Download Leading Canadian Pediatric Hospital Continues to Drive

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Leading Canadian Pediatric Hospital
Continues to Drive Dose Down for Sick Children
At Canada’s premier pediatric hospital—also recognized as
one of the world’s foremost pediatric healthcare institutions—
new CT imaging protocols are lowering patient exposure to
ionizing radiation.
The clinicians at The Hospital for Sick Children (SickKids) in
Toronto, Ontario, treat some of the most seriously ill children in
Canada. In 2009, SickKids became the first pediatric hospital in
that country to install the Discovery* CT750 HD, the world’s first
high definition CT system. For Guila BenDavid, Manager CT Scan,
PET/CT, and Nuclear Medicine, and Karen Thomas, MB, BCh, MA,
MRCP, FRCR, FRCP(C), radiologist, the speed, workflow, and flexibility
of the system were the key reasons why they believed it was
the right CT scanner for their hospital. However, GE Healthcare’s
commitment to lowering CT dose in pediatric patients tipped
the scale.
According to BenDavid, the hospital’s typical imaging pathway
starts with trying to answer clinical question(s) with non-ionizing
examination techniques such as ultrasound. However, if CT is
ultimately the best option for the patient, the hospital balances
dose and image quality to obtain the information required for
patient diagnosis or management.
“With any new CT scanner comes new and improved capabilities,
but users need to optimize technical settings in order to maximize
its diagnostic ability for their patients,” says Dr. Thomas. “We didn’t
just transpose our protocols from our previous scanner to this
new one. Rather, we wanted to take advantage of the scanner’s
advanced technology for thin slice acquisition and improved Z-axis
resolution, utilize the Smart mA/Auto mA, and incorporate ASiR*.
So we essentially started ‘from scratch’ to develop our new
pediatric body CT protocols.”
Adds BenDavid, “We bought a scanner with advanced capabilities,
so what would be the purpose of utilizing it like an older scanner,
using thick slice acquisition? We were very conscious of dose
considerations. We wanted the best of both worlds—advanced
imaging capabilities at the lowest possible dose.”
And so began what became a year-long process of intensive
development and evaluation of new protocols at SickKids, which
continues today with further modifications in dose-lowering
techniques.
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A
B
Figure 1A, B. Coronal MPR images from a routine
CT chest/abdomen/pelvis on a nine-year-old
child (22.5 kg) showing simultaneous multicompartmental vascular opacification using a
newly developed, dual bolus, intravenous contrast
injection technique. Low attenuation liver was
attributed to fatty change. Scan performed at
100 kVp using AutomA, Noise Index 20, and 30%
ASiR. CTDIvol 3.8 mGy, DLP 199 mGy•cm, Effective
dose estimate 4.7 mSv (using pediatric conversion
coefficients, Kalender 2010).
Protocol development and evaluation
Optimizing protocols to achieve ALARA is more challenging in
pediatrics than in adults, explains Dr. Thomas. Pediatric patients
vary in size from the smallest newborn baby to adult-sized
teenagers. Depending on age, torso shape also evolves from
almost cylindrical in infants to more oval-shaped in school-age
children and then to adult morphology in teenagers, and this
must be taken into consideration when using the Smart mA/
Auto mA system. Children tend not to have much abdominal
fat, which impacts image contrast. Finally, children can have
difficulty staying still in the scanner and so system speed and
adaptability are important. These factors combined require
a large number of gradations in scanning parameters.
For the first six months after installing the new system, the team
underwent an empirical process involving day-to-day observations
of protocol adaptations—based on four iterations of routine chest
and abdominal scanning protocols. With each iteration, the staff
reviewed the protocols and evaluated various aspects for each
weight group, including: image quality, the pattern of tube current
modulation as assessed by the console-displayed “mA Table”,
and dose as indicated by the Dose Length Product (DLP). These
efforts were supplemented by work with anthropomorphic
phantoms to further investigate choice of pitch and the impact
of scan length on beam width selection.
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A GE Healthcare CT publication • www.ctclarity.com
As a result of these observations, “Our knowledge of how this
scanner worked and the impact of altering different technical
parameters considerably increased,” says Dr. Thomas. For
example, before scanning a patient, the technologists could
see how the Smart mA/Auto mA system was going to adapt to
the patient’s size and shape. With each patient, the technologist
recorded how the “mA table” behaved for a specific Noise Index
level, noting the highest and lowest mA and the pattern of
modulation achieved.
“From this process alone, the technologists learned that they
should be paying attention to the projected pattern of tube current
modulation and DLP,” says BenDavid. “They could determine
if the DLP was too high or if modulation was not going to be
optimal even before starting the scan.”
By the end of the first year of evaluation, SickKids developed
new protocols for the Discovery CT750 HD that took into account
differences in patient size and shape. The new protocols also
better utilized the system’s wide array of capabilities. Their body
CT protocols include 12 weight categories from newborn to over
100 kg, with adaptations in kVp (small children are now scanned
at 80 or 100 kVp and older children and teenagers at 120 kVp),
Smart mA/Auto mA parameters, detector width, and acquisition
field of view (FOV).
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Figure 2. It is important to put young patients at ease prior to a CT scan.
A critical element, says Dr. Thomas, was the settings for the
Automatic Exposure Control. “There are five parameters that can
be prescribed—the Noise Index, minimum mA, maximum mA,
and whether to use Smart mA, Auto mA, or both. Each of these
elements requires optimizing for each weight group or patient
shape,” she explains.
Adds BenDavid, “Because it was not implemented on our previous
scanner, it was a substantial learning curve on our part to learn
how to use it to its maximum potential.”
SickKids realized that greater image noise can be tolerated as
patient size increases and so they developed a scale of Noise
Indices based on patient size, specific to chest or abdomen, and
designed for use with thin collimation scanning. Their “final”
protocols take a variable approach to infants, small children,
larger children, and teenagers—although as Dr. Thomas notes,
“protocols are never final; we are always looking to further
improve them.” In infants they found a “set mA” approach more
effective, and they use tube current modulation for children over
12 kg. The rise in mA, which can increase dose through the pelvis
in older children, is a known result of dose modulation systems
and was addressed by prescribing maximum mA settings to
specifically limit this.
“Another practical point we learned was to be meticulous in
prescribing the coverage of the scan when using the Smart mA/
Auto mA because the highest mA values were often at the
periphery of our scan region,” says Dr. Thomas. “Unnecessary
additional coverage of a few centimeters can have a significant
dose impact, which might be up to 10% of the total dose to a
child.” The technologists learned to be as precise as possible
in determining scan coverage, BenDavid adds.
Detector beam width was tailored to patient size. Based on
phantom studies and patient projected DLP studies, on systems
without dynamic Z-axis tracking and with a scan length above
20 to 25 cm, the DLP is similar when using a 40 mm beam width
or a 20 mm beam width. However, below this scan length, the
DLP is lower using a 20 mm beam width, and therefore this
became the preferred beam width for short coverage cases
where scan time was not critical. “We looked at the typical scan
length for chest and abdomen examinations in various patient
weight categories and incorporated this knowledge into our
protocols,” says Dr. Thomas.
Since the acquisition FOV determines the selection of bowtie filter
and maximum kW tube output, it was necessary to select the
scan FOV according to patient size in order to maximize image
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quality, adds Dr. Thomas. “Our goal was to push the scanner as
far as possible to lower dose and optimize our protocols before
adding ASiR.”
The last consideration before ASiR was the administration of
intravenous contrast for extended length scans such as chest,
abdomen, and pelvis. “By creating a robust protocol using the
Smart mA/Auto mA system, we can scan the chest and abdomen
in one helical acquisition at a lower dose compared to our previous
‘two-helical’ technique,” says Dr. Thomas. “We designed a new
dual bolus technique, with two boluses of contrast separated by
a saline flush. With this injection technique and a single helical
scan, we can now achieve simultaneous pulmonary arterial,
systemic arterial, and portal venous vascular opacification,
which has further improved our image quality.”
Adding ASiR to the mix
Finally, ASiR^ was added to the protocol development mix. Although
the implementation of ASiR at SickKids is still a work-in-progress,
the team has been able to increase Noise Indices to obtain
further dose savings. Typically, SickKids uses between 20% and
50% ASiR for body CT examinations, increasing the percentage
with patient size. With continued phantom evaluations, Dr. Thomas
anticipates obtaining more objective data on the effect of varying
blends of ASiR over a range of kVp and mA settings in order to
facilitate a more structured approach to increasing its use
in clinical practice.
Figure 3. 3D angiogram of the hand in an infant with severe soft tissue
and bony fusion (syndactyly), demonstrating detailed anatomy of the
digital arteries for pre-operative planning. Scan performed at 80 kVp,
60 mAs, CTDIvol 7.2 mGy, DLP 75 mGy•cm (16 cm phantom).
Advances in CT angiography
In addition to “routine” body scanning, the team at SickKids has
been pleased with the improvement in image quality of the CT
angiograms that they can now provide to referring physicians.
“We have been able to develop new clinical applications,” says
^In clinical practice the use of ASiR may reduce patient CT dose depending on the clinical task, patient
size, anatomical location and clinical practice. A consultation with a radiologist and a physicist should
be made to determine the appropriate dose to obtain diagnostic image quality for the particular
clinical task.
“One of the greatest advantages of this GE scanner in
pediatric imaging is the degree to which we can prescribe
changes to each protocol parameter, especially the
degree to which the Smart mA/Auto mA parameters are
user defined.”
Dr. Karen Thomas
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Dr. Thomas, “including high resolution images of very small
vessels such as the digital (hand) arteries in babies.”
Notable results
The leap to the Discovery CT750 HD provides a noticeable difference
in image quality, and the ability to design and tailor protocols was
another tremendous advantage of the new system at SickKids.
“One of the greatest advantages of this GE scanner in pediatric
imaging is the degree to which we can prescribe changes
to each protocol parameter, especially the degree to which
the Smart mA/Auto mA parameters are user defined,” says
Dr. Thomas.
Other features stand out as beneficial for the team at SickKids,
including the protocol password protect—particularly useful
when technologists are unfamiliar with the Discovery CT750 HD.
Another key benefit, says BenDavid, is the user interface and
ability to move from screen to screen very quickly. “System
speed is very important when scanning children who often
move during a scan, and thus may have required sedation.”
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Similarly, GE Healthcare provides reliable and timely support;
BenDavid found GE responds quickly to support requests,
especially when the iLinq is utilized.
“It’s not always just about the end product, but what it takes
to get there,” BenDavid says. Dr. Thomas adds, “GE Healthcare
helped us achieve the low doses we wanted. We had the
impression that our work was also a key priority for GE.”
The adaptation of imaging protocols to children of all ages has
long been the philosophy of SickKids Diagnostic Imaging
Department. BenDavid explains, “As new technology emerges,
we strive to make it available to our patients, while ensuring
that we optimize and adapt it to achieve the best balance
of image quality and radiation dose for the particular needs
of children.”
Thanks to the collaboration with GE and dedicated effort of
the entire imaging team at SickKids, their new CT scanner
has become a successful part of realizing this philosophy. n
Karen Thomas, MB BCh MA MRCP FRCR FRCP(C), Radiologist, Hospital for Sick Children, Modality Lead, Body CT, Assistant Professor, University of
Toronto. Dr. Karen Thomas graduated from the University of Oxford Medical School, England and obtained the Membership of the Royal College of
Physicians (Paediatric) in 1993, Fellowship of the Royal College of Radiologists, UK in 1998 and the Fellowship of the Royal College of Physicians and
Surgeons of Canada in 2005. She has been a member of the body imaging group at the Hospital for Sick Children, Toronto, Canada since 2001.
She has developed a wide-ranging research interest in the practical, strategic, and educational aspects of radiation safety and dose reduction
in pediatric imaging with clinical impact on dose reduction and optimization of practice within the diagnostic imaging department at SickKids in
addition to contributing toward current international efforts to improve physician, manufacturer, and public awareness.
Dr. Thomas has received several Caffey awards from the Society for Pediatric Radiology for her work. She has been an invited speaker on various
aspects of pediatric radiation safety, including participation in the Image Gently 2011 ALARA CT Dose Reduction Conference, keynote radiation
safety speaker at the 2011 International Pediatric Radiology Congress, and presentations to the Ontario Hospitals Association, Ontario Association
of Radiologists and Radiological Society of North America.
Guila BenDavid, MRT(R), Manager – CT, Nuclear Medicine, PET/CT, Diagnostic Imaging. BenDavid joined SickKids in 1990
and in 1996, assuming the role of CT Manager. In 2010, BenDavid also took on the role of Manager in the Nuclear Medicine
department BenDavid is a member of several committees throughout SickKids and is considered a leader in Canada for
radiation dose reduction. Acquiring optimal images without administering more radiation than necessary is a priority for
BenDavid. She provides education to her Child Health Network colleagues and beyond through lectures at conferences, and
educational facilities. BenDavid has shared her expertise around the world with prestigious institutions throughout the US
and Canada, as well as in the UK, Turkey, Brazil, and India. BenDavid has been very involved in the Ontario government’s
initiative to reduce CT dose for pediatric patients, and is consulted by other centers to provide expert advice in
pediatric protocols.
The Hospital for Sick Children (SickKids) is recognized as one of the world’s foremost pediatric healthcare institutions
and is Canada’s leading center dedicated to advancing children’s health through the integration of patient care, research,
and education. Founded in 1875 and affiliated with the University of Toronto, SickKids is one of Canada’s most researchintensive hospitals and has generated discoveries that have helped children globally. Its mission is to provide the best
in complex and specialized family-centered care; pioneer scientific and clinical advancements; share expertise; foster
an academic environment that nurtures health-care professionals; and champion an accessible, comprehensive
and sustainable child health system. SickKids is proud of its vision for Healthier Children. A Better World. For more
information, please visit www.sickkids.ca.
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