Radiographic Film / Screen Technology

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RADIOGRAPHIC FILM

Film base

Film emulsion

Latent image

Silver halide vs. metallic silver

Premise

•Radiographic materials are sensitive to light (photosensitive) and x-ray photons

•Film may be manufactured to be insensitive to portions of the light spectrum (wavelengths of light)

•Radiographic film has a base material onto which is coated one emulsion layer (single-emulsion film) or two emulsion layers, one on each side of the base (double-emulsion or duplitized film)

Radiographic Film Base

•To provide a support for the emulsion

•Desirable properties include; flexible yet tough, stable, rigid, and uniformly lucent

•Usually includes a blue dye to tint therefore reducing eye strain

•A special light-absorbing layer to prevent crossover effect

•Crossover control layer is removed during processing

•Single-emulsion film may incorporate an antihalation coating to absorb reflected light at the film base-air interface; this phenomenon is known as halation

•Antihalation coating is removed during processing

–Carlton & Adler P. 272 Figure 19-3

Radiographic Film Emulsion

•Emulsion is composed of a homogeneous mixture of gelatin and silver (Ag) halide crystals

•Gelatine is the suspending medium for silver (Ag) halide crystals or grains

•Silver halide crystals or grains are the photosensitive agents

Latent Image

•The x-ray beam is modified or modulated after it passes through the patient

•The modulated beam interacts with compounds in radiographic film that serve to produce the latent image.

•An invisible image formed on film as a result of exposure to radiation, and which may later be made visible by photographic development/processing

Silver halides vs. Metallic silver

Fixing Agent Light Radiographic Image
Silver halide Converted into soluble   compounds Sensitive to light •Silver halide has been   removed•Transparent areas
Metallic silver Unaffected Unaffected Blackened areas

FILM STORAGE AND HANDLING

Unexposed Film

Exposed Film

Length of time x-rays are stored

Handling and Storage of Unexposed Film

•X-ray films are sensitive to light, heat, humidity, chemical contamination, mechanical stress and x-radiation

•Storage conditions must protect unexposed film from stray radiation, chemical fumes and light

–The optical density from the base material and film fog (B+F) must not exceed 0.30 OD

–Radiation exposure must be limited to 0.1 mGy (SC35 P. 21)

•Film is sold in light-proof and moisture-proof packaging and may be handled in dark safelight conditions (caution: ensure correct safelight filter)

•Boxes of film must be stored on-edge away from chemical fumes, at a temperature range of 18⁰ C to 23⁰ C and humidity between 40% and 60%

•Boxes of film must be used before their expiry date

•Loaded cassettes must be stored in a shielded area and must be limited to 0.5 μGy (SC35 P. 21)

Storage Conditions for Exposed Film

•Adequate ventilation

•Satisfactory illumination

•Dust free environment

•Smoke detectors and secure locks

•Metal shelving units subdivided into compartments for easier storage of film file folders (containing patients x-rays)

Length of Time X-rays are Stored

•Depending on the institution the length of time an original radiograph is stored is usually 5-8 years

•Obstetrical ultrasound are kept for 25 years

•Pediatric radiographs are kept until the patient’s 25th birthday

•Unusual or rare pathology

INTENSIFYING SCREENS

Purpose

Screen base

Phosphor material

Screen / film combinations

Relative speed

Resolution

Cassettes

Purpose:

–To decrease x-ray dose to the patient

–To decrease motion blur or unsharpness

•Converts x-ray energy to visible light

•Amplifies the effect of the x-ray beam

–Each x-ray is converted into many light photons

–X-ray film is more sensitive to light than x-rays

Intensifying Screen Base

•Provides mechanical support for the active phosphor layer

•Polyester is the preferred material

–Characteristics include; flexible yet tough, rigid, chemically inert, and uniformly radiolucent

•A reflective layer (magnesium oxide or titanium dioxide) may be added to redirect the light towards the film

•An absorptive layer (coloured dye) may be added to absorb light going away from the film

Phosphor or luminescent material

•Active layer of the radiographic intensifying screen

•Emits light (fluorescence) when stimulated by x-rays

–Recall Roentgen’s discovery of X-rays when he observed the luminescence of barium platinocyanide

•Calcium tungstate (CaWO4) was the original phosphor material; rare earth elements (gadolinium, lanthanum, and yttrium) are used in newer, faster screens

Phosphor Materials in Radiography

1.High atomic numbers are very efficient at x-ray absorption; called quantum detection efficiency (QDE)

2.Emits a large amount of light per x-ray absorption; called conversion efficiency (CE)

3.Light emitted must match the sensitivity of the x-ray film; called spectral matching

4.Fluoresce strongly with little afterglow (phosphorescence)

Screen / Film Combinations

•Designed to compliment each other to produce a quality image

–Intensifying screens manufactured to emit a certain wavelength of light require films to have enhanced sensitivity to the same wavelength

Characteristics of film/screen combinations

•The two important characteristics are speed and resolution

–The speed of an imaging system in inversely proportional to the dose

•As the speed of a system increases the dose required to obtain the desired optical density decreases

–The term “relative speed” (RS) is assigned to film/screen combinations

•RS of 200 is a slower system when compared to a system having a RS of 400

Relative Speed

•When changing from one imaging system to another, the speeds of the two imaging systems must be considered to ensure proper selection of exposure factors

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Resolution

•The ability of an imaging system to demonstrate detail varies on its purpose

–Mammography demands the visualization of fine structure versus abdominal radiography which does not require as detailed imaging

•Also described as recorded detail or sharpness

•Measured as line pairs per millimeter (lp/mm), line spread function (LSP), and modulation transfer function (MTF) (Note, we will study these concept further in Unit 4 – Analyze Image for Quality and Diagnostic Purposes)

•Under ideal conditions the naked eye can resolve ~ 10 to 20 lp/mm

–Direct exposure (non-screen) film = 100 lp/mm

–Detail screens = 15 lp/mm

–High-speed screens = 7 lp/mm

•Inverse relationship between RS and resolution

–As the speed of the imaging system increases the resolution decreases

•Film/screen contact has a direct effect on image resolution

–Poor film/screen contact decreases resolution and image density (C&A P. 323 Figure 22-5)

•Quantum mottle occurs when an insufficient quantity of photons are used to create an image

–The image appears “grainy” and may be undiagnostic; the technologist controls the quantity of photons by selecting an appropriate mAs (C&A P. 323 Figure 22-6)

Clinical Choices

•Chest radiography

–Imaging of high subject contrast areas (lung tissue and bone) requiring a short exposure time (minimize motion unsharpness)

•Abdominal radiography

–Imaging of low subject contrast areas (organ densities are similar)

•Pediatric radiography

–Radiosensitivity of tissue and potential of motion unsharpness

Cassettes

•A lightproof, portable, rigid holder that contains the intensifying screens and radiographic film

•The front cover (x-ray tube side) designed for minimum attenuation of the x-ray beam; graphite carbon fibre materials versus aluminum or plastic

•The back cover is usually made of a heavier metal or may have a sheet of lead foil for the purpose of absorbing back-scatter radiation

•A compression device (foam pressure pad) to provide good contact between screen and film

CARE OF SCREENS

Proper handling

Regular cleaning

Screen-film contact test

Care of Screens

•Screens should be handled carefully; rough handling may lead to scratches or damage resulting in radiographic artefacts

–Place the film inside the cassette when loading

–Do not dig the film out with your fingernail when unloading

–Do not leave the cassette open; susceptible to dust or darkroom chemicals

¨Cleaned regularly with manufacturers recommended preparation often containing an antistatic compound

¨Screens must be completely dry before reloading

¨Screen-film contact checked with a wire mesh

¨Lighter areas indicate poor film-screen contact; may be necessary to replace the cassette


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