Uses

Direct-action film is used for intraoral radiography where the need for excellent image quality and fine anatomical detail are of importance.

Sizes

Various sizes of film are available, although only three are usually used routinely (see Fig. 6.1).

Fig. 6.1 The typical sizes of barrier-wrapped direct-action radiographic film packets available. A Small periapical/ bitewing film. B Large periapical/bitewing film. C Occlusal film.

The film packet contents

The contents of a film packet are shown in Figure 6.1. It is worth noting that:

• The outer packet or wrapper is made of non-absorbent paper or plastic and is sealed to prevent the ingress of saliva.

• The side of the packet that faces towards the X-ray beam has either a pebbled or a smooth surface and is usually white.

  Page 42 

• The reverse side is usually of two colours so there is little chance of the film being placed the wrong way round in the patient’s mouth and different colours represent different film speeds.

Fig. 6.2 The contents of a film packet. A The outer wrapper. B The film. C The sheet of lead foil. D The protective black paper.

• The black paper on either side of the film is there to protect the film from:

– Light

– Damage by fingers while being unwrapped

– Saliva which may leak into the film packet.

• A thin sheet of lead foil is placed behind the film to prevent:

– Some of the residual radiation that has passed through the film from continuing on into the patient’s tissues

– Scattered secondary radiation, from X-ray photon interactions within the tissues beyond the film, scattering back on to the film and degrading the image.

• The sheet of lead foil contains an embossed pattern so that should the film packet be placed the wrong way round, the pattern will appear on the resultant radiograph. This enables the cause of the resultant pale film to be easily identified (see Ch. 18).

The radiographic film

The cross-sectional structure and components of the radiographic film are shown in Figure 6.3. It comprises four basic components:

Fig. 6.3 Diagram showing the cross-sectional structure of double emulsion radiographic film.

Film orientation

The film has an embossed dot on one corner that is used to help orientation. Its position is marked on the back of the packet or can be felt as a raised dot on the front. The side of the film on which the dot is raised is always placed towards the X-ray beam. When the films are mounted, this raised dot is towards the operator and the films are then arranged anatomically and viewed as if the operator were facing the patient.

Uses

Film/screen combinations are used as image detectors whenever possible because of the reduced dose of radiation to the patient (particularly when very fine image detail is not essential). The main uses include:

Indirect-action film construction

This type of film is similar in construction to direct-action film described above. However, the following important points should be noted:

The relative spectral sensitivity of these four different film emulsions is shown in Figure 6.4.

Fig. 6.4 Graph showing the relative spectral sensitivity of standard silver halide (BLUE), modified silver halide (ULTRAVIOLET), orthochromatic (GREEN) and panchromatic (RED) film emulsions.

Optical density (OD)

Optical density is the term used for describing the degree of film blackening and can be measured directly using a densitometer. In diagnostic radiology the range of optical densities is usually 0.25–2.5. There are no units for optical density.

Characteristic curve

The characteristic curve is a graph showing the variation in optical density (degree of blackening) with different exposures. Typical characteristic curves for direct-action (non-screen) and indirect-action (screen) film are shown in Figure 6.5. This curve describes several of the film’s properties.

Fig. 6.5A A typical characteristic curve of indirect-action radiographic film, showing the main regions of the curve including background fog density, toe and shoulder. B A typical characteristic curve for direct-action film.

Background fog density

This is the small degree of blackening evident even with zero exposure. This is due to:

If the film has been stored correctly (see later), this background fog density should be less than 0.2 (see Fig. 6.5).

Film speed

This is the exposure required to produce an optical density of 1.0 above background fog (see Fig. 6.6). Thus, the faster the film, the less the exposure required for a given film blackening and the lower the radiation dose to the patient.

Fig. 6.6 The characteristic curve of an indirect-action (screen) film showing the film speed — the exposure required to produce an optical density of 1.0 above background fog.

Film speed is a function of the number and size of the silver halide crystals in the emulsion. The larger the crystals, the faster the film but the poorer the image quality.

In clinical practice, the fastest films consistent with adequate diagnostic results, either D speed or more usually nowadays the faster E or F speed, should be used.

Film sensitivity

This is the reciprocal of the exposure required to produce an optical density of 1.0 above background fog. Thus, a fast film has a high sensitivity.

Film latitude

This is a measure of the range of exposures that produces distinguishable differences in optical density, i.e. the linear portion of the characteristic curve (see Fig. 6.7). The wider the film latitude the greater the range of object densities that may be seen.

Fig. 6.7 The characteristic curve of an indirect-action film showing film contrast and latitude.

Film contrast

This is the difference in optical density between two points on a film that have received different exposures (see Fig. 6.7).

Film gamma and average gradient

Film gamma is the maximum gradient or slope of the linear portion of the characteristic curve. This term is often quoted but is of little value in radiology because the maximum slope (steepest) portion of the characteristic curve is usually very short.

Average gradient is a more useful measurement and is usually calculated between density 0.25 and 2.0 above background fog (see Fig. 6.8).

Fig. 6.8 Characteristic curves showing A film gamma and B average gradient of an indirect-action (screen) film.

Thus the film gamma or average gradient measurement determines both film latitude and film contrast as follows:

Resolution

Resolution, or resolving power, is a measure of the radiograph’s ability to differentiate between different structures that are close together. Factors that can affect resolution include penumbra effect (image sharpness), silver halide crystal size and contrast. It is measured in line pairs (lp) per mm. Direct-action film has a resolution of approximately 10 lp per mm and indirect-action film a resolution of about 5 lp per mm.

Action

Two intensifying screens are used — one in front of the film and the other at the back. The front screen absorbs the low-energy X-ray photons and the back screen absorbs the high-energy photons. The two screens are therefore efficient at stopping the transmitted X-ray beam, which they convert into visible light by the photoelectric effect (described in Ch. 2). One X-ray photon will produce many light photons which will affect a relatively large area of film emulsion. Thus, the amount of radiation needed to expose the film is reduced but at the cost of fine detail; resolution is decreased. The ultraviolet system was developed to improve resolution by reducing light diffusion and having virtually no light crossover through the plastic film base (see Fig. 6.10).

Fig. 6.10 Diagram showing the action of conventional calcium tungstate and ultraviolet systems. Note the small cone of ultraviolet light with no cross-over through the film base, compared to the large cone of blue light and marked cross-over from the calcium tungstate phosphors. These differences result in better resolution and image sharpness with ultraviolet systems.

Useful definitions

The following terms are used to describe intensifying screens:

Fluorescent materials

Three main phosphor materials are, or have been, used in intensifying screens:

Rare earth and related screens

Modern screens employ these phosphors which produce very fast screen speeds, enabling a substantial reduction in radiation dose to patients, without excessive loss of image detail. The main points can be summarized as follows:

l The rare earth group of elements includes:

– lanthanum (Z = 57)

– gadolinium (Z = 64)

– terbium (Z = 65)

– thulium (Z = 69).

• The term rare earth is used because it is difficult and expensive to separate these elements from earth and from each other, not because the elements are scarce.

• These phosphors only fluoresce properly when they contain impurities of other phosphors, e.g. gadolinium plus 0.3% terbium. Typical screens include:

– Terbium-activated gadolinium oxysulphide (Gd₂O₁S:Tb)

– Thulium-activated lanthanum oxybromide (LaOBr:Tm).

• Terbium-activated screens emit GREEN light, while thulium-activated screens emit BLUE light (see Fig. 6.11).

• Yttrium (Z = 39), the rare earth related phosphor, in the form of pure yttrium tantalate (YtaO₄) emits ULTRAVIOLET light (see Fig. 6.11).

• Rare earth and related screens are approximately five times faster than calcium tungstate screens. The amount of radiation required to produce an image is therefore considerably reduced, but they are relatively expensive.

  Page 47 

• Several different screens of each phosphor, each producing a different image system speed, are available:

Fig. 6.11A Graph showing the relative spectral emissions of different types of intensifying screens. B Graph showing the spectral sensitivity of different types of film combined with the relative spectral emissions of different types of intensifying screens.

Screen type	Image system speed
Detail or Fine	100
Fast detail or Medium	200
Rapid or Fast	400
Super rapid	800

• It is important to use the appropriate films with their correctly matched screens.

Types

Cassettes are made in a variety of shapes and sizes for different projections. A selection is shown in Figure 6.12.

Fig. 6.12 Various cassettes for different radiographic projections. A Oblique lateral cassette. B Intraoral occlusal cassette. C Flat panoramic cassette. D Skull cassette. E Curved panoramic cassette.

Construction

Despite their different shapes, the construction of the cassettes is very similar. They consist usually of a light-tight aluminium or carbon fibre container with the radiographic film sandwiched tightly between two intensifying screens (see Fig. 6.13). Any loss in film/screen contact will result in degradation of the final image.

Fig. 6.13A A standard 18 × 13 cm cassette opened up showing the white intensifying screens and the film. B Diagram showing the cross-sectional components in a cassette.

Film storage

All radiographic film deteriorates with time and manufacturers state expiry dates on film boxes as a guide. However, this does not mean that the film automatically becomes unusable after this date. Storage conditions can have a dramatic effect on the deterioration rate. Ideally films should be stored:

Screen maintenance

Intensifying screens should last for many years if looked after correctly. Maintenance should include:

These aspects are discussed further in Chapter 18.

Uses

Both types of sensors can be used for intraoral (periapical and bitewing radiograph) and extraoral radiography including panoramic and skull radiography. Only phosphor storage plates are available for occlusal and oblique lateral radiography as it is currently too expensive to manufacture sufficiently large solid-state sensors.

Intraoral sensors

The intraoral sensors are small, thin, flat, rigid rectangular boxes usually black in colour and similar in size to intraoral film packets as shown in Figure 6.14. They vary in thickness from about 5–7 mm. Most sensors are cabled to allow data to be transferred directly from the mouth to the computer. Several systems are now available, examples include Gendex Visualix®, Planmeca dixi²® and Kodak RVG 6000 (see Fig. 6.14).

Fig. 6.14 Examples of modern solid-state sensors. A Planmeca dixi² ® and conventional film packets to show their comparative size. B Gendex Visualix®. C Kodak RVG 6000.

(kindly provided by Mr R. France)

For ease of clinical use the sensor cables are usually 1–2 m long and plug into a remote docking station which can be conveniently attached to the tubehead supporting arm (see Fig. 6.15). A separate cable then connects the docking station to the computer.

Fig. 6.15 Examples of docking stations. A Planmeca dixi²® attached to an X-ray tubehead. Note the little holder for conveniently supporting the sensor (arrowed). B Gendex Visualix®. The sensor plugs into the arrowed port. The open arrowed cable connects to the computer

(kindly provided by Mr R. France).

A cable-free system is also available. The Schick CDR Wireless™ sensor transmits radiowaves from the mouth to a remote base station which is connected by a cable to the computer. This removes the inconvenience the cable can create clinically, but additional electronics make the sensor slightly more bulky.

The solid-state sensors are NOT autoclavable. When used clinically they all need to be covered with a protective plastic barrier envelope for infection control purposes (see Ch. 9).

CCD (charge-coupled device)

Individual pixels, consisting of a sandwich of P and N-type silicon, are arranged in rows and columns called an array or matrix, above which is a scintillation layer made of similar materials to the rare-earth intensifying screens. The basic design is shown in Figure 6.16. The X-ray photons that hit the scintillation layer are converted to light. The light interacts via the photoelectric effect with the silicon to create a charge packet for each individual pixel, which is concentrated by the electrodes.

Fig. 6.16 Diagrams illustrating the basic construction of an intraoral CCD sensor. A The imaging surface showing the pixel array. B Sensor from the side showing the scintillation layer. C An individual pixel consisting of a sandwich of N- and P-type silicon.

The charge pattern formed from the individual pixels in the matrix represents the latent image. The image is read by transferring each row of pixel charges from one row to the next. At the end of its row, each charge is transferred to a read-out amplifier and transmitted down the cable as an analogue voltage signal to the computer’s analogue-to-digital converter, often located in the docking station. Each sensor consists of between 1.5 million and 2.5 million pixels and pixel sizes vary from 20 microns to 70 microns.

Extraoral sensors

Extraoral sensors contain CCDs in long, thin linear arrays. They are a few pixels wide and many pixels long. The CCD array is incorporated into two different designs of sensor:

Fig. 6.17A The original Trophy Digipan® CCD sensor for retro-fitting to panoramic equipment. B Diagram showing the basic with a long thin array of CCDs.

Fig. 6.18A Specifically designed Planmeca dimax³® sensor for cephalometric radiography (see Ch. 15). B Diagram showing the basic design with two long thin arrays of CCDs.

Although the outward appearances of these sensors is very different, both designs work in a similar fashion. The long narrow pixel array is aligned with a narrow slit-shaped X-ray beam and the equipment scans across the patient. This scanning motion takes several seconds to scan the skull and is discussed in more detail in Chapter 15.

Plate construction and design

The plates typically consist of a layer of barium fluorohalide phosphor on a flexible plastic backing support, as shown in Figure 6.21.

Fig. 6.21 Diagram showing the cross-sectional structure of a typical phosphor imaging plate.

As with using film, image production is not instantaneous with this type of image receptor. Two distinct stages are involved, namely: