugar levels will be low and so the patient will be prone to hypoglycaemic attacks.

• Change patients into gowns where appropriate.

• Remove essential jewellery only.

Scan room care

• Owing to the vertical movements of the table, minimal assistance is required for patients.

• Make use of accessories such as straps and cushions to maintain patient comfort throughout the scan.

• Always give clear instructions and ensure that they are understood. This may mean practising the required breathing technique.

• As well as oral recorded instructions some scanners have a lighting system to indicate the breathing phase, which is of value for patients who are hard of hearing.

• Intravenous contrast may be administered by a pump injection or hand injection. Patients should be warned that they might experience feelings of warmth throughout their body and a metallic taste in their mouth.

• Rarely a patient will react to contrast (Fig. 18.5). This can range from vomiting through to anaphylactic shock. Basic life support procedures should be practised and essential equipment should be to hand at all times.

• Recognise reactions and take appropriate action; for example, patients may only notice urticaria when they are getting dressed and they may draw attention to this.

Figure 18.5 Reactions to contrast medium.

Adapted from Charing Cross Hospital, with permission.

PROTOCOLS

At the core of every CT department are the scanning protocols. These are agreed with the consultant radiologist, lead radiographer and the radiation safety advisor. These protocols must conform to the IR(ME)R regulations and they are designed to give maximum diagnostic information with the least radiation possible. Protocols will vary from one department to another and within departments from one machine to another.