Imaging Accessories PDF

Imaging Accessories Radiographic Film X-ray film displays the radiographic image Radiographic film has two parts: the base and the emulsion Most film have two layers of emulsion; called double- emulsion film Between the emulsion and the base is a thin coating of material called the ad...

Imaging Accessories Radiographic Film X-ray film displays the radiographic image Radiographic film has two parts: the base and the emulsion Most film have two layers of emulsion; called double- emulsion film Between the emulsion and the base is a thin coating of material called the adhesive layer, which ensures uniform adhesion of the emulsion to the base. Film thickness is approximately 150 to 300 µm. Film Structures Adhesive layer The layer between the emulsion and the base , which ensures uniform adhesion of the emulsion to the base. This layer allows the emulsion and the base to maintain proper contact and integrity during use and processing. Film Structures Base The base is the foundation of the radiographic film. Provides surface and support for the emulsion. The base is 150 to 300 µm thick, semirigid, lucent, and made of polyester. Polyester can withstand higher temperatures and is more fireproof. It must have strength, but it should be flexible. The base is usually tinted blue to reduce light glare. Film Structures Emulsion The emulsion is the heart of the radiographic film. The x-rays or light from the intensifying screens interact with the emulsion and transfer information to the film. A homogeneous mixture of gelatin and silver halide crystals and is about 3 to 5 µm thick. The gelatin provides mechanical support for silver halide crystals by holding them uniformly dispersed in place. Film Structures The emulsion is enclosed by a protective covering of gelatin called the overcoat. The overcoat protects the emulsion from scratches, pressure, and contamination during handling, processing, and storage. Silver Halide Crystal The active ingredient of the radiographic emulsion. In the typical emulsion, 98% of the silver halide is silver bromide; the remainder is usually silver iodide. 98% of silver halide is Silver Bromide and 2% Silver Iodide. These atoms have relatively high atomic numbers (ZBr = 35; ZAg = 47; ZI = 53) compared with the gelatin and the base (for both, Z ≈ 7). The interaction of x-ray and light photons with these high-Z atoms ultimately results in the formation of a latent image on the radiograph. Silver Halide Crystal May be tabular, cubic, octahedral, polygonal in shape. Tabular shape used most commonly for general radiography.  About 0.1µm thick and 1µm in diameter. Latent Image Invisible image produced on the film after exposure prior to development.  The latent image is the invisible change in the silver halide crystals.  The interaction between the photons and the silver halide crystals produces the latent image. Latent Image Formation This interaction is sometimes referred as the photographic effect. 1. A Radiation interaction releases electrons. 2. Electrons migrate to the sensitivity center(contaminant in the silver halide crystal,usually silver sulfide). 3. At the sensitivity centre, atomic silver is formed by attracting an interstitial silver ion--- latent image centre. Latent Image Formation 4. The process is repeated many times resulting in the build up of silver atoms. 5. The remaining silver halide is converted to silver during processing. 6. The resulting silver grain is formed. Silver halide that is not irradiated remain inactive. The irradiated and non- irradiated silver halide produces the latent image. Types of Film Screen Film Used with radiographic intensifying screens. Screen films most commonly used.  Screen film used with intensifying screens.  Single emulsion- emulsion on one side of base.  Double emulsion used with two screens. Types of Film Direct Eposure Film or Non-screen Film Used without intensifying screen Has thicker emulsion and more cystals Not sensitive to light Not commoly used because of increased patient dose Special purpose films such as that used in mammography, video recording, duplication, subtraction, cineradiography, and dental radiology). Handling and Storage of Film X-ray film is a sensitive radiation detector and it must be handled in an area free of radiation.  Film storage must be shielded.  The darkroom adjacent to the x-ray room must be shielded. Improper handling of the film will result in poor image quality due to artifacts.  Avoid bending, creasing or rough handling of the film. Avoid sharp objects contacting the film. Handling and Storage of Film Hands must be clean and dry.  Avoid hand creams, lotions or water free hand cleaners. Static electricity or a dirty processor can cause artifacts. Handling and Storage of Film Heat and Humidity Film is sensitive to heat and humidity.  Heat and humidity causes fog or a loss of contrast. Film should be stored at less than 20º C (68ºF) Humidity should be between 40% and 60%. Handling and Storage of Film Light Film must be handled and stored in the dark.  Low level diffuse light causes fog.  Bright light causes gross artifact.  Luminous watches, cell phone and darkroom light leaks should be avoided. Handling and Storage of Film Films should be used no longer than the stated Shelf life. The oldest film in stock should always be used first. Expired film results in loss of speed and contrast and an increase in fog. Radiographic Cassette A radiographic film cassette is a rectangle or square plastic or metallic container used to hold x- ray films (exposed or unexposed) and intensifying screens in close and uniform contact with one another.  The front cover, the side facing the x-ray source, is made of material with a low atomic number such as plastic and is designed for minimum attenuation of the x-ray beam.  Attached inside of the front cover is the front screen, and attached to the back cover is the back screen. The radiographic film is sandwiched between the two screens.  Between each screen and the cassette cover is a compression device, such as radiolucent plastic foam, which maintains close screen-film contact when the cassette is closed and latched.  The back cover is usually made of heavy metal to minimize backscatter. The x-rays transmitted through the screen-film combination to the back cover more readily undergo photoelectric effect in a high-Z material than in a low-Z material. Functions 1. Hold intensifying screens and protect them from damage. 2. Exclude all light from entering the cassette and fogging the film. 3. Maintain a close and uniform contact between the film and screens. 4. Exclude dust and dirt from the sensitive screens. 5. Act as a medium from exposure upto further processing of film. Features of an ideal cassette 1. Lightweight to facilitate easy handling and carrying.2. 2. Easy to open and close under low light conditions. 3. No sharp edges or corners which might injure patients or staff. 4. Strong and rigid to withstand physical damage from daily wear and tear. 5. The front part must provide minimum beam attenuation, be of uniform thickness and have no irregularities. Features of an ideal cassette 6. Internal rear surface must have an adequate layer of lead foil attached to minimize the risk of back scatter. 7. Cassette construction should be so that there is uniform contact between film and screen by the use of foam sponges. 8. Availability in range of film sizes. Construction Consists of a front and a back hinged at one edge. Thin sheet of lead foil attached on the back side which is further attached to a plastic foam pressure pad and an I.S. The front, also referred to as cassette wall contains front i.s. and a short lead blocker used for patient identification.  A range of locking methods are used,from spring clips to sliding locking bars,which serve to exclude light and along with foam pads maintain close contact between film and screens. All the internal metal or plastic surfaces are given black coating to prevent internal light reflections. To ensure good film-screen contact a slightly curved cassette back is used which minimizes photographic unsharpness. Materials used in cassette construction Carbon fiber material is commonly used because of the following advantages: Strength Light weight Low beam absorption The use of carbon fibre cassettes can mean significant reduction in patient dose because it absorbs only approximately half the number of x-rays that an aluminum or plastic cassette does. Cassette back May be of metal or plastic construction and lined with lead foil to protect from back scattered radiation from bucky tray or other surface. As recommended it should have a lead eq. of at least 0.12 mm when used with 150 kv equipment. Cassette Fittings Clips or fasteners- usually stainless steel Hinges - metal or plastic Presssure pad - plastic foam sponge Care of Cassettes Should be treated gently Number of cassettes carried at a time should be limited. Should be carried securely between body and arm, with fingers holding their bottom edge. If stored on edge, it should be ensured that they are as near vertical as possible Contact with fluids should be avoided Intensifying Screen It’s a part of cassette - converts x-ray energy into visible light spectrum. First developed by Thomas Edison in 1897 Initial screens used Calcium tungstate (CaWO4) Advantages of Calcium tungstate (CaWO4) It emits light in the violet-to-blue region. The sensitivity of conventional radiographic film is highest in the violet-to- blue region of the spectrum. If the screen phosphor emitted green or red light, its IF will reduced because it requires a greater number of light photons to produce a latent image. The light of the screen emission would be mismatched to the light sensitivity of the film. The light emitted by calcium tungstate screens is readily absorbed in radiographic film. Advantages of Calcium tungstate (CaWO4) Reduces x-ray dose to the patient Short exposure time - reduced motion blur. Rare Earth Screens Newer phosphor materials of choice for most radiographic applications. The term rare earth describes those elements of group IIIa in the periodic table that have atomic numbers of 57 to 71. These elements are transitional metals that are scarce in nature. Rare Earth Screens These screens are faster than those made of calcium tungstate, rendering them more useful for most types of radiographic imaging. Use of rare earth screens results in a lower patient dose, less thermal stress on the x-ray tube, and reduce shielding for x-ray rooms. Rare earth intensifying screens have increased speed because of the following characteristics: – The percentage of x-rays absorbed by the screen is higher. This is detective quantum efficiency (DQE). – The amount of light emitted for each x-ray absorbed also is higher. This is conversion efficiency (CE). Materials Used in Rare Earth Screens Gadolinium Lanthanum Yttrium The compositions of the four principal rare earth phosphors are: terbium-activatedgadolinium oxysulfide (Gd2O2S: Tb) terbium-activated lanthanum oxysulfide (La2O2S: Tb) terbium-activated yttrium oxysulfide (Y2O2S: Tb) lanthanum oxybromide (LaOBr). Layers of the Intensifying Screen 1. Protective Coating – The layer of the radiographic intensifying screen closest to the radiographic film is the protective coating. – It is 10 to 20 µm thick and is applied to the face of the screen to make the screen resistant to the abrasion and damage caused by handling. Layers of the Intensifying Screen – This layer also helps to eliminate the buildup of static electricity and provides a surface for routine cleaning without disturbing the active phosphor. – The protective layer is transparent to light. Layers of the Intensifying Screen 2. Phosphor – The active layer of the radiographic intensifying screen – The phosphor emits light during stimulation by x-rays. – Phosphor layers vary in thickness from 50 to 300 µm, depending on the type of screen. Layers of the Intensifying Screen – The phosphor converts the x-ray beam into light. – The active substance of most phosphors before about 1980 was crystalline calcium tungstate embedded in a polymer matrix. – The rare earth elements gadolinium, lanthanum, and yttrium are the phosphor material in newer, faster screens. Layers of the Intensifying Screen – The action of the phosphor can be seen by viewing an opened cassette in a darkened room. The radiographic intensifying screen glows brightly when exposed to x-rays. Layers of the Intensifying Screen 3. Reflective Layer – The layer between the phosphor and the base. – Approximately 25 µm thick – Made of a shiny substance such as magnesium oxide or titanium dioxide. – When x-rays interact with the phosphor, light is emitted isotropically. Layers of the Intensifying Screen – Isotropic emission refers to radiation emitted with equal intensity in all directions. – 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. Layers of the Intensifying Screen – The reflective layer enhances the efficiency of the radiographic intensifying screen, nearly doubling the number of light photons that reach the film. Layers of the Intensifying Screen 4. Base – The layer farthest from the radiographic film is the base. – The base is approximately 1 mm thick and serves principally as a mechanical support for the active phosphor layer. – Made of polyester. Screen Characteristics Any material that emits light in response to some outside stimulation is called a phosphor, and the emitted visible light is called luminescence. A number of stimuli causes luminescence in materials, such as: electric current (the fluorescent light), biochemical reactions (a lightning bug), visible light (a watch dial), and x-rays (a radiographic intensifying screen), cause luminescence in materials. Screen Characteristics Types of luminescence: 1. Fluorescence - the process in which visible light is emitted only while the phosphor is stimulated 2. Phosphorescence - the process in which the phosphor continues to emit light after stimulation.  Three primary characteristics of radiographic intensifying screens: screen speed image noise spatial resolution Speed Ranges from 50 (slow, detail) to 1200 (very fast). Screen speed is a relative number that describes how efficiently x-rays are converted into light. The screen speed is determined by the relative number of x-rays that interact with the phosphor and how efficiently x-ray energy is converted into the visible light that interacts with the film. Properties of radiographic intensifying screens that affect screen speed Properties of radiographic intensifying screens that affect screen speed controlled by the radiologic technologist. – These include radiation quality, image processing, and temperature. Image Noise Appears on a radiograph as a speckled background. Occurs most often when fast screens and high-kVp techniques are used. Noise reduces image contrast. Higher conversion efficiency results in increased noise. Spatial Resolution Spatial resolution refers to how small an object can be imaged. A photograph in focus shows good spatial resolution; one that is out of focus shows poor spatial resolution and therefore much image blur. Radiographic intensifying screens have the disadvantage of lower spatial resolution compared with direct-exposure radiographs.  When x-rays interact with the screen’s phosphor, the area of the film emulsion that is activated by the emitted light is larger than with direct x-ray exposure. Which results in reduced spatial resolution or increased image blur.  High-speed screens have low spatial resolution, and fine- detail screens have high spatial resolution. Spatial resolution improves with smaller phosphor crystals and thinner phosphor layers. Care of Screen When loading cassettes, do not slide in the film. A sharp corner or the edge can scratch the screen. Place the film inside the cassette. Do not dig the film out of the cassette with fingernails. Remove the film by rocking the cassette on the hinged edge and letting it fall to your fingers. Do not leave cassettes open because the screens can be damaged by whatever might fall on it, be it dust or darkroom chemicals.  Radiographic intensifying screens must be cleaned periodically. – The frequency of cleaning is determined primarily by two factors: the amount of use and the level of dust in the work environment. Use special screen cleaning materials. Maintaining good screen-film contact. Thank you...