managing patient dose in msct

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?


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






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


• What are the risks?



• What are the risks?


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





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.


selecting short or long rotation time ?

mAseff =mAs

pitch


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


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


selecting high or low pitch value ?

Beam Collimation


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





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





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


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


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


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.


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/

managing patient dose in msct

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