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IAEA International Atomic Energy Agency Optimization of Protection in Computed Tomography (CT)-What...
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Transcript of IAEA International Atomic Energy Agency Optimization of Protection in Computed Tomography (CT)-What...
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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Outline
• Introduction
• Factors affecting image quality
• Factors affecting dose
• Optimization
18: Optimization of Protection in CT Scanner 5
IAEA 18: Optimization of Protection in CT Scanner 6
• 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
18: Optimization of Protection in CT Scanner 7
• 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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Factors affecting image quality
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• 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
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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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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
• The faithful representation of shapes and contrast throughout the image
Non-Uniform Contrast
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Spatial Uniformity
• The faithful representation of shapes and contrast throughout the image
Non-Uniform Shape
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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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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
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Monitoring of dose
• An issue to be considered
• Factors should be
adjusted to produce
necessary image
quality without resulting
to unnecessary dose
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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
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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
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• 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
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• 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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Effect of scan parameters on CTDIvol
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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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Effect of scan parameters on CTDIvol
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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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Effect of scan parameters on CTDIvol
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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
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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Helical considerations
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• 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
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•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
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• 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
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• 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)
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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
IAEA 18: Optimization of protection in CT scanner 42
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
18: Optimization of Protection in CT Scanner 43