quality assrance for ct simulators aapm tg-66
TRANSCRIPT
Samir Laoui, Ph.D. University of California, Irvine
Quality assurance for
computed-tomography simulators
and the computed tomography-
simulation process
History of x-ray Computed
Tomography
CT Imaging is a Two-Step Process
• STEP 1 – Acquire the Data
– Acquire a set of Attenuation/Transmission profiles as the camera
rotates about a central axis
• STEP 2 – Reconstruct Slices
– Convert transmission profiles into spatial distribution
– The numbers reported are CT numbers in Hounsfield Units (HU),
integer values normalized to water and in the range –1000 to +3000
Window and Level
Window and Level: Level = Mid-point
Window = Range
CT Generations
85 cm bore scanner
Image reconstruction
oMathematically, CT image reconstruction is a
linear inverse problem
o Reconstructed image represents a distribution of
radiation-ray linear attenuation coefficients
(Lambert–Beer’s law)
oData recorded on an x-ray detector array are
actually x-ray intensity values after an x-ray beam
traverses an object
FOURIER SLICE THEOREM
parallel-beam projection
Fourier transform
Problem:
Interpolation in the frequency domain
is not as straightforward as in the
spatial domain. Inappropriate
interpolation may cause severe image
artifacts
FILTERED BACKPROJECTION
ALGORITHM oActually, almost all the current commercial CT
scanners use point sources such as an x-ray tube
and produce divergent x-ray beam projections in
either the fan beam (2D) or cone beam (3D)
geometry
Consist of a flat table, laser patient
positioning, marking system, X-
ray tube, detector array and a
computer workstation
CT Components
Some of CT-scanner components
o X-ray tube in a CT simulator must be designed to withstand high heat
input and have a rapid heat dissipation associated with a large number
of images
o Heat anode storage (~ 5MHU, Millions of Heat Units)
o Anode cooling rate of about 0.5 MHU/min
o Collimator and attenuator
o Attenuators are used to harden the beam
o Collimator used to produce a narrow beam
o Flat top table similar to radiation therapy treatment table
o Computer that can reconstruct the projections
o External patient marking/positioning lasers
Simulation process
o Consists of the following:
o Imaging
oContouring of the targets and normal structures
o Involves fusion with other imaging studies
oPlacement of the treatment isocenter and the beams
oDesign of treatment
oGeneration of DRRs
oDocumentation
Quality assurance program goals
o Safety of patients, public, and staff
o QA program must test the integrity of the
following properties
I. High quality images
II. Geometrical and spatial integrity
III. Known CT number to electron density relationship
o Integrity of data transfer
o Ability to detect errors that compromise
accuracy of contouring
CT scanner QA
o The CT-scanner evaluation process consists of
o Evaluation of patient dose from the Ct scanner
o Radiation safety
o Electromechanical components
o Image quality
CT dosimetry and Radiation Safety
o AAPM Report 39
o Not significant
o However, dose assessment is mandatory during acceptance
testing and periodic QA
o Can be a concern of JCAHO
o Door interlock in not necessary (As in the case of UCI)
CT Dosimetry
o The primary measured value is known as the CT Dose Index
CTDI and represents the integrated dose, along the z axis, from
one axial CT-scan (one rotation of the x-ray tube)
o The Code of Federal Regulations, 21 CFR 1020.33 definition:
‘‘the integral of dose profile along a line perpendicular to the
tomographic plane divided by the product of the nominal
tomographic section thickness and the number of tomograms
produced in the single scan;’’
Performance of electromechanical
components o Patient marking/positioning lasers
oGantry lasers: identify the coronal and axial planes
oWall mounted lasers: Patient marking
oOverhead mobile sagittal laser: defines the sagittal and
axial planes
Electromechanical components
Couch and tabletop
o Flat tabletop should be leveled and orthogonal
with respect to the imaging plane
o Table vertical and longitudinal motion according to
digital indicators should be accurate and
reproducible
o Table indexing and position under scanner control
should be accurate
o Flat tabletop should not contain any objectionable
artifact producing objects (screws, etc.)
Couch and tabletop QA
Radiation Beam Width
o Collimation:
o The majority of CT-scanners collimate the radiation beam in the
longitudinal direction
o The actual width of the imaged slice, which is affected by the post-
patient collimation, is assed by measuring the Sensitivity Profile
Width
o If the radiation profile width is wider than indicated, unnecessary
radiation will be delivered to the patient
X-RAY GENERATOR TESTS
o Typical tests of the x-ray generator include
oEvaluation of the peak potential kVp (+/- 2kV)
omAs, linearity and reproducibility
o the integral exposure mR should be a linear function of mAs
oHalf-value layer HVL (mm of Al)
oTime accuracy
Image Quality tests
o Suboptimal image quality may cause the omission
of a portion of the target volume or inadvertent
delineation of normal structures as target volumes
o It is imperative that the image performance of a
CT-scanner used for CT simulation be maintained
as optimally as possible
Random uncertainty in pixel value
(noise) o The variation in pixel intensities has random and
systematic components. The random component of
image nonuniformity is noise.
o Image noise determines the lower limit of subject
contrast that can be distinguished by the observer
Systematic uncertainty—field
uniformity o Systematic variation may be due to system
malfunction, or physical or design limits
o CT images should be free of systematic artifacts,
and an image of a uniform phantom should have
uniform appearance without streaking and artifacts
(should be within 10 HU)
Contrast and Resolution
o Resolution: Ability of the system to record
separate images of small objects that are placed
very close together
o Contrast: Ability of a system to resolve adjacent
objects with small density differences
Spatial integrity
o Radiation treatment planning relies on accurate
reproduction of true patient dimensions and shape
in CT images
o Image distortions can potentially cause dosimetric
errors by causing delivery of inappropriate
radiation doses or treatment of the wrong area
o CT-simulation images should accurately reproduce
true patient anatomy within +/- 1 mm
oQA using a phantom of known dimensions
Software QA
o Spatial/geometry accuracy tests
oCorrect data transfer
o Image registration
o Image reconstruction
oTests should verify that the software accurately
reconstructs know phantoms in the 3 views
DRRs
o Evaluation of digitally reconstructed radiographs
Spatial and contrast resolution
Geometric and spatial accuracy
McGee et al Phantom
CT simulation process evaluation
o It is necessary to look at the entire simulation
process and evaluate its functionality
o The CT-simulation program should include written
procedures that must be reviewed annually
o Evaluation of immobilization devices
o Scan limits: Scan limits should be specified by the
physician and should encompass volume long
enough to create DRRs with enough anatomical
information
o Scan protocols should be defined
Conclusion
o Task group report addresses quality assurance
process for CT-simulation
o The QA program is designed to improve accuracy
of patient treatments and efficiency of the
treatment planning process while keeping dose
ALARA
oQA program should evolve and adapt as the device
used for CT simulation process change
Thank you