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Advances in CT Technology to Reduce Advances in CT Technology to Reduce Patient Risk Patient Risk
J.A. Seibert, Ph.D.President, AAPM
Department of Radiology
Sacramento, California USA
AAPM / SEFM SymposiumAAPM / SEFM SymposiumPATIENT SAFETY IN COMPUTED TOMOGRAPHYPATIENT SAFETY IN COMPUTED TOMOGRAPHY
SevillaSevilla –– 12 May 201112 May 2011
What are the risks of undergoing a CT exam?
• Radiation dose• Deterministic effects (e.g., CT perfusion studies)
• Stochastic risks and cancer induction
• Other risks• Contrast agent delivery
• Low quality, non‐diagnostic studies
• Poorly trained technologists / operators
• Lack of periodic calibration and quality control
CT scanners: Top‐of‐the‐Line Technology
GE DiscoveryGE DiscoveryCT750 HDCT750 HD
Philips Philips BrillanceBrillanceiCTiCT
Siemens Siemens Definition FlashDefinition Flash
ToshibaToshibaAquilionAquilion OneOne
Fast kV switchingGemstone Detector
ASiRHD mode
256 slices8 cm volume
iDose0.27 s rotation
2 x 128 sliceHigh pitch spiral
IRIS0.75 ms temp res
320 detector rows16 cm volume
AIDRDynamic Volumes
Notable FeaturesNotable Features
CT Imaging chain hardware improvements
TubeTubeFiltrationFiltration
CollimationCollimation
DASDAS
Beam ShapingBeam Shaping
Detector ArrayDetector Array
Imaging chain hardware improvements
TubeTube
Flying focal spot, high heat load, large continuous Flying focal spot, high heat load, large continuous mAmA
Low HighkV
Imaging chain hardware improvements
FiltrationFiltration
Additional Filtration:
Low HighkV
Spectrum Lower energies removed
Imaging chain hardware improvements
Beam ShapingBeam Shaping
Courtesy of PhilipsCourtesy of Philips
OptimizeOptimizedistributiondistributionof Dose toof Dose tothe Patientthe Patient
Imaging chain hardware improvements
CollimationCollimation
Correct forCorrect forOverbeamingOverbeaming &&OverOver‐‐ranging:ranging:
~5~5‐‐6% lower 6% lower dosedose
Courtesy of GECourtesy of GE
Imaging chain hardware improvements
Detector ArrayDetector ArrayCeramic detectorsCeramic detectors
fast decayfast decayshort afterglowshort afterglow
high stopping powerhigh stopping powerhigh light outputhigh light output
DASDAS
High frequency High frequency readoutreadout
Low electronic Low electronic noisenoise
Imaging chain hardware improvements
Reduces scatterReduces scatterImproves SNRImproves SNRLowers doseLowers dose
~ 15%~ 15%
2D anti2D anti‐‐scatterscatter
collimatorcollimator
Courtesy of PhilipsCourtesy of Philips
CT Dose Reduction Strategies
• Automatic Exposure Control (dose modulation)
• XY and Z mA modulation
• ECG gating
• Prospective cardiac acquisition• Helical• Axial
• Advanced reconstruction methods
Automatic Exposure Control: mA modulation
Angular Angular (XY)(XY)
Longitudinal Longitudinal (Z)(Z)
Combined Combined (XYZ)(XYZ)
GEGE Smart ScanSmart Scan Auto Auto mAmA Smart Smart mAmA
PhilipsPhilips ACS/DOMACS/DOM N/AN/A ZZ--DOMDOM
SiemensSiemens Care DoseCare Dose N/AN/A Care Dose 4DCare Dose 4D
ToshibaToshiba N/AN/A Sure Exposure 3DSure Exposure 3D Sure Exposure 3DSure Exposure 3D
Optimized dose for image quality: current selection & modulationOptimized dose for image quality: current selection & modulation
AEC – Dose Modulation
• mAmodulation based on localizer or current scan (for next scan)
Constant mAReduced dose level based on scoutReal-time angular dose modulation
Dose reductions of Dose reductions of 10 10 –– 40%40%
are typical over constant are typical over constant mAmACourtesy ToshibaCourtesy Toshiba
Cardiac ECG modulation
Courtesy ToshibaCourtesy Toshiba
9090‐‐95%95%relativerelativedose dose
reduction reduction achieveableachieveable
CT Reconstruction
• Filtered backprojection (FBP) is a closed analytical solution to reconstruction, and ignores noise
• Iterative reconstruction attempts to solve the “noisy” backprojection/reconstruction problem
• Adaptive “statistical” models ‐ fast
• Model‐based + statistical models ‐ robust
Measuredprojections
ReconstructedObject
FBPEngine
Iterative reconstruction
CurrentEstimate
Projections
Synthesizedprojection
Measuredprojection
Comparisons
Statisticsmodel
Objectmodel
ObjectUpdate
candidate
System Statistics
CurrentEstimate
Models Projections
X‐ray model
Image model
Detector model
Synthesizedprojection
Measuredprojection
Comparisons
Statisticsmodel
Objectmodel
ObjectUpdate
candidate
System StatisticsSystem optics
Statistical methodStatistical methodFast, but limitedFast, but limited
Statistical Statistical plusplusModelModel‐‐based based
methodmethodComputationally Computationally expensive, but expensive, but more robustmore robust
Adaptive Statistical Iterative Techniques
• Requirements:• Software and hardware
• Extra time for computation
• Protocols can remain unchanged• Less noise for large patients
• Protocols can use lower mA• Same noise for lower dose
GE: GE: ASiRASiR –– Adaptive Statistical iterative ReconstructionAdaptive Statistical iterative ReconstructionPhilips: Philips: iDoseiDose –– iterative Doseiterative DoseSiemens: Siemens: IRIS IRIS –– Iterative Reconstruction in Image SpaceIterative Reconstruction in Image SpaceToshiba: Toshiba: AIDR AIDR –– Adaptive Iterative Dose ReductionAdaptive Iterative Dose Reduction
“Blending” FBP + adaptive reconstruction
• Too much processing creates noise correlation, and “plastic‐appearing” images
• Typical use: fractionally blend FBP and IR images (values of 30 – 40% IR to FBP)
30% 30% ASiRASiR 60% 60% ASiRASiR 90% 90% ASiRASiR 100% 100% ASiRASiR
Blended Iterative Reconstruction and FBP
• IMPLEMENTATION OF Iterative Reconstruction:
• Do not change too fast
• Incrementally blend
• Allow radiologist to get used to noise pattern
• Maximum blend found to be about 40% for ASiR
• Allows noise index (GE) to be increased from 12.15 to 15.62 with 30% blend, and to 16.78 with 40% blend for C/A/P exam, thus achieving substantially lower dose
30% 30% ASiRASiR 100% 100% ASiRASiR
IRISIRIS‐‐60% less dose60% less dose
FBPFBP‐‐ full dosefull dose FBP: FBP: CTDIvolCTDIvol = 19 = 19 mGymGy
ASiRASiR: : CTDIvolCTDIvol = 6 = 6 mGymGy
FBPFBP‐‐ low doselow dose
AIDR processingAIDR processing
CLINICAL EXAMPLESCLINICAL EXAMPLES
Model‐based Iterative Reconstruction
CourtesyCourtesy
ToshibaToshiba
GE: GE: MBIR MBIR –– ModelModel‐‐Based Iterative ReconstructionBased Iterative ReconstructionPhilips: Philips: iDoseiDose44 –– iterative Dose 4iterative Dose 4Siemens: Siemens: SAFIRE SAFIRE –– SinogramSinogram Affirmed Iterative ReconstructionAffirmed Iterative ReconstructionToshiba: Toshiba: AIDR AIDR –– Adaptive Iterative Dose ReductionAdaptive Iterative Dose Reduction
Model‐based iterative reconstruction
FBPFBP VeoVeo (MBIR)(MBIR)GEGE
FBPFBP SAFIRESAFIRESiemensSiemens
Reported is up to 60%Reported is up to 60%dose reduction compareddose reduction compared
to typical FBPto typical FBP
Focus is on reducing Focus is on reducing reconstruction timereconstruction time
Input Technique Error Notification
• MITA and manufactures developing “DOSE CHECK”
• Identifies potentially high technique factors before scanning
• User sets internal reference standard
• Notification and Alert status are triggered on the CT console prior to imaging the patient
MITA: Medical Imaging & Technology Alliance
From Rich Mather presentation
From
Rich Mathe
rpresen
tatio
n
Dose reduction: a multifaceted challenge
• Review the protocols• Clinician, technologist, physicist• Periodic reviews for consistency, dose
• Lock down the protocols• Once they have been reviewed and verified• Password protection on scanners
• Dose Reporting• IHE REM profile
• Dose reviews
26
Dose and risk reduction strategies
• It’s more than scanner technologies…..
• Many individuals• Radiologist, technologist, medical physicist• Applications specialist, development engineer• Academic and industry researchers / cooperation
• Many considerations• Development of CT protocols• Independent assessment of CT devices & applications• Accreditation and periodic quality control
• The ultimate scenario • Right scan on the right patient with the right dose by using the right protocol, and optimized software andhardware
Summary
• Knowledge of CT dose reduction strategies is essential for all personnel
• Implementation can produce significant savings in dose and improved image quality
• Correct use of technology requires didactic education, hands‐on training and continuous re‐training of users
Acknowledgements
• Rich Mather, Toshiba Medical Systems • Many of the slides borrowed from his presentation at NIH “sub mSv” conference in March 2011
• CT manufacturers marketing materials • GE, Siemens, Philips, Toshiba
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