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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