FIG 21.16 Cardiac computed tomography (A) Axial image of a left pulmonary artery sling Arrow points to a nasogastric tube in the esophagus Arrowhead points to a narrowed trachea (B) Threedimensional volume-rendered computed tomography angiogram of an aortic coarctation stent (arrow) and a small poststent pseudoaneurysm (arrowhead) ▪ Vascular rings and slings ▪ Pulmonary venous anomalies (including pulmonary venous stenosis) ▪ Metallic implants ▪ Complex CHD in neonates and infants ▪ Coronary arteries72 The main disadvantage of cardiac CT is radiation exposure, which represents a greater concern in children Novel CT scanners enable acquisition of goodquality images with a minimal radiation exposure (approximately 1 mSv or less) Cardiac CT requires the use of iodine base contrast media, which are contraindicated when there is known allergy or severe renal impairment Radiation Dose Optimization in Children The current multidetector CT scanners and sequences have allowed a significant decrease in the radiation exposure The radiation dose is directly proportional to the tube current, the ECG gating technique used, and the duration of acquisition In traditional retrospective ECGgated acquisition, the radiation exposure is higher because the heart is irradiated for the entire duration of the cardiac cycle, with modulations in either diastole or systole, depending on the heart rate It is mostly used in adult patients and can be used both with single source or dual source CT In single source CT the table advancement in the scanner is slow (low pitch, approximately 0.2) to enable the same slice to be irradiated multiple times The disadvantage of this technique, which allows great spatial resolution, is that the radiation exposure is very high, and this has limited applications in children.73 The use of adaptive pitch in combination to dual source CT could potentially reduce the dose exposure, maintaining a good image quality even in presence of high heart rate.74 An increase in the pitch from 0.2 to 0.5 reduces the acquisition time and slice overlap, allowing a consistent reduction of the radiation dose.74,75 Prospective ECG triggering is a technique that allows dose reduction by irradiating the heart only in a predetermined phase of the cardiac cycle In children with heart rate greater than 80 beats/min, the data need to be acquired at end systole to minimize cardiac motion and an absolute trigger delay is set The application of iterative reconstruction techniques could improve further the image quality by reducing the noise and is a useful tool in pediatric setting.76 Comparing retrospective and prospective gated CT for assessment of coronary arteries in pediatric population, the effective radiation dose was significantly higher in the retrospective gated scans (3.8 mSv vs 0.7 mSv in children from 0 to 4 years).76 The disadvantage of the prospective triggering data is that the information acquired is specific for a single phase of the cardiac cycle, and thus evaluation of ventricular function and volumes is not feasible Non–ECG-gated CT is the most commonly used technique in infants and children It allows good spatial resolution and minimizes dose exposure The data can be easily used to reconstruct 3D images However, the image quality does not always allow optimal visualization of the coronary arteries Combination of two different protocols have been proposed with acquisition of a non–ECG-gated scan of the entire thorax followed by an ECG-gated acquisition targeting only the coronary arteries.77 The development of new dual source CT scanners has allowed a further reduction of the dose when prospective triggering is applied—with an effective dose of approximately 0.3 mSv, which is equivalent to the annual background radiation exposure.78 Contrast Agents in Cardiac Computed Tomography Cardiac CT in children and adults with CHD are almost always performed with administration of iodinated contrast, typically 1 to 2 mL/kg.71 The predominant risks associated with contrast media administration are allergic reaction and contrast-induced nephropathy (CIN) Adverse reaction to nonionic iodinate contrast media can vary from mild symptoms (such as itching rash, nausea, and warm sensation), moderate symptoms (including hypotension or hypertension, bradycardia or tachycardia, cutaneous reactions, and dyspnea) to severe reactions (laryngeal edema, anaphylaxis) In a large cohort of 12,494 young patients (up to 21 years of age) who underwent CT, 57 adverse reactions were identified (0.46% of the population).79 The risk of CIN is higher in patients with renal impairment Hydration before the contrast administration, as well as withdrawal of nephrotoxic drugs, are recommended to minimize the risk of CIN.71 ... image quality by reducing the noise and is a useful tool in pediatric setting.76 Comparing retrospective and prospective gated CT for assessment of coronary arteries in pediatric population, the effective radiation dose was significantly