CT Quality Control

8/3/2019 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

CT Quality Control

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