Ct quality control
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This document discusses quality control procedures for CT scanners, including checking image quality metrics like resolution, noise, and CT number accuracy using phantoms. Regular quality control is recommended to establish baselines, identify potential problems early, and reduce downtime. Various tests are described to check parameters like resolution, noise, CT numbers, distance measurement accuracy, slice thickness, table movement, and laser alignment using specialized phantoms and protocols.
Ct quality control
- 1. Quality Control For CT Scanners SAJID EJAZ DEPARTMENT OF RADIOLOGY
- 2. Quality Control in CT A good idea? Yes Required for accreditation? Sometimes Improves image quality? Sometimes Depends on starting point Reduces downtime? Sometimes Depends on starting point Possible recognition of problems before clinical manifestations total failure may reduce unscheduled downtime
- 3. High Contrast Resolution Baseline established when scanner in good operati ng condition Subsequent test compared to baseline
- 4. CT Quality Control Standard protocol required same kVp, mAs, scan diameter, slice width, algorith m, focal spot, filter, etc When performed At regular intervals After major service tube change
- 5. CT # of Water What do we check? Average (ideally 0 by definition) Standard Deviation (ideally 0) Flatness ROI’s at various locations in phantom Results depend upon protocol kVp, mA, scan time slice width phantom size ROI position Problem causes Mis-calibration Noise / drift in detector system
- 6. CT # of Water Can repeat for various Slice thicknesses Reconstruction algorithms Centering positions Phantom diameters Fields of view
- 7. CT Number Accuracy Phantom with several materials of known CT #’s Do ROI’s at each and check CT # accuracy
- 8. High Contrast (or Limiting) Reso lution Phantom rows of equally spaced high contrast objects holes get smaller from row to row Bars of decreasing size & separation Determine smallest row of holes where all holes can be clearly seen smaller holes = better resolution Visual scoring
- 9. Low Contrast Resolution Phanto m holes get smaller from column to column Holes have less contrast from row to row Visual scoring
- 10. Low Contrast Resolution Noise limited less noise = more rows visualized Standardize mAs more mAs less noise better low co ntrast resolution reconstruction algorithm / filter smoothing reduces noise improves low contrast resolution
- 11. Low Contrast Resolution Possible sources of failure: anything that can increa se noise decreased dose to detectors reduced tube output electronic noise detectors amplifiers A - D convertor
- 12. Distance-Measuring Accuracy Phantom object with prec isely known object distan ces Use distance-measuring s oftware Check vertical, horizontal & diagonal Check monitor & hard co py
- 13. Monitor Performance / Hard Copy Output Standard gray scale ima ge needed computer generated (S MPTE pattern) Check monitor & hard c opy 5% patches Contrast patches Resolution bars Hard copy Measure optical densit y of steps with densito meter
- 14. Slice Localization / Thickness Accuracy Phantom with slanted wire Measure length Regularly spaced (in z direction) obj ects count Align phantom slice with laser Make single scan with known slice t hickness
- 15. Slice Localization / Thickness Accuracy
- 16. Table Indexing X-ray cassette on table Load table with weight Select multiple scans small beam width large mm table increment
- 17. Table Indexing Image should show series of exposed b ands Measure distance between bands Possible causes of failure slippage of table drive mechanism table mis-calibration
- 18. Table Backlash Does table return to same position from both directions? Test Apply masking tape to table Load table with weight Note numerical position Mark tape at starting position usin g laser position Move table away from starting posit ion & then back to same numerical location Check laser vs. mark on tape
- 19. Laser Accuracy Align phantom on table m atching outer laser to exter nal marks on phantom “Zero” table Scan phantom “0” z-location should sho w correct phantom position on image Repeat for internal laser
- 20. CT Noise Characteristics Water phantom Multiple scans changing only mAs Measure standard deviation of CT #’s using ident ical ROI’s Noise proportional to standard deviation
- 21. CT Noise Characteristics Excessive noise can be caused by detector problems electronic noise in detector amplifier circuits reduced output per mAs