John Damilakis, PhDAssist. Professor of Medical Physics
University of Crete, Iraklion, Crete, [email protected]
Managing patient dose in MSCT
Aim
• Are the doses from a MSCT examination high?
• What are the radiation risks?
• How can we manage patient dose?
• Are the doses from a MSCT examination high?
Patient sample (n = 250)
18-28
Age group (years)
10 %
30 %
28-38 38-48 48-58 58-68 68-78 >78
20 %
Scans per patient
1 2 3 4 5 6 7
Patie
nt f
ract
ion
(%)
Number of scans
10
40
Scans per anatomic region
0.5
1
1.5
2
# o
f sc
ans
HEAD THORAX LUMBAR ABDOMEN LOWERSPINE ABDOMEN
Mean number of scans per patientN
o o
f sc
ans
per
patie
nt
2.5
0.5
1.5
Males Females
Total
Contrast
Anatomic regions scannedPa
tient
fra
ctio
n (%
)
Number of anatomic regions scanned
10
70
60
50
1 2 3 4
Effective dose per scan and anatomic region
3
6
9
12
mSv
HEAD THORAX LUMBAR ABDOMEN UPPERSPINE ABDOMEN
Mean effective dose per patient
per scanper patient
mSv
15
3
9
Males Females
Are CT doses comparable to background radiation ?
Average background dose :
3 mSv / year (chronic exposure)
Average CT dose :
14 mSv / examination (acute exposure)
Normalized effective dose vs. age
Medical Physics (in press)
Nor
mal
ized
dos
e
Patient age
Head - neck
0 2 4 6 8 10 12 14 16 18
150
140
130
120
110
100
90
Nor
mal
ized
dos
e
Patient age0 2 4 6 8 10 12 14 16 18
450
400
350
300
250
200
150
Chest
Nor
mal
ized
dos
ePatient age
0 2 4 6 8 10 12 14 16 18
950
880
810
740
670
600
530
Trunk
Nor
mal
ized
dos
e
Patient age
0 2 4 6 8 10 12 14 16 18
750
680
610
540
470
400
330
Abdomen - Pelvis
Normalized effective dose vs age
5 y
580 µSv/mGy
5-year-old Abdominal scan
120 kV, 35 mAs, BC = 24 mm, pitch = 1, rsw = 5.0 mm
ED = ND x CTDI = 580 x 6.4 = 3.7 mSv
Nor
mal
ized
eff
ectiv
e d
ose
(µSv
/ mG
y)
0 2 4 6 8 10 12 14 16 18
750
680
610
540
470
400
330
Patient age (y)
Effective dose per scan from pediatric MSCT (abdomen)
2 4 6 8 10 12 14 16 18
20
4
12mSv
8
16
Patient Age (years)
Effective dose per scan from pediatric CT (head & neck / trunk)
mSv
2 4 6 8 10 12 14 16 18
20
4
12
8
16
Patient Age (yr)
head
trunk
Thyroid doses from head & neck CT
0 5 10 15
0.4
0.3
0.2
0.1
0.0Nor
mal
ized
th
yroi
d
dose
Brain
0 5 10 15
0.4
0.3
0.2
0.1
0.0Nor
mal
ized
th
yroi
d
dose
Paranasal sinuses
0 5 10 15
0.08
0.06
0.04
0.02
0.00Nor
mal
ized
th
yroi
d
dose
Inner ear
Patient Age
0 5 10 15
1.50
1.45
1.40
1.35
1.30Nor
mal
ized
th
yroi
d
dose
Neck
Patient Age
European Radiology, 2006 Sep 21; [Epub ahead of print]
Thyroid dose per scan from pediatric head and neck MSCT
Neck, spiral
Brain, spiralBrain, seq
Dos
e m
Gy
50
10
30
20
40
2 4 6 8 10 12 14 16 18Patient Age (yr)
Aim
• Are the doses from a MSCT examination high?
• What are the radiation risks?
• How can we manage patient dose?
• Are the doses from a MSCT examination high?
• What are the radiation risks?
Biological effects of radiation
Deterministic effects :
Stochastic effects :
As dose increases, the probability of the effect
occurring increases.
Stochastic effects are assumed to have no threshold.
They are characterized by a threshold dose, below
which the effect does not occur.
Carcinogenesis
Opacities
SourceSource : BEIR V: BEIR V
Risk coefficients for fatal cancer
Age at acute exposure (yr)
Ris
k p
er U
nit
Dos
e (%
per
Sv)
10 20 30 40 50 60 70 80
3
6
9
12
15 5-year-old
Dose from an abdominal scan: 3.7 mSv
5 y
13.5 % per Sv
Risk = 13.5 x 3.7 x 10-3 % = 0.05 %
Risk of radiation-induced fatal cancer from MSCT (abdomen)
Est
imat
ed R
isk
(%)
10 20 30 40 50 60 70 80 Age at acute exposure (yr)
0.25
0.05
0.15
0.10
0.20
0.30
The probability of radiogenic risk for cancer is not negligible
The number of CT examinations is increasing worldwide
Variety of examinations is increasing
3.6
33
1980 1998 2007 Year
Numberof CTexaminations(millions)
?
Aim
• Are the doses from a MSCT examination high?
• What are the risks?
• How can we manage patient dose?
• Are the doses from a MSCT examination high?
• What are the risks?
• How can we manage patient dose?
How can we manage patient dose?
Proper selection of scanning parameters
Use of technologic innovations
Protection of radiosensitive organs
Justification
Justification
BENEFITS
RISKS
Justification
RISKS
BENEFITS
How do we know if CT is the mostappropriate examination ?
An ACR committee has developed criteria for determining
appropriate imaging examinations for diagnosis and
treatment of specified medical conditions.
These criteria are intended to guide radiologists, radiation
oncologists, and referring physicians in making decisions
regarding radiologic imaging and treatment.
www.acr.org
How can we manage patient dose?
Proper selection of scanning parameters
Use of technologic innovations
Protection of radiosensitive organs
Justification
Parameters that affect CT dose
kV, mAs
Filtration
Beam shaping filter Collimation
Detection system efficiency
Scanning length, Reconstruction slice width, Pitch, Scanner geometry, Algorithms
A comprehensive evaluation of the dosimetric characteristics
of a CT scanner is needed.
Some years ago, we used to follow rules for an optimized
CT dose reduction in patients.
A dose-effective use of any scanner can only be established
with on-site measurements of its dosimetric characteristics.
Rotation Time
Do we optimize a MSCT examination by
selecting short or long rotation time ?
Rotation time decreased
A: The shortest rotation time should be selected to
minimize motion artifacts.
mA increased
mA & automatic change from small to large focal spot
European Radiology, 16:2575-85, 2006
nCTD
Iw (
mG
y/10
0 m
As)
7.5
8.5
9.5
10.5
0 100 200 300 400 500Tube current (mA)
210
When a high tube load is required, an increased
rotation time should be preferred in order to
avoid the automatic selection of the large focal
spot.
Do we optimize a MSCT examination by
selecting short or long rotation time ?
Pitch
mAseff =mAs
pitch
Do we optimize a MSCT examination by
selecting high or low pitch value ?
A: Higher pitch is associated with a reduced dose to the
patient because of a shorter exposure time.
European Radiology, 16:2575-85, 2006
CTD
Iv (
mG
y)
60
65
70
75
0.2 0.4 0.6 0.8 1.0 1.2Pitch
High or low pitch value ?
mAseff =mAs
pitch
Medical Physics 32:1621-1629, 2005
z-overscanning
z-overscanning
In spiral CT, the tissue volume
of patient irradiated differs
from the volume imaged.
z – overscanning (overranging)
BC = 16 x 1.5
Ove
rsca
nnin
g
25
50
75
100
0 2 4 6 8 10RSW (mm)
z – overscanning (mm)
Pitch = 0.5
Pitch = 1.0
Pitch = 1.5
Medical Physics, 32:1621-1629, 2005
BC = 16 x 0.75
Ove
rsca
nnin
g
25
50
75
100
0 2 4 6 8 10RSW (mm)
z – overscanning (mm)
Pitch = 0.5Pitch = 1.0
Pitch = 1.5
Medical Physics, 32:1621-1629, 2005
van der Molen, A. J. et al. Radiology 2006;0:2421051350
z - overscanning
van der Molen, A. J. et al. Radiology 2006;0:2421051350
z - overscanning
z - overscanning
When radiosensitive organs are marginally included in
the examination field, proper selection of BC, RSW and
pitch is needed to restrict z - overscanning.
The relative contribution of the extra exposure due to
z–overscanning may be considerable especially when the
planned image volume is limited.
z - overscanning
Thick beam collimation (24 mm) increases patient
dose due to z-overscanning.
For a given beam collimation, an increase in
the RSW increases patient dose. High pitch
values increase dose due to z-overscanning.
High pitch values increase dose due to:
• automatic selection of focal spot size
• z-overscanning
Do we optimize a MSCT examination by
selecting high or low pitch value ?
Beam Collimation
Do we optimize a MSCT examination by
selecting wide or narrow beam collimation ?
A: The wider the beam the smaller the percentage of
wasted radiation due to overbeaming. Therefore, we
optimize a MSCT examination by selecting wide
collimation.
z axis
Overbeaming
wasted radiation
=
Overbeaming
z axis
Overbeaming
However, wide collimation limits the width of the thinnest sections
that can be reconstructed.
The wider the beam the smaller the percentage of wasted radiation.
Beam Collimation
Scans at thick BC’s are to be preferred on the
basis of protecting the patient from radiation.
Narrow collimations should be avoided as they
are less dose effective, unless their use is dictated
by the clinical need for thin reconstructed slices.
16 x 1.5
DOSE
OVERSCANNING OVERBEAMING
DOSE OVERBEAMING OVERSCANNING
16 x 0.75
Thick beam collimation (24 mm) increases patient dose due to overscanning
Beam Collimation
• Head examinations: 16 x 1.5 mm
Recommended beam configuration
for Siemens Sensation 16:
Medical Physics, in press
• Body examinations: 16 x 0.75 mm
16 x 1.5
16 x 0.75
Do we optimize a CT examination by selecting
wide or narrow beam collimation ?
• Head examinations: 16 x 1.5 mm
• Body examinations: 16 x 0.75 mm
Overbeaming and z - overscanning are two
competing effects regarding patient radiation
burden.
Motion artifacts can be avoided by selecting:
• short rotation time
What is the proper selection of scanning
parameters to avoid motion artifacts ?
However, the dose to the patient increases !
• high pitch value
• wide beam collimation
However, the dose to the patient increases !However, the dose to the patient increases !
‘Standard’ rules to reduce dose
• Scan minimal length
• Reduce mAs without compromising image quality
Efforts must be made to
restrict the scan length to
that clinically essential.
• Reduce number of multiple scans
How can we manage patient dose?
Proper selection of scanning parameters
Use of technologic innovations
Protection of radiosensitive organs
Justification
a
b
mA modulation: Performance evaluation
aOR =
b
Submitted for publication
mA modulation
Oval ratio
Anatomic position (mm)
0 150 300 450 600 750 900
2.5
2
1.5
1
0.5
Ova
l ra
tio
% Dose Reduction
- 10- 50510152025303540
% D
ose
Red
uctio
n
10-year-oldO
val
ratio
% D
ose
Red
uctio
n
Anatomic position (mm)
5-year-old
1-year-old
neonate
Helical Mode 16x1.5 Sequential Mode 12x1.5
mA modulation
mA modulation
The dose reduction achieved with tube mA modulation
is not substantial for neonates and young children.
mA-modulation should be considered as a complementary
means to reduce dose and should not replace other
dose reduction methods, especially in young children.
Software tools for noise simulation
What is the effect of a possible reduction of mA on image quality?
By courtesy of IMP, Erlangen
Software tools for noise simulation
By courtesy of IMP, Erlangen
How can we manage patient dose?
Proper selection of scanning parameters
Use of technologic innovations
Protection of radiosensitive organs
Justification
Bismuth
shielding
Eur Radiol 16:2334-2340, 2006
The threshold for ophthalmologically detectable opacities
has been reported to be 0.5–1.3 Gy. These values refer to
adult individuals and therefore may be lower in infants.
ICRP 60, 1990 & NRPB Vol 7, Nr 3, 1996
Dose to the eye lens per scan: 0.07 Gy in CT scanning
of sinuses and 0.13 Gy in CT of orbital trauma.
NCRP Publication 87, 2000
Eye lens dose from pediatric CT
Εye bismuth shielding
Medical Physics 32, 1024-1030, 2005
Dose reduction factors (%) of eye lens dose
CT examination Infants 1 year 5 years 10 years 15 years
Scanning of orbits 33.1 35.7 37.4 37.1 35.2
Scanning of the head 31.4 32.8 33.1 34.7 33.0
Angled scan. excl. orbits < 1 <1 <1 <1 <1
A considerable reduction in eye lens dose may be
achieved by using orbital bismuth shielding during
pediatric head CT scans. However, this shielding
should not be used in children when the eyes are
excluded from the primarily exposed region.
Protection of radiosensitive organsEye Shielding
z – overscanning and eye lens dose
Medical Physics, 33:2472-2478, 2006
In helical mode, the proximity of eye lenses to the
boundaries of planned image volume in combination
with the additional exposure due to z-overscanning,
can result in a significant increase in the lens dose.
z – overscanning and paediatric patients
-1 0 1 2 3
Nor
mal
ized
eye
len
s do
se (
mG
y/m
Gy)
Distance from first scan line (cm)
0.5
1.0
1.5
2.0
axial scanning
helical, pitch = 1
helical, pitch = 0.5
Medical Physics, 33:2472-2478, 2006
What is the distance of eye lens from thefirst slice of the volume to be imaged ?
2745Total
33IV (distance = 2 to 3 cm)
69III (distance = 1 to 2cm)
1221II ( distance = 0 to 1cm)
612I (distance = -1 to 0 cm)
Number of helical examinations
Number of axial examinations
Category
Medical Physics, 33:2472-2478, 2006
II ( distance = 0 to 1cm) 21 12
z – overscanning and paediatric patients
It is more dose efficient to use axial mode acquisition
rather than helical scan for pediatric head studies, if
there are no overriding clinical considerations.
Protection of radiosensitive organs
Messages to take home
Radiation dose from MSCT examinations is not
comparable to background radiation.
The probability of radiogenic risk for cancer
from MSCT examinations is not negligible.
Messages to take home
A dose-effective use of a MSCT scanner can
only be established with on-site measurements
of its dosimetric characteristics.
The relative contribution of the extra exposure
due to z-overscanning may be considerable.
Messages to take home‘Standard rules’ can be used to reduce dose
Justify CT examinations
Scan minimal length
Reduce mAs without compromizing quality
Reduce number of multiple scans
Avoid radiosensitive organs
HANDOUTS:
URL ADDRESS: http://medicalphysics.med.uoc.gr/handouts/