Chapter 12 -- Screen-Film Radiography (Bushong)

Image-Forming

x-rays exit the patient and expose the radiographic intensifying screen placed in the protective radiographic cassette; the radiographic intensifying screen emits visible light, which exposes the radiographic film placed between the two screens; although some x-rays reach the film emulsion, it is primarily light from the radiographic intensifying screens that eposes the radiographic film

image-forming x-rays

are those that exit the patient and interact with the image receptor

exit x-ray beam

refers to the x-rays that remain as the useful beam exits the patient; it consists of x-rays scattered away from the image receptor and image-forming x-rays

image receptor (IR)

the medium that converts the x-ray beam into a visible image

radiographic film has two parts:

the base and the emulsion; in most x-ray film the emulsion is coated on both side, so it is called double-emulsion film; between the base and emulsion is a thin coating of material called the adhesive layer, which ensures uniform adhesion of the emulsion to the base

overcoat

the emulsion is enclosed by this protective covering of gelatin

150-300

the base of radiographic film is _____ um thick, semirigid, lucent, and made of polyester

dimenstional stability

is the property that the base of radiographic film maintains its size and shape during use and processing

cellulose nitrate

replaced the glass plates that became unavailable due to WW1; had one serious deficiency: it was flammable; improper storage and handling of some x-ray film files resulted in severe hospital fires during the 1920's and early 1930's; by the mid 1920's film with a "safety base," cellulose triacetate was introduced; cellulose triacetate is has similar properties to cellulose nitrate but is not as flammable; in the early 1960's a polyester base was introduced;now polyester base has taken the place of cellulose triacetate

polyester base

is more resistant to warping from age and is stronger than cellulose triacetate, permitting easier transport through automatic processors; its dimensional stability is superior; polyester bases are thinner than triacetate bass (about 175 um) but are just as strong

emulsion

is the heart of the radiographic film; consistes of a homogeneous mixture of gelatin and silver halide crystals; it is coated evenly with a layer that is 3 to 5 um thick

gelatin

the principal function of gelatin ____ is to provide mechanical support for silver halide crystals by holding them uniformly dispersed in place

silver bromide: silver iodine

98%

2%

sensitivity center

Physical imperfections in the lattice of the emulsion layer that occur during the film manufacturing process; during exposure photoelectrons and silver ions are attracted to these sensitivity centers, where they combine to form a latent image center of metallic silver

screen film

most commonly used film, used with intensifying screens

Standard Film Sizes

English Units (in) SI Units (cm)

7 x 7 18 x 18

8 x 10 20 x 25

10 x 12 24 x 30

14 x 14 35 x 35

14 x 17 35 x 43

true

True/False: large-grain emulsions are more sensitive than small-grain emulsions

crossover

is the exposure of an emulsion caused by light from the opposite radiographic intensifying screen

rare-earth

______ screens are made with rare earth elements -- those with atomic numbers of 57-71

reciprocity law

exposure = intensity x time = constant optical density; the balancing relationship between exposure length and aperture opening when various combinations equal in theory the same amount of light; reciprocity law fails when film is exposed to light from radiographic intensifying screens

lower

very long or very short exposure times produce a lower/higher OD than that predicted by the reciprocity law

2

the fog level for unprocessed film is aprroximately _ uGy^a (0.2 mR)

latent image

the _______ is the invisible change that is induced in the silver halide crystal

ion

an ___ is an atom that has too many or too few electrons and therefore has electric charge

true

true/false: the result is the same whether the interaction involves visible light from a radiographic intensifying screen or direct exposure by x-rays

intensifying screen

the radiographic _______ amplifies the effect of image-forming x-rays that reach the screen-film image receptor

phosphors

the _____ converts the x-ray beam into light

isotropic emission

_____ refers to radiation emitted with equal intensity in all directions

the active layer of the radiographic intensifying screen:

is the phosphor

who developed calcium tungstate

Thomas A. Edison; within a year of Roentgen's discovery of x-ray

use of rare earth screens results in

lower patient dose, less thermal stress on the x-ray tube, and reduced shielding for x-ray rooms

between the phosphor and the base of intensifying screen is a:

reflective layer, approximately 25 um thick, that is made of a shiny substance such as magnesium oxide or titanium dioxide; when x-rays interact with the phosphor light is emitted isotropically; less than half of this light is emitted in the direction of the film; the reflective layer intercepts light headed in other directions and redirects it to the film; the reflective layer enhances the efficiency of the radiographic intensifying screen, nearly doubling the number of light photons that reach the film

luminiscent

______ materials emit light of a characteristic color

two types of luminescence

fluorescence: visible light emitted only during stimulation

phosphorescence: continues emission after stimulation

screens are fluorescent primarily

intensification factor

is a measure of the speed of the screen

screen speed

range from 50 (slow, detail) to 1200 (very fast)

IF

= exposure required without screen/exposure required with screens

thinner

film used with screens has (thinner?thicker?)emulsion layer than direct exposure film

true

true/false: noise reduces image contrast

image noise

appears on a radiograph as a speckled background and occurs most often when fast screens and high kVp techniques are used

DQE

refers to how well an imaging system converts the exit beam into an output image; the percentage of x-rays absorbed by the screen is higher; higher DQE does not increase noise

DQE= # x-rays absorbed/# incident x-rays x 100

CE

conversion efficiency; the amount of light emitted for each x-ray absorbed also is higher

CE= emitted light/x-rays absorbed x 100

increase

higher conversion efficiency results in ___ noise

rare earth radiographic intensifying screens have increased speed because of two important characteristics:

DQE and CE

both of which are higher compared with other types of screens

quantum mottle

when a grainy or mottled (spots) image is created; a principle component of image noise

contrast resolution

refers to the ability to image similar tissues

spatial resolution

refers to how small an object can be imaged; in screen-film radiography is limited principally by effective focal spot size

Image Detail

= spatial resolution + contrast resolution

true

T/F: generally, conditions that increase the IF reduce spatial resolution

T

T/F: in mammography the screen is positioned in contact with the emulsion on the side of the film away from the x-ray source to reduce screen blur and improve spatial resolution

TRUE

T/F: screen-film compatibility is essential; use only those films for which the screens are designed

SPEED

rare earth radiographic intensifying screens have the principal advantage of ______

rare earth radiographic intensifying screens obtain their increased sensitivity through:

higher x-ray absorption (DQE) and more efficient conversion of x-ray energy into light (CE)

increase

the combination of improved CE and higher DQE results in the increased?decrease? speed of rare earth radiographic intensifying screens

true

t/f: do not dig the film out of the cassette with your fingernails

developing

is the stage of processing during which the latent image is converted to a visible image

fixing

the silver halide that was not exposed to radiation is the process of clearing it from the emulsion and hardening the emulsion to preserve the image

synergism

occurs when the action of two agents working together is greater than the sum of the action of each agent working independently

glutaraldehyde

lack of sufficient ______ may be the biggest cause of problems with automatic processing; is a hardener that is in the developer solution

time, temperature, and concetration

film development is governed by three physical characteristics

archival quality

refers to the permanence of the radiograph: the image does not deteriorate with age but remains fixed

silver sulfide stain

is the most common cause of poor archival quality

3

the temperature of the wash water should be maintained at approximately __ degrees C (5 degrees F) below the developer temperature

shorter

the (shorter/longer) dimension of the film should always be against the side rail, so the proper replenishment rate is maintained

true

T/F: cleaning the tanks and the transport system should be a part of the routine maintenance of any processor

true

T/F: most processing faults leading to damp film are because of depletion of glutaraldehyde, the hardener in the developer

true

T/F: a finished radiograph that is damp easily picks up dust particles that could result in artifacts

fluorescence

visible light emitted only during stimulation

phosphorescence

continues emission after stimulation screens are fluorescent primarily