CT Quality Control
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Transcript of CT Quality Control
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QC for CT-Simulator
In this tour we will check the following :1. Image quality
2. CT Dose
3. Mechanical performance
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Image quality
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Head Foot
4 cm
20cm
ModelModelModelModel
Image quality
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Model 1alignment , slice thickness and CT
numberMust see all fourBBs(in Modules 1 & 4)
Longer wire musthave same numberof lines above andbelow (1)
Wires are 0.5 mmapart in z-direction
Image quality
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CT Number Calibration
According to NCRP No.99
Water CT# = 0 2
Air CT# = -1000 3
Image quality
Polyethylene -97 HUWater 0 HUAcrylic +120 HUBone +910 HU
Air -1000 HU
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Image quality
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Image quality
Model 2
The low contrast phantomThere are four cylinders foreach of the following
diameters:2 mm, 3 mm, 4mm, 5 mmand 6mm.The space between eachcylinder is equal to the
diameter of the cylinder.
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Image quality
View the best image
A: Record the mean counts in the ROI inside the 25mm rod
B: Record the mean counts in the ROI outside the 25 mm rodSD: the Standard Deviation from the ROI outside the 25 mm rod
CNR = |A-B|/SD 1
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Image quality
The noise can be assessed by scan the phantom andcalculate the standard deviation CT of the HUs in aregion-of-interest (ROI)
Noise is expressed relative to the linear attenuation
coefficient of water
wm
wm
CTCTCS
w
ROICSSD
Noise
%100
Mean = 0 3
SD = 3.4 0.4
Based on GE operator manual
Noise
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Module 3
uniformity and distance
Image quality
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Image quality
The uniformity can be assessed by measuring the average HU in
smaller ROIs about 5 mm in radius that are located in the center andperiphery of the phantom ( 3 , 6 , 9, 12 oclock positions ). The largest
difference between any peripheral HU and the central HU is
determined.
Image uniformity
CT number variation for Water equal 0 5 HU
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two small targets fortesting in-plane
distancemeasurementaccuracy.
Image quality
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Image quality
This phantom is used toassess high contrast (spatial)resolution. It contains eightbar resolution patterns: 4, 5,6, 7, 8, 9, 10 and 12 lp/cm .
Model 4
High contrast resolution
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12
10
9
8
7
6
5
4
Image quality
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Image quality
artifacts
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2 standard CT dosimetry phantoms were adopted by theFood and Drug Administration and still are used today.contain several 1-cm-diameter holes for insertion ofdosimeters.
CTDI Phantoms32cm diam. X 15cm16cm diam. x 15cm
CT - Dose
CT Dose
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edge
edge
Middle
Middle
100mm Ion Chamber
CTDI Phantoms32cm diam. X 15cm16cm diam. x 15cm
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Kel : electrometer reading calibration factor
Ctp : temperature pressure correction factor
Nx :chamber exposure correction factor (R/c)
f: exposure-to-dose conversion factor, D= fX
100 mm :ionization chamber length
beam width = n T
)(..
)(100.100
mmwidthbeamtotal
mmfNCKreadingmeterCTDI
xtpel
Procedure simply is :
Phantom is place in the center ofimaging plane and aligned withcentral axis of the scanner bylasers and single scan isacquired .
CT Dose
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Tolerance
Reference
SourceTolerance limitsTest objective
Performance
Parameter
AAPM Report
Task Group 66
20% of
manufacturerspecifications
verify safe dose
delivered fromthe scanner
Patient dose from
CT-scan
CT Dose
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The CTDI varies across the field of view (FOV). For example,for body CT imaging, the CTDI is typically a factor or twohigher at the surface than at the center of the FOV.
Weighted CDTIW
The average CTDI across the FOV is estimated by theWeighted CTDI (CTDIw)
CT Dose
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CT Dose
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pitch is defined as the ratio of the table travel perrotation (I) to the total beam width (Nx T)
it does not indicate the total energy deposited into the scan
volume because it is independent of the length of the scan. Thatis, its value remains unchanged whether the scan coverage is
10 or 100 cm. It estimates the dose for a 100-mm scan length
only, even though the actual volume-averaged dose will
increase with scan length
CTDIvol is a useful to compare CT protocols betweenscanners
CT Dose
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The DLP reflects the total energy absorbed attributableto the complete scan acquisition , and thus thepotential biological effect .
Dose-Length Product (DLP)
DLP = CTDIvol * L , where L is the Z-length of the scan
CT Dose
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Limitation
For body scan lengths of 250 mm or more, theaccumulated dose closely approaches the limiting
equilibrium dose. However, CTDI100 underestimatesthe equilibrium dose CTDI (or MSAD for pitch of unity)by a factor of ~ 0.6 on the central axis and by ~ 0.8 onthe periphery .therefore the DLP by factor of ~ 0.7 .
In order to measure the equilibrium dose, a bodyphantom length of ~ 400 mm is required.
CT Dose
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Mechanical performance
Positioning lasersWe can perform this test in two ways
By using phantom and BBs on up , down , left and right
Mechanical
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Using film
Mechanical
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Using phantom
By making pin holes mark along illuminateLine .The deviation should be within 2 mm
Mechanical
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Mechanical
On the displayed CT image,the four (1mm) holes should be visible.
On Film,
we measure the deviation of pin-hole center from scanplane center. The deviation should be within 2 mm
Reference Source:
AAPM Report No. 39 and GE Operator Manual
Tolerance
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Mechanical
Couch deflection
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Mechanical
Couch deflection
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Mechanical
Couch deflection
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Mechanical
Couch height
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Mechanical
Couch height
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Mechanical
Couch distance accuracy
C
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Mechanical
Couch distance accuracy