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Quality Control in Diagnostic Radiology

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Quality Control in Diagnostic Radiology

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    1. Quality Control in Diagnostic Radiology

    2. Factors driving Q.C. Why do we do it? Legal Requirements Accreditation JCAHO ACR Clinical improvement equipment performance image quality

    3. Q.C. Goals Minimize dose to patients staff Optimize image quality Establish baselines More on this in a moment

    4. Why is Q.C. Important?

    5. QC can detect Malfunctions Unpredictability may be hard to isolate clinically Inefficient use of Radiation high fluoroscopic outputs Radiation not reaching receptor inadequate filtration oversized collimation

    6. Goals of a Q.C. Program Obtain acceptable image with least possible radiation exposure to patients staff Attempt to identify problems before they appear on patient films without QC problems only detected on patient films

    7. “Acceptable” Image Image containing information required by radiologist for correct interpretation goal: minimize exposure while maintaining acceptability high exposure images often have excellent appearance Low noise

    8. Q.C. & Baselines Baselines quantitative data on equipment obtained during normal operations Baselines useful for troubleshooting isolating problem component, for example generator processor Allows efficient use of engineering / repair personnel

    9. X-Ray Quality Control Filtration Focal Spot Size Collimation Maximum Fluoroscopic Output Calibration Verification Phototimer Performance

    10. Why is Filtration Important? Tube emits spectrum of x-ray energies Filtration preferentially attenuates low energy photons low energy photons expose patients do not contribute to image low penetration

    11. Half Value Layer (HVL) We don’t measure filtration We measure HVL HVL: amount of absorber that reduces beam intensity by exactly 50%

    12. Half Value Layer Depends upon kVp waveform (single/three phase) inherent filtration Minimum HVL regulated by law Maximum HVL regulated only in mammography

    13. Radiographic HVL Setup

    14. Checking HVL Compliance (Radiographic) How much aluminum must be placed in beam to reduce intensity by exactly 50%?

    15. Checking HVL Compliance (Radiographic) Is this machine legal? 2.5 mm Al minimum filtration at 90 kVp

    16. Fluoroscopic HVL Setup

    17. Fluoroscopic HVL Set desired kilovoltage manually measure exposure rates instead of exposure Move absorbers into beam as needed

    18. Focal Spot Size We measure apparent focal spot Trade-off smaller spot reduces geometric unsharpness larger spot improves heat ratings

    19. Focal Spot Size (cont.) Focal spot size changes with technique Standard technique required 75 kV (typical) 50% maximum mA for focal spot at kV used direct exposure (no screen) NEMA Standards defines tolerances

    20. Focal Spot Measuring Tools Direct Measurement Pin Hole Camera Slit Camera Indirect Measurement of Resolving Power Star Test Pattern Bar Phantom

    21. Direct Focal Spot Measurement Measure focal spot directly in each direction Use triangulation to correct for distances formula corrects for finite tool size two exposures required for slit

    22. Star Test Pattern Measures resolving power infers focal spot size Dependent on focal spot energy distribution measure largest blur diameter (in each direction) magnification use equation to calculate focal spot size

    23. Bar Phantom Measures resolving power Find smallest group where you can count three bars in each direction

    24. Bar Phantom Setup

    25. Radiographic Collimation X-Ray / Light Field Alignment Beam Central Axis should be in center of x-ray beam Collimator field size indicators PBL (automatic collimation) field automatically limited to size of receptor Bucky Alignment Using longitudinal bucky light & transverse detent, x-ray field should be centered on bucky film

    26. X-Ray / Light Field Alignment Mark light field on table top with pennies

    27. Radiographic X-Ray / Light Field Alignment

    28. Fluoroscopic Collimation image field is scale seen on monitor expose film on table above scale compare visual field (monitor) with x-ray field on film must check all magnification modes

    29. Fluoroscopic Collimation

    30. Fluoroscopic Collimation

    31. Maximum Fluoro Output put chamber in beam on tabletop block beam with lead above chamber fools generator into providing maximum output 10 R/min. limit for ABS fluoro

    32. Maximum Fluoro Output

    33. Calibration Performance Parameters Timer Accuracy Repeatability Linearity/Reciprocity Kilovoltage accuracy mA must be measured invasively

    34. Calibration mR/mAs should stay constant for all combinations of mA & kVp at any particular kVp

    35. Calibration mR/mAs should stay constant for all combinations of mA & time at any particular kVp

    36. Phototiming (check with output or film) Reproducibility Density Controls Field Placement Field Balance

    39. Phototimer Field Placement / Balance Placement cover desired field with lead select field as indicated Balance no fields covered select field as indicated

    40. Phototimer Field Placement / Balance

    41. Phototiming checked with Exposure Index kV Response phototimer pick-up attenuation may vary with kV phototimer must track kV response of rare-earth film Rate Response Check with varying phantom (lucite) thickness mA

    43. The End

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