Film Screen Combinations

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Unit III

Creating the Image


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Chapter 23

Film/Screen Combinations


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Copyright 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED. 4

Objectives

Relate emission spectra of intensifying

screens to specific types of radiographic

film Explain radiographic film/screen

combination relative speed numbering

systems


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Objectives

Calculate relative speed conversions

from one film/screen combination to

another


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Objectives

Describe methods of measuring

resolution:

Basic description of line pairs permillimeter

Line spread function

Modulation transfer function


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Objectives

Relate film/screen contrast to latitude

Determine appropriate film/screen

combinations for various clinicalsituations


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Copyright 2006 by Thomson Delmar Learning. ALL RIGHTS RESERVED.8


Best combination: Film and screens are

designed to complement each other to

produce the highest image quality with

the lowest patient dose.

Film and screens must match each

other to achieve diagnostic quality

images.


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If screens are designed to emit a light of

a certain wavelength, then the film must

be designed to enhance sensitivity to

the same wavelength.

Mismatching of film and screens

often increases patient dose. *


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Emission Spectra

Different elements used in intensifying

screens emit a different color light on the

electromagnetic spectrum after interaction

with x-ray photons (pg. 34) Rare earth elements emit a green light. The

film matching must be sensitive to green

emitted light. Blue light sensitive film will

not match green light emitting screens. *


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Emission Spectra

Green sensitive film is not sensitive to

the blue-violet light as blue sensitive

film, But is sensitive to the entire range

of phosphor emissions including yellow-

green wavelengths.*


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Emission Spectra

Selection of film/screen combinationsfor clinical situations should be basedon the combined qualities of both

screen and film. (See Clinical Choicespgs. 338 339)

Angio

GI & GU Ab,skeletal, chest, peds


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Emission Spectra

Clinical selection cannot beaccomplished by evaluating film andscreens separately.

Qualities to be considered are speed,resolution, contrast, and latitude.*

Film/screen combination relationships

are the same as the interrelationships ofthese factors for film.


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Emission Spectra

The most common decision that must

be made when selecting film/screen

combinations is image resolution versus

patient dose.*


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Characteristics

The most important characteristics

of film/screen combinations are:

Speed Resolution

Contrast

Latitude


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Speed

The speed of an imaging system

depends on:

Silver halide phosphor/crystal size* Silver halide phosphor/crystal layer

thickness*

Efficiency of the crystal/phosphor in

emitting photons (screens) or capturingphotons (film)*


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Speed

Relative speed numbers (RS) are used

to rate film/screen combinations not

units.*

The numbers are based on a relative

value of 100 for calcium tungstate IS

with medium contrast and latitude blue

sensitive film. This was know as parspeed.


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Speed

It has been suggested that the base RS

100 be calibrated equal to 1.28 mR to

produce film speed point (OD 1.00 +

base + fog)


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Speed

Relative speed numbers are usually

established at 70-80 kVp with 80

preferred.* When kVps below 70 or

above 100 are used, Relative speed

may be less consistent.

Looking at table 23-2 you can see that

relative speed can be calculated by theformula:


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Speed

This makes the relationship between

the relative speed number and

film/screen sensitivity inversely

proportional.


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Speed

Sensitivity in mR=128

RS


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Speed

Relative speed numbers are accurateenough to be used by technologist toconvert exposure technique settings

from one situation to another. The formula for converting mAs from

one relative speed to another is:

mAs1 RS2 mAs2 RS1

=


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Speed

Relative speeds combination of 400 is

the most common compromise between

patient dose and resolution in the

hospital settings.*

Higher RS reduce patient dose,

decrease latitude, and decrease

resolution.


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Resolution

Recorded detail, sharpness, and

resolution are measured as:

Line pairs/mm (lp/mm)*

Measures the minimum size and space

between objects that can be visualized on

the final image


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Resolution

Modulation Transfer Function* MTF provides the best measurement of the

resolving ability of a screen/film combination. Itmeasures the information lost between the subjectand the image receptor. It is determined by thelight diffused between screen and film. It is ascreen function.

Line spread function (LSF)*

Measures the ability of a film/screen system toaccurately represent or measure the boundaries ofan image.*


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Contrast

Is primarily the contrast and function of

the film.

Intensifying screens also exhibitcontrast. Rare earth phosphors contrast

is slightly higher than calcium tungstate.


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Latitude

Primarily dependent upon film latitude

and is directly related to contrast.


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Latitude

Latitude and contrast are inversely

related. Narrow latitude film exhibit a high scale of

contrast.

Wide latitude film exhibit a low scale of

contrast.

High speed film/screen systems tend to have

lower resolution (decrease detail)*


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Selecting Film/Screen Combinations

The selection of appropriate film/screen

combinations requires consideration of

the various film and screen

characteristics.


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Clinical Choices

Film and intensifying-screen

manufacturers publish film/screen

combination charts that compare all

possible combinations of their films and

intensifying screens.


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Angiography

Higher speed rare earths provide

several advantages*

Decreased patient dose Allows for shorter exposure times

Maintenance of resolution

Quantum mottle is less of a problem.


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Gastrointestinal and Urologic

Radiography

High subject contrast due to use of

contrast media

Allows for use of faster film/screencombination*


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Abdominal Radiography

Low subject contrast

Makes slower film/screen combination

preferable*


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Skeletal Radiography

Extremities

Fine/detail screens or slower systems are

desirable

Pelvic and lumbar studies

Higher speed film/screen combo systems

are desirable due to large part thickness.


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Chest Radiography

High kVp chest radiography is achieved

more efficiently with calcium tungstate

screens.*


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Pediatric Radiography

High speed film/screen combinations

Allow for reduction in technical factors

reducing patient dose.

Shorter exposure times

Will limit repeats due to patient motion

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Film Screen Combinations