Cardiac CT angiography in children with congenital heart disease

European Journal of Radiology 82 (2013) 1067–1082

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European Journal of Radiology journal homepage: www.elsevier.com/locate/ejrad

Review

Cardiac CT angiography in children with congenital heart disease Suvipaporn Siripornpitak a,∗ , Ratanaporn Pornkul a , Pongsak Khowsathit b , Thanarat Layangool c , Worakan Promphan c , Boonchob Pongpanich b a b c

Division of Diagnostic Radiology, Department of Diagnostic and Therapeutic Radiology, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, Bangkok, Thailand Pediatric Cardiac Unit, Department of Pediatrics, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, Bangkok, Thailand Pediatric Cardiology Unit, Queen Sirikit National Institute of Child Health, Bangkok, Thailand

a r t i c l e

i n f o

Keywords: Cardiac CT(A) Congenital heart disease Children Cardiac imaging

a b s t r a c t Cardiac imaging plays an important role in both congenital and acquired heart diseases. Cardiac computed tomography (angiography) cCT(A) is a non-invasive, increasingly popular, complementary modality to echocardiography in evaluation of congenital heart diseases (CHD) in children. Despite radiation exposure, cCT(A) is now commonly used for evaluation of the complex CHD, giving information of both intra-cardiac and extra-cardiac anatomy, coronary arteries, and vascular structures. This review article will focus on the fundamentals and essentials for performing cCT(A) in children, including radiation dose awareness, basic techniques, and strengths and weaknesses of cCT(A) compared with cardiac magnetic resonance imaging (cMRI), and applications. The limitations of this modality will also be discussed, including the CHD for which cMRI may be substituted. © 2011 Elsevier Ireland Ltd. All rights reserved.

1. Introduction Congenital heart disease (CHD) remains a major cardiac problem in the pediatric population. Cardiac imaging plays an important role in establish the diagnosis, interventional management, follow-up after palliative or corrective surgery. Various imaging modalities for the diagnosis of CHD progress from plain chest radiograph, 2-dimensional echocardiography and conventional cardiac catheterization to non-invasive advanced cardiac imaging, 3- and 4-dimensional echocardiography, transesophageal echocardiography, cardiac MRI (cMRI), and multidetector CT (MDCT). Echocardiography remains a first-line non-invasive imaging tool for establishing the diagnosis and follow-up in most patients. However, echocardiography has inherent limitations, including a limited acoustic window. This is particularly the case with post-surgical sternal wires and mediastinal scar tissue, and extracardiac vascular structures. Echocardiography is also an operator-dependent imaging tool with poorer spatial resolution than CT or CT angiography CT(A) [1,2]. Cardiac catheterization, the gold standard for cardiac imaging with hemodynamic assessment, is an invasive method which may cause death in up to 1% of neonates with complex CHD [2,3]. Its role has largely been

∗ Corresponding author at: Division of Diagnostic Radiology, Department of Diagnostic and Therapeutic Radiology, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, 270 Rama VI Road, Bangkok 10400, Thailand. Tel.: +66 2 201 1243; fax: +66 2 201 1297. E-mail address: [email protected] (S. Siripornpitak). 0720-048X/$ – see front matter © 2011 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejrad.2011.11.042

replaced by other advanced non-invasive imaging tools. Currently, cardiac catheterization is reserved for hemodynamic assessment, in cases with pulmonary hypertension and complex CHD in whom the data regarding pulmonary vascular resistance, oxygen saturation and chamber pressure are essential for surgical planning, and in whom interventional treatment is necessary [1]. Cardiac CT(A) (cCTA) is a non-invasive tool, with high image spatial resolution, and powerful 3-dimensional post-processing image reconstruction. This provides excellent anatomic information that can replace echocardiography and cardiac catheterization, particularly in the evaluation of extracardiac vessels and coronary arteries [4–6]. However, cCT(A) had poorer temporal resolution relative to other cardiac imaging modalities such as echocardiography, cardiac catheterization and cMRI [1]. A comparison of cardiac imaging modalities, echocardiography, cardiac catheterization, and cCT(A) is presented in Table 1. This review article will focus on the fundamentals and essentials for performing cCT(A) in children, including radiation dose awareness, basic CT(A) techniques, applications, and strengths and weaknesses of cCT(A) comparing with cMRI. We also discuss the limitations of this modality for evaluation of some CHD where cMRI could be a reasonable substitution. 2. MDCT scanner requirements for evaluating children with congenital heart disease A 64-slice MDCT is the minimum solution recommended for evaluating CHD and coronary artery disease in children because of reduction of the radiation dose and improvement of image

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Table 1 Imaging performance of cardiac CT(A) compared with echocardiography and cardiac catheterization. Parameters

Echocardiography

Catheterization

Cardiac CT(A)

Invasiveness Morbidity Acoustic window limitation Temporal resolution Spatial resolution 3-D post-processing Operator dependent Acquisition time Radiation exposure Risks of contrast medium Availability

No No Yes Highest Fair Yes Yes Depends on operator No No Most

Yes Yes No High Highest Yes No Long Yes Yes Least

No Yes No Fair High Yes No Very short Yes Yes Average

Table 2 Suggested cardiac CT protocols with weight-based radiation adjustment. Suggested cardiac CT protocols with weight-based radiation adjustmentc

Recommended parameters based on applications with a 64 s-MDCT (Siemens Medical Solutions)d

Weight (kg)

Weight (kg)

kVp

Non-ECG gated (mAs)

ECG-gated (mAs)

<20 21–25 26–30 31–35 36–40 41–45 46–50 51–55 56–60 61–65 66–70 71–75 76–80 81–85 86–90 91–100 101–110 111–120 120–140

80 80 80 80 80 80 80 80 80 80 80 80 80 100 100 100 100 100 100

15 20 20 25 25 30 30 35 35 40 40 50 50 60 60 70 80 90 100

50 75 75 100 100 125 125 150 150 175 175 200 200 250 250 300 350 375 400

0–3 4–6 7–10 11–15 16–20 21–25 26–30 31–35 36–40 41–45 46–50 51–55 56–60 61–65 66–70 71–75 76–80 81–85 86–90 91–100 >100

a b c d

Non-ECG gated

ECG-gated

kVp

Effective tube current (mAs/slice)a

kVp

Effective tube current (mAs/slice)b

80 80 80 80 80 80 120 120 120 120 120 120 120

80 100 130 160 190 220 80 100 100 120 120 140 140

80 80 80 80 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 140

280 320 400 520 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 800

Pitch 0.676, rotation time 0.5 s. Pitch 0.2, rotation time 0.42 s. Modified from Ref. [2]. Modified from Ref. [4].

Table 3 Suggested parameters for non-ECG cardiac CT based on applications with a 320-slice MDCT (Aquilion One, Toshiba Medical Systems, Japan).a Weight (kg)

kVp

mA

DLP.e (mGycm)

k factorb

Radiation dose (mSv)

≤3 >3–6 >6–9 >9–12 >12–15 >15–20 >20–30

80 80 80 100 100 100 100

100–120 120–150 120–150 150–180 150–180 160–200 200–400

10.9–33.0 32.9–41.2 23.7–48.3 33.1–108.3 33.1–108.3 53.2–120.3 110.4

0.039 0.039 0.026 0.026 0.018 0.018 0.013

0.42–1.29 1.28–1.61 0.61–1.26 0.86–2.82 0.59–1.95 0.96–2.16 1.65

a b

Based on single institution, using 0.35 s rotation scan time and in a volume mode. k factor used for calculation radiation dose followed the age base.

Table 4 Suggested parameters for ECG-gated acquisitions based on applications with a 320-slice MDCT (Aquilion One, Toshiba Medical Systems, Japan).a Weight (kg)

kVp

mA

RRb

DLP.e (mGycm)

k factorc

Radiation dose (mSv)

≤10 >10–15 >15–20 >20–40 >40–60

80 100 100 100 100

200 200 250 300 400

2RR 2RR 2RR 1RR 1RR

243.1 212.3 275.4 258.1 113.81

0.026 0.026 0.018 0.013 0.014

6.32 5.52 4.95 3.35 1.63

a b c

Based on single institution, using 0.35 s rotation scan time and in a volume mode. Numbers of RR collection in a prospective ECG gating mode. k factor used for calculation radiation dose followed the age base.

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Fig. 1. Comparison of image quality from 320 slice-MDCT (0.35 s rotation scan time), using the different kVp and mA in a girl with underlying pulmonary atresia. (A and B) Coronal reformation and volume rendered (VR) CTA images using 80 kVp, 120 mA and (C and D) coronal reformation and VR CTA images using 100 kVp, 200 mA. The images of the major aortopulmonary collaterals (*) obtained by using the lower kVp and mA (A and B) are diagnostic although there is less contrast resolution compared with the image obtained with higher kVp and mA (C and D). # = pulmonary artery.

quality by using noise reduction algorithms. With the development of fast gantry rotation speed and wider collimation coverage in 64-slice MDCT, the scan duration is reduced, temporal resolution is increased, and hence motion artifacts are decreased [4]. The newer generation MDCT scanners, 256 to 320/640-slice MDCT and dual source MDCT, result in approximately 0.3 second (s) scan time, allowing a reduction in radiation exposure of approximately 50–70% compared with the 64-slice MDCT [4,5]. z-Axis volume coverage is improved up to 12 cm for 256- and up to 16 cm for 320/640-slice MDCT in a single gantry rotation with isotropic resolution, eliminating the restriction of 64-slice MDCT [4]. The improvement of gantry rotation time and increased volume coverage reduce the scan time, improve temporal resolution and minimize radiation exposure [4]. Therefore, the newer scanners have the potential to acquire images without sedation or breathhold [7]. Functional assessment is only available with 256- and 320/640-slice MDCT [8]. However, there is no definite indication to perform functional assessment with cCT(A) in pediatric and adolescence with CHD since radiation exposure is a major concern, and serial/multiple scans would be necessary for this task.

3. Sedations and patients’ preparations Sedation prior to scanning prevents agitation during contrast delivery to children, avoiding the need for repeated examinations with increasing the radiation burden. In our experience with a 320slice and a 64-slice MDCT, sedation is not necessary in neonates, calm babies, and children older than six years of age. In our institution, the sedation protocol includes intravenous Midazolam diluted at a dose of 0.1 mg/kg/dose prior to the scan. Additional sedative drugs such as a diluted Fentanyl at a dose of 1 mg/kg/dose can be used, depending on the individual case. But there are different approaches, some use Propofol or other drugs for (deep) sedation, particularly if a breath-hold is required. The cardiovascular anesthesiologist or pediatric cardiologist is responsible for sedation and monitoring the patients. Vital signs and blood oxygen saturation must always be monitored. In contrast to cMRI, general anesthesia for breath-holding is rarely required when performing cCT(A). It may be indicated only in patients younger than eight years of age who undergo evaluation of the coronary arteries (e.g., Kawasaki disease or

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Fig. 2. Systemic arterial supplies to the lungs in a 15-year-old female with underlying pulmonary atresia and ventricular septal defect. (A) Axial CTA image at the level of pulmonary artery reveals absence of the main pulmonary artery. Note the confluence of the hypoplastic right pulmonary artery (RPA) and the left pulmonary artery (LPA). (B) Volume rendered CTA image in the anterior view shows a major aortopulmonary collateral (M) which originates from the anterior wall of aorta (AO) and gives two branches which supply the left lung (M2) and the right lung (M3). The M2 branch communicates with the LPA (arrowheads) while the M3 branch supplies the right lung directly. Another collateral (M1), originating from the left subclavian artery (LSA), is also shown. AA = ascending aorta, DA = descending aorta, SVC = superior vena cava.

anomalous coronary arteries) or post-surgery queries related to (re)implanted coronary arteries (e.g., arterial switch in transposition of great arteries). Other institutions recommend general anesthesia in children younger than seven years of age [9]. Older children who undergo evaluation of the coronary arteries require only breath-hold during the acquisition; evaluation of extracardiac structures including the aorta and pulmonary vessels does not strictly require breath-holds. However, in all these queries, breathhold is recommended in older children to improve image quality. In neonates and younger infants, age adapted feeding strategies can prove very helpful and will help to keep the child calm during an acquisition, additionally wrapping the child in a blanket to diminish motion artifacts (“feed and wrap approach”). Other means of immobilization can be helpful, too. 4. Contrast medium injection and venous assessment Non-ionic, low- or iso-osmolar iodinated contrast agents are preferable because they are safer [10]. Impaired renal function is an absolute contraindication for iodinated contrast medium

(CM) administration. The risk of contrast-induced nephrotoxicity is increased in neonates due to the immaturity of renal function [5], so the volume of CM should be kept as small as possible but should be enough to maintain interpretable image quality, particularly considering the relatively higher circulating blood volume per body weight (BW) in kilogram (kg) in this age group. With the use of 320-slice MDCT, the total amount of CM is routinely based on the BW, routinely 1.0 ml/kg of 300 mg iodine/ml CM for the noncoronary imaging and 1.0–1.2 ml/kg of 350–370 mg iodine/ml CM for CT(A) of the coronary arteries. Others recommend the dose of 2 ml/kg of 300 mg iodine/ml CM and 1.7 ml/kg of 350 mg iodine/ml CM for children under 20 kg BW or 1.7 ml/kg of 300 mg iodine/ml CM and 1.4 ml/kg of 350 mg iodine/ml CM for children between 21 kg and 80 kg BW [4]. Generally, the faster the MDCT acquisition the smaller the volume of CM required, as it is useless to keep on injecting CM once the scan has actually started. Cardiac CT(A) usually requires a high flow rate of the intravenous iodinated CM administration, therefore the site of peripheral intravenous catheter should be carefully examined to prevent contrast leakage. The antecubital vein is the most preferred access site.

Fig. 3. Successful coil embolization of a large major aortopulmonary collateral artery (MAPCA) in a 16-year-old male with underlying Tetralogy of Fallot (TOF). (A) Volume rendered (VR) CTA image in the anterior view reveals presence of the main pulmonary artery (PA) and confluence of the pulmonary artery. Note a large MAPCA (M) originates from the descending aorta (DA) and supplies the right lung. (B) VR CTA image of the MAPCA in the posterior oblique view reveals presence of focal stenosis (arrow 1) proximally and communicates (arrow 2) with the descending branch of the right pulmonary artery (*). (C) VR CTA image in the posterior view demonstrates occlusion of the MAPCA (M) (arrow 1), indicative of successful coil embolization. LA = left atrium, RPA = right pulmonary artery, LPA = left pulmonary artery, # = ascending branch of RPA.

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Fig. 4. An anomalous coronary artery in a 7-year-old male with underlying Tetralogy of Fallot (TOF). (A and B) Volume rendered CTA images in the superior (A) and anterior (B) view show a single coronary artery originating from left aortic sinus. The right coronary artery (RCA) is the 1st branch and runs between the aorta (AO) and main pulmonary artery (PA) before entering the right atrioventricular groove. The 1st branch of the RCA is a prominent conal branch (arrowheads). The single coronary artery continues as a left coronary artery (LCA), where it bifurcates into the left anterior descending artery (LAD) and the left circumflex artery (LCX). A prominent conal branch (arrowhead) originates from RCA runs across the right ventricular outflow tract (RVOT). RA = right atrium, RV = right ventricle, LV = left ventricle.

Fig. 5. An anomalous coronary artery in a 16-year-old man with double outlet right ventricle and pulmonic stenosis, post Cooley shunt. (A) Axial CT scan shows single coronary artery (arrowhead) originates from left aortic sinus (arrowhead). (B) Volume rendered CTA image in the anterior view shows the single coronary artery continues as a left anterior descending artery (LAD). The right coronary artery (RCA) also originates from the single coronary artery and runs across the aortic root before entering right atrioventricular groove. Note a prominent conal branch (MB) runs across right ventricular outflow tract (RVOT). The Cooley shunt (*) is a shunt connecting between the ascending aorta and pulmonary artery (PA). AO = aorta, LAA = left atrial appendage, RA = right atrium, RV = right ventricle, LV = left ventricle.

Selection of the intravenous catheter size depends on assessable venous puncture site, usually 22G–24G for younger children, 20G–22G for older children and 18G or larger for adolescents. Practically, the peripheral venous site should be on the contra-lateral side of the suspected pathology to prevent streak artifact from iodinated CM, or meticulous bolus timing with a thorough saline flush clearing the feeding vessel from high concentrated CM must be granted. A peripheral venous access in the leg is preferable for conditions that require assessment of any atypical venous supply (such as after Fontan operation, Kawashima procedure, heterotaxy associated azygos continuation of an interrupted inferior vena cava), since this will ensure a homogeneous contrast enhancement in these veins and can help to avoid repeating scans due to delayed venous enhancement from post-operative anatomical changes or complications. Others recommend placing the peripheral venous

catheter in the ipsilateral hand or forearm of the affected side for evaluation of the upper extremity venous system; then a low concentration or even diluted CM might become necessary [4]. A different approach for sufficient opacification of all systems within one acquisition might be a split bolus technique or a simultaneous injection from two different injection sites. The rate of contrast injection varies and depends on the size of the angio-catheter and quality of the venous access; we usually choose the highest rate possible. The routine injection rate is from 0.4 to 1.5 ml/s through a smaller gauge catheter and from 2.0 to 4.0 ml/s through a larger gauge catheter [11]. A dual power injector with saline flush is preferred over hand injection because constant CM delivery results in better contrast enhancement and thus a better contrast to noise ratio [4,5]. The volume of saline flush in a non-ECG gated acquisition is about 5–10 ml plus another 10 ml

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Fig. 6. Aneurysms of the coronary arteries in a 12-year-old male with underlying Kawasaki disease. Volume rendered CTA image in the superior view shows multiple fusiform, large coronary aneurysms at the left main coronary artery (LM), left anterior descending artery (LAD), right coronary artery (RCA), and left circumflex artery (LCX). RV = right ventricle, LV = left ventricle, RA = right atrium, LA = left atrium.

to compensate for the volume of the extension and connection tubes. In coronary imaging, we routinely increase the saline volume to 10–20 ml to properly wash-out any CM from the right heart chambers, plus another 10 ml to compensate for the volume of the extension and connection tubes. The optimal timing of image acquisition is crucial and more complicated in children with CHD due to their abnormal hemodynamic status. Scan initiation and CM injection synchronization is accomplished with real-time bolus tracking either visually with a manual start or automated using a time-density analysis by placing the region of interest (ROI) in the proximal descending thoracic aorta to monitor the vessel attenuation in a real-time. The reason why we place the ROI at the proximal descending thoracic aorta is because it is a vascular structure that can be easily visualized and it is located adjacent to the pulmonary artery and vein. Since complex CHD requires anatomical details of both arterial and venous systems, we typically use a “manual” bolus tracking method; an acquisition is triggered when CM opacification within both the aorta and adjacent pulmonary vessels is achieved. Other authors advocate the use of a preset threshold opacification of the ROI at 100–200 HU to initiate the acquisition, which will only work for larger sized vessels and additionally depends on shunt volume and CM concentration [11]. Additionally, some scan delay (i.e., the time interval between the time of optimum vascular enhancement and the time acquisition is triggered) must be considered: with the 64-slice MDCT there is an approximately 2–4 s scan delay due to inherent interscan and image reconstruction delays. However, there is no scan delay with the 320-slice MDCT except the time used for instructing the patients to perform breath-hold, as well as for potential tube repositioning and re-adaptation (e.g., single slice to volume mode) which can take up to 4 s. Make sure that for these tracking maneuvers, no continuous high dose radiation is applied, as usually a lower dose with interval imaging only at the (central) slice in the area of interest is sufficient and can significantly reduce the applied effective dose. 5. Radiation awareness The most important consideration regarding cCT(A) in children with CHD is how to keep the radiation dose as low as reasonably

Fig. 7. Post coronary artery bypass graft (CABG) in a 16-year-old male with Kawasaki disease. (A) Volume rendered (VR) CTA image of coronary artery in the anterosuperior view shows CABG to posterior descending artery branch of right coronary artery (arrow, PDA-RCA). (B) VR CTA image in the anterior view shows a fusiform aneurysm (arrowhead) at proximal left anterior descending aorta (LAD). RA = right atrium, LA = left atrium, LV = left ventricle, LM = left main coronary artery, LCX = left circumflex artery, PL = posterolateral branch of right coronary artery.

achievable (ALARA) with the balance of interpretable image quality which partly depends on individual radiologist preference and experience – but eventually needs to be diagnostic, not the optimum. Image quality is determined by spatial and contrast resolutions and image noise. The spatial resolution depends on the detector width or beam collimation while the contrast resolution is governed by the peak kilovoltage (kV) and iodine attenuation [4,12]. The thinner collimation improves spatial resolution to isotropic image resolution while increasing the necessary radiation dose. The spatial resolution degrades with the greater pitch value, but with the benefit of lower radiation dose and shorter scan duration [12]. We routinely use a pitch value of one when performing non-gated cCT(A). Contrast resolution is degraded when choosing the lower kV, however radiation dose is significantly reduced. Iodinated CM has the peak attenuation at 80 kV, so that cCT(A)techniques will often try to use lower kV as long as penetration is sufficient. One should keep the kV as low as possible

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Fig. 8. Supracardiac type total anomalous pulmonary venous return in a 16-day-old male who presented with dyspnea. (A) Axial CTA image shows small left ventricle (LV) and left atrium (LA) with enlarged right atrium (RA) and right ventricle (RV). Noted no demonstrable pulmonary veins drain into left atrium. The common pulmonary vein (CPV) lies close to the posterior wall of LA. (B) Volume rendered CTA image in the posterior oblique view shows all pulmonary veins (1–4) drain to the CPV which continue drains into a dilated vertical vein (VV), and subsequently drains into a left innominate vein (LINV), where it empties into the superior vena cava (SVC). Note a severe stenosis at junction between the CPV and the VV (arrow) associated with post stenotic dilatation of the VV. LVOT = left ventricular outflow tract, LSPV = left superior pulmonary vein, AO = aorta, IVC = inferior vena cava, RPA = right pulmonary artery, LPA = left pulmonary artery.

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Fig. 9. Infracardiac type total anomalous pulmonary venous return in a 19-day-old male with underlying heterotaxy syndrome. (A) Volume rendered CTA image in the coronal plane shows all pulmonary veins (1 = right superior pulmonary vein, 2 = right inferior pulmonary vein, 3 = left inferior pulmonary vein, and 4 = left superior pulmonary vein) abnormally form a common pulmonary vein then drains downwardly into a vertical vein (VV). The vertical vein descends below the diaphragm and drains into nearly close ductus venosus which still demonstrates the streak of contrast medium passes through the narrow ductus venosus (arrowhead). There is a large collateral vein (COV) connects (arrow) to a vertical vein which serves as a site for diverting blood that cannot pass through the narrow ductus venosus. (B) Subvolume maximum intensity projection CTA image in the sagittal plane shows the streak of contrast medium passes from the nearly close ductus venosus (arrowhead) and drains into inferior vena cava (IVC). AO = aorta.

because the radiation dose increases exponentially with kV [12]. Image noise degrades image resolution and noise will increase when reducing the tube voltage and detector width which can be compensated by increasing the tube current (mA) and exposure time (s) to provide adequate numbers of photons used to generate sufficient image resolution. Tube current is linearly related to radiation dose [12]. Note that the 320-slice MDCT (Toshiba) and Siemens MDCT are exceptions [4]. Methods to reduce radiation exposure include [2,4,9,12,13]:

(3) reduce kV to 80 in children with a weight less than 25 kg for non-gated acquisition; (4) adjust tube current (mA s) according the body weight; (5) deliver iodinated CM in a proper fashion as detailed above; (6) increase pitch and table speed; (7) avoid ECG-gated acquisitions – particularly retrospective gating; preserve this technique practically for coronary artery imaging only; (8) avoid multiphase examinations.

(1) volume coverage as small as necessary; (2) remove all metallic instruments from the chest wall if possible, particularly for scanning with automated tube current modulation;

For non-gated neonatal acquisitions, kV should be kept at 80 kV because radiation dose is reduced by 65% when using constant current settings, without substantial loss of image quality; furthermore iodine has its peak attenuation at this level allowing for

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Fig. 10. A 2-year-old male with persistent levoatrial cardinal vein (A and B). (A) Volume rendered CTA image in the posterior view and (B) a curve multiplanar reformation CTA image show right superior (1) and right inferior (2) pulmonary veins drain directly to left atrium (LA) while the left inferior (3) and left superior (4) pulmonary veins form a common trunk before draining to the LA. Noted the levoatrial cardinal vein (LACV) connects between the left superior pulmonary vein (4) and left innominate vein (LINV). SVC = superior vena cava, AO = aorta, RPA = right pulmonary artery, LV = left ventricle, RA = right atrium, RV = right ventricle.

an optimal dose-contrast relation [13]. The recommendation for adjusting the tube current is 10 mA s per kg BW up to 6 kg [9]. The radiation dose with prospectively ECG-gated acquisitions is about three times higher than the non-gated acquisition, owing to the requirement of higher kV and mA s, and the pitch value of less than 1 [2]. In addition, retrospective ECG-gated acquisitions are about 3–4 times the radiation dose of the prospective ECG-gated acquisition [5]. ECG-pulse tube current modulation should be applied to reduce the radiation dose up to 30% [12]. In a neonate with a heart rate of 120 beat per minute (bpm), the radiation saving reaches zero [2]. A beta-blocker may not be applied in most children and this limits the application of the prospective gating acquisition. In addition, some authors have mentioned the limitation of applying the tube current modulation technique in neonates due to inhomogeneity of the tissue in the x and y-axes, and the short scan length (in the z-axis) causing increasing image noise [2]. Therefore, the coronary artery CT(A) should be avoided in newborn and young infants because the faster the heart rate, the more the radiation exposure. However, Stolzmann et al. demonstrated that when performing

Fig. 11. Double aortic arch in a 4-month old female presented with persistent respiratory wheezing. (A) Volume rendered CTA image in the posterior oblique view demonstrates double aortic ach. The larger arch (#) is on the right side and in the higher position while the smaller arch (*) is on the left side and in the lower position forming a “vascular ring”. Note a focal stenosis of the smaller arch (arrow). (B) Coronal reformation in the lung window demonstrates distal tracheal stenosis (arrow) due to an external compression by the larger arch (#) on the right and smaller arch (*) on the left. AA = ascending aorta, DA = descending aorta.

prospective ECG-gating with a dual-source MDCT, there was no significant difference in image quality, but there was significant reduction in radiation dose compared to 100 kV (1.2 mSv ± 0.2) and 120 kV (2.6 mSv ± 0.5) [14]. Recent study determining coronary artery anatomy after Jatene operation performed with 64slice MDCT had a radiation dose of 4.5 ± 0.5 mSv [15]. In another study with the use of a gated acquisition on a dual-source scanner, the radiation dose was 5 ± 3.9 mSv [16]. To date, there are no large published data of radiation dose of coronary artery imaging in children with a 320-slice MDCT. The suggested radiation parameters for weight-based voltage and tube current from one institution compared with the recommended parameters for a 64-slice MDCT (Siemens Medical Solution, Malvern, PA, USA) are listed in Table 2 [2,4]. The suggested radiation parameters and dosing in patients with CHD using a 320-slice MDCT from a single institution are listed in

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Fig. 12. Jatene operation in a 2-year-old male with underlying D-transpositional of great artery (D-TGA). (A) Axial CTA image at the level of great vessels shows ascending aorta (AA) lies posterior to the pulmonary artery bifurcation with compression on the proximal left pulmonary artery (LPA) by the relocated aorta. Note a focal stenosis of main pulmonary trunk (PA) at the bifurcation. (B) Volume rendered CTA image in the anterosuperior view reveals the focal stenosis of the LPA and PA in relationship with the aorta and heart. The stenosis LPA lies anterior to the relocated aorta. There is dilatation of the right ventricular outflow tract (RVOT). RPA = right pulmonary artery, LCCA = left common carotid artery, LSA = left subclavian artery, RINNA = right innominate artery, RV = right ventricle, LV = left ventricle, DA = descending aorta.

Tables 3 and 4. The data is based on our 276 patients from February 2009 to February 2011, with the age and weight ranging from <1 day to 19 years and from 4.6 kg to 88 kg BW, respectively. The study divided into non-ECG gated acquisition of 236 studies (Table 3) and ECG-gated acquisition of 40 studies (Table 4). The parameters should be altered and varied, depending on the scanner, CM administration, and underlying problem or query. One should always consider the difference in image resolution when applying 80 or 100 kV (Fig. 1). 6. Cardiac CT(A) techniques The cCT(A) comprises two acquisition modes, non-ECG gated and ECG-gated acquisitions. Since echocardiography is an excellent tool for delineation of the intracardiac anatomy and pathology, non-ECG gated acquisition is therefore mainly used for evaluation

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of extracardiac structures while the ECG-gated acquisition is used primarily for coronary artery imaging. Non-ECG gated acquisition of the heart should be acquired with an attempt to reduce motion and pulsatile artifacts by choosing the highest appropriate pitch, wider collimation coverage, and fastest available gantry rotation speed [4,8]. The scanning range coverage depends on the clinical indications. We routinely cover the upper abdomen in the heterotaxy and limit the coverage to the chest in other CHD. There are two modes of ECG-gated acquisition, prospective and retrospective ECG-gating, either with or without applying tube current modulation. A retrospective ECG-gated acquisition acquires the data for the whole R-to-R interval of a cardiac cycle and hence provides multi-segment reconstruction, better temporal resolution, and global left ventricular function [5,14]. This acquisition results in three to four times the radiation dose of a prospectively ECG-gated acquisition, but provides reliable image quality at heart rates up to 170 bpm [5]. A tube current modulation mode is a radiation dose saving mode for the retrospective ECG-gating in which the tube current will be reduced in the phase where image quality is not required to be the optimum [14,17]. This mode should be applied for all children whenever possible. A prospective ECG-gating mode or “step and shoot” acquisition is a dose saving mode which acquires data in a narrow predefined cardiac phase and therefore applies radiation only at a preselected part of the cardiac cycle [14]. Selecting the proper cardiac phase is important, usually at during late-diastole in adults and end-systole in pediatrics [9,18,19]. It limits the functional evaluation due to suboptimal images when low tube current is applied [14]. Image degradation occurs when the heart rate is greater than 70 bpm, heart rate variation is greater than 10 bpm, and when the body mass index is greater than 30 kg/m2 [6,9,18]. For coronary artery imaging, volume coverage should be limited from the subcarinal area to the cardiac apex. With a 320-slice MDCT, 320 × 0.5 mm detector elements, 350 ms rotation time and a 16-cm anatomic coverage, coronary artery imaging in children can be accomplished with single shot and hence diminish radiation exposure. We routinely attempt to acquire only one or two heart beats irrespective of how high the heart rate, in order to reduce radiation dose to children – although singlebeat acquisition is recommended only when the heart rate is less than 65 bpm, a two-beat acquisition is recommended when the heart rate is about 65–75 bpm and a three-beat acquisition is recommended when the heart rate is greater than 75 bpm [20]. The images are routinely acquired in a retrospective gating manner with tube current modulation mode if the heart rate is high and in a prospective gating mode if the heart rate is below 80 bpm. Children have an intrinsically faster heart rate, potentially causing artifacts and image degradation. However, preliminary data reveal that dual-source MDCT, 256- and 320-slice MDCT can provide a high heart rate independent image quality with the retrospective ECG-gating mode which cannot be provided with 64-slice MDCT [8,21].

7. Reading cardiac CT(A) The basic segmental approach described by Van Praagh is fundamental for interpretation of the anatomy of children with complex CHD [22]. The approach consists of three anatomical descriptions (the viscero-atrial situs, the ventricles, and the great vessels morphology) and two connections (atrioventricular and ventriculoarterial). The pulmonary veins, systemic veins, and associated other cardiac anomalies should be carefully evaluated. There are limitations in evaluation of the valves apparatus in non-ECG gated images. The details of how to determine the anatomy of the segmental approach are not included in this review.

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Fig. 13. Post-Rastelli operation in a 16-year-old-woman with situs inversus, double outlet right ventricle (DORV), and valvular pulmonic stenosis. (A) Subvolume maximum intensity projection CTA image in the coronal oblique view shows evidence of both great arteries connected to the right ventricle (RV), consistent with DORV. The conotruncal septum (arrow) is situated between the enlarged aortic valve (two arrowheads) and the hypoplastic pulmonic valve (one arrowhead). There are atrio-ventricular and ventriculo-arterial discordance with conduit (C) connections between the left ventricle (LV) and the distal main pulmonary artery (PA), hemodynamic is as corrected transposition of great artery. Note a focal stenosis at the distal anastomosis between the thick calcified conduit (C) and PA. (B) VR CTA images in the anterior oblique view without post-processing removal of the conduit shows the left circumflex artery (LCX) and left anterior descending artery (LAD) directly originate from the anterior aortic sinus and have normal course. (C) VR CTA images in the anterior oblique view with the post-processing removal of the conduit shows the right coronary artery (RCA, arrow) has its course underneath the conduit (C) which was removed during post-processing reconstruction. AO = aorta, RA = right atrium, LA = left atrium, LAA = left atrial appendage.

8. Cardiac CT(A) compared to cardiac MRI While both cCT(A) and cMRI give excellent spatial resolution, cCT(A) has higher spatial resolution while cMRI has a better temporal resolution [2,23]. Cardiac CT(A) is a rapid scanning tool with powerful 3-dimensional post-processing image reconstruction options. It is a more applicable imaging tool for critically ill children and in the emergency setting – compared with cMRI which has a more complex setting and long scanning time [5,11,24]. Although, radiation exposure and CM risks are a weaknesses of cCT(A), this modality typically avoids the risks of general anesthesia. Cardiac MRI has the risk of nephrogenic systemic fibrosis (NSF) from administration of gadolinium-based agent [1,24], although reported cases of NSF in children are rare [14,24]. Cardiac CT(A) can be an alternative imaging tool in children who have pacemaker, internal cardiac defibrillator, or aneurismal clip (all are contraindicated for MR scanning). While cardiac valve prostheses, surgical clips, and occlusion devices are not contraindicated for cMRI, cCT(A) is sometimes preferable because these implants may cause signal loss and limited image quality with cMRI [24]. Cardiac CT(A) provides anatomical evaluation and has great benefit for small size vessels that require high spatial resolution, e.g. major aortopulmonary collateral arteries (MAPCAs) and coronary

arteries, while cMRI provides both anatomic and hemodynamic information (e.g. ventricular and valvular function, flow and shunt assessment, myocardial perfusion and viability) [1]. Cardiac CT(A) is the modality of choice for evaluation of lung and airway abnormalities which may occur concomitantly with complex CHD [5,24]. Therefore, cCT(A) gives a more comprehensive assessment particularly in post-operative patients in whom assessment of both lungs and airway are important to determine the source of complications [11]. Cardiac MRI is better in defining intracardiac structures, compared with non-EKG gating cCT(A) (Table 5) [5,11]. 9. Applications of cardiac CT(A) in congenital heart disease 9.1. Pulmonary arteries and systemic-blood-supply to the lungs The anatomical detail of the pulmonary artery (PA) and its branches is important in Tetralogy of Fallot (TOF), because there is either central or peripheral pulmonary artery stenosis in about 10% of the patients [11,25]. Pulmonary atresia is also a common CHD referred for evaluation of the PA and systemic-blood-supply to the lungs [11]. CT pulmonary angiogram (CTPA) is useful for PA assessment because the echocardiography can be limited in evaluation of the branch pulmonary arteries, and pulmonary angiography

Table 5 Comparison between cardiac CT(A) and cardiac MRI in evaluation of CHD. Parameters

Cardiac CT(A)

Cardiac MRI

Radiation exposure Risks from enhancing agents Require for general anesthesia Examination time Cardiac pacemakers and defibrillators Spatial resolution (mm) Temporal resolution (ms) Coronary artery evaluation Functional evaluation Flow quantification Myocardial perfusion and viability Intracardiac anatomy Mediastinum, lungs and airways Evaluation of calcified conduit Critically ill children Ease in emergency access

Yes Yes Less frequent Very short (10–20 s) Not contraindicated 0.4–0.6a 90–180 Better Require further evaluation No Require further evaluation Inferiorb Yes Yes More applicable Less complex setting

No Yes More frequent Time consume (≥50 min) Contraindicated 1.0–2.0 20–50 With limitation Modality of choice Modality of choice Modality of choice Superior No With limitation Less applicable More complex setting

a b

64-Slice MDCT. Non ECG gating CT.

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Table 6 Indications for cardiac CT(A)and cardiac MRI in evaluation CHD.

Intracardiac morphology 1. Heterotaxy and complex CHD 2. Criss-cross AV connection 3. Conjoined twin Aorta 1. All arch anomalies 2. Coarctation of aorta, pre-post-repaired 3. Vascular ring 4. Vasculopathy such as Marfan syndrome Coronary arteries 1. Anomalous coronary arteries 2. Kawasaki disease 3. Post surgery coronary arteries 3.1 Repaired coronary fistula 3.2 Coronary artery bypass graft Pulmonary arteries 1. TOF and pulmonary atresia 2. MAPCAs and collaterals to lungs 3. Peripheral pulmonary arteries 4. Post stent placement 5. Pulmonary sling Pulmonary veins 1. TAPVR or PAPVR or mixed form 2. Abnormal pulmonary veins such as stenosis or hypoplasia 3. Scimitar syndrome Post-palliative surgery 1. Blalock-Taussig and central shunts 2. Kawashima operation 2. Glenn operation Post-corrective surgery 1. Repaired TOF 2. Fontan operation 3. Rastelli operation 4. Ross operation 5. Jatene operation Intracardiac lesion and myocardium 1. Cardiac tumor 2 Cardiomyopathy 3 Myocardium viability study 4. LV aneurysm

cCT(A)

cMRI

√ √

√ √ √

√ √ √ √

√ √

√ √ √ √ √

√

√ √ √ √ √

√ √

√ √

√

cMRI can determine only proximal segment

cCT(A) has better spatial resolution

Evaluation of lung pathology

√ √ √

√ √ √

√

√ √ √ √ √

can be limited in the obstructed right ventricular outflow portion since the catheter cannot pass beyond the atretic pulmonic valve. Accurate assessment of PA status can be achieved by nonECG gated acquisition with additional oblique reformatted images along both the right and left PA. Important information includes the presence and confluence of the central PA, size and arborization, status of the peripheral PA, and presence of dual pulmonary circulation (Figs. 2 and 3), which all play a major role in determining the prognosis and mode of surgical treatment. CTPA can detect PA not demonstrated during cardiac catheterization and can accurately detect the diminutive central PA [11,26] (Fig. 2A). The 3D post-processing images provide diagnostic mapping to guide the interventionists and help cardiovascular thoracic surgeons plan with coil embolization or unifocalization. In addition, CTPA is useful in following patients with post-coil embolization (Fig. 3C). Abnormal systemic-blood-supply to the lungs occurs mainly in pulmonary atresia with poor development of PA as another source of pulmonary blood supply [27]. The source of abnormal systemic arterial supplies to the lungs are derived from: (1) patent ductus arteriosus; (2) major MAPCAs which may or may not communicate with the native PA (Figs. 2B and 3B); (3) bronchial arteries; (4) direct origin of the PA from the ascending aorta (anomalous origin of PA from the ascending aorta); and (5) other systemic arteries such as connection between the coronary arteries and PA [11,27]. All types of collaterals can be depicted by cCT(A) as accurately as by conventional angiography [11,26].

cMRI provides functional evaluation Evaluation of airway compromise Evaluation of branch vessel and coronary artery

Evaluation of airway compromise

√

√

Benefits

√ √ √ √

9.2. Coronary arteries Although, non ECG-gated cCT(A) is sufficient to visualize the origin of the coronary arteries in children, ECG-gating is the method of choice to retrieve motion-free images. Anomalous coronary arteries can be found either as isolated conditions or more frequently in association with CHD [14]. A variety of coronary anomalies detected by coronary CT(A) have been reported in many studies, including a recent study of 3236 patients [28]. Anomalies of the course of the coronary arteries vary in their clinical course. The interarterial course is considered a malignant type as it may cause sudden death due to extrinsic vascular compression [29] (Fig. 4A). Approximately 2–9% of patients with TOF have an anomalous origin of the coronary artery, of which the left anterior descending coronary artery (LAD) originating from the right coronary artery (RCA) and crossing the right ventricular outflow tract (RVOT) is a major concern for surgical correction [30]. Other anomalous coronary arteries found in TOF are a prominent conal branch from RCA (Fig. 4B), paired anterior descending arteries originating from the left and right coronary arteries, and single coronary artery which gives both RCA and LAD (Figs. 4 and 5). Mucocutaneous lymph node syndrome, known as Kawasaki disease, is an acute self-limited, multisystemic panarteritis of unclear etiology. Cardiac sequelae include coronary artery aneurysm or ectasia, thrombosis, stenosis or occlusion of coronary arteries, and myocardial infarction [31,32]. Involvement of the coronary arteries

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Fig. 15. Waterston–Cooley shunt in a 32-year-old female with underlying TOF. Volume rendered CTA image in the anterosuperior view shows a Waterston–Cooley shunt seen as short segment shunt (arrow) between the ascending aorta (AO) and right pulmonary artery (RPA). Note the kinking and focal narrowing of RPA at mid part of the RPA with post-stenotic dilatation of the distal RPA. LPA = left pulmonary artery, PA = main pulmonary artery.

usually occurs in cases of delayed treatment with ␥-globulin. Approximately 49% of the patients who have involvement of the coronary artery regress spontaneously within 2 years after onset [33]. Coronary aneurysms are categorized as small (<5 mm internal diameter), medium (5–8 mm internal diameter), and giant (>8 mm internal diameter) [31,34]. The disease affects mainly the proximal coronary artery. The left main coronary artery is most commonly involved [35]. Coronary artery CT(A) is useful to assess the diameter, extension and location of coronary artery aneurysm, and to depict the presence of any intraluminal thrombus or stenosis [32] (Fig. 6). In our practice, coronary artery calcification mapping is routinely performed to determine thick calcified aneurysms. A thick calcified aneurysm at the inlet and outlet portions of the aneurysm may result in failure to detect the coronary artery stenosis at both the inlet and outlet portions due to blooming artifact. This group of patients should be referred for invasive coronary angiogram, rather than continuing with non-invasive CT(A). In addition, the coronary CT(A) is also useful in patients after treatment of coronary artery bypass grafting (Fig. 7). 9.3. Pulmonary venous return

Fig. 14. Ross operation in a 12-year-old male with underlying severe aortic valvular stenosis. (A) Axial CTA image at the aortic root shows autograft aortic valve connecting to the left ventricle (LV) (arrows). (B) Coronal oblique CTA image shows autograft aortic valve (arrowhead under AO) with dilated aortic root (AO). The small homograft pulmonary valve connects to the pulmonary artery (PA) (arrowhead under PA). (C) Sagittal CTA image shows relationship of the right ventricle (RV) and homograft pulmonic valve (PMV) with narrowing of the supravalvular portion (arrow), consistent with supravalvular pulmonic stenosis. Note post-stenotic dilatation of the main pulmonary artery (PA) and the left pulmonary artery (LPA). LA = left atrium.

Cardiac CT(A) is well established for the evaluation of extracardiac vascular structures, either partial or total anomalous pulmonary venous return, and conditions where pulmonary veins drain to structures, other than the left atrium. Cardiac CT(A) can be used as an alternative tool to echocardiography and catheterization, because it provides information useful for surgical planning: e.g., the site of abnormal venous drainage, pulmonary venous obstruction, and associated other abnormalities of the heart and lung [11,36] (Figs. 8 and 9). 3D-post processing using volume rendering and subvolume maximum intensity projections facilitate the understanding of the complex and tortuous vascular course (Figs. 8B and 9). 9.4. Systemic venous return Many children with CHD have a persistent left superior vena cava (SVC) which is usually associated with the presence of a right

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Fig. 16. Assessment of pulmonary regurgitation (PR) in a 7-year-old male who underwent total correction for TOF. (A) A magnitude image at the level of right ventricular outflow tract (RVOT) shows hyposignal turbulent flow in the RVOT (arrowhead) indicative of PR. (B) A phase image shows two directions of flow in the RVOT, the retrograde flow is seen as intermediate signal (arrowhead) while the antegrade flow is seen as dark signal due to the direction of the velocity encoding. (C) Flow volume curve versus time reveals a PR, the regurgitant volume calculated as the area under the baseline during the diastole.

SVC. The left SVC usually drains to the coronary sinus [37]. Echocardiography reveals this anatomical information; therefore cCT(A) plays a lesser role in this anomaly. Nevertheless, cCT(A) has its role in evaluation of other anomalous systemic veins, e.g. interruption of the inferior vena cana with azygos continuation and a patent levoatrial cardinal vein [11,38]. The levoatrial cardinal vein is an anomalous vein connecting either the left atrium or the pulmonary vein to a systemic vein, and is usually associated with hypoplasia of the left heart [38]. Cardiac CT(A) is an alternative tool to angiography, which may misdiagnoses this as supracardiac partial anomalous pulmonary venous return [38] (Fig. 10). 9.5. Upper airways evaluation Cardiac CT(A) has benefits over cMRI for evaluation of the airway. Compression of the airways due to vascular anomalies is seen in aortic arch anomalies, e.g., double aortic arch, pulmonary artery sling, and pulmonary or aortic dilatation [9]. Minimum intensity projection reconstructions of the airway and coronal reformation aid in the diagnosis and determination of the severity of airway compromise (Fig. 11).

9.6. Surgical repair and complications Arterial switch (the Jatene operation) is reserved for dextrotransposition of the great arteries (D-TGA) and has replaced the atrial switch procedure due to the lower incidence of arrhythmias. The procedure consists of switching of the aorta and the main PA and transferring the coronary arteries to the neoaortic root [39]. The most frequent complication is narrowing of the PA, more commonly found in the left PA [39,40] (Fig. 12). Other complications include left main bronchus compression and coronary artery stenosis [40]. The prevalence of coronary events with a mean follow-up period of 58 months after operation in a series of 1,198 survivors was 7.2% [41]. Cardiac CT(A) provides comprehensive information of coronary arteries, aorta, PA and airway. Determination of coronary stretching, kinking, or compression by adjacent structures should be carefully reviewed [39,40]. Additional myocardial imaging studies such as rest or stress cMR should be performed if suspicious for myocardial damage [39]. The Rastelli operation consists of creation of a tunnel connection between the right ventricle and PA using a patch. Surgery is used in D-TGA and heart conditions with RVOT obstruction. The main

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Fig. 17. Assessment of differential pulmonary blood flow in a 17-year-oldmale who underwent repaired TOF. (A) 3D volume-rendered MR angiography of pulmonary artery in superoanterior orientation shows focal stenosis at the proximal part (arrowhead) of the left pulmonary artery (LPA) associated diffuse hypoplasia. The right pulmonary artery (RPA) is normal. Note that diffuse small size of the peripheral left pulmonary arteries with preferential pulmonary blood flow to the right lung. Large aneurysm of the right ventricular outflow tract (RVOT) and mild dilatation of main pulmonary artery (PA) are shown. (B and C) Flow volume curves versus time of the RPA (B) and LPA (C) reveal difference in pulmonary blood flow seen as greater area under the initial positive part in systole in the RPA compared with that of the LPA (C), reflecting a larger amount of blood flow to the right lung and corresponding to the anatomy of the pulmonary artery. RV = right ventricle.

complication is conduit obstruction and calcified degeneration of the conduit [39] (Fig. 13). Cardiac CT(A) allows early detection of the important complications such as conduit stenosis, ventricular outflow tract obstruction, and demonstrates the anatomy of a calcified conduit in relation to the sternum and the adjacent coronary arteries (Fig. 13). Ross operation replaces the pathological aortic valve with a pulmonary autograft valve, and replaces the pulmonic valve with a pulmonary homograft [42]. Although this single-valve disease is treated with a double-valve procedure, it is still the surgical choice for infants and young children with severe aortic valve stenosis since it avoids the use of anticoagulants and early repeated surgery due to inability of the growth of any homograft prostheses. Long term complications include dilatation of the autograft causing aortic regurgitation [43], homograft pulmonic valve stenosis and RVOT obstruction which is usually found in patients with rapid growth [42] (Fig. 14). Cardiac CT(A) can depict anatomical complications

and provide evaluation of coronary arteries, but it has limitations in the evaluation of valvular and ventricular function. Either echocardiography or cMRI may offer superior imaging of valvular and ventricular function. Waterston–Cooley anastomosis is a palliative shunt connecting the ascending aorta and right PA (RPA). A complication with this shunt is traction of the RPA causing distortion or stenosis of the RPA and preferential flow to the other lung [44] (Fig. 15). 10. Cardiac MRI as an alternative tool to cardiac CT(A) Both cCT(A) and cMRI have their strengths and weaknesses, thus the choice and selection of the modality depends primarily on the clinical questions (Table 6). In general, the indications for both modalities can be grouped into intra-cardiac and extra-cardiac morphologies. Intra-cardiac pathology is usually best determined by echocardiography, only a few conditions require for additional

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studies. Evaluation of extra-cardiac vessels is the most common indication for cCT(A) and MRI [24]. All aortic arch anomalies can be determined by both modalities, except for the vascular ring. Evaluation of airway compromise in the vascular ring is important and cCT(A) is the preferable modality. Cardiac CT(A) is also preferable for evaluation of diseases related to aortopathy involving small branch vessels and coronary arteries. Pulmonary artery and pulmonary vein abnormalities can be evaluated with both modalities, however cCT(A) is better for conditions that require good spatial resolution, e.g. MAPCAs or pulmonary vein branch stenosis. Evaluation of the systemic veins, either pre-operative or post-operative, e.g. Glenn shunt, Fontan and Kawashima operation, can be performed with either tool [5]. Whereas cCT(A) is useful for evaluation of small pulmonary arteriovenous malformations (e.g., in children who present with cyanosis after a Kawashima operation) or for a calcified conduit. However, cMRI is preferable when direction and amount of blood of the venous flow or collaterals influence the treatment plan, such as a dilated azygos vein. In addition, cMRI can avoid multiple scans sometimes needed with cCT(A). For the Jatene operation, cCT(A) is useful for evaluation of coronary implantation, while cMRI is useful for evaluation of ventricular function and myocardial viability. Cardiac MRI is currently the most accurate modality to quantify RV volume and mass; therefore it is the modality of choice for following children with CHD, which requires determination of function of both ventricles, such as in repaired TOF [45]. Cardiac MRI is an alternative tool in evaluation of valvular function and MR velocity mapping gives an accurate measurement of pulmonary regurgitation volume [46] (Fig. 16). MR velocity mapping is a non-invasive and radiation-free method for assessing differential branch pulmonary blood flow and can be used as the modality of choice to avoid radiation burden from radionuclide lung scans [47] (Fig. 17). Additionally, selection of either cCT(A) or cMRI in various CHD conditions and queries also depends on institutional preference. 11. Conclusion Cardiac CT(A) is a very rapid imaging procedure giving excellent spatial resolution. Post-processed three-dimensional images facilitate the understanding of the complex anatomy. The strengths of cCT(A) complement the weaknesses of cMRI, particularly in patients in whom cMRI is contraindicated. Cardiac CT(A) is an important non-invasive diagnostic tool for evaluating CHD, particularly extra-cardiac vessels and coronary arteries. Cardiac CT(A) is not the modality of choice for evaluation of ventricular and valvular function, blood flow and shunt quantification, and myocardial viability, which can be imaged by radiation-free cMRI. Furthermore; cMRI can be useful for longitudinal follow-up particularly in terms of radiation protection. Conflict of interest None. Acknowledgements The authors would like to thank Professor Michael Riccabona, MD for his critical proofreading and editing of the manuscript and suggestions for this review article. References [1] Bailliard F, Hughes ML, Taylor AM. Introduction to cardiac imaging in infants and children: techniques, potential, and role in the imaging work-up of various cardiac malformations and other pediatric heart conditions. Eur J Radiol 2008;68(2):191–8.

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[37] [38]

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