Published on December 22, 2008
X-ray Imaging : X-ray Imaging Sharif Qatarneh Medical Physics Division Department of Radiation Oncology King Hussein Cancer Center, Jordan. X-ray Film : X-ray Film Film consists of: Emulsion layer (10µm thick) on both sides of a polyester base (150-200 µm thick). Protective and antistatic coating. Emulsion layer contains Silver halide grains (1µm diam.), which can be sensitized by radiation. Absorbed light photons liberate (e-) in the silver grain. Liberated (e-) combines with silver ions (Ag+). X-ray exposure transforms silver atoms to millions of Ag+ ions. Formation of a latent image of silver. Film Development : Film Development Conversion of invisible silver latent image into permanent visible image. Alkaline developer solution adds (e-), which neutralizes the (Ag+) ions in the film converting the ions into silver atoms. A developed silver grain appears black. Un-exposed grains are also developed at a very slow rate. The development process, including developer chemistry and temperature, affects the quality of image. Increasing developer temperature or time ? increasing film contrast and density. Film Processing : Film Processing Film processors roll film through developer, fixer and washing solutions. Film is washed to remove developer. Fixing solution (acetic acid) develops some and remove other silver grains. Fixing is necessary to make film stable and unaffected by light. Film is then washed from chemicals and dried by heaters or IR. Film Quality can be affected by: Dirty or un even processing rollers (artifacts). Static electricity (artifacts). Temperature of developer. Film Density : Film Density Optical density (OD) is a measure of film blackening (Directly related to number of photons that reach the film). OD = Log10 (I0/It). I0 is incident light intensity on film. It is transmitted light intensity through film. Useful range of OD = 0.3 – 2. Maximum possible OD of exposed film = 3.5 OD units. A densitometer is used to measure OD. Logarithmic scale (like the physiological eye response). Transmittance = (100 x It/I0) %. It is fraction of incident light that passes through the film. Indirectly proportional to OD. Useful range of Transmittance = 50% – 1%. Hot bright light is needed to achieve OD > 2.2. Characteristic Curve : Characteristic Curve Relationship between exposure and OD can be represented by a characteristic curve (H & D Curve). Low exposure region (toe). High exposure region (shoulder). Base: Density of film base alone, which absorbs small fraction of incident light. Fog: Level of blackening caused by few developed grains, without any radiation exposure. Base plus Fog level: The film density in the absence of any radiation exposure (typical range = 0.1 – 0.2 OD units). Film speed. Less exposure ? Fast film. More exposure ? Slow film. Characteristic Curve : Characteristic Curve Characteristic Curve & Contrast : Characteristic Curve & Contrast Useful range of OD = 0.3 – 2. Exposure, Density, Contrast : Exposure, Density, Contrast Characteristic Curve Gamma : Characteristic Curve Gamma Maximum slope curve. Gamma relationship with OD? Gamma relationship with Contrast? X-ray film vs. Screen : X-ray film vs. Screen X-Ray After processing Convert the x-ray energy into a visible light spectrum Light spectrum X-Ray Film Intensifying Screens : X-Ray Convert the x-ray energy into a visible light spectrum Light spectrum receiver screen Intensifying Screens Intensifying Screen : Intensifying Screen Used to reduce the dose to the patient required to create an image. Short time of exposure (important to minimize the patient motion). Double photosensitive emulsion. Two intensifying screens in the cassette. The light spectrum from the intensifying screen forms a latent image. Construction : Construction Layers of the intensifying screen: Base Reflecting layer. Phosphor layer. Plastic protective coat. Protective layer Phosphor Base Reflecting layer Construction : Construction Layers of the intensifying screen: Base a high-grade cardboard or polyester plastic. Construction : Construction Layers of the intensifying screen: Reflecting layer. Reflect the light back towards the front of the screen. It is made of a white substance. Construction : Construction Layers of the intensifying screen: Phosphor layer. contain phosphor crystals. Applied over the reflected coat. phosphor layer thicker for high speed screens and thinner in detail screens. Construction : Construction Layers of the intensifying screen: Plastic protective coat. Prevent static electricity. Physical protection for the delicate phosphor layer. Cleanable surface without any damage to the phosphor layer. Phosphor : Phosphor Crystalline Calcium Tungstate (CaWO4) (old). Should be absolutely free from any contaminant. It produces light in the blue region of the visible spectrum. Film sensitive to this wavelength unlike human eye. wavelength blue IS emission Eye sensitivity Film Sensitivity Intensifying Action : Intensifying Action Few number of x-ray photons to a many number of light photons. Intrinsic conversion efficiency: the efficiency with which the phosphor converts x-ray to light ~5% = light energy liberated / x-ray energy absorbed X-ray photons light Image Quality: Contrast : Image Quality: Contrast Object contrast: the attenuation characteristics of a target object in comparison to those of adjacent structures surrounding the target object. Object contrast depends on: Object density, thickness and atomic number. Increasing density and thickness of target object (compared to adjacent structures) increases its contrast. Subject contrast: the difference in x-ray intensities transmitted through a target object and the adjacent structures. Subject contrast is primarily affected by x-ray energy (Lower energy results in high subject contrast- PE effect). But higher kilovolt peak x-rays are required and those result in more Compton scatter (Difficult to achieve high subject contrast). Image Quality: Image Contrast : Image Quality: Image Contrast Object and subject contrast are essential factors for image contrast. Image contrast: the difference in intensity of a target object in the image compared to adjacent structures. Image contrast in films. In screens and films, image contrast depends on film density. Underexposed or overexposed films have little contrast. Film contrast relates to the observed density difference measured on the film for a given exposure difference. Film contrast can be determined by the slope of the characteristic curve. Maximum slope represents film gamma. Average slope between two specified film densities represents film gradient. (Gradient > 1 amplifies subject contrast). Contrast agents (air, barium, iodine) improve subject contrast. Image Resolution : Image Resolution Spatial resolution: the ability of an imaging system to resolve two adjacent high contrast objects as discrete entities. It is affected by many factors, primarily: Focal spot size, screen thickness, patient motion. Spatial resolution is expressed by line pairs per mm or (lp/mm). Blurring (Lack of sharpness): Focal spot blur: blurred edge of image (penumbra). Screen blur: caused by light diffusion in the screen. Thick screens (fast) have more light diffusion and more blur. Thin screens (slow, detail) have excellent spatial resolution but poor x-ray absorption efficiency. Motion blur: Motion smears out the object in the exposed film. Shorter exposure times using fast screens/films reduce motion blur. Noise: the random fluctuation of image intensity about some mean value following uniform exposure.