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2
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- Why AEC in CT?
- Principles of AEC in CT
- Implementation of AEC in CT
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4
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- Adjust tube current (mA) for variations in patient attenuation to
achieve required image quality
- The driving force behind development of AEC systems in CT has been dose
reduction
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- CT offers ideal opportunity for tailoring mA to changes in patient
attenuation
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- The mA can be adjusted at three levels:
- for overall patient size
- for varying attenuation along z-axis
- for varying angular attenuation
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- Benefits of AEC:
- More uniform image quality (noise)
- Reduced dose to less attenuating regions
- Reduced load on x-ray tube
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- Obtaining attenuation data and calculating required mA
- patient size and z-axis
- angular
- How much is the mA adjusted for changing patient size?
- Do we want to keep image quality constant for different sizes?
- Defining image quality requirements
- What image quality are we aiming for?
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- Acquisition of attenuation data
- SPR performed → attenuation data at each z-position
- Water equivalent diameter calculated for each level
- max attenuation level compared to a standard size
- allows relative mA to be calculated
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- If adjusting for overall patient size mA calculated for level of maximum
attenuation is used throughout the examination
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- For different patient sizes the appropriate mA will be used
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- For z-axis modulation the attenuation at each level is calculated
relative to maximum
- For each rotation the appropriate mA will be used
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- Method 1: Prospective calculation from SPR
- x & y dimensions of ellipse calculated
- from information in
attenuation profile
- tube current varied sinusoidally during rotation
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- Method 2: ‘On line’ modulation
- uses attenuation data from previous rotation
- adapts tube current to patient attenuation ‘on the fly’
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- Noise in image is governed by most attenuating projections
- Reducing mA from AP direction
does not change noise significantly but reduces dose
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- To maintain constant image noise need constant signal to detectors
- Half value layer (HVL) of CT beam in tissue » 4 cm
- Double mA for every increase of 4 cm
- Halve mA for every decrease of 4 cm
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- Do we want to maintain constant noise with changing attenuation?
- Smaller patients require lower noise
- With larger patients can accept more noise
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- AEC system requires a reference level from which to adjust the mA
- This must be defined by the user
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- You can have perfect adaptation of mA to patient attenuation
- Inappropriate setting of image quality can result in dose increase
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- Two approaches used on AEC systems to define image quality:
- standard deviation of CT numbers (noise level)
- reference mA: mA for standard patient required to give appropriate
image quality
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- AutomA: Patient size and z-axis
- SmartmA: Angular modulation
- can be selected additionally*
- uses prospective attenuation from single
- Scout View
- mA adjusted to maintain ~ constant noise
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- Specify a ‘noise index’ (NI)
- NI defined as s.d. of CT numbers in water phantom with ‘standard’
algorithm
- Set min & max mA
- Patient s.d. ~ matches noise index for standard algorithm
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- Different algorithms: patient s.d. will not match the noise index
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- Increasing Noise Index (NI):
- increases noise
- decreases dose
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- SUREExposure 3D
- incorporates all three levels of modulation
- angular (x-y) modulation: ON/OFF
- uses prospective attenuation from Scanogram
- Two Scanograms required
- mA adjusted to maintain ~constant noise
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- Specify s.d. level (or ‘image quality level)
- patient mA calculated to achieve this noise level at any scan parameter
settings
- Set min & max mA
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- CARE Dose 4D: all three levels of AEC applied
- some exceptions
- e.g.adult head protocols: z-axis only
- Angular modulation uses ‘on-line’ attenuation data
- Use same kV for Topogram as for scan
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- Adapting mA for attenuation variation
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- Specify ‘Quality reference mAs’ in each protocol
- effective mAs for required image quality in standard patient
- Effective mAs is determined only by ‘Quality reference mAs’ and patient
size
- Independent of scan parameter settings
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- ACS: Automatic Current Selector
- Z-DOM: Z-axis modulation
- must be used initially with ACS
- D-DOM: Angular modulation
- D-DOM can be used independently or with ACS
- uses ‘on-line’ modulation
- D-DOM & Z-DOM cannot be implemented simultaneously
- Aims to keep image quality fairly constant with varying attenuation
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- Specify mAs/slice in protocol
- defines image quality (s.d.) in water phantom for settings in protocol
- following SurView mAs/slice for similar s.d. in patient is calculated
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- To obtain correct attenuation data from SPR always centre the patient
carefully
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- Ensure nothing but the patient is in the beam
- Always check CTDIvol info
- Check system is not over-ranging – may not be able to achieve the range
of mA values required
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40
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- Manufacturers differ in their approach to AEC
- Know your AEC system: read manual, talk to applications specialist
- AEC systems can increase as well as decrease dose
- Define image quality requirements carefully for each protocol
- Review image quality and dose continuously
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- The manufacturers for providing information & material; in
particular:
- Thomas Toth & Sandie Jewell, GE
- Iris Sabo-Napadensky & Derek Tarrant, Philips
- Christoph Suess & Susie Guthrie, Siemens
- Henk de Vries & Craig Hagenmaier, Toshiba
- The physicists & radiographers who gave me their time & time on
their scanners; in particular:
- Grace Keltz, St. George’s Hospital
- Claire Skinner, Royal Free Hospital
- CT department, Royal National Orthopaedic Hospital
- Lynn Martinez & Nina Arcuri, Royal Mardsen Hospital
- Trupti Patel, Harefield Hospital
- Sue & Jim, my colleagues at ImPACT, for helpful comments
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Notes