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Radiographic Quality

Recorded Detail Laura Herz , BA, RT(R). Radiographic Quality. Recorded Detail. Definition: degree of geometric sharpness or accuracy of the structural lines actually recorded in the image One of two geometric properties of radiographic image quality Also known as: Definition Sharpness

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Radiographic Quality

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  1. Recorded Detail Laura Herz, BA, RT(R) Radiographic Quality

  2. Recorded Detail • Definition: degree of geometric sharpness or accuracy of the structural lines actually recorded in the image • One of two geometric properties of radiographic image quality • Also known as: • Definition • Sharpness • Resolution • Detail

  3. Recorded Detail • Poor resolution on the radiographic image is seen as a lack of sharp definition • Penumbra: geometric unsharpness around the periphery of an image • All radiographic images have less recorded detail than the actual object being imaged • Radiographer’s Goal: to control the degree of unsharpness or penumbra so it does not interfere with image diagnosis

  4. Unit of Recorded Detail • Primary unit of resolution: line pairs per millimeter (lp/mm) • Human visual acuity: 5 lp/mm

  5. Assessing Recorded Detail • Simple Evaluation: • Trabeculae of bone is an excellent guidepost to image resolution • Absence of blur/motion • Mathematical Evaluation: • Line Spread Function (LSF): complex mathematical measurement of the boundaries of an image • Modulation Transfer Function (MTF): mathematical assessment of the performance of an imaging system • MTF = information recorded ÷ information available

  6. Factors Affecting Recorded Detail • Motion • Materials • Geometry

  7. Motion • Motion fails to permit enough time for a well-defined image to form • Voluntary • Under direct control of patient • Involuntary • Not under direct control of patient; heartbeat, peristalsis, tremors, babies • Equipment

  8. Motion • Voluntary motion controlled by: • Communication • Instructions should be clear, concise, and understandable • Immobilization/comfort devices • Foam pads, angle sponges, blankets, sand bags, tape compression, commercial immobilizers, human immobilizers • Reduction in exposure time

  9. Motion • Involuntary motion controlled by: • Reduction in exposure time

  10. Reducing Exposure Time Shortens length of exposure to compensate for involuntary and/or voluntary motion

  11. Increase mA to Reduce Exposure Time • Density can be maintained with an increase in mA and a proportional decrease in time • Each exposure will then use the same mAs • mAs = mA x time • 100 mAs = 100 mA x 1 sec • 100 mAs = 400 mA x .25 sec mAs mA X time

  12. Increase Screen Speed to Reduce Exposure Time • Each screen speed produces a different density on the radiograph • Greater the screen speed, the greater the density • Greater screen speed requires a decrease in mAs to maintain density  decrease in mAs indirectly causes a decrease in exposure time Old screen speed New mAs = New screen speed Old mAs

  13. Increase Screen Speed to Reduce Exposure Time • You take a radiograph utilizing 40 mAs and a 200 screen speed. In order to produce the same density with a 400 screen speed, what would your mAs need to be?

  14. Decrease SID to Reduce Exposure Time • Inverse Square Law • Intensity of radiation is inversely proportional to the square of the distance from the source of radiation • There is an inverse relationship between the SID and the radiation intensity at the film Intensity 1 Distance 2² = Intensity 2 Distance 1²

  15. Decrease SID to Reduce Exposure Time • To maintain density, if the SID decreases, the mAs must be decreased  decrease in mAs indirectly causes a decrease in exposure time • Density Maintenance Formula Old mAs Old distance² = New distance² New mAs

  16. Decrease SID to Reduce Exposure Time • You take a radiograph utilizing 400 mA at .05 seconds at a 72” SID. A new radiograph is taken at a 36” SID. What would the new mAs need to be to maintain density? • What would be the new time?

  17. Increase kVp to Reduce Exposure Time • Kilovoltage (kVp) is the controlling factor for radiographic contrast • Determines the quality or penetrating power of the beam • There is a direct relationship between kVp and density

  18. Increase kVp to Reduce Exposure Time • To maintain density, if the kVp decreases, the mAs should increase • To maintain density, if the kVp increases, the mAs should decrease  decrease in mAs indirectly causes a decrease in exposure time • 15% Rule for maintaining density: • If the kVp is changed by 15%, the mAs should be changed by a factor of 2 • If kVp is decreased by 15%, mAs should be doubled • If kVp is increased by 15%, mAs should be cut in half

  19. Increase kVp to Reduce Exposure Time • You take a radiograph utilizing 100 mA at 1 second with 70 kVp. You increase your kVp to 80. What is your new mAs? • What would be the new time? • You further increase your kVp to 92. What is your new mAs? • What would be the new time?

  20. Materials • Film • Intensifying Screens • Poor Film-Screen Contact

  21. Materials • Film • Always some loss of detail with the use of film, however, all films employed in radiography are capable of resolving far greater lp/mm than the human eye is capable of visualizing

  22. Materials • Intensifying Screens • Phosphor size and phosphor layer have an influence on recorded detail • As phosphor size and layer thickness increase, screen speed increases • As phosphor size and layer thickness increase, recorded detail decreases

  23. Materials • Intensifying Screens • Quantum Mottle: grainy or blotchy appearance caused by insufficient photons (insufficient mAs) striking the image receptor; refers to the fact that the image is intensified (converted to light and made brighter) rather than produced only by x-ray photons • Often noticed with increasing screen speeds or increasing kVp

  24. Materials • Poor Film-Screen Contact • Increased distance between the intensifying screen phosphors and the film • Amplifies existing unsharpness and reduces recorded detail • Calculating the effect on recorded detail: X Old Unsharpness Poor Film-Screen Contact²

  25. Geometry • Focal Spot Size • SID • OID

  26. Geometry • Focal Spot • Area on the anode of the x-ray tube that is hit with electrons from the filament • All x-ray photons in the beam originate from this point

  27. Geometry

  28. Geometry • If the focal spot is smaller, the image will be less spread out and will appear sharper on the radiograph • If the focal spot is smaller, the greater the heat applied to that point on the anode, which results in limitations when selecting the focal spot size • Tube Rating Charts • Anode Cooling Charts

  29. Geometry • SID • Recorded detail is increased/improved when the source to image receptor distance (SID) is increased

  30. Geometry • OID • Recorded detail is increased/improved when the object to image receptor distance (OID) is decreased • Calculating Geometric Unsharpness: Focal Spot Size x OID SOD

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