qaulity control of ct
DESCRIPTION
Performance Testing of CT (Computer Tomography) using ACR (American College of Radiology) PhantomTRANSCRIPT
08/22/08 1
Performance Testing and Dosimetry in Computer
Tomography
Asim Gilani
Radiation Protection Unit
Medical Physics Department
Dubai Hospital
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Annual Performance Evaluation
•Alignment light accuracy
•Alignment of table to gantry
•Table/gantry tilt
•Slice localization from localization image (SPR)
•Table incrementation accuracy
•Slice thickness
•Image quality
– High contrast (spatial) resolution
– Low contrast resolution
– Image uniformity
– Noise
– Artefact evaluation
•CT number accuracy and constancy
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Annual Performance Evaluation
•Display devices– Video display
– Hard-copy display
•Dosimetry-Computed Tomography dosimetry index (CTDI)
-Patient radiation dose for representative examinations
•Safety evaluation– Visual inspection
– Audible/visual signals
– Posting requirements
– Scattered radiation measurements
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ACR Performance Phantom
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Positioning Accuracy
CT # Accuracy
Slice Width Accuracy
Low Contrast Resolution
High Contrast (Spatial) Resolution
Image Uniformity and Noise
Image Artifacts
Distance Measurement Accuracy
The ACR CT phantom was designed to examine a broad range of scanner parameters
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4 3 2 1
20 c
m
4 cm
Head Foot
AlignmentAlignment
CT # CT #
Slice widthSlice width
Low Low contrast contrast
resolutionresolution
Uniformity Uniformity & noise & noise
Distance Distance accuracy accuracy
& SSP& SSP
High High contrast contrast
resolutionresolution
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•Align Module 1 to lasers
•Scan with Hi Resolution Chest protocol
Test 1: Phantom and Scanner Test 1: Phantom and Scanner AlignmentAlignment
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ACR Performance Phantom
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Air ≈ -1000 HU
Water ≈ 0 HU
Polyethylene ≈ -97 HU
“Bone” ≈ +910 HU
Acrylic ≈ +120 HU
MODULE 1
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MODULE 1
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High-resolution chest technique
Must see all four BBs
Longer wire must have same number of lines above and below (±1)
Wires are 0.5 mm apart in z-direction
WW = 1000WL = 0
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•Prescribe a scan at center of Module 4 i.e., 120 mm superior
•Scan with Hi Resolution Chest protocol
Test 2: Table Distance Travel Test 2: Table Distance Travel TestTest
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12
5 9
4
6
7
8
10
Bar patterns:lp/cm MODULE 4
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DATA FORMDATA FORM
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Test 3: CT Number Accuracy and Test 3: CT Number Accuracy and Slice ThicknessSlice Thickness
•Align Module 1 to lasers
•Scan with Adult Abdomen protocol & all kVp’s available
•Record all HU and slice thickness
•Also scan with 3, 5, 7 mm & hi resolution chest slice thickness
•Record water HU and slice thickness for each scan.
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Adult abdomen technique
CT number measured in 5 materials
CT number of water measured at several scan widths and kVps
WW = 400WL = 0
“Bone”
Water
Poly
Acrylic Air
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Adult abdomen technique
Scan width measured at several scan widths
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DA
TA
FO
RM
DA
TA
FO
RM
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Test 4: Low Contrast ResolutionTest 4: Low Contrast Resolution
• Scan Module 2 with Abdomen and Head protocols
• Record the smallest rod seen
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25 mm
6 mm
5 mm
4 mm
3 mm
2 mm
Low contrast = 6 HU ± 0.5 HU
MODULE 2
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Adult abdomen and head techniques
Record diameter of the smallest set of rods for which all 4 rods can be seen
ROI check of absolute contrast using large rod WW = 100
WL = 100
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DATA FORMDATA FORM
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Test 5: Uniformity, Noise, Test 5: Uniformity, Noise, Artifacts, and Distance AccuracyArtifacts, and Distance Accuracy
• Scan Module 3 with Abdomen protocol• Record HU & SD in center• Record HU at 12, 3, 6, 9 o’clock• Examine for artifacts and record• Check distance accuracy (optional)
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MODULE 3
100 mm
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Adult abdomen and head techniques
Measure uniformity and noise
Assess for artifacts
Measure distance accuracy(optional)
WW = 100WL = 0
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DATA FORMDATA FORM
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• Scan Module 4: – Hi Resolution Chest protocol– Abdomen protocol
• Record limiting lp/mm
Test 6: High Contrast Test 6: High Contrast ResolutionResolution
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12
5 9
4
6
7
8
10
Bar patterns:lp/cm MODULE 4
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Adult abdomen, adult head and high-resolution chest techniques
Record the first highest frequency bar pattern for which the bars and spaces merge
WW = 100WL 1100
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WW = 400WL = 0
CT ART?
At soft-tissue window settings these streaks are visible and not of concern
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DATA FORMDATA FORM
Questions
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• ACR Manual
ReferencesReferences
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Thank You