Image receptors

Published on 13/06/2015 by admin

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Chapter 4

Image receptors

This chapter summarizes the various image receptors used in dentistry to detect X-rays. These include:

• Radiographic film

– Direct action or packet film

– Indirect action film used in conjunction with intensifying screens in a cassette

• Digital receptors

– Solid-state sensors

– Phosphor plates.

Radiographic film

Radiographic film has traditionally been employed as the image receptor in dentistry and is still widely used. There are two basic types:

• Direct-action or non-screen film (sometimes referred to as wrapped or packet film). This type of film is sensitive primarily to X-ray photons.

• Indirect-action or screen film, so-called because it is used in combination with intensifying screens in a cassette. This type of film is sensitive primarily to light photons, which are emitted by the adjacent intensifying screens. They respond to shorter exposure of X-rays, enabling a lower dose of radiation to be given to the patient.

Direct-action (non-screen) film

Indirect-action film

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:

• Extraoral projections, including:

– Oblique lateral radiographs (Ch. 12)

– All skull radiographs (Ch. 13)

– Panoramic radiographs (Ch. 15)

– All routine medical radiography.

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 silver halide emulsion is designed to be sensitive primarily to light rather than X-rays.

• Different emulsions are manufactured which are sensitive to the different colours of light emitted by different types of intensifying screens (see later). These include:

– Standard silver halide emulsion sensitive to BLUE light

– Modified silver halide emulsion with ultraviolet sensitizers sensitive to ULTRAVIOLET light

– Orthochromatic emulsion sensitive to GREEN light

– Panchromatic emulsion sensitive to RED light.

The relative spectral sensitivity of these four different film emulsions is shown in Fig. 4.4.

Fig. 4.4 Graph showing the relative spectral sensitivity of standard silver halide (BLUE), modified silver halide (ULTRAVIOLET), orthochromatic (GREEN) and panchromatic (RED) film emulsions.

• It is essential that the correct combination of film and intensifying screens is used.

• There is no orientation dot embossed in the film so some form of additional identification is required, e.g. metal letters, L or R, placed on the outside of the cassette or electronic marking.

Characteristics of radiographic film

This section summarizes the more important theoretical terms and definitions used to describe how radiographic film responds to exposure to X-rays.

Optical density (OD)

< ?xml:namespace prefix = "mml" />OD=logIncident light intensityTransmitted light intensity

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 Fig. 4.5. This curve describes several of the film’s properties.

Background fog density

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

• The colour/density of the plastic base

• The development of some unexposed silver halide crystals.

If the film has been stored correctly (see later), this background fog density should be less than 0.2 (see Fig. 4.5).

Film speed

This is the exposure required to produce an optical density of 1.0 above background fog (see Fig. 4.6). Thus, the faster the film, the less the exposure required for a given film blackening and the lower the radiation dose to the patient.

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. 4.7). The wider the film latitude the greater the range of object densities that may be seen.

Film contrast

This is the difference in optical density between two points on a film that have received different exposures (see Fig. 4.7).

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. 4.8).

Fig. 4.8 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:

• If the gamma or average gradient is high (i.e. a steep gradient), that film will show good contrast, but will have less latitude.

• If the film gamma or average gradient is low (i.e. a shallow gradient), that film will show poor contrast but will have wider latitude.

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.

Intensifying screens

Intensifying screens consist of fluorescent phosphors, which emit light when excited by X-rays, embedded in a plastic matrix. The basic construction and components of an intensifying screen are shown in Fig. 4.9.

Fig. 4.9 Diagram showing the cross-sectional structure of a typical intensifying screen.

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. 4.10).

Useful definitions

The following terms are used to describe intensifying screens:

• Conversion efficiency – the efficiency with which the phosphor converts X-rays into light

• Absorption efficiency – the ability of the phosphor material to absorb X-rays

• Screen efficiency – the ability of the light emitted by the phosphor to escape from the screen and expose the film

• Intensification factor (IF)

IF=Exposure required when screens are not usedExposure required with screens

• Screen speed – the time taken for the screen to emit light following exposure to X-rays. The faster the screen, the lower the radiation dose to the patient

• Packing density – the ability of the phosphor to pack closely together resulting in thin screens and less light divergence.

Fluorescent materials

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

• Rare earth phosphors including gadolinium and lanthanum

• Yttrium (a non-rare earth phosphor but having similar properties)

• Calcium tungstate (CaWO₄).

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:

• 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. 4.11).

Fig. 4.11 A 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.

• Yttrium (Z = 39), the rare earth related phosphor, in the form of pure yttrium tantalate (YtaO₄) emits ULTRAVIOLET light (see Fig. 4.11).

• 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.

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

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.

Calcium tungstate screens

This was the original material used but it is no longer recommended. The main points can be summarized as follows:

• The speed of these screens depends upon:

– The thickness of the phosphor layer

– The size of the phosphor crystals

– The presence or absence of light-absorbing dyes within the screen

– The conversion efficiency of the crystals

• The faster the screen, the lower the radiation dose to the patient but the less the detail of the final image

• All calcium tungstate screens emit BLUE light and must be used with blue-light-sensitive monochromatic radiographic film (see Fig. 4.11).

• Slower than rare earth screens.

Cassettes

Types

Cassettes are made in a variety of shapes and sizes for different projections. A selection is shown in Fig. 4.12.

Fig. 4.12 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 usually consist of a light-tight aluminium or carbon fibre container with the radiographic film sandwiched tightly between two intensifying screens (see Fig. 4.13). Any loss in film/screen contact will result in degradation of the final image.

Fig. 4.13 A 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.

Important practical points to note

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:

• In a refrigerator in cool, dry conditions

• Away from all sources of ionizing radiation

• Away from chemical fumes including mercury and mercury-containing compounds

• With boxes placed on their edges, to prevent pressure artefacts.

Screen maintenance

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

• Regular cleaning with a proprietary cleaning agent

• Careful handling to avoid scratching or damaging the surface

• Regular checks for loss of film/screen contact.

These aspects are discussed further in Chapter 17.

Digital receptors

There are two types of direct digital image receptors available, namely:

• Solid-state

• Photostimulable phosphor storage plates.

Uses

Both types of sensors can be used for intraoral (periapical and bitewing radiography) 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.

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. They vary in thickness from about 5 to 7 mm as shown in Fig. 4.14. Most sensors are cabled to allow data to be transferred directly from the mouth to the computer. Several systems are now available.

Fig. 4.14 Examples of modern solid-state sensors. A Planmeca dixi²® and conventional film packets to show their comparative size. B Small and large Sirona solid-state sensors showing their thickness.

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. 4.15). A separate cable then connects the docking station to the computer.

Fig. 4.15 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. 8).

Construction and design

The sensors consist of tiny silicon chip-based pixels and their associated electronics encased in a plastic housing. Underlying technology involves either:

• CCD (charge-coupled devices)

• CMOS (complementary metal oxide semiconductors).

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 Fig. 4.16 and the complex electronic circuitry required is shown in Fig. 4.17. 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. 4.16 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.

Fig. 4.17 X-ray image of a Sirona sensor showing the complex internal electronic circuitry.

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 µm to 70 µm.

CMOS (complementary metal oxide detectors)

These sensors are similar in construction to CCDs and consist of an array of pixels but they differ from CCDs in the way that the pixel charges are read. Each CMOS pixel is isolated from its neighbour and directly connected to a transistor. The charge packet from each pixel is transferred to the transistor as a voltage enabling each individual pixel to be assessed separately.

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:

• Flat cassette-sized sensors designed to be retro-fitted into existing film-based panoramic equipment to replace conventional cassettes.

• Individually designed sensors as part of completely new solely digital panoramic or skull equipment such as the Planmeca dimax^3® (see Fig. 4.18).

Fig. 4.18 A Specifically designed Planmeca dimax³® sensor. B Diagram showing the basic design with two long thin arrays of CCDs. The lower array is used for panoramic radiography (see Ch.15), both arrays are used for cephalometric radiography (see Ch.14) and the two images obtained are *stitched* together to create one large image.

Although the outward appearances of these sensors is very different, both designs work in a similar fashion. A 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 Chapters 14 and 15.

Photostimulable phosphor storage plates

These digital sensors consist of a range of imaging plates that can be used for both intraoral and extraoral radiography. The plates are not connected to the computer by a cable. Several systems are available and include the DentOptix™ (Gendex) and the Vistascan™ (Durr) and Digora® Optime (intraoral) and PCT (extraoral) (Soredex).

A range of intraoral and extraoral plate sizes are available with these systems, identical in size to conventional periapical, occlusal, oblique lateral, panoramic and skull films (see Fig. 4.19). Once cleared (erased), the plates are reusable. Intraoral plates need to be inserted into protective barrier envelopes for control of infection purposes (see Fig. 4.20A).

Fig. 4.19 The different sized DenOptix™ plates for panoramic, skull and intraoral radiography.

Fig. 4.20 A The white imaging side of a DenOptix™ phosphor plate. B The reverse side of the plate. C The plate being inserted into the protective barrier envelope – note the reverse side of the plate is visible through the clear side of the envelope. D The plate in the envelope ready for clinical use.

Plate construction and design

The plates typically consist of a layer of barium fluorohalide phosphor on a flexible plastic backing support, as shown in Fig. 4.21.

Fig. 4.21 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:

• The phosphor layer absorbs and stores the X-ray energy that has not been attenuated by the patient.

• The image plate is then placed in a reader where it is scanned by a laser beam. The stored X-ray energy in the phosphor layer is released as light which is detected by a photo-multiplier tube and converted into a voltage which is relayed to the computer and displayed as a digital image. This is described in more detail in Chapter 6.

To access the self assessment questions for this chapter please go to www.whaitesessentialsdentalradiography.com

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