Chapter 11 Film Screen Systems

 

 

   

 Screen and film technology expanded rapidly in the past few years with changes in speed of the new systems and changes in degree of film detail. These recent developments have confronted the radiographer with a wide assortment of possible film-screen combinations. Systems can be partially evaluated by considering their advertised speed, however, it is necessary to use the system under specific conditions in your practice to determine if sharpness or detail of radiographic images and speed are satisfactory to meet your unique requirements.

Assessment of various film-screen systems should include examination of the relative speed. This could be done with comparative exposures of an aluminum wedge or from subjective evaluation of radiographs made of an animal model. The radiographs should be made with the same diagnostic unit and processed with the same processor. A densitometer can be used in measuring background density and the optical density of the radiographic images of the aluminum wedge. If a densitometer is used, it is possible to compute the average gradient between optical densities of 0.25 and 2.00 and gain information relative to the latitude of the combination. Evaluation of the system should also include some method of evaluation of the degree of film quality. Use of a star test pattern provides a method of evaluation of the degree of resolution or detail present on radiographs made with each system.

The average practice will not find it practical to perform all of these studies on every combination of film and screen. As yet, studies are not available in the literature that provide all of the answers for each system. In some instances, the fastest system possible must be used because of the type of anticipated studies and relatively low powered equipment. In another practice, the high mA capability plus a requirement to radiograph only small animals may make it unnecessary to consider any of these fast systems at all. As with most aspects of radiography, you lose something each time you think you are gaining. In selection of faster systems you may lose quality in the radiograph as you gain the ability to make exposures that you would otherwise not be able to make with the existing equipment.

A numerical value can be assigned to each film type and each type of intensifying screen. Unfortunately this is often done in such a manner that only products of a given company can be compared (Table 11-1). Comparisons of film-screen systems produced by different companies are more difficult (Table 11-3,11-4,11-5).

More specific information is given in the table concerning Kodak products (Table 11-1). The speed of X-OMAT RP combined with medium speed calcium tungstate screens is equal to 100 and is the base for comparison of other film screen speeds.

Table 11-1

RELATIVE SPEED OF FILM-SCREEN SYSTEMS

Intensifying Screens

 The number of film-screen combinations that are possible can be seen by examination of the options offered by E.I. DuPont. The relative speed is listed for each combination. Low numbers indicate slower combinations (Table 11-1).

In addition to different speeds, different film-screen combinations offer different alternative in film contrast, film latitude, and image sharpness. These combinations only take into consideration blue light emitting screens. If new orthochromatic films or green-light-emitting rare earth screens or blue sensitive film, the number of combinations becomes even larger.

Relative speeds of various combinations of non-calcium tungstate intensifying screens are a function of kVp as determined by settings on the x-ray generator. These screens exhibit higher sensitivity at 80 kVp than at 60 kVp, suggesting that at 80 kVp a significant portion of the x-ray photons are more energetic than the k absorption edge of the critical phosphor element. From 80 to 120 kVp the sensitivity of the screens does not exhibit a similar rise but instead decreases somewhat.

Table 11-3

COMPARATIVE FILM-SCREEN SYSTEMS*

BLUE SYSTEMS

*rare earth type *Thompson, et al., 1977

Table 11-4

COMPARATIVE FILM-SCREEN SYSTEMS*

Screen/Film 

*(Koblik et al., 1981)

+These combinations developed 30 F below recommended level

Table 11-5

COMPARATIVE FILM-SCREEN SYSTEMS

* Calcium tungstate screens

** Ortho film

*(Rossi, et al., 1976)

Table 11-6

RARE EARTH SYSTEMS

Advantages

• greater x-ray absorption efficiency
• greater photon-to-light conversion
  • shortened exposure time
  • permits use of smaller x-ray generators(lower mA requirement)
  • less scatter radiation
  • lower radiation exposure to patient
  • reduced x-ray tube load (longer tube life)
  • increased film resolution
  • thinner phosphor layer
  • smaller crystals in film and screens
  • permits use of smaller focal-spot size
  • shortened exposure time (less patient motion) 

Disadvantages

• initial expense
• increase quantum mottle (loss of radiographic quality)
• may need green sensitive x-ray film
• technique chart more complex since screen response is kVp dependent

These new systems:

• a) offer improved detail,
• b) offer increased versatility with low-powered equipment because of increased speed,
• c) reduce patient exposure,
• d) reduce exposure times resulting in less motion unsharpness,
• e) reduce kVp resulting in enhanced subject contrast, and
• f) reduce mA permitting more frequent use of small focal spots.

Disadvantages are:

• a) dependence of relative speed of the screen on x-ray photon energy making technique conversion changes (kVp) more difficult,
• b) increased emulsion sensitivity to normal darkroom light illumination requiring modification, and
• c) increased cost of rare earth screens (Table 11-6).

Comparison of blue ortho film systems suggests that "Green light" systems offer:

• 1) gain in speed,
• 2) loss in image quality,
• 3) require a different safelight,
• 4) may limit film selection and
• 5) require purchase of new screen pairs.

"Blue light" systems may offer almost the same speed and provide certain advantages in

• 1) the continued use of conventional safelight, and
• 2) the full compatibility with existing films.

References:

: Medical Radiography and Photography Vol. 53, Nov. 1, 1977

• Grum, F., Costa, L. F., and Donovan, J.L.: Measured Light-Emission Efficiency and Quantum Yield for X-Ray Screens and Phosphors. J. Opt. Soc. Amer. 59:848, July 1969.
• Imhof, H. and Doi, K.: Application of Radiographic Magnification Technique with an Ultra-high-speed Rare-earth Screen/Film System to Oral Cholecystography. Radiol. 129:173-8, 1978
• Koblik, P., Hornof, W.J. and O’Brien, T.R.: Rare Earth Intensification Screen for Veterinary Radiography: An evaluation of two systems. Vet. Rad. 21:224-32, 1980.
• Ludwig, G.W. and Prener, J.S.: Evaluation of Gd2O2S:Tb as a Phosphor for the Input Screen of X-Ray Image Intensifier. IEEE Tran. Nucl. Sci., NS-19:3, August 1972.
• Ovitt, T.W., Moore, R. and Amplatz, K.: The Evaluation of High-speed Screen-film Combination in Angiography. Radiol. 114:449-52, 1975.
• Reynolds, J., Skucas, J. and Gorski, J.: An Evaluation of Screen Film Speed Characteristics. Radiol. 118:711-3, 1976
• Rossi, R.P., Hendee, W.R. and Ahrens, C.R.: An Evaluation of Rare Earth Screen/Film Combinations: Radiol. 121:465-71, 1976
• Stables, D.P., Rossi, R. P., Caruthers, S. D. and Anderson, N.: The Application of Newer Screen-film Systems to Excretion Urography. Amer. J. Roentgenol. 128:617-9, 1977
• Stromberg, B., Olsson, S. E. and Lundgren M.: Rare Earth Intensifying Screens in Veterinary Radiology. J. Sm. Anim. Pract. 19:689-96. 1978
• Thompson, T. T., Radford, E. L. and Kirby, C. C.: A Look at Rare Earth and High Speed Intensifying Screens. Applied Radiology, p. 71, March/April, 1977
• Thornbury, J. R., Fryback, D. G., Patterson, F. E. and Chiavarini, R.L.: Effect of Screen/Film Combinations on Diagnostic Certainty: Hi-plus/RPL versus Lanex/ortho G in Excretion Urography. Amer. J. Roentgenol. 130:83-7. 1978.
• Trefler, M. and Ungaro, J. W.: conversion of a Radiology Department to Rare-Earth Screen-Film Systems. Proceedings of the Society of Photo-optical Instrumentation Engineers 173:52-7, 1979.
• Wagner, R.F. and Weave, K.E.: Prospects for X-ray Exposure Reduction Using Rare Earth Intensifying Screens. radiol. 118:183-88, 1976. 

  

KVP RANGE AFFECTS SCALE OF CONTRAST