IAEA International Atomic Energy Agency Optimization of Protection in Computed Tomography (CT)-What can radiographers do? IAEA Regional Training Course.

IAEAIAEA

International Atomic Energy Agency

Optimization of Protection in Computed Tomography (CT)-What can radiographers

do?

IAEA Regional Training Course on Radiation Protection of patients for Radiographers, Accra, Ghana, 11-15 July 2011

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Introduction

18: Optimization of Protection in CT Scanner 2

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Multislice CT scanner

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The CT image is mathematically reconstructed from the measured data.

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Outline

• Introduction

• Factors affecting image quality

• Factors affecting dose

• Optimization


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• CT scanning offers immense medical benefits (improved low contrast resolution)

• Patient doses can be high in CT

• CT trauma: 59 mGy (normal chest 0.33 mGy x ~ 150)

• Image quality in CT is often higher than necessary for diagnostic confidence

• Need for patient dose management

• Radiographer plays a big role in this endeavor

…………………Introduction

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…….Introduction


• CT scanner is a complex equipment

• User must understand relationship between scan protocols and patient dose and image quality

• Such understanding is not intuitive/training & experience needed

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•Scanning protocols can not simply be transferred between different manufacturers

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Factors affecting image quality


• Spatial resolution• Contrast discrimination• Spatial uniformity• Noise• Pixel size• Slice thickness• mAs• Tube voltage• Reconstruction algorithm• Sampling frequency• Pitch (in multislice)• Patient size

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Spatial resolution


The ability to resolve neighbouring structures•Size of detectors• Number of detectors• X-ray focus• Distance between source and detector• Sampling frequency

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Spatial Resolution

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Contrast Discrimination

• The ability to detect differences between neighbouring structures of similar density (CT number)

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Spatial Uniformity

• The faithful representation of shapes and contrast throughout the image

Identical

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Spatial Uniformity


Non-Uniform Contrast

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Spatial Uniformity


Non-Uniform Shape

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Noise

What is noise? Anything that contributes to the degradation of CT images

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Noise

• Decreases the quality of the image

• Makes diagnosis harder by• Masking information

• Not presenting correct information

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Other Factors - Image Quality & Patient Dose

• Pixel Size

• Slice Thickness

• mAs

• Algorithm

• Sampling Frequency

• Artefacts

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Slice Thickness

• Thin slice -> smaller signal

• Smaller signal -> more noise

• More noise -> poor image

• Solution increase dose!!

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mAs

• Current directly proportional to intensity of X-rays

• Low current, low intensity

• Low intensity, low signal

• Low quality image produced

• Solution increase dose!!

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mAs

100 mAs 50 mAs

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Paper of Interest

• http://www.oucom.ohiou.edu/ou-microct/Downloads/Tradeoffs_in_CT_Image_Quality_and_Dose_9794-13379.pdf

• Good paper on CT image quality & dose tradeoffs


Monitoring of dose

• An issue to be considered

• Factors should be

adjusted to produce

necessary image

quality without resulting

to unnecessary dose


Monitoring of dose

• CTDI: special quantity to express radiation Dose in CT• Useful since it is not easy to measureactual dose to internalorgans• Not a measure of patient dose


Dose Length Product

• DLP is a practical

way for expressing

total radiation dose

deposited in body

A measure of patient

dose/risk

DLP=CTDIvol x scan

length

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Factors affecting dose


• Tube current (mA)• Scan (rotation) time (s)• Tube voltage• Beam (slice) width (mm)•Helical pitch• Number of slices/tube rotations• Number of slices/tube rotations

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Effect of scan parameters on CTDIvol


• mAs and scan (rotation) time

-At a given pitch, CTDIvol increases linearly with mA and time e.g. 2 x mAs= 2x CTDIvol - Given 100 mA 1s, one can double mAs by 200 mA x 1s or 100 mA x 2s)

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• CTDIvol increases with kV (approx. kV squared) - Variation of dose with kV if other parameters are constant kV relative CTDIvol 80 0.4 120 1.0 140 1.4• Beam width: CTDI increases if beam width wider than nominal imaged width (otherwise remains constant)

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For some image noise, dose can increase with decreasing kV

kV CTDIvol per unit noise

head body

80 1.6 2.6

110 1.2 1.2

130 1.0 1.0

e.g. Siemens Emotion 6

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Slice width : indirect effect of imaged slice width on dose•In theory: halve image slice width means double mAs • In practice: have increased contrast so compromise on increase in mAs

Scan length: Number of slices (tube rotations)• CTDIvol is approx. independent of scan length• DLP is directly related to scan length

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Pitch

• Pitch = table travel per rotationnominal slice width

Single Slice Multi Slice

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Pitch

Single Slice CT

Table travel = 5 mm per rotationSlice width = 5 mmPitch = 5/5 =1

Pitch = table travel per rotation nominal slice

width

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Pitch

Multi Slice CT

Pitchx = table travel per rotation xray beam width

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Pitch

Multi Slice CT

20 mm

5 mm

15 mm

Pitchx = 20 /15 = 1.33

Pitchd = 20 / 5 = 4

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Pitch

Pitch = 1, no gaps in helical path

Pitch > 1, gaps

Pitch < 1, overlap

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Helical considerations


• For equivalent scan parameters: helical dose is approx.equal to sequential (axial dose)

• For constant mAs, CTDIvol is inversely proportional to pitch

•On single slice scanners, increasing pitch is used for dose reduction)

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Multi-slice considerations


•Dose differences between single and multi-slice - Use of pitch for dose reduction - Extent of additional rotations in helical scanning -’over-beaming’-penumbra lies outside active detectors

Multi-slice pitch: 200 mAs (200 mA, 1s rotation)• Effective mAs or mAs per slice= True mAs/ pitch• On MSCT, mAs is often adjusted to keep effective mAs constant• Therefore CTDIvol will remain constant with pitch

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Multislice: scan protocol


• Greater flexibility of MSCT allows user to

- increase scan length

- scan more phases in multiphase studies

- increase mAs to keep noise down in thin slices

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Effect of patient size


• Theory of x-ray CT: HVL in tissue is approx. 4 cm• To maintain constant noise, double mAs for extra 4 cm of tissue

• Adjusting mAs for patient size: - small patients: require lower noise -large patients : higher noise accepted

• Some manufacturers recommend doubling mAs for - approx. 10 cm in abdomen scans -approx. 13 cm in lung scans• Automatic tube current control (mA modulation) (20-50% dose reductions without compromising image quality) -manufacturer’s weight/age based pre-programmed protocol - mAs adjusted for lateral patient dimensions

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CH McCallough: Dose optimization in CT: Implementation and clinical acceptance of size based charts (RSNA, 2002)


Patient width* (cm) Relative mAs

> 21-26 0.4

> 26-31 0.5

> 31-36 0.7

> 36-41 1.0

> 41-46 1.4

> 24-51 2.0

* Lateral width based on A-P scout at level of liver

IAEA Radiation Protection in Paediatric Radiology L06. Radiation protection in computed tomography 41

Example of successful story. IAEA TECDOC-1621(2009): Dose reduction in CT

while maintaining Diagnostic Confidence: A feasibility /Demonstration study

• Six hospitals (Canada, Greece, India,Poland,Thailand,UK) developed a relationship between image noise and patient weight.

• Used the relationship to adapt the noise in the image to a pre-selected value.

• Dose reductions: 25-62% (abdomen CT ) + 12-79% (Chest CT)

Paediatric chest CT

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Quality Control Tests In CT

Quality control test description on:

• CT accuracy, uniformity, linearity and noise,

• Low and high contrast resolution

• Alignment, Couch travel accuracy

• Gantry tilt measurement

• Dosimetry

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Summary• Scanning parameters should be based on study

indication, patient size and body region being scanned

• Manufacturer protocols should be the starting point. Any adjustments must be done in consultation of radiologist.

• Image quality in CT is often higher than necessary for diagnostic confidence

• Implementation of QC programme is important for patient dose management

• Training of physicians and CT staff can help in management of protocols and patient dose


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THANK YOU!


IAEA International Atomic Energy Agency Optimization of Protection in Computed Tomography (CT)-What can radiographers do? IAEA Regional Training Course.

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