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Dual-source Computed Tomography in Paediatric Congenital Heart Disease Patients
Dose Consideration The basic protocol of injection of the pulmonary arteries or for
Radiation exposure is a major public health issue. CT contributes systemic vascular enhancement is as follows. Using 80kV, the rate of
greatly to the population dose arising from medical exposure, injection can be as low as 0.5cc per second in neonates with a catheter
accounting for 35% of the dose delivered during diagnostic placed in the vein of the hand. A higher rate may be used in cases
examinations even though it represents only 4% of such studies. where a central catheter (femoral or jugular) is used. A power injector
Multislice CT offers even more diagnostic capabilities but tends to is routinely used to ensure a continuous and regular flow rate. In cases
increase the radiation dose due to routine use of thinner slice of peripheral injection, the rate of injection varies from 0.5 to 1cc/s
thickness, extension of the volume of acquisition or multiple-phase depending on the quality of venous access. The start delay in neonates
acquisitions. Following the ALARA principle (‘as low as reasonably and infants is 15 seconds for peripheral injection and 10 seconds for
achievable’), dose reduction is necessary, but examination quality must central venous injection. To be sure of having vascular contrast during
be preserved without losing diagnostic information.
5,6
the acquisition, we sometimes slightly increase the amount of contrast
medium in order to follow this rule:
The thorax is a low-attenuation region, although substantial dose
reduction during chest CT is feasible because of the high inherent time of injection = start delay + time of acquisition
contrast. In our centre, we decided to apply the ALARA principle as far as
possible in neonates and babies with CHD, and then apply some Using this rule, acquisition is never ‘too late’ for good vascular
systematic rules: enhancement, because acquisition ends with the end of the injection, so
the contrast medium is still in the peripheral veins when acquisition ends.
• systematic use of 80kV settings;
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• adaptation of the mAs to the child’s weight; and Precautions for Venous Access
• only one-phase acquisition when possible. Peripheric venous access is achieved in the paediatric unit. Right-arm
injection is preferable (but not mandatory) to avoid possibly striking
Eighty kilovolt protocols have been successfully performed to scan the artefacts on the innominate left brachio-cephalic vein. In some cases,
thorax of adults weighing less than 75kg without substantial loss of venous connections are congenitally different or surgically modified. It is
image quality.
8
Reducing the kilovoltage from 120 to 80kV decreases the important to have this information, when available, before the scan
radiation dose to 65% of that produced at the constant current setting, procedure as it may change the scan injection protocol. Venous
as radiation dose varies with the square of kV. This setting is sufficient visualisation may be realised at first pass, with a high concentration of
for high-quality images, as long as the mAs are adjusted according to contrast medium, or sometimes later, at the time of venous return. The
the child’s weight. We use 80kV as the standard kilovoltage setting in optimal injection protocol depends on each particular venous anatomy.
children.
7
Current exposure is adapted to bodyweight in neonates and
infants: for non-ECG-gated acquisitions we recommend 10mAs per kg Catheter permeability is checked before the injection. It is essential to
bodyweight up to 6kg. avoid any air injection during the scan procedure; all bubbles should be
removed when connecting the catheter to the power injector. Because
Using this protocol, radiation exposure is estimated to be 1mSv for a many patients with CHD have right-to-left shunt, air injection through
neonate, which is equivalent to the dose delivered by natural radiation venous access could cause systemic air embolism, with possibly fatal
over a six-month period. Radiation dose due to CT acquisition may be consequences. Extravasation of contrast may occur, with an incidence of
lower than the radiation dose delivered during conventional 1.4% in our centre in 2007. These rare complications were treated
angiography.
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Radiation dose associated with ECG-gated multislice CT is immediately without any consequence.
higher using the current retrospective mode: for example, in our
institution, in a one-year-old baby thermo-luminescent measurements Sedation
provide a dose level of 3.4mGy using ECG-gated acquisition. Coronary In our experience, general anaesthesia is never necessary. In neonates
angiography in the same patient was associated with a very similar we do not use any sedative drugs. In infants, we recommend oral or
radiation level (3.1mGy). Anatomical data acquired from CT may be intra-rectal sedation (or both) before the CT procedure to prevent baby
judiciously used to limit the number of views acquired using agitation during the acquisition, which may be responsible for poor
angiography, and can sometimes replace conventional angiography.
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image quality and, sometimes, need for re-examination. Sedation is
CT may then be advantageous in terms of reducing global radiation not always mandatory if the baby is quiet. Experienced technologists
exposure in CHD patients. are necessary in the CT room for good management of the babies:
precise knowledge of managing babies and a gentle attitude are of
The other advantage of 80kV settings is the ability to reduce the amount primary importance.
of contrast medium injected, because a lower kilovoltage is more
sensitive to contrast (iodine has a high atomic number) than the standard Our sedation protocol in infants includes intra-rectal midazom at a dose
120kV setting. of 0.3mg/kg given 15 minutes before examination. Additional sedative
drugs may be useful (hydroxyzine at a dose of 1mg/kg orally one hour
Injection Protocol before examination). With experienced technologists, the mean total
Dose injection must be adapted to the baby’s weight: in our institution, examination time in the CT room is between 15 and 20 minutes.
we currently use 2cc per kg of Iopromide (Ultravist) at a concentration of Qualified medical monitoring may sometimes be necessary during the
300mg/ml. We did not record any serious adverse reactions over a seven- examination, depending on the clinical condition of the baby. In all cases,
year period including more than 800 babies. oxygen saturation should be closely monitored.
INTERVENTIONAL CARDIOLOGY 17
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