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.

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