1. David E. Hintenlang, Ph.D., DABR
CT Scanner QC
Presented by:
David E. Hintenlang, Ph.D., DABR
Medical Physics Program Director
Department of Nuclear and Radiological Engineering Department of Biomedical Engineering
American College of Medical Physics 2004 Annual Meeting
Scottsdale Arizona

2. CT Quality Control - Motivations
Appropriate diagnostic benefit vs risk
Image Quality
Noise
Contrast
Spatial resolution
Radiation Risk
Dosimetry indices (ie. CTDI)

3. Presentation Overview
Introduction & Background
General categories of QC tests
Specific QC tests and procedures
QC Program recommendations & comparisons
Acceptance tests
“Annual” QC tests
“Daily” QC tests
Conclusions & References

4. Background Expanded CT capabilities over last decade
Helical and Multislice CT
Decreases scan times - Changes in patient dose
Increased utilization of CT
Complex systems, high utilization, large contribution to patient dose demands a continuous and robust QA program

5. QC Objectives
Complexity of system offers opportunities for many individual QC tests
Level of testing balanced against QC Objectives
QC Categories:
Acceptance Testing
Annual Testing
Daily QC
ACR Accreditation

6. Categories of QC Tests Safety Mechanical accuracy Imaging Performance
Display System

7. General QC Tests : Safety
Electrical
Mechanical
Laser
Radiation

8. General QC Tests: Mechanical Accuracy
Ensure that aligned patient anatomy is imaged
Alignment light accuracy
Alignment of table and gantry
Table and gantry positioning
Slice localization from scout images

9. General QC Tests: Image Quality
Noise and Field Uniformity
CT Number Linearity
Low Contrast Detectability
Spatial Resolution
Display and Hard Copy Image Quality

10. General QC Tests: Dosimetry
CTDI
Head & Body
Patient Dosimetry

11. Alignment Laser Accuracy
Internal & External scan plane lights
Verify correct distance between internal & external scan plane lights
Wrap film around centered phantom
Make pin hole marks along illuminated lines

12. Scan 1 slice @ 1mm thickness
Repeat for external alignment laser
Measure distance between line of pierced holes and center of exposed slice
Misalignment should be less than 2 mm

13. Table - Gantry Alignment
To ensure that longitudinal axis of table is aligned with isocenter
Measure to locate & mark central axis of table at:
head end
middle
foot end
Hang plumb bob from center of gantry - to table top
Measure distances between plumb bob and central axis of table
Repeat for various gantry angles
Table should be aligned within +/- 5 mm

14. Table Incrementation Accuracy
Evaluate accuracy/ reproducibility of longitudinal table motion
Tape ruler along table edge
Tape pointer on frame around mid-point of ruler
Load table
Move table mm in one direction & return
Repeat for a total of 3-times in each direction
Average & Standard Deviation should be less than 3 mm

15. Table Incrementation Accuracy & Collimation (axial scan)
Evaluate accuracy & reproducibility of longitudinal table motion & width of x-ray beam
Wrap film around centered head phantom
Scan 5 slices for several combinations of slice thickness and gap.
Measure slice thickness and gaps from film
Mark table frame at one end of the phantom
Return to original table position and note any difference from original position
Interslice gap should have < 0.5 mm discrepancy
Slice thickness should be accurate to within 1 mm

16. Table Incrementation, scanned volume, helical pitch accuracy
Evaluate accuracy & reproducibility of longitudinal table motion & helical pitch
Wrap film around centered head phantom
Mark table frame at one end of phantom
Record indicated table position
Obtain volume scans for various slice thicknesses and pitch
Measure from film: Total length scanned
Spiral slice thickness
Spiral gaps (pitch)

17. kVp Accuracy Lock x-ray tube position
Center voltage divider using CT specific filer pack in center of x-ray beam
Check kVp’s at : 80, 100, 140 kVp
Tube potential should be within +/- 5%

18. Half-Value Layer Lock x-ray tube position
Set scanning parameters per manufacturer (80 kVp and mAs to obtain 200 mR)
Mount CT chamber parallel to table long axis, centered on isocenter
Vary Al thicknesses
Should be at least 2.3 mm 80 kVp

19. Exposure Reproduceabiity & Linearity
Head phantom in head holder on table
Insert CT ion chamber in center of phantom
Perform 4 scans at 80 kVp, 100 mA, 1s
Record exposure for each
Repeat at: 200 mA, 300 mA
Repeat at : 120 kVp, 140 kVp

20. Radiation Profile Width
Evaluates the accuracy of pre-patient collimator settings
Tape film around centered head phantom
positioned flat on table with 0o gantry angle
Scan one slice at each slice thickness
Lowest kVp and tube current available
space about 2 cm apart
Measure width of each slice
Should be within 1 mm of selected thickness

21. Slice Sensitivity Profile
Evaluate the actual width of the imaged slice
Set standard scanning parameters
Scan phantom insert with “ramps” for each available slice thickness
For each image determine average pixel values of:
a) metal ramp and b) acrylic background
Set window width < 10 HU of min width setting
Set window level at average of ramp and background values
Measure width of ramp and multiply by tanq

23. Image Quality Measures
Noise and Field Uniformity
CT Number Linearity
Low Contrast Detectability
Spatial Resolution
Reconstruction times
Scout view accuracy

24. Phantoms & Test Tools
Testing phantoms available from manufacturers & third parties
Early CT phantoms may not provide sensitivity require to test modern CT systems

25. Noise & Field Uniformity Axial Scan
Assess noise level under simulated clinical conditions
Set standard scanning parameters
Position a centered homogenous phantom
Scan and select ROI’s
Obtain mean pixel value and standard deviation for each ROI
Std. Dev. Central ROI < 10 HU
Max difference between peripheral
and central ROI’s < +/- 5 HU

26. Noise & Field Uniformity Volume Scan
Assess noise level under simulated clinical conditions
Set standard scanning parameters
Position a centered homogenous phantom
For each voume scanned measure 3 reconstructed slices
center & both ends of volume
Scan and select ROI’s
Obtain mean pixel value and standard deviation for each ROI
Std. Dev. Central ROI < 10 HU
Max difference between peripheral
and central ROI’s < +/- 5 HU

27. CT Number Linearity
CT numbers should be consistent from system to system
Set standard scanning parameters
Image phantom insert with 5 rods of different materials
Determine mean pixel values of each object & background
Perform for low, mid, & high kVp settings
CT number should be within 50 HU for each material

28. Low Contrast Detectability
Evaluate scanner low contrast object discrimination
Set standard scanning parameters
Scan low-contrast detectability insert
Determine mean value of each hole & surrounding material
Contrast = difference in HU of objects & background
Determine Number of objects visible
Should meet manufacturer’s specifications

29. Spatial Resolution Evaluate high contrast spatial resolution
Set standard scanning parameters
Scan Spatial Resolution Insert
Determine highest frequency group visually resolved for each image
Should meet manufacturer specifications

30. Spatial Resolution MTF
Derive the MTF as a measure of limiting spatial resolution
Scan spatial resolution insert
For each group of high contrast resolution patterns determine the mean value & std. deviation.
Determine & plot MTF
Repeat using bone reconstruction filters
Should meet manufacturer’s specifications

31. Image Reconstruction Time
Evaluate reconstruction times
Set parameters for typical head scan
Scan anthropomorphic head phantom
Measure time required to obtain 1 image slice
Subtract out data collection time
Repeat for anthropomorphic chest phantom and typical body slice
Should meet manufacturer specifications

32. Scout Views Evaluate scout view production
Position anthropomorphic head phantom
Perform scout view at 120 kVp, 160 mA, 1 mm thickness
Scan length 0  500 mm
Perform scout views for 0, 45 and 90 degree gantry angles
Record time required to perform and print images
Repeat with anthropomorphic chest phantoms

33. Display and Hard Copy Image Quality
Ensure that information in the video signal is accurately transferred to display media
Reduce room illumination to normal viewing level
Display SMPTE Pattern
Adjust brightness and
contrast controls
5% and 95% patches should be visible

34. Dosimetry Measurement: CTDI
Determine a measure of radiation dose to tissue under different scan conditions
Set standard scanning parameters (i.e. head)
Position CTDI head phantom
Scan with CT ion chamber inserted sequentially in each of 5 positions
Calculate CTDI using peripheral and central values
Repeat with CTDI body phantom
Repeat for various scanning parameters as desired

35. CT Equipment Quality Control Program
ACR Accreditation Program for CT
QC program established & implemented under supervision of a qualified medical physicist
Requirements
Acceptance testing upon installation
Annual medical physicist survey
Continuous Quality Control

36. Acceptance Testing
Series of measurements performed to verify that a CT system conforms to required specifications
Specific tests/technical specifications should be included in bid request or purchase agreement
Many more combinations of operating parameters exist than can be practically tested
Must acquire sufficient data to adequately characterize system performance

37. Considerations for Acceptance Tests
Determine most important performance specs to test in view of:
Clinical needs
Scanner capabilities
Devise a short list of 4-5 “standard” scanning conditions:
Standard body
Standard head
Spine
Pediatric Body
Pediatric Head
Pertinent performance tests should be performed for each standard condition to provide a performance baseline
Select a few additional scanner conditions that test performance limits

38. AAPM Report #39 Recommendations
Essential Tests
Localization light accuracy
Table Incrementation
Radiation Profile
Sensitivity Profile
Image Noise
Field Uniformity
High Contrast Resolution
Display & Hard Copy Image Quality
Radiation Dose
Optional Tests
Alignment of table to gantry
Table gantry tilt
mAs Linearity
kVp Accuracy
Scan Field Dependence (QCT)
Alternative Tests
Slice localization from Scout image
High contrast resolution – MTF
Low contrast detectability

39. Annual ACR Requirements
Localization light accuracy
Alignment of table to gantry
Table gantry tilt
Slice localization from Scout image
Table Incrementation
Slice thickness -Sensitivity Profile
Image Noise
Field Uniformity
High Contrast Resolution
Low Contrast Detectability
Artifact Evaluation
CT Number Accuracy & Linearity
Display & Hard Copy Image Quality
Radiation Dose – CTDI
Patient Radiation Dose for select exams
Safety Evaluation
Visual inspection
Audible/visual signals
Posting requirements
Scattered radiation measurements
Other regulatory tests

40. Monthly to Semi-Annual Tests per AAPM Report # 74
Localization light accuracy
Slice localization from Scout image
Table Incrementation
Slice thickness -Sensitivity Profile
Image Noise
Field Uniformity
High Contrast Resolution
Low Contrast Detectability
Artifact Evaluation
CT Number Accuracy & Linearity
Display & Hard Copy Image Quality
Radiation Dose – CTDI
Dosimetry from Scout Image
Safety Evaluation
Visual inspection
Audible/visual signals
Posting requirements
Scattered radiation measurements
Specified by ACR but not AAPM 74
Patient Radiation Dose for select exams
Alignment of table to gantry
Table gantry tilt

41. Continuous Quality Control
Required for all CT units
Qualified medical physicist should determine tests, frequency, and responsible individual
Facility Considerations
CT Usage
On-site radiological technologist should be identified with responsibility for conducting routine QC

42. Continuous Quality Control Tests
Localization light accuracy
Slice thickness
Image Noise
Field Uniformity
High Contrast Resolution
Low Contrast Detectability
Artifact Evaluation
CT Number Accuracy
Display & Hard Copy Image Quality

43. Daily Tests- AAPM Report #74
CT Number Accuracy
Image Noise
Field Uniformity
Artifact Evaluation
All can be obtained from same CT phantom scans
Sensitive to a wide variety of scanner problems

44. Aids to Daily QC i.e. AutoQA LiteTM 1
A software program to evaluate and monitor performance of CT scanners
Provides database tool for tracking a limited number of tests:
Image Noise
CT Number accuracy
Field Uniformity
Slice Width
Spatial Resolution
1. The Institute for Radiological Image Sciences, Inc. , MedSoft Technologies, Inc.

45. AutoQA Lite Overview Provides automated analysis of CT phantom images
CatPhan series
ACR Accreditation phantom
Toshiba InnerVision
Vendor specific (GE, Siemens, and Philips)
Operates on PC with Dicom 3.0 CT image formats

46. L

47. Conclusions ACR provides an excellent outline for CT QC Programs
A wide variety of tests exist
Medical Physicist has latitude to define requirements for “continuous” QC testing

48. CT QC References: American College of Radiology Website
(links to “accreditation”)
AAPM Report No. 39, Specification and acceptance testing of computed tomography scanners, American Institute of Physics, 1993.
AAPM Report No. 74, Quality control in diagnostic radiology, Medical Physics Publishing, 2002.
ImPACT, CT Scanner Acceptance Testing, Version 1.02, 2001
The ACR CT Accreditation Program and the Medical Physicist, Maynard High, ACMP 2003 Annual Meeting