Quality Assrance for CT simulators aapm TG-66

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