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CT Scanner QC

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Department of Nuclear and Radiological Engineering. Department of Biomedical Engineering ... Expanded CT capabilities over last decade. Helical and Multislice CT ... – PowerPoint PPT presentation

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Title: CT Scanner QC


1
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 _at_ 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 300-500 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 lt 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 kVps 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 Al _at_ 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 lt 10 HU of min width setting
  • Set window level at average of ramp and
    background values
  • Measure width of ramp and multiply by tanq

22
(No Transcript)
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 UniformityAxial Scan
  • Assess noise level under simulated clinical
    conditions
  • Set standard scanning parameters
  • Position a centered homogenous phantom
  • Scan and select ROIs
  • Obtain mean pixel value and standard deviation
    for each ROI
  • Std. Dev. Central ROI lt 10 HU
  • Max difference between peripheral
  • and central ROIs lt /- 5 HU

26
Noise Field UniformityVolume 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 ROIs
  • Obtain mean pixel value and standard deviation
    for each ROI
  • Std. Dev. Central ROI lt 10 HU
  • Max difference between peripheral
  • and central ROIs lt /- 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 manufacturers 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 manufacturers 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
  • www.acr.org (links to accreditation)
  • AAPM Report No. 39, Specification and acceptance
    testing of computed tomography scanners, American
    Institute of Physics, 1993.
  • www.aapm.org/pubs/reports/
  • AAPM Report No. 74, Quality control in diagnostic
    radiology, Medical Physics Publishing, 2002.
  • www.aapm.org/pubs/reports/
  • ImPACT, CT Scanner Acceptance Testing, Version
    1.02, 2001
  • www.impactscan.org/acceptance.htm
  • The ACR CT Accreditation Program and the Medical
    Physicist, Maynard High, ACMP 2003 Annual Meeting
  • www.acmp.org/meetings.html
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