| The following tables illustrate the main factors and how these affect radiographic definition and contrast. Show Factors affecting definition FactorEffectExplanationFilm speed (grain size)Slower film (finer grain) = better definitionBoundaries are defined by grains: smaller dots = sharper boundaryEnergy (kV, kVp, keV, MeV)Lower energy = better definitionLower energy = smaller area affected by a photon/electron strikeScreensClose contact = better definitionAffected area is enlarged by any gap between screens and emulsionVibrationAny vibration reduces definitionApplicable to any radiographGeometry of image formationLonger ffd, smaller s or f, smaller ofd = better definitionInteraction of ffd, f or s, ofd in determining Ug = (f x ofd) / (ffd-ofd)DevelopmentOptimum time is requiredUnder-development is uneven, developing only part of a boundary; over-development leads to irregular growth along a boundary of black metallic silver into the less dense areaFactors affecting contrast FactorEffectExplanationPhotographic densityHigher density = higher contrastDerived from the slope of the characteristic (sensitometric) curve: minimum density determined from contrast, maximum from adequate transparencyFilm speed (grain size)Slower film (finer grain) = better contrastMaking grain size smaller greatly slows the film: to reduce the slowing, more silver salt is added to the emulsion giving higher contrastEnergy (kV, kVp, keV, MeV)Lower energy = better contrastEnergy affects subject (object) contrast: at lower energies, a given thickness increase (or physical density increase) results in greater absorptionScreensLead screens = less scatterScatter has a longer path length through the lead, and scatter is more readily absorbed per mm of path length, than the primary beamVariations in object thickness or physical densityLarge difference in thickness or physical density = higher contrastSubject (object) contrast is the difference in the amount of radiation penetrating adjacent areas of the objectDevelopmentOptimum time is requiredUnder-development gives low density and unevenness of density; over-development results in the preferential darkening of areas that should be light Receptor Contrast: It refers to a receptor's ability to adequately show the information that the photons transmitted through the subject. In conventional radiography, the contrast depends on the size of the grains, the development time, the concentration and temperature of the developing solution, and overall film density.2,4,7,12As there is limited usage of conventional film, we will not discuss the details of these factors. In digital imaging, contrast depends on the bit-depth of the receptor. Bit-depth refers to the number of possible grey values stored in an image. The higher the bit-depth, the more gray values it can show. A 1-bit image can only show two colors, black and white. 1-bit can only store one of two values, 0 (white) and 1 (black). An 8-bit image can store 256 possible gray values, while a 12-bit image can display 4096 gray values.1,4,9Radiographic contrast is the density difference between neighboring regions on a plain radiograph. High radiographic contrast is observed in radiographs where density differences are notably distinguished (black to white). Low radiographic contrast is seen on radiographic images where adjacent regions have a low-density difference (black to grey). Contrast scaleAs radiographs have varying regions of density, one cannot simply make assumptions based on a small region of interest. It is due to this that the radiographic contrast of an entire image is referred to as 'long-scale' or 'short-scale.' Short-scale contrastShort-scale radiographs are considered 'high-contrast' whereby density differences albeit greater, overall possess fewer in density steps (lesser shades of grey). Long-scale contrastLong-scale radiographs are considered 'lower-contrast' whereby density differences are less noticeable however possess many more shades of grey. Long-scale radiographs are preferred while examing the lung fields, where subtle changes in density are pertinent to a diagnostic image. ADVERTISEMENT: Supporters see fewer/no ads Contrast controlKilovoltageRadiographic contrast is dependent on the technical factors of the radiographs taken. The kilovoltage (kV) during the radiographic examination will determine the primary beams' energy; higher energy effects increased penetrating power. A primary beam with greater kV results in an overall rise in penetration through all tissues (decrease in attenuation differences), therefore resulting in a lower contrast radiograph. Hence the high kV technique of the chest x-ray is employed to present a more uniformly dense image to better appreciate the lung markings. A 15% increase in kV will essentially correlate to an increase in density similar to double the mAs 2. Scatter radiationScatter radiation will decrease the contrast of the radiograph. Factors that contribute to scatter radiation are increasing volume of tissue, tube kilovoltage, the density of matter, and field size.3 Ways to reduce scatter include close collimation, grids, or air gap technique. What is the technical factor affect radiographic contrast?In conventional radiography, the contrast depends on the size of the grains, the development time, the concentration and temperature of the developing solution, and overall film density.2,4,7,12As there is limited usage of conventional film, we will not discuss the details of these factors.
What influences the factor of radiographic contrast?Frequently Asked Questions. What 4 factors affect the proper scale of radiographic contrast?Radiographic contrast is influenced by (1) subject contrast, (2) kVp level, (3) scatter radiation, (4) film type, and (5) film fog.
Which factors influence the contrast in an image?Which factors influence the contrast in an image? Select all that apply. Contrast is the difference between adjacent densities on a radiographic image.. Absorption of scatter radiation.. Control of the quantity of x-ray photons.. Control of the penetrating ability of the beam.. Determination of resolution of the digital images.. |
