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An Introduction to TG-142 Imaging QA Using Standard Imaging Products
Mark Wiesmeyer, PhD, DABRTechnical Product ManagerStandard Imaging, Inc.
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Goals
• Understand the nature and intent of TG‐142 imaging metrics,
• Introduce some imaging phantoms used by PIPSpro,
• Understand how our software works,
• Develop an intuitive appreciation of the mathematics.
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TG-142, Table VI – Imaging QADaily Monthly Annual
• Mechanicals
• Imaging Dose
• Positioning
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PIPSpro - Imager QA Metrics
* The Phantom Laboratory** Elekta Acceptance Test*** Not available with 503…missing the 515.
Metric QC‐3, QCkV‐1Catphan
503, 504, 600*Geometric Distortion – Volumetric XUniformity X XNoise X XContrast X XSpatial Resolution X XHounsfield Unit Constancy XLow‐Contrast Visibility** XLow‐Contrast Detectability*** X
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QC-3 Phantom
• Noise• Contrast• Spatial resolution• Uniformity*
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QCkV-1 Phantom
Essentially identical to QC‐3, but for kV and adds contrast detail regions.
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Catphan Phantom Family
Catphan M1 M2 M3 M4 M5503 404 528 486 Elekta504 528 404 515 486‐2 Varian600 404 591 528 515 486 Siemens
Module Inserts
The following diagrams were taken from Catphan manuals, which are all available online.
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Catphan 503
Brackets to attach to box and adjustment screws.
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Catphan 504
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Catphan 600
What distinguishes the phantoms are the analysis modules and their placement….
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Noise
∑
• Noise is calculated as the standard deviation of the pixel values of a region in an image.
• Noise represents randomdetector signal fluctuations.
• Noise is assumed to be “stationary”.
Which image is noisier?
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Uniformity
• (Non)uniformity represents systematic detector signal fluctuations.
• (Non)uniformity can be removed (improved) through system calibration.
Which image is more uniform?
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CTP 486 - Image Uniformity Module 503, 504, 600
The mean value of each region of interest (ROI) is used to quantify uniformity. The standard deviation (Sigma) is used to quantify noise
Hounsfield Numbers
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Integral Non-Uniformity
• PIPS shows CT#’s plotted on the same graph.
• Note that CBCT images tend to be more noisy than CT images.
“Cupping” or “capping” of the CT number may indicate the need for recalibration.
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A Basic Equation
Same equation as for beam flatness.
2
2
" "" "
IntegralNon‐Uniformity
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Single Image – Uniformity and Noise
1 100
A Percent Integral Uniformity value of 100% theoreticallymeans that there is no variability within the useful field of view.
∆ ∆ ∆ ∆ ∆5
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Dual Image – Noise Only
- 1996
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Noise (Dual Image) ∆1
2 1
Assumption: The “images” are the same, the noise is different.
Std. Dev. = 0.0224
Subtracting images lets us estimate noise in the region that we are analyzing.
12∗ 0.0224 0.0158
0.0146 0.01562 0.0151
~4% error
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PIPSpro - Noise
12∗ ∆ ∆ ∆ ∆ ∆ ∆
6
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PIPSpro - Contrast to Noise Ratio
Mean of the pixel values in the “lightest” region of an image
Mean of the pixel values in the “darkest” region of an image
Standard deviation of the pixel values in the “darkest” region of an image
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Modulation Transfer Function (MTF)• The MTF shows the frequency
response of the imaging system.
• Higher frequencies are required for greater resolution.
• Thicker screens have better absorption properties but exhibit poorer spatial resolution (lower dose, increased scatter).
• Film (by itself) has far better spatial resolution than that of intensifying screens. FIGURE 6‐8, Bushberg, The Essential
Physics of Medical Imaging, 2nd Ed.
Frequency Response ~ Δ Spatial Resolution
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What I learned to pass my boards.
“The MTF is the magnitude of the Fourier transform of the point or line spread function— the response of an imaging system to an infinitesimally small point or line of light.”
Dirac Delta Function
Amplitu
de
Distance
Point Spread Function
Amplitu
de
Distance
Amplitu
de
Spatial Frequency
MTF
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PIPSpro – MTF
Where is my delta function?
CTP 528 – High Resolution Module
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SWMTF – Key Concepts• In imaging, for square wave
patterns, as spatial frequency increases, variance decreases.
• The change in variance is proportional to the change in frequency response.
• The SWMTF is less susceptible to noise than Fourier transform‐based methods.
• Provides a “Relative” MTF.
Original Phantom
Largest Bars
Variance from Noise
Total Variance
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CTP 528 – High Resolution Module503, 504, 600
* 100%
Largest Bars For square wave patterns, as spatial frequency increases, variance decreases.
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Sigma ~ 0
Frequency response can be simulated by convolution of a Gaussian with “line pairs”.
The change in variance is proportional to the frequency response.
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Sigma = 1
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Sigma = 2
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Sigma = 3
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Sigma = 4
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Sigma = 5
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CTP 404 – Hounsfield Number Constancy
503, 504, 600
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HU Constancy
The “Expected” values may differ for your CBCT imager.
• Need for calibration• Imager limitations
Baselining functions in PIPSpro.
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CTP 515 - Low Contrast Module504, 600
These are normal results for CBCT low contrast detectability. 0.3% and 0.5% contrast may be detected with fan beam CT’s (or maybe higher MU’s).
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Low Contrast Detectability
• Detectability occurs (is defined) when the “contrast to noise ratio” is greater than or equal to 4 (psychophysical basis).
• Subslice targets are ignored by PIPSpro.
/ 4
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Imaging Pointers
• Use imaging protocols similar to what you would use clinically.
• If you want to trend your QA metrics, always use the same imaging protocols, including fields of view.
• Smaller fields of view yield better results because pixel density per area increases with decreased field of view.
• Be sure to set up your phantoms correctly and accurately.
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Bibliography
Stock M, Pasler M, Birkfellner W, Homolka P, Poetter R, Georg D. Image quality and stability of image‐guided radiotherapy (IGRT) devices: A comparative study. Radiother Oncol. 2009 Oct;93(1):1‐7
Lehmann J, Perks J, Semon S, Harse R, Purdy JA. Commissioning experience with cone‐beam computed tomography for image‐guided radiation therapy. J Appl Clin Med Phys. 2007 Jul 17;8(3):2354.
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Scaling (Planar Only)- FC-2 Phantom
The MLC phantom or FC‐2 phantom can be used for this purpose as can any other phantom that has fiducials set at a known distance from one another.
10cm
x 10 cm
15cm
x 15 cm