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Managing patient dose in MSCT
John Damilakis, PhD
Assist. Professor of Medical Physics
University of Crete, Iraklion, Crete, Greece
damilaki@med.uoc.gr
Aim
• Are the doses from a MSCT examination high?
• What are the radiation risks?
• How can we manage patient dose?
Patient sample (n = 250)
30 %
20 %
10 %
18-28 28-38 38-48 48-58 58-68 68-78 >78
Age group (years)
Scans per patient
40
Patient fraction (%)
10
1 2 3 4 5 6 7
Number of scans
Scans per anatomic region
2
1.5
# of scans
1
0.5
HEAD THORAX LUMBAR ABDOMEN LOWER
SPINE ABDOMEN
Mean number of scans per patient
2.5
Total
No of scans per patient
Contrast
1.5
0.5
Males Females
Anatomic regions scanned
70
60
50
Patient fraction (%)
10
1 2 3 4
Number of anatomic regions scanned
Effective dose per scan and anatomic
region
12
9
mSv
6
3
HEAD THORAX LUMBAR ABDOMEN UPPER
SPINE ABDOMEN
Mean effective dose per patient
15
per scan
per patient
mSv
9
3
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
150 450
Chest
Normalized dose
Normalized dose
Head - neck
140 400
130 350
120 300
110 250
100 200
90 150
0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18
Patient age Patient age
750 950
Normalized dose
Normalized dose
680 Abdomen - Pelvis 880 Trunk
610 810
540 740
470 670
400 600
330 530
0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18
Patient age Patient age
Medical Physics (in press)
Normalized effective dose vs age
750 5-year-old Abdominal scan
Normalized effective dose (µSv / mGy)
680 120 kV, 35 mAs, BC = 24 mm, pitch = 1, rsw = 5.0 mm
610
580 µSv/mGy ED = ND x CTDI = 580 x 6.4 = 3.7 mSv
540
470
400
5y
330
0 2 4 6 8 10 12 14 16 18
Patient age (y)
Effective dose per scan from
pediatric MSCT (abdomen)
20
16
mSv
12
8
4
2 4 6 8 10 12 14 16 18
Patient Age (years)
Effective dose per scan from
pediatric CT (head & neck / trunk)
20
trunk
16
mSv
12
head
8
4
2 4 6 8 10 12 14 16 18
Patient Age (yr)
Thyroid doses from head & neck CT
Normalized thyroid dose
Normalized thyroid dose 0.4 0.4
Brain Paranasal sinuses
0.3 0.3
0.2 0.2
0.1 0.1
0.0 0.0
0 5 10 15 0 5 10 15
Normalized thyroid dose
0.08 1.50
Normalized thyroid dose
Inner ear Neck
0.06 1.45
0.04 1.40
0.02 1.35
0.00 1.30
0 5 10 15 0 5 10 15
Patient Age Patient Age
European Radiology, 2006 Sep 21; [Epub ahead of print]
Thyroid dose per scan from
pediatric head and neck MSCT
50
ral
, spi
ck
40
Ne
Dose mGy
30
20
10
Brain, spiral
Brain, seq
2 4 6 8 10 12 14 16 18
Patient Age (yr)
Aim
• Are the doses from a MSCT examination high?
• What are the radiation risks?
• How can we manage patient dose?
Biological effects of radiation
Stochastic effects : Carcinogenesis
As dose increases, the probability of the effect
occurring increases.
Stochastic effects are assumed to have no threshold.
Deterministic effects : Opacities
They are characterized by a threshold dose, below
which the effect does not occur.
Risk coefficients for fatal cancer
Risk per Unit Dose (% per Sv)
15 5-year-old
13.5 % per Sv
Dose from an abdominal scan: 3.7 mSv
12
Risk = 13.5 x 3.7 x 10-3 % = 0.05 %
9
6
3
5y
10 20 30 40 50 60 70 80
Age at acute exposure (yr)
Source : BEIR V
Risk of radiation-induced fatal
cancer from MSCT (abdomen)
0.30
0.25
Estimated Risk (%)
0.20
0.15
0.10
0.05
10 20 30 40 50 60 70 80
Age at acute exposure (yr)
The probability of radiogenic risk for cancer is not negligible
The number of CT examinations is increasing worldwide
?
33
Number
of CT
examinations
(millions)
3.6
1980 1998 2007 Year
Variety of examinations is increasing
Aim
• Are the doses from a MSCT examination high?
• What are the risks?
• How can we manage patient dose?
How can we manage patient dose?
Justification
Proper selection of scanning parameters
Use of technologic innovations
Protection of radiosensitive organs
Justification
BENEFITS
RISKS
Justification
RISKS
BENEFITS
How do we know if CT is the most
appropriate 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?
Justification
Proper selection of scanning parameters
Use of technologic innovations
Protection of radiosensitive organs
Parameters that affect CT dose
Beam shaping filter Collimation
kV, mAs
Filtration
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 ?
A: The shortest rotation time should be selected to
minimize motion artifacts.
Rotation time decreased mA increased
mA & automatic change from small to large focal spot
10.5
nCTDIw (mGy/100 mAs)
9.5
8.5
7.5
210
0 100 200 300 400 500
Tube current (mA)
European Radiology, 16:2575-85, 2006
Do we optimize a MSCT examination by
selecting short or long rotation time ?
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.
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.
mAs
mAseff =
pitch
High or low pitch value ?
75
CTDIv (mGy)
70
65
mAs
mAseff =
pitch
60
0.2 0.4 0.6 0.8 1.0 1.2
Pitch
European Radiology, 16:2575-85, 2006
z – overscanning (overranging)
z-overscanning
In spiral CT, the tissue volume
z-overscanning
of patient irradiated differs
from the volume imaged.
Medical Physics 32:1621-1629, 2005
z – overscanning (mm)
Pitch = 1.5
BC = 16 x 1.5
100
Pitch = 1.0
Overscanning
75
Pitch = 0.5
50
25
0 2 4 6 8 10
RSW (mm)
Medical Physics, 32:1621-1629, 2005
z – overscanning (mm)
BC = 16 x 0.75
100
Overscanning
75
50 Pitch = 1.5
Pitch = 1.0
25 Pitch = 0.5
0 2 4 6 8 10
RSW (mm)
Medical Physics, 32:1621-1629, 2005
z - overscanning
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
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.
Do we optimize a MSCT examination by
selecting high or low pitch value ?
High pitch values increase dose due to:
• automatic selection of focal spot size
• z-overscanning
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.
Overbeaming
Overbeaming =
wasted radiation
z axis z axis
Overbeaming
The wider the beam the smaller the percentage of wasted radiation.
However, wide collimation limits the width of the thinnest sections
that can be reconstructed.
Beam Collimation
Narrow collimations should be avoided as they
are less dose effective, unless their use is dictated
by the clinical need for thin reconstructed slices.
Scans at thick BC’s are to be preferred on the
basis of protecting the patient from radiation.
Beam Collimation
Thick beam collimation (24 mm) increases patient dose due to overscanning
D
O
S OVERSCANNING OVERBEAMING
E
16 x 1.5
D
O
S
OVERBEAMING OVERSCANNING
E
16 x 0.75
Recommended beam configuration
for Siemens Sensation 16:
• Head examinations: 16 x 1.5 mm
16 x 1.5
• Body examinations: 16 x 0.75 mm
16 x 0.75
Medical Physics, in press
Do we optimize a CT examination by selecting
wide or narrow beam collimation ?
Overbeaming and z - overscanning are two
competing effects regarding patient radiation
burden.
• Head examinations: 16 x 1.5 mm
• Body examinations: 16 x 0.75 mm
What is the proper selection of scanning
parameters to avoid motion artifacts ?
Motion artifacts can be avoided by selecting:
• short rotation time
• high pitch value
• wide beam collimation
However, the dose to the patient increases !
‘Standard’ rules to reduce dose
• Scan minimal length
Efforts must be made to
restrict the scan length to
that clinically essential.
• Reduce mAs without compromising image quality
• Reduce number of multiple scans
How can we manage patient dose?
Justification
Proper selection of scanning parameters
Use of technologic innovations
Protection of radiosensitive organs
mA modulation: Performance
evaluation
a
b
a
OR =
b
Submitted for publication
mA modulation
Oval ratio % Dose Reduction
40
2.5 35
% Dose Reduction
30
2 25
Oval ratio
20
1.5 15
10
5
1 0
-5
0.5 - 10
0 150 300 450 600 750 900
Anatomic position (mm)
10-year-old
% Dose Reduction
5-year-old
Oval ratio
1-year-old
neonate
Anatomic position (mm)
mA modulation
Helical Mode 16x1.5 Sequential Mode 12x1.5
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?
Justification
Proper selection of scanning parameters
Use of technologic innovations
Protection of radiosensitive organs
Bismuth
shielding
Eur Radiol 16:2334-2340, 2006
Eye lens dose from pediatric CT
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
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
Εye bismuth shielding
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
Medical Physics 32, 1024-1030, 2005
Protection of radiosensitive organs
Eye Shielding
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.
z – overscanning and eye lens dose
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.
Medical Physics, 33:2472-2478, 2006
z – overscanning and paediatric patients
Normalized eye lens dose (mGy/mGy)
2.0
axial scanning
helical, pitch = 1
1.5
helical, pitch = 0.5
1.0
0.5
-1 0 1 2 3
Distance from first scan line (cm)
Medical Physics, 33:2472-2478, 2006
What is the distance of eye lens from the
first slice of the volume to be imaged ?
Category Number of axial Number of helical
examinations examinations
I (distance = -1 to 0 cm) 12 6
II ( distance = 0 to 1cm) 21 12
III (distance = 1 to 2cm) 9 6
IV (distance = 2 to 3 cm) 3 3
Total 45 27
Medical Physics, 33:2472-2478, 2006
Protection of radiosensitive organs
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.
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/
