Radiographic Image Production. Radiographic Image Production. Principles of Imaging Science I (RAD 119) Film, Screens, and Cassettes

Transcription

1 Principles of Imaging Science I (RAD 119) Film, Screens, and Cassettes Radiographic Image Production X-ray photons emitted from the x-ray tube interact with the body, exit the patient (exit beam) and interact with the image receptor (image forming x-rays) PE Absorption Compton Scatter Radiographic Image Production Image receptor converts the exit beam into a latent image through fluorescence of intensifying screen phosphors X-ray film is one medium that captures the diagnostically useful information to render a diagnosis Image intensifier and associated TV monitor Laser imaging (CT, MRI, CR) 1

2 Photosensitive Film First image receptor chosen by Röntgen No longer most common image receptor because of predominance of digital imaging in 21 st century Knowledge of film/screen technology still required of radiographers Radiographic Film Construction Base Adhesive layer Emulsion Supercoat (overcoat) Radiographic Film Base Foundation of radiographic film Flexible, yet durable Uniformly radiolucent Film must retain its size and shape throughout use and processing Thickness ranges from micrometers Modern base is thin sheet of polyester Dimensional stability to minimize distortion Semi-rigid for viewbox placement Blue tint Reduces eyestrain Increases diagnostic accuracy Crossover Light from one screen exposed opposite emulsion 2

3 Film Emulsion (3-5 um) Latent image produced by energy deposited in silver halide crystals Distribution of exposed and unexposed crystals in undeveloped film Double layer Film Emulsion Gelatin Provides support for silver halide crystals Permits crystals to be suspended evenly Radiolucent, low Z# Allows processing chemicals to penetrate through Silver halide crystals Silver (+), Bromine (-), Iodine (-) combine in crystal lattice structure Silver Bromide Silver Iodide or Silver Chloride Energy deposited in silver halide crystals represents anatomy Processing Latent image converted to manifest image LATENT IMAGE FORMATION Latent image: Invisible image; distribution of exposed and unexposed crystals in undeveloped film Gurney-Mott Theory X-rays cause ionization of the of AgBr crystal Free electrons gravitate to the sensitivity speck Positive Ag+ crystals attracted to negatively charged sensitivity speck 3

4 LATENT IMAGE FORMATION Supercoat/Overcoat Protective layer of gelatin Provides sturdiness to unexposed radiographic film Antistatic Reduces damage from scratches, pressure, or contamination during storage, handling, and processing Film Storage and Handling Prior to exposure: Storage On end Age Expiration dates Rotate stock Heat Below 68ºF (20ºC) 4

5 Film Storage and Handling Prior to exposure: Humidity 30-60% Light Radiation Film Identification Each exposure must include medical record information Date of exposure Full name of patient Name of institution Name of ordering physician Patient identification number Type of exam Screen Film Most widely used type of film Placed between intensifying screens in a cassette Other Types Direct Exposure Mammography Laser Duplicating Spot Standard Screen Film Sizes English Unit (inches) SI Unit (cm) 18 X 43 8 X X X X X X X X X 43 5

6 Screen Film Characteristics Speed based upon sensitivity of x-ray & light for Image Receptor (IR) Film speed has limitations, due to ideal thickness of emulsion layer. Light is absorbed quickly on outermost layer and will not expose portion closest to base if too thick Double emulsion increases speed Contrast High, Medium, Low High: Sm silver crystals, uniform size Low: Lg silver crystals, non-uniform size Multiple Latitude available (inversely related to contrast) Screen Film Characteristics Spectral Matching Response to spectrum of light emitted by screen Rare earth phosphors Z# Emit UV, blue, green and red Screen film responds to UV & blue, not green, yellow, or red unless spectral dye is added Radiographic film is either blue-sensitive or green sensitive Proper matching avoids technique error Spectral Matching Blue Sensitive Film Blue or UV emitting screens If used with Green emitting screens, IR speed is reduced Green Sensitive Film Green emitting screens Radiographic films are either bluesensitive or green-sensitive and they require amber-and red-filtered safelights, respectively. 6

7 Screen Film Characteristics Safelight Filters Used in darkroom processing to prevent exposure to white light 15 watt bulb, > 5 feet from work surface Provides sufficient illumination to process film Incandescent bulb with colored filter Amber filter: Blue sensitive film Fog a green sensitive film Red filter: Blue or green sensitive film (orthochromatic) Screen Film Characteristics Crossover Light from intensifying screens crosses the base and exposes the opposite side emulsion. Image blur results Screen Film Crossover Minimized by: Tablet or cubic silver halide crystals Crossover control layer between base and emulsion Absorbs crossover light Remains as separate, distinct layer Removed during processing 7

8 Screen Film Characteristics Reciprocity Law Short or long exposure times reduce speed due to exposure from intensifying screen light Increases patient exposure due to increase in mas needed Relative speed ratings 1 ms = 95% 10 ms = 100% 100 ms = 100% 1 sec = 90% 10 sec = 60% Intensifying Screens Used to reduce patient radiation dose without compromising anatomic detail Converts image forming x-ray photon energy to light energy forming the latent image through amplification 99% of image comes from light conversion 1% of image comes from direct interaction between x-rays and film Screen-film is placed between two intensifying screens within the cassette Cassette is light tight film holder Open 2 3 to retrieve or replace film Screen Artifacts Construction Base Reflective layer Phosphor layer Protective coat 8

9 Screen Construction Protective Coating Thin plastic outer layer Protects phosphor layer from abrasion Resists abrasion, eliminates static build-up Artifact on film Phosphor Layer Emits light when exposed with x-ray energy Active layer High atomic # (Y, La, Gd) Reflective Layer - Magnesium oxidide, titanium dioxide Redirects isotopic light photons back to film Base Polyester support inner layer Supports phosphor layer Intensifying Screen Reflective Layer Screens without reflective layers are not as efficient as those with reflective layers because fewer light photons reach the film. Allows for reduction of patient dose Luminesence Luminescence occurs when an outer-shell electron is raised to an excited state and returns to its ground state with the emission of a light photon Types Fluorescence Visible light is emitted only while the phosphor is stimulation Phosphorescence Visible light continues to be emitted after stimulation Screen lag or afterglow 9

10 Screen Characteristics Speed Quantifiable value used to denote the conversion efficiency of x-ray energy to light energy Phosphor type Calcium Tungstate (RSS ) 5% conversion efficiency o Blue emission Rare Earth (RSS ) 15% - 20% conversion efficiency o Ytrium, lanthanum, gadolinium o Blue or green emission Phosphor size Phosphor layer thickness Reflective layer CONVERSION EFFICIENCIES OF RARE EARTH AND CALCIUM TUNGSTATE Beam Absorption Calcium tungstate Absorb 20-40% of beam Rare earths Absorb 50-60% of beam 10

11 Film/Screen Contact Cassette Front surface (x-ray tube side) made of low atomic # material (Carbon fiber Z#6) Back surface made of high atomic # material to absorb backscatter radiation Cross-sectional view of cassette containing front and back screens and loaded with double-emulsion film. 11