Chapter 10

Intensifying Screens

 

   

Intensifying screens consist of a thin layer of tiny phosphor crystals mixed with a suitable binder and coated in a smooth layer on a cardboard or plastic support. The coating over the layer of crystals is protected by a cleanable surface. The basic principle in the action of intensifying screen is utilization of a phosphor (Greek = light bearer) that converts energy carried by an x-ray photon into visible light. The purpose of this screen is to reduce radiation exposure required to produce a diagnostic radiograph. This results in usage of lower mAs settings and frequently is advantageous because of the ability to utilize shorter exposure times. Patient dosage is reduced, motion unsharpness is not as severe a problem, and film contrast is improved through use of intensifying screens(Table 10-1).

The luminescent chemical used must:

10.1 Construction

An intensifying screen has four layers:

The total thickness of a typical intensifying screen is about 0.4 mm (Fig 10-1). 

The base is often mylar with a thickness between 157 and 250 microns. 

Table 10-1

ADVANTAGES OF THE USE OF INTENSIFYING SCREENS

 Reflecting layer. The reflecting layer is made of a white substance such as titanium dioxide and is spread between base and the binder in a thin layer so as to reflect back toward the front of the screen any light photons that are directed toward the base layer and would be lost as far as photographic activity is concerned.

 Phosphor layer in binder. The binder holds the phosphor crystals and must: 1) not discolor with age, 2) have no adverse effect on the film emulsion, 3) remain flexible so that cracks do not develop, and 4) be coated to the base in a uniform unchanging layer. It is easy to appreciate that any change in thickness of the thin layer of binder, any cracks or breaks in the layer, or change of color of the binder overlying the crystals would influence the amount of light produced by interaction of x-rays on the crystals and markedly alter the uniform pattern of film exposure.

 Protective coat. The protective coating is placed on the outer surface of the screen and provides protection for the crystals. The coating must resist marking and abrasive wear and must permit easy cleaning. Any debris on the surface of a screen influences the ease with which fluorescent light can reach the film. Extraneous material between the phosphor and the film inhibits passage of light and produces film artifact. Because of this, it must be possible to frequently clean the surface of the screens and do so in a manner that causes no deterioration in the protective coating. The coating is approximately 0.7-0.8 mil in thickness.

 

 Par-speed x-ray intensifying screen(cross-section)

 

Table 10-2

ABSORPTION EFFICIENCY

-

-

Calcium tungstate -- slow

20%

Calcium tungstat -- fast

40%

Non-calcium tungstate

60% 

10.2 Intensifying action of screens

Use of intensifying screens permits conversion of a portion of the absorbed x-ray photons into light photons that ultimately create small foci on the radiographic film which leads to film density.

The first action is the absorption activity of the phosphor. Only a portion of the x-ray beam is absorbed by the screens. This is determined by the thickness of the phosphor layer. Increase in the thickness of this layer is the is the major way the speed of calcium tungstate screens has been increased in the past. Improvement in absorption characteristics of the phosphor has resulted in a 30 to 50% increased absorption in rare earth screens (Table 10-2). Absorption is most likely to occur in elements with high atomic numbers and when the x-ray photon energy and the binding energy of the K-shell electron are similar. When this happens, there are a maximum of photoelectric interactions that occur. Diagnostic radiology is usually performed at a range of 60-130 kVp or with an effective energy of the beam of 20-60 keV. Examination of K-edge energies will show why x-ray film has a low x-ray photon absorption efficiency and why both gadolinium and lanthanum have higher efficiencies (Table 10-3).

Another way in which efficiency of a phosphor can be judged is to evaluate the efficiency of conversion of absorbed x-rays to light. This is termed the intrinsic efficiency of a phosphor. Conversion to light is about 5% in calcium tungstate screens and has been increased to about 20% through use of rare earth type screens. Other phosphors such as barium fluorochloride achieve greater intensifying action due to both absorption and conversion.

Because of this intensifying action of the screens,direct film exposure requires about 34 times as many photons as a film-screen combination to obtain the same film density. That translates into an exposure of 3.4 seconds for no-screen film as compared with 0.1 second for a screen type exposure. The definition of the intensification factor of the film is a comparison of the exposure required when screens are not used divided by the exposure required when screens are used (Table 10-4). This is a bit misleading because it requires exposure of the film by direct action of the x-ray photons when, in fact, the screen type film is not as sensitive to x-ray photons as it is to visible light. Later we will compare the exposure required for screen versus non-screen techniques using the film suitable for each type of exposure. 

Table 10-3

K-ELECTRON BINDING ENERGIES(keV)

Photographic film

Ag

25

-

Br

13 

Screens

CaWO4

69.5

-

Lanthanum

38.9

-

Gadolinium

50.2

--

Barium

37.0

-

Yttrium

17.0

Table 10-4

Intensification = exposure without screen

factor exposure with screens

 

10.3 Screen Speed

There are several factors governing the speed of an intensifying screen. One of these factors is luminescent crystal size (Fig 10-2). Within certain limits the larger the crystal, the greater is its fluorescent emission. An x-ray photon striking on any part of a crystal causes the entire crystal to fluoresce. This results in large flashes of light with fewer flashes required to cause total exposure of the film. The result of increasing the speed of the screen through an increase in crystal size is a grainy film that is less acceptable because of a decrease in detail. Within limits, this increase in crystal size is acceptable. Conversely, the use of small crystals produces a film of higher detail but requires the use of larger amounts of radiation (Table 10-5).

Another factor affecting both speed and definition is the thickness of the phosphor layer. Light from deep crystals is reflected and refracted before reaching the surface of the screen and, thus, records a diffuse and unsharp image. The thicker the layer, the more diffuse is the radiographic image but the faster is the screen speed. 

 

EFFECT OF SCREEN CRYSTAL SIZE ON FILM DETAIL.

X-radiation strikes the crystal in the intensifying screen and causes a flouescence that covers an area of the film dependent on the size of the crystal. The smaller the crystal, the better the film detail.

 

Table 10-5

FACTORS AFFECTING SCREEN SPEED

-

Effect on Speed

Increase phosphor layer thickness

increased

 Use of reflecting underlayer

increased

Use of absorbing underlayer

decreased

Use of light absorbing pigment in binder

decreased

ncrease phosphor crystal size

increased

Change to a phosphor with higher density

increased

Change to a phosphor with higher atomic number

increased

Absorption efficiency can be influenced by phosphor thickness with an increase in thickness increasing x-ray absorption. However, there is a loss of radiographic detail with thickening of the phosphor layer. Another method is now available to increase absorption of the photons that does not compromise film quality. This is use of the fact that absorption increases sharply if the input x-ray spectrum and the K-edge (K-shell electron binding energy) of the absorbing material are coincident (Fig. 10-3).

The K-edge of the prominent element in calcium tungstate is 69 kev, gadolinium oxysulfide is 50 kev, and lanthanum oxybromide is 39 kev. The lower K-edge energies of the new phosphors are much closer than calcium tungstate to the input x-ray energies and, thus, the new phosphors have much higher absorption efficiencies.

The conversion efficiency which is the percentage of absorbed x-ray energy that is ultimately emitted as light is 3.5% for CaWO4, 15% for Gd2O2S, and 13% for LaOBr.

The spectral emission emitted by the phosphor is ultraviolet to blue for CaWO4, green for Gd2O2S and blue-green for LaOBr.

Screen speed is also influenced by the reflectance of light from the reflecting coat spread over the base.

It must be remembered that the speed of intensifying screens may only be increased within certain limits. With high speeds the resulting radiographs have a more pronounced aspect of granularity. Quantum mottle, or film granularity, is a definite problem that is associated with use of screens of high speed. It also must be remembered that film-screen combinations can be so fast that presently used reciprocating grids are too slow. It is convenient to use a medium-speed (Par speed) calcium tungstate screen as a basis for comparison of the speed of different types of intensifying screens. 

10.4 Phosphor

The luminescent chemical first suggested by Edison in 1896 was calcium tungstate (CaWO4) and this has been used almost universally since that time. To its credit, caWO4 has a relatively high x-ray absorption coefficient(20%) and is a physically strong material. However, its x-ray to light conversion efficiency is poor being typically in the range of 3 to 5%. Lead-activated barium sulfate (BaSO4:Pb), barium fluorochloride (BaFCl), and strontium/europium-activated barium sulfate (BaSO4:SrEu) have been used in recent years in a few applications in an effort to increase speed of the screens. All of these phosphors emit blue light.

 

 Absorption curves of three phosphors of an 80 kVp x-ray beam attenuated by having passed through a patient’s body. Note the increased absorption at the K-edge of the predominant element (Lanthanum = 39 kev, Gadolinium = 50 kev, and Tungsten = 69 kev)

To maximize absorption of x-ray photons, intensifying screens should be composed of constituents of high atomic number which can be packed to a high density within the screen emulsion. X-ray absorption increases with an increase in atomic number. Some elements pack better than other providing a higher density that absorbs more -xrays.

In the past few years,research has led to the evoluation of new phosphors and new methods of construction.

 

TABLE 10-7

RELATIVE SPEED OF CALCIUM TUNGSTATE SCREENS

Dupont

Relative Speed 

1. Cronex - Detail -very slow

1

2. Cronex - Fast Detail - slow

2

3. Cronex - Par

4

4. Cronex - Hi-speed

7

5. Cronex - High plus

8

6. Cronex -

12

Lightning plus Radelin (U.S. Radium Corp.)

-

1. UD - Ultra Detail - very slow

1.0

2. T-2 - General purpose-medium

2.1

3. TF-2 - High speed - fast

4.2

4. STF-2 - Super high speed

5.55

 TABLE 10-6

NON-CALCIUM TUNGSTATE INTENSIFYING SCREENS.

E.I. DuPont

-

(Film-Cronex-blue sensitive)

Quanta II

blue

BaFlCl:Ev

Quanta III

blue

LaOBr:Tb

Quanta V

green

Gd2O2S:Tb

Hi-speed

blue

BaSO2:Pb

Agfa Gevart

-

(Film - Curix MR4)

MR 50

main blue mainly blue

Y2O2S:Tb or La2O2S:Tb

MR 200

mainly blue

LaOBr:Tb

MR 400

mainly blue

LaOBr:Tb or Y2O2S:Tb

MR 600

mainly blue

LaOBr:Tb

 U.S. Radium

-

-

Rarex B (BMS)

mainly blue

Y2O2S:Tb

Rarex BG

blue/green

Y2O2S:Tb and Gd2O2S:Tb

Rarex BGD

blue/green

Y2O2S:Tb and Gd2O2S:Tb

Rarex BGMS

blue/green

Y2O2S:Tb and Gd2O2S:Tb

Rarex BGHS

blue/green

Y2O2S:Tb and Gd2O2S:Tb

 IIford

(Film-Rapid E Rapid R-blue sensitive)

-

Rapid E

mainly blue

LaOBr:Tb

Siemens

-

-

Titan

mainly blue

LaOBr:Tb 

General Electric

-

-

 BluMax 1

mainly blue

LaOBr:Tb

BluMax 2

mainly blue

LaOBr:Tb 

3M

(Film - XUD, XD, XDL, XM-blue/green sensitive) 

-

Trimax (alpha 2)

green (545 nm)

Gd2O2S:Tb and La2O2S:Tb

Trimax (alpha) 4

green (545 nm)

Gd2O2S:Tb and La2O2S:Tb

Trimax (alpha) 8

green (545 nm)

Gd2O2S:Tb and La2O2S:Tb

Trimax (alpha)12 (SX68)

green (545 nm)

Gd2O2S:Tb and La2O2S:Tb

Trimax (alpha) M

green (545 nm)

Gd2O2S:Tb and La2O2S:Tb

 Eastman kodak

(Film-Ortho G Ortho H-green sensitive)

-

Lanex Regular

blue/green (454 nm)

Gd2O2S:Tb and La2O2S:Tb

Lanex Fine

blue/green (454 nm)

Gd2O2S:Tb and La2O2S:Tb

Kodak X-Omatic

Regular blue (380 nm)

BaSrSO4:Eu

Kodak X-Omatic Fine

blue (360 nm)

BaPbSO4

 Kodak X-Omatic High Detail

blue

BaPbSO4 

Kodak Min-R

green

Gd2O2S:Tb

 Table 10-8

RELATIVE SPEED RADELIN (U.S. RADIUM CORP.) SCREENS.

1. UD*

1.0

2. Rarex BG - Detail

2.7

3. Rarex BG - Mid-speed

6.6

4. Rarex BG - Hi-speed

10.0

5. Rarex B

13.8

*(Calcium tungstate)

This group of screens is referred to as rare earth screens (Table 10-6). This is partially in error because only lanthanum and gadolinium are in the rare earth series of elements. These may be called the Lockheed phosphors because they were developed under AEC contract by Lockheed Aviation Corporation. There are four basic groups of new phosphors.

These phosphors exhibit higher x-ray absorption efficiency and greater x-ray energy-to-light conversion efficiency than conventional calcium tungstate screens.

Light emitted from these new screens is within a narrow band of wave lengths as compared to the broad band emission of calcium tungstate screens. Since light emitted from Gd202S:Tb and La202S:Tb occurs primarily in the green portion of the visible spectrum (550 nm) it is therefore not well matched to be used with conventional blue light sensitive x-ray film. Nevertheless, exposure reductions of two or greater are achievable in the range of 70kVp and above using these screens and standard blue sensitive x-ray film. Significantly larger exposure reductions would be anticipated with the use of faster green sensitive film.

Y202S:Tb and LaOBr emit a significant fraction of light in the blue region of the visible spectrum in addition to the green emission so that these screens may be used with conventional blue sensitive x-ray film efficiently. However, their use with green sensitive film is recommended (Fig. 10-4,10-5).

Advantages of faster screens are

Companies produce intensifying screens of different speeds that are usually sold under a specific trade name. These have an assigned mAs factor or relative speed that permits comparison between the different speeds of screens manufactured by that company. It is, however, difficult   

 

 

  

 Spectral emission of three phosphors as compared with the spectral sensitivity of standard silver halide x-ray film. This demonstrates how it is possible for some phosphors to be limited to use with only one film type while others can be used with both film types.

to gain information that compares the speed of screens produced by different companies. The mAs factor or relative speed may vary according to the kVp used, but in veterinary radiography this usually does not have to be taken into consideration. Variation of response by screens when used with different kVp settings is especially noticeable in groups of non-calcium tungstate screens. The characteristic speeds of calcium tungstate intensifying screens available from two companies are listed (Table 10-7) as are those from a series of non calcium tungstate screens (Table 10-8).  

10.5 Types of Intensifying Screens  

Diagnostic intensifying screens. This type of intensifying screen is used routinely in medical radiography. Intensifying screens are usually mounted in pairs in a cassette. The screen on the front side of the cassette is slightly thinner than the screen on the back side. This tends to even out the exposure of light to both sides of the film. The cassette is a flat, rigid metal case with a bakelite or metal front and a hinged metal lid. The screens are mounted on the inside surface of the cassette so that the screen is in contact with the emulsion on each side of the x-ray film. The lid is equipped with latches that exert just enough pressure to protect against light leaks and to keep uniform contact between the screens and the film. The x-ray energy absorbed by the screen is converted into visible and ultraviolet light that is recorded on the film.  

 

  Spectral emission of three phosphors as compared with the spectral sensitivity of orthochromatic x-ray film. This demonstrates how it is possible for some phosphors to be used with only one film type while others can be used with both film types.

Some screens are used singly such as DuPont extremity B screens . These will be slower but produce a film of greater detail.

The crystal used most commonly today remains calcium tungstate. In excess of 95% of the exposure recorded on the film is due to the light emanating from the calcium tungstate crystals. The remaining 5% exposure to the x-ray film comes directly from the ionizing action of the x-ray photon. 

Fluorescent screens. A second type of intensifying screen is called a fluorescent screen and was commonly used in fluoroscopic examinations of the gastro-intestinal tract in the past. They have largely been replaced by image intensification units. The principle of operation was similar to that of a medical x-ray intensifying screen. Zinc cadmium sulfide (ZnCdS) was a commonly used fluorescent material since it emitted a yellow-green light that was more easily seen in a darkened room with dark adapted eyes. The fluoroscopic screen has the property of phosphorescence; that is, the phosphor continues to be luminescent after the exciting energy is removed. This is a disadvantage in a medical x-ray intensifying screen but is acceptable to a limited extent in a fluoroscopic unit where motion is being observed and afterglow is not objectionable. Since the radiologist viewed the fluoroscopic screen directly, it was necessary to cover the screen with lead glass to provide protection from the ionizing x-rays.

In the fluroscopic examination, the ability to evaluate motion of a contrast agent was most important. Detail was, therefore, not as critical and the crystals used in production of the fluoroscopic screens were larger.

Fluorescent screens deteriorate when exposed to ultraviolet light and experience a marked reduction in efficiency. For this reason all fluorescent screens that are over 20 years of age should be discarded. 

Photofluorographic screens. A third type of intensifying screen was used in the procedure of photofluorography. This is a little used technique today. It provided a technique for mass chest radiography in which exposure to patient and intensifying screen was continuous until the fluorescence from the screen reached a level that triggered a camera shutter. At this time a photograph was made of the image on the screen from the opposite side. The crystals used were zinc sulfide. The technique was widespread in its use because the unit could be made portable and a proper uniform exposure to the film was assured. However, radiation exposure to the patient was high. There are those who would question whether the minimal number of patients diagnosed with lung disease was sufficient to warrant the massive radiation exposure to the general population. 

10.6 Care of Intensifying Screens

Care of intensifying screens is so important in veterinary radiography that it deserves special comment. The screen is held in intimate contact with the film and the film protected from light by the cassette. The front is relatively radiolucent but still rigid and protective. It is possible with use that cassettes become damaged so as to affect the function of the screens. One common fault is that contact between the film and the screens is poor due to warping of the cassette front, sprung or cracked frame, warping of the screens, and worn or bent latches. It is possible for air to be present under the screen at the time of screen installation that causes improper film-screen contact. Often this air bubble moves with repeated closing of the cassette until it no longer causes loss of detail due to the poor film-screen contact. Another problem is wearing or destruction of the felt on the border of the lid that provides a light tight seal when the cassette is closed. If the felt is missing or thin, light will cause a "frame" of black around the radiograph due to exposure of film crystals from room light.

It is obvious in the daily use of cassettes in veterinary radiology that there will be frequent opportunities for the outer surfaces of the cassette to become dirty. These cassettes are then taken into the darkroom and placed on the counter for film unloading. When the cassette lid is opened, a partial vacuum is created and a flow of air from the counter top can carry dirt particles onto the screens. This is probably the most common method of introducing dirt onto the surface of the screens. Another problem in screen care centers around the chemicals and dirt that are carried on the fingers to the screens during removal of the film from the cassette and during reloading of the cassette. Any of these problems results in damage to the surface of the screens if not properly cleaned. Cleaning of the screens is not difficult and can be performed by a commercial screen cleaner or mild soap and warm water. Most commercially produced screen cleaners have anti static agents incorporated so that regular cleaning also helps to avoid production of static electricity. Avoid using denatured alcohol or abrasive products. Ethyl alcohol can be used safely in cleaning screens. It is important to permit the screens to dry prior to reloading with film and closing. Intensifying screens are expensive. They have a long life if properly cared for. It is suggested that they be cleaned weekly or biweekly in a busy practice. In this way, dirt and stains can be removed before they become permanent.

A record should be kept of the dates of screen cleaning. This can be recorded on the back of the cassette or a separate record can be maintained (Fig. 10-6). 

 SCREEN CLEANING RECORD

References

EFFECT OF INCREASING mAs ON FILM DENSITY