CT radiation exposure in multimodality imaging

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15 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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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 FP7
-
Health
-
2009
-
1.2
-
6 (grant agreement number 241608)











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


PEDDOSE.NET Project Partners


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


Use and disclaimer

Multimodality imaging


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)

Introduction

CT radiation exposure in multimodality imaging


Multimodality imaging


PET
-
CT/SPECT
-
CT:


The strengths of both imaging modalities


CT attenuation correction


Precise localization


Higher sensitivity and specificity

(Masciari et al.


JAMA

2008)

Introduction

CT radiation exposure in multimodality imaging


Multimodality imaging




1995


1998


1999


2001


2011


Stand
-
alone PET +



First


PET/CT(128 slices)

AC Ge68 ring source



SPECT/CT



PET/MRI








system




TOF
-
PET/CT




Introduction

First
commercial
PET/CT


PET/CT
prototype

MRI

PET

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


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)

Increasing interest in multimodality imaging

CT radiation exposure in multimodality imaging





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)



Education and training in CT imaging physics

Multimodality imaging

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


Attenuation correction


Anatomical localization


Diagnostic CT


Non
-
enhanced


Contrast
-
enhanced


Single phase


Multiple phase

CT radiation dose level vs. clinical application

(Cuocolo et al


EJNMMI

2010)


Dose

CT radiation exposure in multimodality imaging


CT radiation exposure

Are the CT doses high?

Compound

E(PET)

mSv

E(CT)

mSv

E(PET/CT)

mSv

%CT

Huang et al.

(
Radiology
2009)

18
F
-
FDG

6.2

7.2
-

26

13.4
-

34.2

54
-

76

Brix et al.

(
JNM

2005)

18
F
-
FDG

5.7
-
7.0

16.7
-

19.4

22.4
-

26.4

74

Wu et al.

(
EJNMMI

2004)

18
F
-
FDG

10.7

19.0

29.7

64

Gould et al.

(
JNM

2008)

82
Rb

4.4

3


5.4

7.4


9.8

41
-

55


Reported CT doses in
adult patients

with standard CT protocols


Comparison with
68
Ge 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?

Compound

E(PET)

mSv

E(CT)

mSv

E(PET/CT)

mSv

%CT

Chawla et al.

(
Pediatr Radiol
2010)

18
F
-
FDG

4.6

20.3

24.9

82

Fahey et al.

(
JNM

2009)

18
F
-
FDG

8.4

9.9

18.3

54

Jadvar et al.

(
Sem NM
2007)

18
F
-
FDG

6.4

12.9

19.3

67

Gelfand et al.

(
Sem NM
2007)

18
F
-
FDG

6.8

~13

~19.8

~66


Reported CT doses in
pediatric patients

with standard CT
protocols

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


CT exposure

ImPACT

(
www.impactscan.org
)

CT radiation exposure in multimodality imaging


Basics of CT

CT radiation exposure in multimodality imaging



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)

Basics of CT

CT dose distribution

(HD Nagel et al.


2000)


Narrow X
-
ray fan beam interacts perpendicular to patient’s z
-
axis


Scatter within the patient will be important but does not
contribute in the image

Basics of CT

CT dose profile

(HD Nagel et al.


2000)

CT radiation exposure in multimodality imaging



Scatter fractions of dose profiles are overlapping when making
a full scan

Basics of CT

CT dose profile

CT radiation exposure in multimodality imaging


(HD Nagel et al.


2000)


Scatter fractions of dose profiles are overlapping when making
a full scan


Overlap depends on the helical pitch

Basics of CT

CT dose profile

CT radiation exposure in multimodality imaging


(HD Nagel et al.


2000)


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

Overbeaming

CT radiation exposure in multimodality imaging


Basics of CT

ImPACT

(
www.impactscan.org
)


Relative dose for narrow collimations and narrow slice widths is
significantly higher (especially for <16 slice CT scanners)

Basics of CT

Overbeaming

ImPACT

(
www.impactscan.org
)

CT radiation exposure in multimodality imaging



Additional rotations for helical interpolation (reconstruction)


Especially important for >16 slice scanners

Basics of CT

Overscan

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


Computed tomography dose index = CTDI (mGy)


Quantity representing the mean dose within a CT slice


Definitions

CT centre

CT periphery

CT radiation exposure in multimodality imaging


(HD Nagel et al.


2000)

p
c
w
CTDI
CTDI
CTDI
,
100
,
100
3
2
3
1


Measuring CT dose


Computed tomography dose index = CTDI (mGy)


Quantity representing the mean dose within a CT slice


Definitions

p
c
w
CTDI
CTDI
CTDI
,
100
,
100
3
2
3
1


CT radiation exposure in multimodality imaging


(HD Nagel et al.


2000)

CTDI
vol





CTDI
vol

takes into account effect of helical pitch


CTDI
vol

measured within


16cm PMMA phantom (adult head/pediatric scans)


32cm PMMA phantoms (adult body)


CTDI
vol

indicated on CT console


For pediatric protocols
CTDI
vol

sometimes wrongly indicated
(presented as 32 cm phantom values): large underestimation!



p
CTDI
CTDI
w
vol

Measuring CT dose

CT radiation exposure in multimodality imaging



Dose length product = DLP (mGycm)


Reflects the total CT radiation exposure: DLP=CTDI
vol

x L


Indicated on CT console


From DLP


conversion to effective dose (mSv)

Measuring CT dose

DLP

(G Stamm et al.

CT Expo software)

CT radiation exposure in multimodality imaging


Effective dose

(G Stamm et al.

CT Expo software)

Measuring CT dose

CT radiation exposure in multimodality imaging



Most CT scanners are providing a summary of the scanning
protocol that was used: “dose report”


Number of CT scans


Dose settings (CTDI
vol
)


Total radiation exposure (DLP)

Measuring CT dose

Dose reports





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


Avoid narrow slice reconstructions


Slice thickness

image noise
↑↑

Adjusting scan parameters

ImPACT

(www.impactscan.org)

CT radiation exposure in multimodality imaging


CT dose reduction


Avoid narrow slice reconstructions


Slice thickness

image noise
↑↑


Adjust mA values according to patient size

Adjusting scan parameters

ImPACT

(www.impactscan.org)

CT radiation exposure in multimodality imaging


CT dose reduction


Avoid narrow slice reconstructions


Slice thickness

image noise
↑↑


Adjust mA values according to patient size


Lowering mA will increase noise


mA/4

noise x2

Adjusting scan parameters

ImPACT

(www.impactscan.org)



mAs/2

CT radiation exposure in multimodality imaging


CT dose reduction


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!

Adjusting scan parameters

CT radiation exposure in multimodality imaging


CT dose reduction


“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


Adjusting scan protocol



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 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

Diagnostic reference levels

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 adjustment of mA according to:


the patient size


position of the X
-
ray tube


along the patient’s z
-
axis

Automatic tube current modulation

ImPACT

(www.impactscan.org)

CT radiation exposure in multimodality imaging


Recent developments


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

Automatic tube current modulation

Brink et al.

(Radiology 2008)

CT radiation exposure in multimodality imaging


Recent developments


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

Adaptive collimation

Deak et al.

(Radiology 2009)

CT radiation exposure in multimodality imaging


Recent developments


Iterative reconstruction


filtered
-
back reconstruction CT


Significant noise reduction @ same radiation dose


Same noise level @ lower radiation dose: up to 65% reduction


Iterative CT reconstruction techniques

Hara et al.

(AJR 2009)

I.R. CTDI
vol

= 8 mGy

FB.R. CTDI
vol

= 22 mGy

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