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Next Generation Diagnostic Reference
Levels Keyed to Patient Size
Aaron Sodickson MD, PhD
Disclosure
Research grant from Siemens on Dual
Energy CT
[email protected]
Section Chief, Emergency Radiology
Director, Brigham NightWatch Program
Medical Director of CT, Brigham Radiology Network
Associate Professor, Harvard Medical School
NIH - National Library of Medicine:
R01LM010679-01: Automated Radiation
Monitoring and Decision Support to
Reduce Cumulative Exposure
Brigham and Women’s Hospital
Harvard Medical School
[email protected]
Diagnostic Reference Levels
•  A DRL is intended to represent:
“An investigational level used to identify unusually high radiation
doses for common diagnostic medical X-ray imaging procedures.
DRLs are suggested action levels above which a facility should
review its methods and determine if acceptable image quality can
be achieved at lower doses.
DRLs are based on standard phantom or patient measurements
under specific conditions at a number of representative clinical
facilities. DRLs have been set at approximately the 75th percentile
of measured patient or phantom data. This means that procedures
performed at 75% of the institutions surveyed have exposure levels
at or below the DRL.”
ACR–AAPM PRACTICE GUIDELINE FOR DIAGNOSTIC REFERENCE LEVELS AND ACHIEVABLE DOSES IN MEDICAL XRAY IMAGING, Rev 2013, at:
http://www.acr.org/Quality-Safety/Standards-Guidelines/Practice-Guidelines-by-Modality/~/media/ACR/Documents/PGTS/
guidelines/Reference_Levels.pdf
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NEMA XR-25 Dose Check
•  Derived from 1st 3 years of ACR accreditation data (2002-2005 ),
2005 NEXT data and NCRP Report 172
•  “LAT dimensions are for average patients of the specified age.”
•  “Individual patients should not be compared against these values.”
ACR–AAPM PRACTICE GUIDELINE FOR DIAGNOSTIC REFERENCE LEVELS AND ACHIEVABLE DOSES IN MEDICAL XRAY IMAGING, Rev 2013, at:
http://www.acr.org/Quality-Safety/Standards-Guidelines/Practice-Guidelines-by-Modality/~/media/ACR/Documents/PGTS/
guidelines/Reference_Levels.pdf
[email protected]
AAPM recommended Dose Notification Values
•  2010 standard of the National Electrical
Manufacturers Association, from
collaboration of the CT manufacturers
•  Allows scanners to be configured for:
-  Dose Notification: User warning if configured
CTDIvol or DLP may be exceeded for any scan part
-  Dose Alert: Warning if cumulative CTDIvol or DLP
may be exceeded during entire scan at any
anatomic location
http://www.nema.org/Standards/Pages/Computed-Tomography-Dose-Check.aspx
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•  Intentionally higher than ACR DRLs
•  “The XR 25 standard does not provide a mechanism to take patient
size into account when evaluating dose indices. Hence, for obese
patients, use of CTDIvol or DLP values that exceed the notification
levels is likely to occur but may be completely appropriate.”
•  Intent of the alerts is to “present a ‘time-out’ opportunity for
the user to confirm scan settings.”
http://www.aapm.org/pubs/CTProtocols/documents/NotificationLevelsStatement.pdf
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•  Helpful for high level screening of institutional
techniques
•  But CT technique and patient dose depend
heavily on patient size
•  Meaningful protocol surveillance / optimization
must address this size dependence
•  Outlier detection in the torso is severely limited
without considering patient size
[email protected]
Mean organ dose / CTDIvol across scanners
How then can DRLs and dose
alerts be used?
Patient Perimeter (cm)
Turner, Zhang, Khatonabadi et al. The feasibility of patient size-corrected, scannerindependent organ dose estimates for abdominal CT exams. Med Phys 2011;38(2):820-829
CTDIvol from PE CT
Tube Current Modulation
Appropriate 8-fold
variation in CTDIvol
Modified from: Kalender WA. Computed Tomography, 3rd ed. Publicis Publishing, Erlangen Germany, 2011
Implications of Size Dependence
Size-agnostic alert values do not
make sense for all patient sizes
Sodickson, Weiss. Effects of Patient Size on Radiation Dose Reduction and Image Quality in low-kVp CT
Pulmonary Angiography Performed with Reduced IV Contrast Dose. Emergency Radiology. 2012
CT Protocol Quality Control
CTDIvol of PE CT, ER scanner, by Patient Weight
–  Too low for large patients
–  Too high for small patients
–  Potential to expose small patients
to FAR more radiation than needed
–  Small patients imaged at the
notification value receive better
than needed image quality, at
higher organ dose per CTDIvol
–  False +’ve alerts in large patients
–  False –’ve alerts in small patients
[email protected]
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2
50
Outlier Detection
AAPM adult torso
recommended dose
notification value
Size-Dependent Notification Values
CTDIvol of PE CT, ER scanner, by Patient Weight
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CTDIvol of PE CT, ER scanner, by Patient Weight
Select size-appropriate
values based on any
convenient threshold:
- 75th percentile
- 1.5 stddev above mean
ACR adult
chest DRL
Outliers have higher
than expected CTDIvol
relative to tube current
modulation output of
similarly-sized patients
[email protected]
[email protected]
Size-Dependent Notification Values
•  Size-dependent DRLs:
For accreditation / regulatory purposes, derive
from multi-center data collection WITH size info
What is the best size metric
CTDIvol of PE CT, ER scanner, by Patient Weight
•  It almost doesn’t matter – use what’s
available
–  Weight, BMI
–  AP, LAT diameter
–  Eff diam = sqrt (AP X LAT)
–  Water equivalent diameter
•  At institutional level, helpful to gauge overall
performance, compare devices and protocols
•  Size-dependent Dose Notification Values:
At scanner level, ideal to derive as protocol-type
specific to enable meaningful outlier detection by:
–  Technologist during scanning
–  Radiologist during interpretation / technique review
–  Medical Physicist during regular protocol review
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•  Menke J. Comparison of Different Body Size Parameters for Individual Dose Adaptation in Body CT of Adults. Radiology.
2005;236(2):565.
•  Li B, Behrman RH, Norbash AM. Comparison of topogram-based body size indices for CT dose consideration and scan
protocol optimization. Medical Physics. 2012;39:3456.
•  Wang J, Christner JA, Duan X, Leng S, Yu L, McCollough CH. Attenuation-based estimation of patient size for the
purpose of size specific dose estimation in CT. Part II. Implementation on abdomen and thorax phantoms using cross
sectional CT images and scanned projection radiograph images. Med Phys. 2012 Nov;39(11):6772–8.
•  Ikuta, Warden, Andriole, Khorasani, Sodickson. Estimating Patient Dose from X-ray Tube Output Metrics: Automated
Measurement of Patient Size from CT Images Enables Large-scale Size-specific Dose Estimates. Radiology. 2013
[email protected]
Patient Axial CT Image
Cylinder of Water
Effective Diameter (Deff)
Water-Equivalent Diameter (DW)
AP
HU
HU
HU
HU
HU
HU
HU
HU HU HU HU Lateral
HU
HU
HU
DW
HU
Y
X
Deff =
[email protected]
i=1
AP * Lateral
DW =
∑ HU +1000
i
n
1000
(XY)
4
π
Ikuta, Warden, Andriole, Khorasani, Sodickson. Estimating Patient Dose from X-ray Tube Output Metrics: Automated
Measurement of Patient Size from CT Images Enables Large-scale Size-specific Dose Estimates. Radiology.
2013
[email protected]
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Patient size extraction after the scan
Patient size extraction after the scan
Derived from axial CT images or planning
projection radiographs
Wang J, Christner JA, Duan X, Leng S, Yu L, McCollough CH. Attenuation-based estimation of patient size for the
purpose of size specific dose estimation in CT. Part II. Implementation on abdomen and thorax phantoms using
cross sectional CT images and scanned projection radiograph images. Med Phys. 2012 Nov;39(11):6772–8.
Deff =
AP X LAT
[email protected]
Patient size extraction after the scan
Correcting for Anatomy Truncation
•  These patients have the same Deff
•  Dw is substantially smaller (15%) in the chest
due to reduced attenuation of the lungs
Optimal prospective approach
•  Water equivalent diameter
–  Adjusts for patient attenuation
–  Insensitive to body contours
–  Mirrors underlying scanner TCM calculations
=> Tightest distributions of CTDIvol vs size
•  Based on WED, scanner uses appropriate
size-specific dose notification value to help
confirm appropriate technique
[email protected]
Ikuta, Warden, Andriole, Khorasani, Sodickson. Estimating Patient Dose from X-ray Tube Output Metrics: Automated
Measurement of Patient Size from CT Images Enables Large-scale Size-specific Dose Estimates. Radiology.
2013
[email protected]
Optimal prospective approach
•  Scanners should output water-equivalent
diameter in:
–  DICOM header (slice by slice & overall scan
range average)
⇒  Enables radiologist outlier detection at review
–  DICOM SR dose reports (average and array
by position) to enhance:
⇒  Systematic protocol review
⇒  Patient-specific organ dose calculation
[email protected]
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Acknowledgements
CME question
NIH - National Library of Medicine:
R01LM010679-01: Automated Radiation Monitoring and Decision
Support to Reduce Cumulative Exposure
BWH - Center for Evidence Based Imaging
Ramin Khorasani
Graham Warden
Tim O’Connell
Luciano Prevedello
Jenifer Siegelman
Matthew Raffol
Stacy O’Connor
Ichiro Ikuta
Dick Hanson
Kathy Andriole
Bobby Bransfield
Ali Raja
Eliott Wasser
Cami Farkas
Creating size-dependent CTDIvol alert values and
Diagnostic Reference Levels would most likely:
A)  Enable better prediction of patient doses
B)  Improve compliance with accreditation limits
C)  Decrease variability in CT technique for patients of
different sizes
D)  Reduce the number of alerts in large patients, and
increase the number in small patients
[email protected]
Next Generation Diagnostic Reference
Levels Keyed to Patient Size
Aaron Sodickson MD, PhD
[email protected]
Section Chief, Emergency Radiology
Director, Brigham NightWatch Program
Medical Director of CT, Brigham Radiology Network
Associate Professor, Harvard Medical School
Brigham and Women’s Hospital
Harvard Medical School
[email protected]
[email protected]
[email protected]
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