PEDDOSE.NET Educational Presentation:
CT radiation exposure in
multimodality imaging
Prepared by:
Klaus Bacher, Ghent University, Department of Basic Medical Sciences, Division of Medical Physics
This presentation contains educational material with respect to CT radiation exposure in multimodality imaging.
The presentation was developed in the framework of PEDDOSE.NET project (Dosimetry and health effects of
diagnostic applications of radiopharmaceuticals with particular emphasis on the use in children and adolescents).
The latter project is financially supported by the European Commission under the 7th Framework Program FP7Health-2009-1.2-6 (grant agreement number 241608)
PEDDOSE.NET Project Partners
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Department of Nuclear Medicine, University of Würzburg, Germany:
 M. Lassmann
 U. Eberlein
Department of Radiation Protection and Health, Federal Office for
Radiation Protection, Germany:
 D. Nosske
 J. H. Bröer
Department of Basic Medical Sciences, Division of Medical Physics,
Ghent University, Belgium:
 K. Bacher
 C. Vandevoorde
INSERM UMR892, France:
 M. Bardiès
 P. Santos
CT radiation exposure in multimodality imaging
Use and disclaimer
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•
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This is a PowerPoint file
It may be downloaded free of charge
It is intended for teaching and not for commercial purposes
The presentation was developed in the framework of
PEDDOSE.NET project which is financially supported by the
European Commission under the 7th Framework Program
FP7-Health-2009-1.2-6 (grant agreement number 241608)
CT radiation exposure in multimodality imaging
Multimodality imaging
Introduction
• PET/SPECT:


Functional information
Image:
 “Hot spot”
 Few anatomical landmarks
• CT:



Anatomical detail
High resolution
Wide dynamic range (soft tissue - bone)
(Masciari et al. – JAMA 2008)
CT radiation exposure in multimodality imaging
Multimodality imaging
Introduction
• PET-CT/SPECT-CT:




The strengths of both imaging modalities
CT attenuation correction
Precise localization
Higher sensitivity and specificity
(Masciari et al. – JAMA 2008)
CT radiation exposure in multimodality imaging
Multimodality imaging
Introduction
1995
1998
Stand-alone PET +
AC Ge68 ring source
1999
2001
First
SPECT/CT
system
PET/CT(128 slices)
PET/MRI
MRI
TOF-PET/CT
PET/CT
prototype
2011
PET
First
commercial
PET/CT
CT radiation exposure in multimodality imaging
Multimodality imaging
Increasing interest in multimodality imaging
(Buck et al. – JNM 2010)
CT radiation exposure in multimodality imaging
Multimodality imaging
Increasing interest in multimodality imaging
• Oncology is the most common application in PET/CT




Distinguish malignant from benign disease
Staging and re-staging of disease
Treatment response
Radiotherapy treatment planning
(Cuocolo et al. – EJNMMI 2010)
CT radiation exposure in multimodality imaging
Multimodality imaging
Education and training in CT imaging physics
Increasing interest in multimodality imaging:
 Need for knowledge/experience with physics/technology of CT
to deal with issues related to
 Patient radiation dose
 Image quality
 Based on this education: better justification and optimization op CT
acquisitions in multimodality imaging will be possible
(ICRP 113 – An. ICRP 2009)
CT radiation exposure in multimodality imaging
Is the CT radiation dose contribution
important in multimodality imaging?
CT radiation exposure in multimodality imaging
CT radiation exposure
CT radiation dose level vs. clinical application
• Attenuation correction
• Anatomical localization
• Diagnostic CT


Non-enhanced
Contrast-enhanced
Dose
 Single phase
 Multiple phase
(Cuocolo et al – EJNMMI 2010)
CT radiation exposure in multimodality imaging
CT radiation exposure
Are the CT doses high?
• Reported CT doses in adult patients with standard CT protocols
Huang et al.
(Radiology 2009)
Brix et al.
(JNM 2005)
Wu et al.
(EJNMMI 2004)
Gould et al.
(JNM 2008)
Compound
E(PET)
mSv
E(CT)
mSv
E(PET/CT)
mSv
%CT
18F-FDG
6.2
7.2 - 26
13.4 - 34.2
54 - 76
18F-FDG
5.7-7.0
16.7 - 19.4
22.4 - 26.4
74
18F-FDG
10.7
19.0
29.7
64
82Rb
4.4
3 – 5.4
7.4 – 9.8
41 - 55
• Comparison with 68Ge transmission scan: 0.20-0.26 mSv
(Wu et al. – EJNMMI 2004)
CT radiation exposure in multimodality imaging
CT radiation exposure
Are the CT doses high?
• Reported CT doses in pediatric patients with standard CT
protocols
Chawla et al.
(Pediatr Radiol 2010)
Fahey et al.
(JNM 2009)
Jadvar et al.
(Sem NM 2007)
Gelfand et al.
(Sem NM 2007)
Compound
E(PET)
mSv
E(CT)
mSv
E(PET/CT)
mSv
%CT
18F-FDG
4.6
20.3
24.9
82
18F-FDG
8.4
9.9
18.3
54
18F-FDG
6.4
12.9
19.3
67
18F-FDG
6.8
~13
~19.8
~66
CT radiation exposure in multimodality imaging
CT radiation exposure
Are the CT doses high?
• Estimated cumulative radiation dose from PET/CT in children
with malignancies: a 5-year retrospective review (Chawla et al. –
Pediatr. Radiol 2010)
CT radiation exposure in multimodality imaging
Why are CT doses high?
CT radiation exposure in multimodality imaging
Basics of CT
CT exposure
ImPACT (www.impactscan.org)
CT radiation exposure in multimodality imaging
Basics of CT
CT dose distribution
• A narrow X-ray fan beam interacts perpendicular to patient’s
z-axis
• As during the acquisition the X-ray source rotates around the
patient, a rather uniform dose distribution will be delivered (in
contrast with projection radiography)
(HD Nagel et al. – 2000)
CT radiation exposure in multimodality imaging
Basics of CT
CT dose profile
• Narrow X-ray fan beam interacts perpendicular to patient’s zaxis
• Scatter within the patient will be important but does not
contribute in the image
(HD Nagel et al. – 2000)
CT radiation exposure in multimodality imaging
Basics of CT
CT dose profile
• Scatter fractions of dose profiles are overlapping when making
a full scan
(HD Nagel et al. – 2000)
CT radiation exposure in multimodality imaging
Basics of CT
CT dose profile
• Scatter fractions of dose profiles are overlapping when making
a full scan
• Overlap depends on the helical pitch
(HD Nagel et al. – 2000)
CT radiation exposure in multimodality imaging
Basics of CT
Overbeaming
• The collimation of the X-ray beam on multi-slice systems is
increased such that the penumbra lies beyond the active
detectors and they are all irradiated uniformly
ImPACT (www.impactscan.org)
CT radiation exposure in multimodality imaging
Basics of CT
Overbeaming
• Relative dose for narrow collimations and narrow slice widths is
significantly higher (especially for <16 slice CT scanners)
ImPACT (www.impactscan.org)
CT radiation exposure in multimodality imaging
Basics of CT
Overscan
• Additional rotations for helical interpolation (reconstruction)
• Especially important for >16 slice scanners
ImPACT (www.impactscan.org)
CT radiation exposure in multimodality imaging
How do we quantify CT doses?
CT radiation exposure in multimodality imaging
Measuring CT dose
Definitions
• Computed tomography dose index = CTDI (mGy)
• Quantity representing the mean dose within a CT slice
CT periphery
CT centre
(HD Nagel et al. – 2000)
1
2
CTDI w  CTDI 100 ,c  CTDI 100 , p
3
3
CT radiation exposure in multimodality imaging
Measuring CT dose
Definitions
• Computed tomography dose index = CTDI (mGy)
• Quantity representing the mean dose within a CT slice
(HD Nagel et al. – 2000)
1
2
CTDI w  CTDI 100 ,c  CTDI 100 , p
3
3
CT radiation exposure in multimodality imaging
Measuring CT dose
CTDIvol
CTDIvol 
CTDI w
p
• CTDIvol takes into account effect of helical pitch
• CTDIvol measured within
• 16cm PMMA phantom (adult head/pediatric scans)
• 32cm PMMA phantoms (adult body)
• CTDIvol indicated on CT console
• For pediatric protocols CTDIvol sometimes wrongly indicated
(presented as 32 cm phantom values): large underestimation!
CT radiation exposure in multimodality imaging
Measuring CT dose
DLP
•
•
•
•
Dose length product = DLP (mGycm)
Reflects the total CT radiation exposure: DLP=CTDIvol x L
Indicated on CT console
From DLP → conversion to effective dose (mSv)
(G Stamm et al.
CT Expo software)
CT radiation exposure in multimodality imaging
Measuring CT dose
Effective dose
(G Stamm et al.
CT Expo software)
CT radiation exposure in multimodality imaging
Measuring CT dose
Dose reports
• Most CT scanners are providing a summary of the scanning
protocol that was used: “dose report”
 Number of CT scans
 Dose settings (CTDIvol)
 Total radiation exposure (DLP)
 Interesting information to compare with diagnostic reference
levels
CT radiation exposure in multimodality imaging
How can we reduce CT doses?
CT radiation exposure in multimodality imaging
CT dose reduction
Adjusting scan parameters
• Avoid narrow slice reconstructions

Slice thickness ↓ image noise↑↑
ImPACT
(www.impactscan.org)
CT radiation exposure in multimodality imaging
CT dose reduction
Adjusting scan parameters
• Avoid narrow slice reconstructions

Slice thickness ↓ image noise↑↑
• Adjust mA values according to patient size
ImPACT
(www.impactscan.org)
CT radiation exposure in multimodality imaging
CT dose reduction
Adjusting scan parameters
• Avoid narrow slice reconstructions

Slice thickness ↓ image noise↑↑
• Adjust mA values according to patient size


Lowering mA will increase noise
mA/4 → noise x2

mAs/2
ImPACT
(www.impactscan.org)
CT radiation exposure in multimodality imaging
CT dose reduction
Adjusting scan parameters
• Avoid narrow slice reconstructions

Slice thickness ↓ image noise↑↑
• Adjust mA values according to patient size


Lowering mA will increase noise
mA/4 → noise x2
• Lowering kVp settings is very efficient for dose reduction,
especially in pediatric CT
 Pay attention for system calibration for attenuation correction!
CT radiation exposure in multimodality imaging
CT dose reduction
Adjusting scan protocol
• “Take what you need”
• Avoid multiple CT scan series


Contrast-enhanced diagnostic CT scan may be used for
attenuation
Take into account the fact that diagnostic scans may be
programmed at the department of radiology as well
• Whole body CT dose for attenuation correction <1 mSv is
feasible (Brix et al., JNM 2005)
• Low dose whole body CT protocols with diagnostic information
down to 7 mSv are possible (Huang et al., Radiology 2009)
CT radiation exposure in multimodality imaging
CT dose reduction
Diagnostic reference levels
• Diagnostic reference levels are interesting tools for comparison
of your CT settings
• Unfortunately, the EU CT reference values (1999) are outdated
and the presented dose levels are NOT reflecting good practice
EU DRL for DLP
(mGycm)
2010 DRL for DLP in Belgium
(mGycm)
Head
1050
740
Chest
650
240
Abdomen
800
415
CT radiation exposure in multimodality imaging
Is dose-reducing technology
available for CT?
CT radiation exposure in multimodality imaging
Recent developments
Automatic tube current modulation
• Automatic adjustment of mA according to:



the patient size
position of the X-ray tube
along the patient’s z-axis
ImPACT
(www.impactscan.org)
CT radiation exposure in multimodality imaging
Recent developments
Automatic tube current modulation
• Automatic adjustment of mA aims for


Constant noise level throughout the complete scan range and
within a single scan area
Dose reduction: up to 45% reduction
Brink et al.
(Radiology 2008)
CT radiation exposure in multimodality imaging
Recent developments
Adaptive collimation
• Minimizing the effect of the helical overscan:


Dose reduction of 10% for large scan lengths
Dose reduction up to 38% for short (<12cm) scan ranges
Deak et al.
(Radiology 2009)
CT radiation exposure in multimodality imaging
Recent developments
Iterative CT reconstruction techniques
• Iterative reconstruction  filtered-back reconstruction CT


Significant noise reduction @ same radiation dose
Same noise level @ lower radiation dose: up to 65% reduction
I.R. CTDIvol = 8 mGy
FB.R. CTDIvol = 22 mGy
Hara et al.
(AJR 2009)
CT radiation exposure in multimodality imaging
Summary
• CT radiation exposure in multimodality imaging may be high
• Appropriate justification is needed for setting up a CT scanning
protocol for multimodality imaging taking into account:
 the age of the patient
 required image quality (≠ “best” image quality)
 availability of previous diagnostic CT scans
• Lowering CT radiation dose is feasible:
 using dose-reduction options of CT scanners
 comparing CT dose settings with diagnostic reference levels
CT radiation exposure in multimodality imaging