CT Scanner QC

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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
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CT Quality Control - Motivations

• Appropriate diagnostic benefit vs risk
• Image Quality
• Noise
• Contrast
• Spatial resolution
• Radiation Risk
• Dosimetry indices (ie. CTDI)

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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

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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

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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

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Categories of QC Tests

• Safety
• Mechanical accuracy
• Imaging Performance
• Display System

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General QC Tests Safety

• Electrical
• Mechanical
• Laser
• Radiation

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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

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General QC Tests Image Quality

• Noise and Field Uniformity
• CT Number Linearity
• Low Contrast Detectability
• Spatial Resolution
• Display and Hard Copy Image Quality

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General QC Tests Dosimetry

• CTDI
• Head Body
• Patient Dosimetry

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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

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• 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

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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Image Quality Measures

• Noise and Field Uniformity
• CT Number Linearity
• Low Contrast Detectability
• Spatial Resolution
• Reconstruction times
• Scout view accuracy

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Phantoms Test Tools

• Testing phantoms available from manufacturers
  third parties
• Early CT phantoms may not provide sensitivity
  require to test modern CT systems

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.
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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

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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

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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