Transcript 06. Radiation Protection of Children During Computed Tomography

Radiation Protection in Paediatric Radiology
Radiation Protection of Children
During Computed Tomography
L06
IAEA
International Atomic Energy Agency
Educational objectives
At the end of the programme, the participants
should:
• Recognize that CT is a relatively higher dose
imaging procedure.
• Understand dose management strategies for
computed tomography in children.
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Answer True or False
1. Reduction of kVp in CT reduces the dose.
2. CT contributes 60-70 % of the dose from
radiological examinations in developed countries.
3. The same CT protocol used for children and
adults will result in a higher dose to adults.
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Contents
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Overview of CT systems: SDCT and MDCT.
Dose levels in CT and risk attributable to paediatric CT.
Importance of application of justification in paediatric CT.
Optimization of image quality and patient dose in paediatric
CT.
Selection of appropriate technical parameters.
Use of shielding devices in paediatric CT.
Dose management strategies in paediatric CT.
Requirements for staff: experience and training.
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Computed Tomography
• Computed tomography (CT) is the method that extends the
clinical capabilities of X-ray imaging:
• High contrast sensitivity for visualizing soft tissues.
• Production of configurable data sets.
• Three-dimensional (3D) representations
• Multiplanar depictions
• “Volume” CT
• Dynamic (e.g. perfusion, cardiac) information
• Tissue characterization (dual energy technology)
• Advances in computed tomography (CT) technology have
continued to improve existing and open new clinical
applications.
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Computed Tomography
• Since 1972; then…
Hounsfield
Cormack
Nobel prize for medicine
1979
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Computed Tomography
• ..and now…
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Modern CT Scanners
• Modern CT scanners are 3rd generation, that is the tube
and detectors rotate together around the patient
• Slip ring technology allows for spiral hence volume
scanning
Principle of spiral CT. Patient
is transported trough the
gantry, x-ray tube traces
spiral path around the
patient when acquiring data
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M. Mahesh, MDCT physics, the basic technology, image quality and radiation dose,
Wolters Kluwer, 2009
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Single Detector (SDCT) vs Multi-detector
(MDCT) Computed Tomography
SDCT and MDCT design. The difference is the presence of multiple-row detectors
in the longitudinal direction with MDCT yielding multiple slice options for single rotation
M. Mahesh, MDCT physics, the basic technology, image quality and radiation dose, Wolters Kluwer, 2009
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Multi-detector (MDCT) Computed
Tomography
MDCT detectors
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M. Mahesh, MDCT physics, the basic technology, image quality and radiation
dose, Wolters Kluwer, 2009
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CT and Paediatric Radiology
• The patient dose in CT is an important issue for
children.
• In some centres, the exposure factors used for
scanning children are the same as for adults.
• CT scanning contributes most to collective dose
from exposures from medical imaging due both to
relatively high dose per exam and to the increasing
use of this modality.
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Facts About CT…
• Facts about CT…
• 69 million CT examinations per year for all ages
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in USA in 2007.
Approximately 10% growth rate per year
7 million CT examinations per year in children
40-50 % increase in paediatric CT from 2005/06.
Up to 31% of paediatric body CT examinations
are multiphase in some reports
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Facts About CT…
• The frequency of CT examinations is evenly
distributed at all ages:
• 33% are performed in children under age of 10
• Repeated examination:
• 30% of adults and children have three or more
CT scans
METTLER, F.A., et al., J. Radiol. Prot. 20 4 (2000) 353-359
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CT as a Dose Contributor
CT examinations:
• comprise only 17% of all radiological examinations, but...
• contributes to 49% of the effective dose all radiological
examinations
Radiological examinations
Collective dose
CT
17%
Rest
51%
CT
49%
Rest
83%
Mettler et al. Helath Phys 2008, 95:502-7
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Amount of Radiation Resulting From CT
Examination
Effective Dose (mSv)
Chest X-ray Equivalents
3-view ankle radiography
0.0015
0.07
2-view chest radiography
0.02
1
Radionuclide cystogram
0.18
9
Flouroscopic cystogram
~0.33
~16
Radionuclide bone scan
~5
~250
Brain CT
2
100
Chest CT
up to 3
up to 150
Abdominal CT
up to 5
up to 250
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Frush D, et al, CT and Radiation Safety: Content for Community Radiologists
www.imagegently.org
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Why is this so?
Radiography
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Why is this so?
Dose distribution*
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*in relative units
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Risk of CT Examination
• Unique consideration in children:
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Life time to manifest the bioeffects
More radiosensitive tissues
Dose is considered cumulative over time
Risk is higher for females and younger age groups
• From a single abdominal CT in paediatric age,
lifetime estimated risk for fatal cancer is 1: 1000 1: 2000.
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Risk Versus Benefit
• Important to distinguish between individual risks and
collective, public-health risks
• The individual risks are small, so the benefit / risk ratio for
any child will generally be very large,
• …but the exposed population (~7.0 million children/yr in the
US) is large
• Even a very small individual radiation risk, when multiplied
by a large (and increasing) number of children, is likely to
produce a significant long-term public health concern
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CT in Paediatric Radiology
• The frequency of paediatric CT examinations has
been increasing over the past 20 years
• Reduced requirements for sedation and allowance of
examination of younger, sicker and less co-operative
children
• Increased speed of acquiring diagnostic
information
• Increased number of multiple scans
• Attention must be given to adapting protocols to
suit children taking into account that they are more
sensitive than adults
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In Paediatric Radiology…
• If identical CT head examination protocol is
used:
• Adult dose: 1.5 mSv
• Child dose: 6 mSv
Huda et al. Radiology, 1997, 203:417-22
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In Paediatric Radiology…
• It estimated that between a third and half of the
examinations occurring have questionable
indications.
• Many are conducted using inappropriate technical
factors.
Frush, RSNA, 2006,
Berenner Pediatr. Radio.l 32 (2002) 228 – 231,
Oikarinen et al. Eur Radiol 19 (2009) 1161-5
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Justification and CT
• It is very important that each examination is
rigorously justified, thus…
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Justification for CT: Practical Advice
• Justify CT examination rigorously and eliminate
inappropriate referrals.
• Perform only necessary CT examinations.
• Reduce the number of multiple phase scans.
• Work to account for previous procedures.
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Justification for CT: Practical Advice
• Use referral guidelines and appropriateness
criteria when available
• Use alternative approaches, such as ultrasound,
MRI where appropriate
• Include justification in clinical audit
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How to achieve the objective?
• Respect age-specific pathology and its prognosis.
• Consider potential contribution of the scan to
patient management and outcome.
• Consider the patient’s medical imaging record with
respect to ionizing radiation
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How to Achieve the Objective?
• Respect cost and radiation exposure.
• Replace CT by examination with no or with lower
radiation exposure (e.g. US, MRI).
• Delay/cancel follow-up examination unless a
decision based on scan is needed now.
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Optimisation and CT
One size does not fit all...
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Optimisation and CT
For paediatric CT
examinations, the use of
specific radiographic
technical parameters
should be promoted as:
• Child size the kVp and
mA.
• One scan (single phase) is
often enough.
• Scan only the indicated
area.
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General Recommendation
• You must use paediatric protocols to reduce the
dose for the same image quality as in adults
• Make sure there are no inappropriate high (e.g.
adult) parameter settings behind the name
paediatric protocols
• Plan paediatric scans according to patient’s size
and age
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Generic Requirements for Optimisation
• Inform and prepare the patient and accompanying
person(s).
• Be familiar with CT dose descriptors.
• Realise lower noise usually means higher doses;
accept noise if scan is diagnostic.
• Make sure operating conditions balance image
quality and radiation exposure.
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Generic Requirements for Optimisation
• Optimize scan parameters within the axial plane.
• Optimize a set of tube current settings for
paediatric examinations.
• Optimize scan parameters for volume coverage.
• Scan minimal length and minimise repeated
scanning at identical areas.
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Equipment, Protocol, Dose and Image Quality
• In most children a tube voltage
of 80–100 kVp will suffice,
especially in children with a
body weight <45 kg.
• In adolescents, a tube voltage of
100 kVp for the thorax and 120
kVp for the abdomen is usually
sufficient
• Recent studies with phantoms
suggest that the optimal tube
voltage in children may be even
lower (60kVp) at least for some
indications
Nievelstein, Pediatr Radiol, 2010
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Equipment, Protocol, Dose and Image Quality
• Spiral or helical scanning is
preferable in paediatrics as an
entire volume is imaged
• Short tube rotation times reduce
movement artefacts and provide
more detailed cardiac imaging
• One main benefit for MDCT
scanners is speed of acquisition
rather than dose reduction
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Equipment, Protocol, Dose and Image Quality
• An increase in pitch can result in a shorter scan time and
(in some scanner types) in a dose reduction
• In modern MDCT scanners this may not be the best option
(due to overranging)
• If effective mAs is used, an increase in pitch will result in an
increase in the tube current
• Therefore, it is usually more dose efficient to keep the pitch
as low as possible (<1) and if needed manually decrease
the tube current
Nievelstein, Pediatr Radiol, 2010
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Equipment, Protocol, Dose and Image Quality
• Multi-slice scanners have potential to deliver
higher dose
• by having a wider beam irradiating a number of
detector rows to achieve multiple slices
simultaneously
• as well as owing to more extensive clinical use
• However.…
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Equipment, Protocol, Dose and Image Quality
• Strategies for dose reduction in MDCT:
• Hardware improvements
• Software improvements, as tube current
modulation, image reconstruction algorithms, …
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Equipment, Protocol, Dose and Image Quality
• Modern scanners give automatic
or semiautomatic correction of
tube current (mA) for patient size
(mA modulation).
• Significant dose reduction (20–
50%) without appreciative loss of
image quality.
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Equipment, Protocol, Dose and Image Quality
1. Image thickness:
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•
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Should be chosen depending on the size of
the child and the application
Use maximal acquisition collimation (assuming
this would result in scanning at lower mA)
appropriate for specific diagnosis
Narrow collimation in MSCT and 1 mm slices
on some SDCT result in a higher dose
(increase in mAs to maintain image quality)
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Equipment, Protocol, Dose and Image Quality
2. Pitch:
• SDCT: a pitch factor 1.5 is recommended for
most examinations
•
•
25% reduction in dose compared with using a
pitch of 1
MDCT: reduction in dose due to greater pitch may
not be achieved
•
tube current (mA) can be automatically
adjusted to keep the dose and noise the same
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Equipment, Protocol, Dose and Image Quality
3. Tube potential (kVp)
• There are few advantages to using a high tube potential
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(kV).
Without a reduction in tube current (mA) this leads to a
significantly higher dose.
100 kVp or 80 kVp is usually adequate for children.
Lowering of kVp enhances contrast
10 kg patient transmits 3-4% while an adult transmits less
than 0.1%.
Be aware that images with high noise, even if they do not
look very crisp, may provide the diagnostic information.
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Equipment, Protocol, Dose and Image Quality
4. Lower tube current (mA):
•
•
Lower tube current (mA) should be used for scanning
kids.
• High tube current is required only when there is a need for
high image detail ( in low contrast settings)
Decrease of mA according to body diameter and use
of exposure charts if AEC is not available (dose
reduction 70-80%), Lucaya, et al, 2000, AJR
175:895-92
•
Use of tube current modulation technology results in
dose reduction by 60% for paediatric scanning, Kalra
et al, 2004, Radiology, 233:649-57
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Equipment, Protocol, Dose and Image Quality
5. Gantry Tilt
• A straight gantry results in irradiation of a
smaller volume of tissue compared with a tilted
gantry and is recommended.
• Exception: tilt is used to avoid unnecessary
exposure of sensitive tissues, e.g. in brain CT
for avoiding the orbits.
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Equipment, Protocol, Dose and Image Quality
6. Scan Length
• Scan the minimum length required and be
restrictive in defining upper and lower limits.
• Optimise scan parameters for volume coverage
by using representative volume sample(s) when
the entire volume is not needed (by sequential
scans with gaps) to reduce dose-length product
Vock and Wolf , Dose Optimization and Reconstruction in CT of
children, in Radiation Dose from Adult and Paediatric MDCT,
Springer, 2007
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Equipment, Protocol, Dose and Image Quality
7. Reconstruction Algorithm
• Appropriate reconstruction algorithms, window
levels and window settings should be used
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Equipment, Protocol, Dose and Image Quality
8. Dose Indices
• Protocols must be adjusted by the operator to take into
account the patient's age and weight (size).
• Newer scanners indicate the volumetric CT dose index
(CTDIvol ) and Dose-length product (DLP) on the console
(Requirement from IEC 60601-2-44).
• This allows the user to automatically:
• See the relative effect on dose owing to changes in kVp,
mA, collimation and pitch,
• Estimate the effective dose to patient.
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Radiation Dose Indices for CT
Dose displays on modern multislice scanners:
• Volume CTDI (CTDIvol)
• Dose Length Product (DLP)
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Dose Indices for CT
• CTDI is a local per scan
dose and is dependent on
kVp, mAs and slice
collimation.
• DLP is an integral dose over
the scan length and number
of series and depends on
pitch and dose
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Computed Tomography Dose Indices
• Effective dose, E,
provides risk estimate
which depends on the
body size and organs
imaged as well as on
the integral dose.
• E is calculated as the
product of DLP and
conversion factors
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Shrimpton et al, BJR (2006) 79, 968-980
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Typical Doses in Paediatric CT
Exam type
Relevant
organ
Range of absorbed
organ doses (mGy)
Range of
effective doses
(mSv)
Head unadjusted*
(200 mAs)
Brain
23- 49
1.8 - 3.8
Head adjusted
(100 mAs)
Brain
11 - 25
0.9 - 1.9
Abdomen unadjusted Stomach
(200 mAs)
21 - 43
11 - 24
Abdomen adjusted
(50 mAs)
5 - 11
6 - 12
Stomach
*"Unadjusted" refers to using the same settings as for adults.
"Adjusted" refers to settings adjusted for body weight.
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NCI: www.cancer.gov/cancertopics/causes/radiation-risks-pediatric-CT
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Equipment, Protocol, Dose and Image Quality
9. Viewing Conditions:
• Make sure windows levels and settings are
adequate and that the monitors are calibrated.
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Equipment, Protocol, Dose and Image Quality
10. Shielding:
• Lead shielding can be place over the male gonads if:
• the edge of the volume of investigation is less than 1015cm away
• it does not interfere with the image
Dauer, et al, BMC Medical Imaging 2007, 7:5 doi:10.1186/1471-2342-7-5
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Equipment, Protocol, Dose and Image Quality
10. Shielding:
• The use of reusable bismuth
attenuation shields is possible
for sensitive organs such as
the eyes, gonads, breasts
and thyroid.
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Shielding
• The bismuth eye shield is simple to
place and covers only the eye
• In-plane shields are associated with
greater image noise and streak
artifacts. However, shields reduce
radiation dose. Automatic exposure
control did not increase radiation
dose when using a shield.
Karla et al, Korean J Radiol. 10:156-63,
2009
• This adult patient has a 3 layer
bismuth latex eye shield in place.
While artefact is seen into the globe,
no artefact is transmitted into the
brain. Standoff pads can reduce
surface artefact
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Hopper KD, et al, Am J Neuroradiol
22:1194–1198,2001
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Equipment, Protocol, Dose and Image Quality
11. Training
• The examination should always be supervised
by a radiologist experienced in paediatric
imaging
If all listed factors are taken into consideration, significant
dose reduction can be achieved
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Training
• Following options are available on modern scanners
• Tube current modulation (mA, mA/slice, effective mAs),
pitch, noise level setting, field-of-view for bow tie filter,
kVp, beam (vs slice) collimation…
• This requires a skilled operator:
• Who knows well the model of the scanner using
• Trained in paediatric imaging to adjust the examination
parameters according to examination type, age and/or
size of the child
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Radiologists, Physicists and Technologists’
Responsibilities
• Improve awareness of need to decrease CT
radiation dose to children.
• Be committed to make a change in daily practice
by team work between radiologists, technologists,
referring healthcare providers and parents.
• Medical physicists, radiologists, technologists and
department managers should review vendor or
other CT protocols and “down-size” them for
children.
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Radiologists, Physicists and Technologists’
Responsibilities
• Single phase scans are often adequate
• Pre- and post-contrast or delayed scans rarely add
additional information in children, but can double or
triple the dose.
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Radiologists, Physicists and Technologists’
Advice
• Scan only the indicated area. If a patient has a
possible small dermoid on ultrasound, there may
not be a need to scan the entire abdomen and
pelvis.
• Be involved with your patients. Be the patient’s
advocate. Ask the questions required to ensure
that you “child-size” the scan.
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http://rpop.iaea.org/RPoP/RPoP/Content/index.htm
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http://www.pedrad.org/associations/5364/ig/
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Summary
• CT can be a relatively high dose diagnostic imaging
procedure
• Rigorous justification of CT for children is required
• Good practice in paediatric CT:
• Optimisation of the CT examination protocol based on
patient size (lower kVp and mA)
• Acceptance of images with greater noise
• One scan (single phase) is often enough - Reduce
repeat scanning of identical body areas
• Scan only the indicated area
• Use of shielding devices
• Trained and experiences staff
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Answer True or False
1. Reduction of kVp in CT reduces the dose.
2. CT contributes 60-70 % of the dose from
radiological examinations in developed countries.
3. The same CT protocol used for children and
adults will result in a higher dose to adults.
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Answer True or False
1. Тrue - Reduced kVp reduce the dose in children
while maintaining image quality.
2. Тrue - It is a high dose modality and with 10%
contribution to number of all radiological
examination it gives 60-70% of dose.
3. False- It is opposite, the same protocol will give a
few time higher dose to children.
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References
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BRENNER, D.J., ELLISTON, C.D., HALL, E.J., BERDON, W.E., Estimated risks of radiation-induced
fatal cancer from paediatric CT, Am. J. Roentgenol. 176 (2001) 289-296.
BRENNER, D.J., Estimating cancer risks from paediatric CT: going from the qualitative to the
quantitative, Pediatr. Radio.l 32 (2002) 228 – 231.
FRICKE, B.L., et.al., In-plane bismuth breast shields for pediatric CT: effects on radiation dose and
image quality using experimental and clinical data, Am. J. Roentgenol. 180 (2003) 407 – 411.
HOPPER, K.D.,et al, The breast: in-plane x-ray protection during diagnostic thoracic CT - shielding
with bismuth radioprotective garments, Radiology 205 (1997) 853 – 858.
KILJUNEN, T., JÄRVINEN, H., SAVOLAINEN, S., Diagnostic reference levels for thorax X-ray
examinations of paediatric patients, Br. J. Radiol. 80 (2007) 452-9.
BOONE, J.M., et. al., Dose reduction in paediatric CT: a rational approach, Radiology 228 (2003) 352360.
LUCAYA, J., et. al., Low-dose high-resolution CT of the chest in children and young adults: dose,
cooperation, artefact incidence and image quality, Am. J. Roentgenol. 175 (2000). 985-992.
INTERNATIONAL ATOMIC ENERGY AGENCY, Dose Reduction in CT while Maintaining Diagnostic
Confidence: A Feasibility/Demonstration Study, IAEA-TECDOC-1621, IAEA, Vienna, (2009).
KALRA, M.K., et. al., Techniques and applications of automatic tube current modulation for CT,
Radiology 233 (2004) 649-657.
INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, ICRP Publication 102:
Managing Patient Dose in Multi-Detector Computed Tomography (MDCT), Annals of the ICRP Volume
37/1, Elsevier, (2007).
D. Tack,Pierre A Gevenois, Radiation Dose from Adult and Pediatric Multidetector Computed
Tomography, Springer, 2007
Karla et al, In-plane shielding for CT: effect of off-centering, automatic exposure control and shield-tosurface distance, Korean J Radiol. 2009 Mar-Apr;10(2):156-63..
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Additional material
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Practical Optimisation in Paediatric CT (I)
• Reduce mAs according to body weight/diameter
or composition and/or
• Use dose modulation (angular/longitudinal)
• Use maximal slice reconstruction thickness to
reduce noise and potentially dose appropriate for
specific diagnosis.
• Decrease kVp for thin (small) patients and high
contrast exams (CT angiography, chest,
musculoskeletal )
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Practical Optimisation in Paediatric CT (II)
• Normally use shortest rotation time available.
• Use representative volume sample when entire
volume is not needed.
• Use spiral scan with pitch greater than 1 (eg.: 1.5),
provided this does not automatically increase the
mA.
• Use newer dose reduction strategies such as
iterative reconstruction and adaptive modulation
(to reduce over ranging)
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Practical Optimisation in Paediatric CT (III)
• Be restrictive in defining upper-most and lower•
•
•
•
most scan range
Use localising projection scan extending just
minimally beyond scan limits.
Consider low kVp and single AP topogram
Reconstruct additional thick noise-reduced slices
without increase in exposure.
Avoid major overlap when scanning adjacent areas
with different protocols
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Practical Optimisation in Paediatric CT(IV)
• Avoid additional non-enhanced scans unless
•
•
•
•
specifically justified.
Optimise the protocol to obtain all the information
requested during one scan.
Minimise the number of scans in multi-phase
scanning.
In case of multi-phase scanning use shorter scan
length for additional scans.
Use lower dose for non-enhanced or repeat scans
unless high quality is needed.
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Practical Optimisation in Paediatric CT (V)
• Minimise length of scans and fluoroscopy time in
interventional applications.
• Use low mA with CT fluoroscopy
• Replace test bolus/bolus triggering by standard
can delay unless timing is very critical.
• Use additional protection devices where indicated
such as bismuth shields (lens, thyroid, breast,
gonads).
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Tube Current Modulation Options
Tube current modulation:
•
•
•
•
Based on patient's size
Longitudinal (z-axis)
Angular (xy-axis)
Combined
Thin patient
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Thick patient
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Tube Current Modulation Options
• Dose reduction based on
patient anatomy.
• Lower mA in AP, higher mA in
lateral directions.
Methods
200 mA
180 mA
150 mA
130 mA
150 mA
180 mA
210 mA
200 mA
170 mA
• Patient attenuation measured during scout scan (AP &
Lat) and alter mA for each gantry rotation (Smart mA1,
Real AEC2) or “on-the-fly” (Care dose3)
1
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GE, 2 Toshiba and 3 Siemens MDCT
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Tube potential (kVp)
Decreasing kVp significantly reduces dose, typically:
•
•
•
•
80 kV – 0.5 mSv
100 kV – 1 mSv
120 kV – 1.6 mSv
140 kV – 2.3 mSv
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kV
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Tube potential (kVp)
• CT examinations with a high intrinsic contrast
(chest, bones) justify lowering the tube voltage to
80–100 kVp
Nievelstein, Pediatr Radiol, 2010
• However, bony examinations can
be performed with very
low current 25-70mA
Cook, Imaging, 2001
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Guidelines
• FDA Public Health Notification: Reducing Radiation Risk
from Computed Tomography for Paediatric and Small Adult
Patients, November 2nd, 2001
• National Cancer Institute: Radiation Risks and Paediatric
Computed Tomography (CT): A Guide for Health Care
Providers,
http://www.cancer.gov/cancertopics/causes/radiation-riskspediatric-CT
• Image Gently: http://www.pedrad.org/associations/5364/ig/
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A Practice Quality
Improvement (PQI)
Program in CT Scans in
Children:
• The PQI module capture
how your practice performs
CT scans in children, and
allows you to compare
your practice to “safe
practice” guidelines in the
literature.
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How to Develop CT
Protocols for Children?
• Provide guidance in
developing CT protocols for
children and periodically
verifying that your current
protocols are appropriate
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Example of successful story I
Arch and Frush, AJR 2008;191:611–617:
• Since 2001, kVp and mA settings, two principal parameters determining
radiation dose, have decreased significantly for paediatric body MDCT
• It is a reasonable assumption that these changes are due to efforts to
increase awareness about the risks of radiation
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Paediatric chest CT
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Example of successful story II
Wallace, et al. Proceedings of IRPA 12, Buenos Aires, 2008, FP0227:
• Eight paediatric hospitals
• Training and seminars on optimisation
• Dose reduction greater than 50%
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