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There are several types of image receptors that are available for use in radiography. The most common one is the use of silver halide crystals mounted on a plastic support (conventional x-ray film). Another technique used uncommonly in veterinary radiology is the use of charged selenium plates (xeroradiography) . A third technique that involves equipment and costs beyond veterinary practice is the use of ionization chambers that absorb x-rays and produce voltage pulses that are displayed on a cathode tube (computerized tomography).
The following paragraphs discuss the use of radiographic film in recording the information gained from the study. The other two techniques are discussed under special equipment.
The purpose of using radiographic film is to provide a permanent record containing the maximum amount of diagnostic information that can be recorded. It is possible to evaluate a bone fracture by observing the image on a fluoroscopic screen. However, because of the size of the crystals in the screen, the detail is not great and the maximum amount of information is not available for study. More important, there is no permanent record of the study for later evaluation. For reasons such as these, the radiographic film is used to store the greatest amount of information for evaluation immediately following the study and at a later date as well.
When visible light or x-ray (ionizing radiation) interacts with the silver halide crystals, a latent image is formed. This image remains unchanged until the film is processed. At that time, a permanent record is created on the film. It is important to understand that the crystals in the film emulsion are sensitive to x-rays or other ionizing radiation, visible light, and such things as pressure, bending or moisture.
Film composition is similar to that found in other photographic film. Silver halide crystals are coated in an emulsion on both sides of a polyester base. Some single sided emulsion film is available for use. The purpose of the film base is to provide support for the emulsion. Therefore, it must be stable, inert, and relatively transparent. A thin adhesive subcoat holds the emulsion layer to the film base. The first photographic films used had a paper base. This was followed by the introduction of a glass base in 1847. Eastman developed a nitrocellulose base in 1889 that was explosive and several serious hospital fires resulted. As a result of this, a cellulose acetate base was developed in 1906 that was nonexplosive. Newer films use a polyester base. Most film bases are blue tinted because this is more pleasing to the eye.
Films have been produced that have a different color base. One of these is Medichrome by Agfa-Gevaert. Viewing the films through a set of filters can enhance gradation of shadows. This is a blue film of medium speed with a high exposure latitude that can be exposed either directly or by intensifying screens. The film is free of graininess and can be used with conventional safety lighting. A different developer is required. Processing can be done by hand or in an automatic processor.
Film is sensitive to ionizing radiation or light and numbers of the tiny silver halide crystals in the emulsion are converted to metallic silver by exposure to these forms of electromagnetic radiation and subsequent
reduction in the developer solution. The greater the number of silver halide crystals that have been transferred to metallic silver, the blacker will be the film and the greater the film density. The latent image is formed on screen type film by absorption of a light photon by a grain of silver halide. The energy of the light photon removes an electron as a photoelectron from the halide atom in the silver halide crystal. The electron is caught by a sensitivity trap in the crystal created by an imperfection in the crystal. This holds the electron temporarily in one place. Because of the negative charge of the electron, positively charged silver ions are attracted. A pair of positively charged silver ions join to form a silver atom. This speck of silver becomes the latent image. The silver grows to be come large enough to form a developmental center that can change an entire crystal to silver (Table 8-11).
If the silver halide crystals are not exposed to light or ionizing radiation, they are removed during the fixing portion of film processing leaving only the nearly transparent film base. The number of metallic silver crystals on the film is proportional to the film density.
Table 8-1
FORMATION OF LATENT IMAGE
Br -> Br+ e-
e- -> electron trap
Ag+ -> electron trap
Ag+ e- -> Ag
Ag+ Ag -> Ag (latent image)
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to the quantitative measure of the film blackening. Density is defined by the equation:
where D is density, I0, is the light intensity incident on an particular area of a processed film, and I, is the light intensity transmitted. If transmission of light is 100%, then the log of 1 is 0 (D = 0). If transmission of light is limited to 10%, then the log of 10 is 1 (D = l). If transmission of light is limited to 1%, then the log of 100 is 2.0 (D = 2). If transmission of light is limited to 0.1%, then the log of 1000 is 3.0 (D = 3).
A plot of the log of photographic density represents the most convenient method of demonstrating the response of a film to light or x-rays. This curve is referred to as a densitometric or characteristic curve or the "H and D" curve, after Hurter and Driefield who used it first in 1890. The curve expresses the relationship between the logarithm of the exposure (horizontal axis) and the resulting density (vertical axis) (Fig. 8-1).
Characteristic curves are derived by exposing a type of film to a series of exposures. The film is developed and the resulting density is plotted against the known exposure. Film exposure is usually varied by using a constant kilovoltage and doubling the mAs with each exposure. The log of the relative exposure is plotted versus the log of the density measured from the exposure. Examination of the curve shows that there is a density noted even when the relative exposure is 0. This is due to film base density plus fog which are present, independent of any radiographic exposure. The important part of the curve lies between the toe and the shoulder. In this portion the density is approximately proportional to the log of the relative exposure.
Film contrast Film contrast is the difference between two densities noted on the radiograph. Film contrast depends on:
The shape of the curve shows how much change in film density will occur as the film exposure changes. The slope, or film gamma, is defined as the maximum slope of the curve and is defined as:
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where D2 and D1 represent the densities on the steepest part of the slope that result from the log of the relative exposures E2 and E1. The gamma of films exposed with intensifying screens ranges from 2.0 to 3.5. In an effort to gain more information concerning the entire curve, the concept of average gradient has been used. The density from approximately 0.25 to 2.0 is joined by a straight line and a calculation similar to that used to determine the gamma of the curve is performed on this straight line. A film with an average gradient of over 1 will tend to exaggerate the subject contrast.
The radiograph with optical densities in the region of the toe or the shoulder of the curve furnishes an image with inferior contrast. The exposure range over which acceptable densities are produced is known at film latitude. Latitude refers to the range of log relative exposures (mAs) which will produce densities within an accepted range for diagnostic radiology. This range is usually considered to be between densities of 0.25 and 2.0. Remember that the curve will vary with the kVp used to make the exposures. Higher kVp settings produce a curve with greater gamma. Developing time and temperatures alter the curve also. Longer developing times and use of temperatures recommended produce a lower gamma.
Film Speed Film is manufactured with various speeds through use of silver halide crystals of different sizes in the emulsion. Speed of the film determines the amount of x-ray required to produce an image on the radiograph. Basically, a fast film has
A slow film has:
If film A requires twice the exposure as film B to produce a radiograph of similar density, film B is described as being twice as fast as film A. Film sensitivity is changed after exposure and becomes 4-6 times as sensitive to injury from bending, pressure, heat or chemicals (Table 8-2).
Film Types The two general types of film that are used in diagnostic radiology are screen and non-screen films. Screen type film is manufactured with crystals that are sensitive to fluorescent light that originates from intensifying screens and are less sensitive to direct ionizing radiation. This type of film requires less exposure to produce an image than does direct exposure type film because of intensification due
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Large crystals |
Small crystals |
Grainier image |
Less grainy image |
Requires less exposure |
Requires more exposure |
Poorly defined image |
Sharply defined image |
Less exposure latitude |
Greater exposure latitude |
to fluorescent screens. Some loss in image definition is noted because of this intensification of the x-ray beam through fluorescence.
Non-screen film is a direct exposure type film and is manufactured to be more sensitive to direct ionizing radiation. Since there is not intensification of the primary x-ray beam, greater exposures are required. Resulting radiographs have greater details and this film is of value in study of bone detail and is often used in dental radiography.
Screen Film Screen film has been, for many years, primalily a "blue sensitive" film (Table 8-3). This means
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X-OMAT R* |
Fastest 90 sec processable |
X-OMAT RP* |
Medium speed 90 sec processable |
X-OMAT L* |
Medium speed wide latitude |
X-OMAT S* |
Medium contrast |
X-OMAT G* |
Fastest, high contrast |
Blue Brand* |
Medium speed, medium contrast |
SB |
Medium speed, wide latitude,single emulsion Agfa-Gevaert |
Curix RPl |
Fine grain, medium speed, high contrast |
Curix RPlL |
Medium speed, long latitude |
Curix RP2 |
High speed, high contrast |
Curix MR4 |
Ultra detail |
Ilford |
- |
Rapid R Type |
S High speed |
Rapid R |
Medium speed |
Red Seal |
Very high speed |
Sakura |
- |
QH |
Standard |
QL |
Medium speed, high contrast |
Type A |
High speed, medium contrast |
that the primary photographic response of the film has been to ultra-violet, violet, and blue light that originates from the intensifying screen. Today, newer films are being developed that are sensitive to green light as well as to ultra-violet, violet, and blue light (Table 8-4). Calcium tungstate has been most commonly utilized as a phosphor in intensifying screens because it efficiently converts energy of the x-ray photon into photographically usable ultra-violet, violet, and blue light. A new generation of intensifying screens is now available that utilize rare earth phosphors. Approximately 60% of their emission is within the green portion of the spectrum and about 25% of their emission is within the ultra-violet, violet and blue portion of the spectrum. Both blue and green light emitting screens can be used with blue or green sensitive films . The relative speeds of the systems will vary depending upon the type of combination selected.
To most efficiently expose radiographic film through use of intensifying screens, film must be sensitive to the type of light emitted by the screens. Therefore, film is manufactured so that it is sensitive to a certain color or wavelength of light. The standard silver halide film absorbs light in the light-violet, violet and blue regions of the visible spectrum. This film used with calcium tungstate or barium lead sulfate screens works well because these phosphors emit light in these portions of the spectrum. It is possible to extend the film sensitivity to green wave lengths by covering the silver halide grains in film with a thin layer of dye that absorbs green light and then transfers this absorbed energy to the silver halide grain. This type of film is referred to by the term "ortho." To utilize silver halide film with a green emission light such as would be seen to emit from gadolinium or lanthanum oxysulfide crystals would require that the film be orthodyesensitized to absorb green light. It is also possible to coat the silver halide grain with a different dye that will absorb red light. This is referred to as "pan" film. Film used with calcium tungstate crystal screens is not dye sensitized because the emission of light is in the wavelength that silver halide absorbs
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Kodak |
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Ortho G |
medium-high contrast |
Ortho G |
fast speed, medium contrast |
Min-R |
single coated 3M |
XUD |
ultra detail, high contrast |
XUL |
ultra detail, wide latitude |
XD |
detail, high contrast |
XDL |
detail, medium-low contrast |
XM |
medium speed, high contrast |
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In commonly used x-ray film a considerable fraction of film darkening (up to approximately 40% in terms of total effective exposures) is due to a factor referred to as crossover exposure. In double-coated film with two screens, the crossover effect is attributed to additional exposure of the film emulsion to light emitted by the screen placed opposite that emulsion. The primary cause of this crossover is the incomplete absorption of screen light by the adjacent emulsion. This light is spread due to transmission, scattering, and reflection in the film base and interfaces. This effect increases the film speed, but decreases the sharpness of images obtained with a screen-film system. New films have attempted to better control this cross-ovel by coating below either emulsion and thus absorbing the excess light diffusing through the base to the opposite emulsion. This coating is actually visible as a thin, pink layer in some films. The coating is rendered nonvisible during the normal processing of the film and is not visible on a processed radiograph (Fig. 8-1a) .
In the case of one anticrossover film (3M XUD) the relative speed of the film has been reduced by approximately 40% . However, the image quality is enhanced considerably. This advantage is more apparent in low-scatter
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Kodak Type R (single coated) |
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Type R (double coated) |
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Type M |
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Type M (ready pack) |
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Type T |
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Type AA |
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Type AA (ready pack) |
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Incident x-ray photons |
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X-rays absorbed by film |
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X-rays absorbed by screens |
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Light photons produced |
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Light photons incident on film |
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Light photons absorbed by film |
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Latent images formed |
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situations where high-definition screens are used (extremity studies) (Doi et al, 1981).
Generally, screen film can be processed by manual processing techniques or in automatic processors. As long as temperature and developer time are considered.
Non-screen Film Non-screen film is exposed by direct action of the x-ray photon and may be used for table top examinations of extremities where the requirement for detail is high. Most non-screen film must be processed manually in wet tanks because of the thicker emulsion size
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Sizes Equal |
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35 x 43 cm |
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35 x 35 cm |
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Metric sizes slightly different from inch size |
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18 x 43 cm |
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Metric size without U.S. equivalent |
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30 X 40 cm |
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30 X 35 cm |
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24 X 30 cm |
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18 X 24 cm |
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U.S size without metric equivalents |
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emulsion present on this film. An example of non-screen film requiring direct exposure by x-rays and manual processing is Kodak no-screen medical x-ray film. An example of non-screen type film requiring direct exposure by x-rays and automatic processing is Kodak RP/M x-omat medical x-ray film for mammography.
One problem associated with use of non-screen film is the absence of a strong protective cover for the film. Heavy cassettes protect screen type film but only a piece of heavy paper offers protection for non-screen film. Use of a piece of cardboard of thin Plexiglass protects non screen film from the dog's nails or from the teeth of the small animal during intraoral studies.
Industrial non-screen film is a direct exposure type film that is much slower than that commonly used for diagnostic purposes. It is occasionally used in medicine to radiograph a tissue specimen or is used in radiography of small birds or small mammals where tissue thickness is minimal (Table 8-5).
A comparison of the relative number of x-ray photons and light photons required to form a latent image and the number of photons at the various stages is given for direct exposure and screen type film. (Table 8-6) (Bushong, 1980).
Film Size Film size is available in both metric and U.S. measurements. Some sizes are equivalent while others are not (Table 8-7).
Film Packaging Film may or may not be interleaved with protective paper. Film are available in 25, 50, and 100 sheets per box or 500 sheets per case. There are cost advantages to purchase in larger quantities.
Film Costs Film costs are fluctuating widely, making it impossible to present any meaningful price list. Larger screen type film such as 14' x 17' (35 cm x 43 cm) costs approximately $2.50 per sheet. Smaller screen type film such as 8' x 10' (18 x 24 cm) costs approximately $1.00 per sheet.
Film Handling The manner in which x-ray film is handled is of great importance. This applies to the film at the time of first delivery to your office, the manner in which it is stored, and the care exercised in handling film during the radiographic examination.
Film Storage Film boxes should be stored on end in a room that is cool, in the range of 10° to 15°C and has a low humidity (40% to 60% relative humidity). Avoid storage near a hot water heater, radiator, or steam line. Store film so that there is no pressure on the boxes. Film storage areas must be free of radiation such as might originate from an x-ray machine or any other source of ionizing radiation.
Do not store film where gases or vapors from formalin,hydrogen sulfide, hydrogen peroxide, or ammonia can come in contact with the film. These substances cause fogging of the film.
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The film expiration date is marked on the end of each box. This should be rewritten with large numbers so that boxes are used in a sequence that avoids film remaining unused after the expiration date. The expiration date should be at least 12 months in advance of the date the film is delivered.
Use of blue sensitive x-ray film requires the use of a Wratten safelight filter No. 6B (brown) or equivalent filter. Green sensitive film requires the use of a filter shifted more to the red portion of the spectrum. A filter type GS-l (reddish brown) has been developed to provide darkroom illumination where green sensitive film is in use. A No. 6B filter is not suitable for use with green sensi tive film and a significant amount of fog will result. The GS-l filter permits exposure of films for up to 5 minutes in the darkroom with negligible fogging on green sensitive film. A GS-l filter can be used in darkrooms where both blue and green sensitive films are processed (Fig. 8-2).
