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Image Quality

Image Quality . Bushong Ch. 16 . Objectives. Image Quality – Factors Geometric Factors Subject Factors Artifacts. Image Quality. Is the exactness of the representation of the patient’s anatomy

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Image Quality

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  1. Image Quality Bushong Ch. 16

  2. Objectives • Image Quality – Factors • Geometric Factors • Subject Factors • Artifacts

  3. Image Quality • Is the exactness of the representation of the patient’s anatomy • 3 major factors affecting image quality that is under the control of the technologist: Image Receptor selection/use, Geometric factors & Subject factors.

  4. Judging Image Quality • The most important characteristic of radiographic quality are: Spatial Resolution, Contrast Resolution, Noise & Artifacts

  5. Main Factors Affecting Recorded Detail • kVp & mAs • Motion • Object Unsharpness • Focal Spot Size • SID (Source to Image Distance) • OID (Object to Image Distance) • Material Unsharpness/ Film Screen Combo

  6. Recorded Detail • Other names: - detail -sharpness of detail -definition -resolution -degree of noise - visibility of detail

  7. Resolution • Is the ability to image two separate objects and visually distinguish one from the other. • Spatial resolution is the ability to image small objects that have high subject contrast. Ex: bone-soft tissue interface, breast calcifications, calcified lung nodule • Conventional radiography has excellent spatial resolution

  8. RESOLUTION TEST TOOLS LINE PAIRS/ MM Depicts how well you can see the differences in structures More lines=more detail

  9. Measuring Resolution for an x-ray imaging system

  10. SMPTE Test Pattern

  11. Focal Spot Size • Smaller x-ray beam width will produce a sharper image. • Fine detail = small focal spot (i.e. small bones) • General radiography uses large focal spot • Beam from penlight size flashlight vs. flood light beam

  12. Focal spot size of the cathode

  13. Line-focus principle

  14. Contrast Resolution • Is the ability to distinguish anatomic structures of similar subject contrast. Ex: liver-spleen, gray matter-white matter • Magnetic Resonance Imaging has the highest contrast resolution • Computed Tomography is excellent as well

  15. MRI CT

  16. The contrast of an object is expressed relative to its surrounding background.  That is what determines its visibility.

  17. Radiographic Contrast • Is the product of image receptor contrast and subject contrast

  18. “Noise” • Borrowed from electrical engineering • Audio noise = hum or flutter heard from a stereo • Video noise = “snow” on a TV • Radiographic noise = random fluctuation on the OD of the image

  19. QUANTUM MOTTLENot enough PHOTONS – can create a mottled or grainy image - MORE COMMON IN CR SYSTEMS

  20. Radiographic noise

  21. Radiographic Noise • Four components: • Film graininess, structure mottle, quantum mottle & scatter radiation

  22. Radiographic Noise • Film graininess – distribution & size of the silver halide grains in the emulsion • Structure mottle – speed of the intensifying screen. Phosphor size & DQE/CE • Not under the control of the technologist

  23. Image Noise • Speckled background on the image • Caused when fast screens and high kVp techniques are used. Noise reduces image contrast • The percentage of x-rays absorbed by the screen is the detective quantum efficiency (DQE) • The amount of light emitted for each x-ray absorbed is the conversion efficiency (CE)

  24. Quantum Mottle • An image produced with just a few x-rays will have more quantum mottle. • The use of very fast intensifying screens or not enough mAs or kVp will increase quantum mottle

  25. Quantum mottle

  26. Screen Speed • Efficiency of a screen in converting x-rays to light is Screen Speed.

  27. Speed • Fast image receptors • ? Noise ? Spatial resolution • ? Contrast resolution • Slow image receptors • ? Noise ? Spatial resolution • ? Contrast resolution • See pg 274 for answers

  28. Speed • Low noise = fast or slow speed? • High contrast resolution = fast or slow speed? • Reduced spatial resolution = fast or slow speed?

  29. PARALLAX –each emulsion has an imagesingle image overlaped edges edge sharp less sharp

  30. Other Film FactorsCharacteristic Curve • Is used to describe the relationship between OD and radiation exposure

  31. What is the useful OD range? Characteristic curve of radiographic film

  32. The latitude of an image receptor is the exposure range over which it responds with diagnostically useful OD. • Depending on the manufacturing characteristics radiographic film will respond differently to radiation exposure

  33. F/S vs Digital Dynamic Range

  34. Unexposed film • Appears like a frosted glass window • ODs of unexposed film are due to base density and fog density • Base density – tint added to the base to reduce eye strain and crossover. Has a densitometer value of approximately 0.1

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