INTRODUCTION

Within the imaging department there is a range of equipment and accessory items to assist the practitioner in obtaining optimal quality images. Technological developments have seen the introduction of digital capturing systems (digital radiography, DR) and advancements in operator ergonomics, patient throughput and comfort. Modern general X-ray rooms should be spacious, permitting the examination of a wide spectrum of patients (Fig. 11.1).

Figure 11.1 Modern general X-ray room.

The use of relatively high proton number materials to provide radiation protection measures has generally remained unchanged in the last fifteen years. Walls of a general X-ray room may be coated with barium plaster, and lead shielding is incorporated into access routes. Consideration is also given to the ceilings and floors of any X-ray examination room if the radiology department is situated in proximity to adjoining floors within a hospital.

IMAGING TABLES

The majority of imaging tables utilised within general diagnostic departments have to meet a number of key requirements to ensure patient safety and the provision of optimal image quality. Modern imaging tables are designed to minimise unnecessary strain on practitioners during examinations and provide security and comfort for patients (Fig. 11.2). The availability of a variable height feature permits easy transfer of patients, whether they are walking, in a chair or on a stretcher. A wide range of movement is available, usually via foot control switches situated at the base of the table, and this prevents the need to move the patient around on a draw sheet and minimises injury to the practitioner. Controls positioned at either side of the imaging table also provide a universal approach to transferring and positioning patients.

Figure 11.2 Modern imaging table of a diagnostic imaging room, courtesy of Xograph Healthcare Ltd.

Most modern imaging tables are designed using low attenuation materials, such as carbon fibre, which have the advantages of being lightweight and strong to accommodate the weight of the heavy individuals, typically up to 450 kg.¹ Using materials such as carbon fibre also absorbs less of the primary beam and reduces radiation dose when undertaking grid work. Carbon fibre is also warm to the touch and has rounded edges to prevent physical damage to patients and staff and the build up of electrostatic charge.

All modern imaging tables should encompass a good range of longitudinal and transverse travel and possess a floating top design to minimise physical strain upon the practitioner. Such designs may aid in the ability to undertake prompt and effective examinations; however, the distance between the patient and cassette unit is increased and appropriate measures need to be taken to ensure object magnification and image unsharpness does not occur. This is normally compensated by increasing the source–image distance (SID) when undertaking examinations requiring an oscillating grid mechanism. The practitioner also needs to ensure the various table and X-ray tube interlocks are observed during such examinations; all manufacturers use colour coding to indicate X-ray tube functions.

Modern imaging tables also accommodate direct capture imaging plates, which are linked to a monitor and computer unit for a near instantaneous display. The advent of digital imaging has redefined clinical practice in terms of patient positioning and appropriate use of a DR system. Traditional systems employ integrated conventional cassette units, which also act as a scatter minimisation unit. Such systems are also linked to automatic exposure devices (AED). The use of the AED has become a routine aspect of clinical work. There are normally three AED chambers integrated into the tabletop mechanism. Selection of the appropriate chamber(s) is facilitated by pre-set protocols with a manual override if the practitioner needs to change the programme to provide an optimum image, or if the patient has a metal prosthesis (e.g. hip), affecting the overall quality of the image. The density settings on the control console may be password protected to prevent changes to optimal stored values.²

The use of automatic cassette size sensing (ACSS) techniques ensures the collimation from the light beam diaphragm matches the size of the cassette within the Bucky mechanism. This reduces the area irradiated and produces less scattered radiation, improving image quality as well as limiting radiation dose to the patient and staff. Closer collimation can again be performed, as the diaphragms are not fixed in the position sensed by the ACSS.

TUBE SUPPORTS

Modern general X-ray tubes are supported via a ceiling track mechanism which is able to withstand the rigour and demands of a typical department workload (Fig. 11.3). Most clinical departments utilise ceiling suspended units and employ a ‘cross track’ mechanism to allow the unit to move swiftly over a large floor space. The tube support facilitates movement around the longitudinal, transverse and vertical axes. The telescopic column provides a flexible range of vertical movement and is supported by electromechanical locks. The ceiling in imaging rooms containing a ceiling mounted unit needs to be of a reasonable height, thus permitting the required SID for a range of examinations.

Figure 11.3 Tube support mechanism.

The practitioner is normally provided with colour coded functions on the X-ray tube control unit, which are associated with the same coloured indicators on the ceiling and telescopic support. Some imaging rooms which undertake basic procedures (e.g. chest room) may employ a floor mounted X-ray unit, which is cost effective but

not as flexible as a ceiling mounted system. Such units are limited in their range of movement and the practitioner should take this into consideration when undertaking more complex examinations or with non-ambulant patients. Floor mounted systems are positioned on a track unit which is recessed into the floor and secured to a ceiling track for added stability. This design limits the available floor space and may present health and safety challenges.

The major advantages of a ceiling suspended unit include:

Figure 11.4 Example of wide floor coverage.

High-tension cables are securely encased in protective flexible piping, which is supported by additional ceiling sockets (Fig. 11.5). This enables the practitioner to safely manoeuvre the X-ray unit without the risk of placing unnecessary stress upon the high-tension cables. The supports for the cables must run freely along the track to prevent any mechanical strain when the X-ray tube is rotated.

Figure 11.5 Flexible piping and high-tension support mechanism.

CASSETTES AND FILM HOLDERS

Modern imaging departments utilise some form of digital capturing devices (image receptor), such as computed radiography (CR) or DR. This will have some impact upon the working logistics within a general imaging room. However, some departments may still also utilise conventional film screen cassettes to capture a latent image.

Typically, if a department is employing either conventional cassettes or CR technology, there will be a range of image receptor sizes to accommodate various radiographic techniques. All image receptor devices should be stored away from primary and scattered radiation, ideally in an appropriately designed storage holder. Routine quality control assessments of the image capturing devices should also be performed, in particular the routine secondary erasure of unused CR cassettes. Direct capture units (Fig. 11.6) may either be mobile devices or fixed units (e.g. chest imaging unit). Such units are expensive and require proper storage facilities when not employed.

Figure 11.6 Direct capture image receptor device, courtesy of Canon Europe.

To undertake a range of diagnostic techniques, the modern imaging environment should include various ancillary items. These include cassette holders, foam pads, stationary grid units, sandbags and radiation protection devices (e.g. lead rubber coats). The practitioner is required to utilise a range of skills and knowledge in order to produce optimal quality images. The use of a cassette holder to acquire images may minimise radiation dose to staff or carers.

Cassette holders may take the form of a basic device, such as the example shown in Figure 11.7, and are generally employed for techniques such as lateral horizontal beam hip examinations. However, a modern upright Bucky mechanism with an integrated cassette holder is also a crucial feature in a general imaging room.

Figure 11.7 Lateral cassette holder.

Figure 11.8 demonstrates the advantages of a ceiling mounted X-ray unit and upright Bucky unit. The provision of standard interlocks also ensures set geometry for chest examinations, which need to be at a standardised distance of approximately 180 cm SID. This aids in the minimisation of geometric unsharpness for certain examinations.

Figure 11.8 Upright Bucky unit with cassette holder unit.

A variable height facility supported by either manual or electromechanical locks enables the assembly to be moved up and down to examine a range of procedures, from lateral cervical spines to erect standing knees. The cassette support mechanism may then be removed to facilitate AED or Bucky work. The system can also be rotated 90 ° into a horizontal position if required for adapted techniques, such as a lateral elbow in plaster or wrists on patients who may be immobile. Modern upright DR units (Fig. 11.9), which possess an integrated image capturing device, may also be used for a wide range of examinations and allow flexible rotation. All upright units should possess patient positioning aids, such as handles and chin rests. Also the provision of electromechanical interlocks will prevent any compromise of patient safety in the event of an electrical failure.

Figure 11.9 Digital radiography upright imaging unit, courtesy of Canon Europe.

OPERATOR CONSOLE AND DOSE RECORDING FEATURES

General X-ray units are supported with space saving control consoles (Fig. 11.10), which encompass touch screen facilities. This is coupled with the small space required for a high frequency generator to supply power to the X-ray unit. The use of solid state electronics provides accurate exposure times for examinations and improves image quality at lower patient doses by producing a nearly constant potential voltage waveform which is not possible with a single or three phase power supply.³ The creation of a near constant potential waveform helps to reduce the possibility of movement unsharpness during radiographic procedures. This is especially true for paediatric examinations, where very short exposure times may be required.

Figure 11.10 Operator console and image review workstation, courtesy of Canon Europe.

Most operator consoles have preset anatomically programmed (APR) exposure values, which are stored on a microprocessor. The pre-set radiographic exposures provide prompt access to a digital bank of typical values used in clinical practice. However, practitioners still require the knowledge and skills to adjust these pre-set values in order to provide optimal image quality at the lowest possible dose.⁴

Typical power ratings for a high voltage generator for general radiography procedures are in the region of 30–55 kW. This is generally lower than for units such as interventional systems, which require greater power supplies due to the demands placed on them.⁵

Practitioners have a duty to ensure minimal exposure values are used in order to produce optimal diagnostic radiographs.⁶ The development of radiological equipment has assisted in the promotion of minimal exposures, optimal image production and the provision of a safe working environment. Rooms are designed to ensure the operator console is at a safe distance from the examination table and erect Bucky unit. Unlike in mobile equipment, the exposure button is attached to the console, thus preventing an exposure unless the practitioner is safely behind the protective lead glass screen.

It is a legal requirement to record radiation doses to patients, and whilst an integrated dose area product (DAP) meter does not reduce radiation dose, it does facilitate easy recording of the entrance dose to the patient being examined in that room. This allows the practitioner to measure the dose and ensure it is within the diagnostic reference level (DRL) required by the Ionising Radiation (Medical Exposure) Regulations 2000.⁶ The meter can easily demonstrate the effect of close collimation and this can encourage good working practices by the practitioner. DAP units display the dose given to the patient in milligrays (mGy) or centigrays (cGy) and this must be recorded for each study undertaken.

The provision of features such as exposure hold, which prevents an exposure-taking place for a number of reasons, minimises a potentially unsafe examination from taking place.⁷ Reasons may include the door to the radiology department being opened during the examination, the X-ray tube not being correctly centred to the Bucky tray, no cassette in the Bucky tray or a cassette in both the upright and table Bucky systems.

MOBILE EQUIPMENT

Radiography may be performed outside the realm of the X-ray department, encompassing a range of examinations, environments and patients. Although the practitioner when performing mobile examinations may employ various adaptations upon radiographic techniques, the fundamental principles of radiation protection, image quality and patient care remain pinnacle to this service.

Mobile radiography may be performed in a wide range of environments within a hospital, including an accident and emergency department, intensive therapy unit (ITU), coronary care unit (CCU), special care baby unit (SCBU) and theatre. Working in ITU, SCBU or general ward environments requires some examinations to be performed in often small, difficult and busy environments. The practitioner is primarily responsible for obtaining a diagnostic image whilst maintaining the radiation protection of staff and patients.

Mobile X-ray units are rechargeable (capacitor discharge) in design, being generally safer than mains units, which draw power for exposures from wall sockets. This reduces the necessity for the provision of dedicated electrical sockets on wards such as ITU and CCU, where interruptions to the local power supplies are generally not recommended. However, a mains powered unit is lighter in design and has a smaller footprint, which aids with manoeuvrability of the system.³

Most mobile units are APR in design and encompass the majority of features located on static units (e.g. DAP meters) and have similar power outputs to general imaging rooms. Generators incorporated within mobile units work up to 40 kW peak power, allowing exposures from 40–110 kV and 0.2–360 mA s. This adds to the flexibility of mobile units, allowing them to perform a wide variety of different procedures that require a range of exposure factors, including very short exposure times (1 ms), which is a crucial consideration for various scenarios, such as trauma, CCU and ITU where involuntary patient movement (i.e. respiratory) may be an issue.

Features of a typical mobile X-ray unit are demonstrated in Figure 11.11. Mobile units may vary in design, with some units possessing extendible tube supports or adjustable-height mobile units. Extendable tube supports or adjustable-height mobile units should aim to provide a good range of movements, including a generous source–image distance (SID) and subtle adjustments for minor angle corrections.

Figure 11.11 Typical design features of a mobile X-ray unit.

Modern systems should be easy to use, flexible and provide high quality images. Typically, most modern mobile units are compact in design to enable movement in confined spaces, such as in isolation rooms. Most mobile X-ray units are designed to enable the provision of an optimal imaging service and are supplied with similar X-ray tubes to those in general X-ray rooms, with typical anode angles of 15 °, and provide 8500 revolutions per minute during an exposure.⁶ The mobile X-ray tube and its housing are customised for its role, needing to provide the same level or radiation protection as those in the department but at a minimal weight. Modern mobile units use a single focal spot size of 0.8 mm, giving a compromise between acceptable geometric unsharpness and tube life.

The practice of mobile radiography places a great demand upon mobile X-ray units to move easily within small spaces, such as between beds and in cubicles and also over great distances between wards. Some modern machines have integrated a ‘tilt-up footstep’ to allow smooth transfer over thresholds and, with polyurethane tyres, are designed to glide across most floor surfaces that would be encountered throughout the hospital. Mobile machines also have built-in braking systems, usually activated by releasing the drive handle. This avoids the demand on practitioners to physically bring the machines to a halt themselves, which may otherwise have considerable manual handling consequences.

Additional features aiding manoeuvrability include a lightweight construction of the base unit and a design that enhances the operator’s visibility when travelling. This is a crucial issue during transportation, where mobile units may be in close proximity with members of the public within the hospital. A small electrical motor can be used to provide a motorised drive within the unit to help promote the ease of movement.⁵ Radiography is a profession that is particularly conscious of the potential manual handling risks that can be incurred while undertaking such a physical occupation on a day-to-day basis.² Modern advances are aimed at improving the ergonomic design of mobiles so that examinations can be carried out easily, without any undue stress being placed upon the practitioner.

A cordless exposure device is now available, which has a proprietary infrared coding frequency and provides the operator with the ability to increase their distance up to almost 11 m.⁶ The remote control provides the greatest radiation protection possible to the operator, especially when working in restricted environments, as it has the ability to function around corners. In common with conventional exposure hand switches, the remote control allows the operator to check the collimation at a distance before exposing.

The development of mobile DR units (Fig. 11.12) with amorphous silicon has enabled the instantaneous viewing of images (typical ‘exposure-to-view’ time = 5 s), whilst being remote from the X-ray department. The advent of DR imaging has also removed the necessity for traditional chemical processors and enhanced the storage/transportation of images. Fundamental image viewing and manipulation may be achieved using a touch screen encompassed within the mobile unit.

Figure 11.12 Digital radiography mobile unit.

With the near instant availability of digital images and the ability to post-manipulate at the site of exposure, additional pressures are being placed upon practitioners to provide information in order to ensure effective patient management. It is imperative that practitioners ensure the focus primarily relates to the welfare of the patient during portable examinations, whilst appreciating technology advancements.

FLUOROSCOPY EQUIPMENT

The practitioner may use fluoroscopy equipment within the imaging department or in a theatre environment. Traditionally, the fundamentals of obtaining images within a fluoroscopy environment have involved the use of an image intensification unit to convert transmitted incident X-ray photons, which have emerged from the patient, into an electrical output signal. Fluoroscopy examinations result in an instantaneous ‘real time’ image being produced and include examinations that may employ the use of barium or iodine contrast agents, such as enemas and meals.

The process of converting an incident X-ray photon into an electrical signal involves a number of stages within a conventional image intensification unit. This process is depicted in Figure 11.13 and is performed in fluoroscopy and mobile units, which employ a ‘drum’ type structure aligned to an X-ray tube.

Figure 11.13 Basic process of traditional fluoroscopy image creation.

The practitioner may encounter two different types of fluoroscopy unit within the imaging department. These are under- or overcouch units (Fig. 11.14). Undercouch fluoroscopy units are designed with the X-ray tube unit underneath the imaging table and are the most popular within imaging departments. With undercouch units, the image intensification unit incorporates a handheld movement unit, allowing the practitioner to move the unit over the patient. A lead rubber protective skirt minimises the amount of primary and scattered radiation reaching the operator. Overcouch units only permit limited movement, owing to the X-ray tube being positioned over the patient, but are useful for examinations with certain patient groups, such as paediatrics.

Figure 11.14 A Undercouch fluoroscopy unit. B Overcouch fluoroscopy unit.

The basic purpose of an image intensification unit is to convert a weak input signal into a strong output signal within a glass vacuum environment. This conversion process begins at the input phosphor screen within the image intensification unit, which converts the incident X-ray photons into light signals. Traditionally, caesium iodide (in the form of rod shaped crystals) has been used for this purpose, as it possesses high absorption properties and is therefore able to ‘stop’ the high-energy X-ray photons. The caesium iodide crystals are supported on a glass plate and the emitted light is absorbed by a photocathode device and subsequently converted into a weak electrical signal. The number of emitted photoelectrons is proportional to the initial brightness of the input screen.⁸

The weak electrical signal is attracted to the output end of the image intensification unit (anode) by applying a potential difference across the input and output ends of the unit. This is similar to an X-ray tube, but the potential difference is much smaller in an image intensification unit (Fig. 11.15). The initial weak electrical signal is increased by the use of focussing electrodes, which are negatively charged and repel the electrons (which are also negatively charged). This has the effect of condensing the electrical signal, which eventually reaches the output phosphor.

Figure 11.15 Typical design of a traditional image intensification unit.

In traditional fluoroscopy systems, the output phosphor re-converts the electrical signal into a visible light signal. This signal subsequently appears on the viewing monitor, or may be recorded using an appropriate image-capturing device (e.g. SVHS). Modern fluoroscopy devices employ analogue to digital converter (ADC) electronics to store captured images and permit the operator to either save data onto a local or centralised system or print selected images. Advantages of digital images include the ability to manipulate and enhance the original data and potentially improve overall image quality.

Fluoroscopy equipment utilises a number of additional safety features not normally located on standard radiographic equipment. Such features include:

Figure 11.16 Freestanding mobile operator console with integrated lead glass screen.

The fixed relationship between the X-ray tube and image intensification unit requires the patient to be positioned for certain projections. However, with the advent of modern C-arm design fluoroscopy units and the use of ultra-thin carbon fibre tabletops, the imaging unit may be positioned around the patient and can also be used for special projections, such as lateral decubitus images. The C-arm fluoroscopy design has been utilised in the theatre environment for many years and mobile image intensification units provide an essential service for various examinations, such as orthopaedic operations and surgical procedures. Advancements in technological design have provided operators with ‘flat panel’ image intensification units (Fig. 11.17) to replace those in the traditional ‘drum’ style. These incorporate amorphous silicon and so are similar to technology encountered in direct capture general radiography units. Such units are true digital systems, converting the incident X-ray photons into an electrical signal.

Figure 11.17 Modern ‘flat panel’ design mobile image intensification unit.

All modern mobile image intensification units employ a high frequency generator power supply and provide the operator with a wide range of exposure manipulation features, which may be rapidly adapted to suit the radiological procedure. The nominal output ranges vary according to manufacturer but typical values range from 3 to 8 kW.¹ Such values are lower than traditional fluoroscopy units; however, image quality is not compromised on modern digital units. Most modern systems also employ dual focal spot sizes (typically 0.6 and 1.4 mm), which are automatically adjusted according to the type of examination being performed. A broad filament is employed when high demands are placed upon the unit to provide greater amounts of X-rays. Although a number of design features have been developed on mobile image intensification units, the majority of fundamental systems still appear to house stationary anodes.

Depending upon the range of examinations being performed, different field sizes (typically 23 cm and 30 cm) are available. Larger field of view intensification units may be required for cardiac (i.e. pacing examinations) examinations. A zoom factor is normally also available for magnification purposes. A range of colour coded interlocks, measurement scales, multiple glide rails, angle indicators and laser positioning all aid the practitioner in the ensuring optimal image quality and minimal doses to patients and staff.

The operating of large electrical equipment in an environment such as a theatre introduces high risks to staff. There are many areas to consider, including the use of electricity around many compressed and combustible gases and possibly in the presence of large volumes of free fluid. The electrical supply to the unit’s generator is of a high demand and therefore can only be taken from a stable circuit that can withstand it. The positioning of sockets needs to be carefully considered as they need to be placed high, but this can make them difficult to access by shorter members of staff. Conversely, if too low, they become susceptible to fluid on the floor and become hazardous to theatre staff, resulting in the plugs being kicked and cables being damaged.

Manual handling issues are becoming of great importance, with many studies demonstrating the risk incurred to their musculoskeletal system by healthcare professionals when undertaking their daily working practices.⁶ Hospital trusts nationwide are now incorporating mandatory manual handling training to promote safe working practices in an attempt to protect their staff.

The majority of modern image intensification systems encompass a wide range of image manipulation features, which commonly include virtual collimation (which permits the operator to collimate the last obtained frame in preparation for the next exposure), image rotation (virtual), zoom, flip, edge enhancement, image subtraction and a wide range of contrast and brightness tools. Advanced tools such as measurement scales, fluoroscopy reply (i.e. dynamic imaging), road mapping, noise reduction and image reversal features are also common on most systems.

With regards to radiation protection, all fluoroscopy equipment contains a number of devices similar to those found on general X-ray units, such as DAP meters. Some fluoroscopy procedures may be lengthy (e.g. tibial nailing) and consequently possibly require long screening times. It is therefore imperative that dose saving features are an essential feature on any modern image intensification unit and encompass a number of ergonomic features to enable the production of high quality images.¹ The use of a ‘pulsed’ stream of electrons across the X-ray tube generates a non-continuous stream of photons, which are subsequently incident upon the patient. Pulsing technology is available on all modern image intensification equipment and should be employed whenever the operator is positioning the patient or obtaining a check image (e.g. k-wire examination in theatre). Continuous beam transmission gives higher quality images at a higher dose whereas pulse imaging gives a lower dose, but as a consequence, image quality is compromised. In some situations the images gained are of high enough quality as to negate the need for postoperative films, thus reducing dose to the patients. Some degradation to image quality may occur as a result of pulsing techniques; however modern systems provide some form of compensatory factor in an attempt to preserve image quality.

CARE OF EQUIPMENT

It is the professional responsibility of the practitioner to ensure the equipment utilised in the production of quality images is safe and regularly checked. Incorrect handling may result in excessive wear and tear, such as damage to interlocks or brakes and the tube housing. Staff working within general imaging departments should undertake routine equipment checks, which may encompass:

SECONDARY RADIATION GRIDS

Grids are a highly effective way of reducing the amount of scattered radiation reaching the image receptor (Fig. 11.18). Grids are made of strips of lead (or other attenuating material) interspaced with a radiolucent material, such as aluminium or carbon plastic fibres. The attenuating material will intercept the scattered radiation before it reaches the film, increasing the quality of the image. Up to 97% of scattered radiation can be removed through the use of a grid.

Figure 11.18 Function of a grid absorbing scattered radiation whilst allowing the useful transmitted beam to pass through.

Grids fall into a number of different categories describing their construction.

TERMS USED WITH GRIDS

Grid ratio

The grid ratio describes the ratio of the grid height to the width of the interspace material (Fig. 11.19). Grid ratios range from 4:1 to 16:1. High-ratio grids remove more scattered radiation, increasing the radiographic contrast, and are ideal for high kV examinations. Low ratio grids are used for mammography examinations where a typical ratio value is 5:1.

Figure 11.19 Grid ratio.

Grid factor

The grid factor describes the increase in exposure need when using a grid. The grid will decrease the intensity of the radiation reaching the image receptor. This cannot be avoided and to counteract the effect the exposure factors used will need to be increased by the grid factor. This is provided with the grid and is usually between two and six.

Grid cut-off

If certain types of grid are used at the wrong focus-to-grid distance or are not in the correct orientation to the primary beam then the beam will be ‘cut-off’. This will be seen on the resultant image with a small area of the image appearing normal but the surrounding area being under exposed, with either visible lines or completely white where the attenuating material has absorbed the useful beam. This can be avoided by using grids at the correct distance and orientation. The correct orientation and distance will be marked on the grid.