Technical Aspects:the machinethe image

LeeAnn Pack DVM

Exposure settings: 4 components

• kVp

• mAs

• Time

• Focal film distance (FFD)– All can be changed alone or in combination

Make it just right

• Make a film more black – Increase the kVp, mAs or time– Decrease the FFD

• Make a film less black– Decrease the kVp, mAs or time– Increase the FFD

Questions to Ponder

• Is there adequate penetration of the part imaged?

• How much contrast is needed on the film (some inherent?)?

• Is motion a strong possibility during the exposure?

• Does the tube move?

Inverse Square Law

• Decrease distance by ½ intensity is increased by 4 times

• Increase distance by ½ intensity is increased by 4 times

• Technical applications

• Radiation safety

Cassettes

• Contain single or double screen

• Plastic, cardboard, metal

• Must be durable

• Film placed between the screens in the cassette

• Foam between cassette and screen

• Keeps out unwanted light

Intensifying Screens

• Absorb X ray photons and convert them to light

• Reduces amount of radiation needed

• Contain phosphors– Calcium tungstate blue light– Rare earth phosphors green light

• More efficient

Screen Speed

• Determined by the efficiency of X ray absorption and conversion

• Thicker phosphor layer and larger crystal produce a greater amount of light faster

• Slow detail photonsresolution• Medium par average• Fast less resolution but not as many

photons needed

Screen Speed/Resolution

• 100,200,400,800

• Smaller number being a slower high resolution screen and the higher numbers are a faster screen

• Screen speed and resolution are inversely proportional

Screen Cleaning

• Keep free of debris• Sharp white artifact• Screen cleaner• Soft lint free cloth• Never use soap and

water• Stand on end to dry

Radiographic Film

• Composition– Base supports the emulsion – blue– Adhesive – base to emulsion– Emulsion – gelatin with silver halide

crystals which is sensitive to radiation– Protective coat protects from damage

during handlin and processing

Radiographic Films

• Single or double emulsion

• Light sensitivity

• Film speed– Film speed is inversely related to exposure

needed to produce a given degree of blackness

– 50 speed is half as fast as a 100 speed

Radiographic Film

• Film latitude

• Range of exposures which can be used to achieve an acceptable film density

• Wide range = high latitude– Image contrast suffers

• Narrow range = low latitude– Good image contrast

Radiographic Density

• Subject density– Additive – thick parts absorb more

• Summation – know definition

– Relative• What are the surroundings?• Silhouette sign – know definition

• Over exposed = film to black

• Under exposed = film not black enough

Factors that affect film density

• Subject density – we cant change this

• mAs – biggest factor

• kVp – increases the penetrability

• Distance – ISL

• Development time and temp

• Scatter and fog

Radiographic Contrast

• Difference of densities of the different parts on the image

• Subject contrast

• Film contrast

• Scatter and fog

Turn that dial: Density

• The 16-20 rule kVp by 20% will double film density kVp by 16% will half film density– Double mAs double density– Half mAs half density– Combos:

• Inc kVp by 20% and dec mAs by half• Dec kVp by 16% and double mAs

Contrast

• Short scale lot of contrast few grey– High mAs and low kVp

• Long scale many shades of grey– Low mAs and high kVp

• Low contrast = long scale = high latitude = low mAs = high kVp

• High contrast = short scale = low latitude = high mAs = high kVp

Magnification

• Enlargement of the image relative to the actual size– Causes loss of detail and blurring

• Subject film distance– Dec SFD decreases magnification

• Focal film distance– Inc FFD decreases magnification

Distortion

• Image which does not represent the true shape of the object– Unequal projection of an object

• Femurs

– Location of an object within the beam• spine

Scatter radiation

• What is scatter?– Bounce around undergo numerous

interactions

• Scatter increases with:– Increased thickness of the patient– Increased X ray beam energy– Amount of patient exposed (collimate)

Beam Limiting Devices

• Smaller fields = less scatter and less radiation expsoure

• Cones, cylinders, shuttered collimators

• They do not shape the beam only exclude the part we don’t want to use

• Light to illuminate field

Filtration• Inherent

– Glass housing, oil and tub housing window– Filter only low energy photons

• Added– Aluminum filter– Will not remove the high energy photons

• Polychromatic beam• Reduces patient dose

Grids

• Used to keep scatter from hitting film

• Perpendicular radiation gets through, off angle photons do not

• Used on parts > 10-11cm

• Film contrast is improved

• Required more X ray exposure

Grid ratio

• Height of lead strips:width between the strips

• The higher the GR, the greater the ability of the grid to remove scatter

• 4:1 – 16:1

• Lines per inch– Good and bad

Grid Cutoff

• Grid must be precisely under the center of the central ray of the X ray beam along the central axis of the grid

• Know what the types of grid cutoff look like

• Bucky-Potter moving grid

Fog

• Light fog

• Storage fog

• Safelight fog

• Chemical fog