In the current era, TOF is almost universally amenable to surgical repair with good long-term outcome. This, however, requires a thorough pre-operative anatomic description of central and branch pulmonary arteries and associated defects, like additional muscular VSD, ductus arteriosus (PDA), Major Aortopulmonary Collateral Arteries (MAPCAs) .
MSCT, including CT angiography (CTA), provides an important diagnostic tool for evaluation patient with CHD even in small infants with its high scanning speed, superior spatial resolution and improved capabilities for concurrent assessment of cardiovascular structures and lung parenchyma. It can provide accurate three-dimensional depictions of complex cardiac morphology of TOF and extra-cardiac association, tracheobronchial tree and pulmonary parenchymal assessment. It can also permit evaluation of associated coronary artery anomalies and aorto-pulmonary collateral vessels [15, 16].
320-MDCT system allows axial volumetric scanning with 160 mm of coverage in the z direction as the detector element consists of 320 × 0.5 mm detector permitting imaging the heart in a single 0.35-s gantry rotation without table movement. It offers advantages of decreased motion artifacts, reduced scan durations and need for sedation. It also reduced contrast agent volume requirements as well as patient radiation exposure due to lack of z-axis overranging and overlapping helical rotations in addition to minimal penumbral overbeaming with volumetric scanning. It allows excellent anatomic imaging of the whole heart, great vessels and the coronaries .
Retrospective ECG gating with a 320-slice scanner utilizing low dose protocol was performed in all cases complying with ALARA (As Low As Reasonably Achievable) principle, in the form of reduction of Kvp and automatic adjustment of tube current according to body weight. Low radiation dose protocol of 80 kV in 48 patients and 100 kV in 2 older patients with automatic adaptation of the mAs. The average dose length product was 72.066 ± 14.048 mGy-cm and the effective radiation dose was 1.960 ± 0.657 mSv.
This was significantly lower than study by Singh et al.  and Lin et al.  who used 64 MDCT in their studies with adapting dose parameters to body habitus and dose saving strategies with the mean of the effective dose reduction was 4.5 mSv and dose length product 115–218 mGy cm This may be attributed to the advantage of volumetric scanning over helical scanning in reducing the time of scanning and the radiation dose. However Al-Mousily et al. , who used prospective cardiac gating on a 320-slice CT, their study reported more dose reduction to 0.8 ± 0.39 mSv and it is owing to use of prospective ECG gating which has the advantage of markedly reduced radiation exposure as the patient is irradiated only at selectable heart phases than the radiation used in our study in retrospective ECG gating.
This study included 22 patients with classic TOF (TOF/PS), 18 patients with TOF pulmonary atresia (PA/VSD) variant, 3 patients with TOF CAVC variant and 7 patients with TOF DORV variant.
According to the status of native pulmonary arteries, MAPCAs and PDA, PA-VSD is classified to Type A where native pulmonary arteries are present and the pulmonary vascular supply through PDA (no MAPCAs), Type B where native pulmonary arteries and MAPCAs are present and Type C with no native pulmonary arteries and the pulmonary supply through MAPCAs only . Out of the 18 patients of PA-VSD variant. 12 patients were of type A, 5 were of type B and only one case was of type C.
Abnormal situs is a rare finding in TOF patients as mentioned by Zakaria et al. . We encountered only one case situs ambiguous (left isomerism). This finding was only depicted by MDCT and had not been detected in echocardiography. This could be attributed to the superior role of cross-sectional imaging in cardiac, bronchial and abdominal viscera evaluation that help in situs determination.
Regarding TOF cardinal features; with evaluation of interventricular communication outlet subaortic peri-membranous cono-truncal VSDs found in 47patients (94%). 3cases showed large inlet VSD extending to the outlet region seen in the TOF CAVC variant. Identifying presence of additional muscular VSD is such an important issue as additional VSD must be closed during surgical repair to avoid pulmonary over flow after surgical correction . In our study 4 patients had additional muscular VSD detected by MDCT, Echocardiography missed one of them and detected only 3. This matches with explanation of Shaaban et al.  in their study that these VSD usually require special attention as they may be sometimes difficult to identify by echocardiography because the large cono-truncal VSD equalizes LV and RV pressures making no significant pressure gradient across these additional VSDs. However, Moustafa et al.  in their study detected additional muscular VSD in 4 cases using MDCT opposite 5 cases by echocardiography attributing their finding to the to the ability of color Doppler to detect very small intermuscular defect rather than MDCT.
Regarding right ventricular hypertrophy (RVH), we noted that all cases showed RVH by MDCT matching with Moustafa et al.  in their study.
Assessment of degree of aortic overriding is important for surgical planning of VSD patch closure as patients with DORV will need much larger patch to baffle the blood from the left ventricle to the aortic root . In our study we reported equal alignment of the aorta to both ventricles in 33 patients (66%) representing the most frequent alignment, more than 50% alignment to the left ventricle was found in 10 (20%) patients. 7 patients (14%) showed more than 50% alignment to the right ventricle in TOF/DORV variant. Our results were matching with what Moustafa et al.  reported in their study where equal aortic overriding was the most encountered and found in (60/77 patients representing 78% however followed by more from the RV in 15 patients (19.5%) then more to the LV alignment only in 2 patients (2.6%) in contrast to our findings where left ventricular alignment was encountered in more cases than right ventricular alignment.
With studying level of obstruction in every patient, 47 patients had sub-valvular pulmonary stenosis representing the most commonly affected level. This came in agreement with what Hrusca et al.  stated in their study where they studied pulmonary artery anomalies in tetralogy of Fallot using non-ECG-gated CT angiography and found that 24 patients (68.57%) had infundibular stenosis. Also, Singh et al.  as well as Moustafa et al.  reported in their recent studies that sub-valvular (infundibular) stenosis was the most commonly affected level of their patients.
We also found that combined level of obstruction was much more common than isolated level of obstruction with combined sub-valvular, valvular, and supravalvular stenosis was detected in 40% of patients matching with Moustafa et al.  findings in their study. In the study performed by Singh et al. , it was encountered that infundibular with valvular stenosis was the most common type of RVOT obstruction (47.28%) followed by isolated infundibular stenosis.
In our study MDCT and Echocardiography showed 100% agreement in detection of sub-valvular stenosis as well as valve atresia however MDCT missed diagnosis of valve stenosis in four patients compared to Echocardiography. Less agreement with statistically significant difference between the two modalities in detecting the supra-valvular stenosis where echocardiography missed detection of supra-valvular RVOTO in MPA, RPA and LPA levels. These findings came in agreement with Hu et al.  who reported 100% accuracy of MDCT in detection of supra-valvular pulmonary stenosis comparing their result to cardiac catheterization.
Regarding pulmonary arteries characterization, a statistically significant difference was found between both Echo, and MDCT in assessment of main, right and left pulmonary arteries. MDCT could efficiently detect supra-valvular and distal stenosis in main pulmonary artery as well as ostial stenosis in their branches while echocardiography failed to detect them. Also, Echocardiography failed to detect hypoplastic main and branch pulmonary arteries in other cases as well as missed depiction of confluent hypoplastic branch pulmonary arteries and reported them as atretic. Apostolopoulou et al.  in their study recommended MDCT in evaluation of complex pulmonary artery defects in TOF patients for better visualization pulmonary arteries anatomy, size, and arborization matching with our results. Raimondi et al.  as well stated in their study the role of CT scans in definition the distal anatomy of pulmonary branches.
In the current study level of obstruction at MPA (stenosis, hypoplasia or atresia) was found in 30(60%) patients followed by RPA and LPA with 18 (36%) patients for each. These findings came in agreement with Moustafa et al.  and Zakaria et al.  who reported incidence of combined MPA stenosis and its branches 17% as the most common pulmonary artery anomaly. While it disagrees with Sheikh et al.  who stated that isolated left pulmonary stenosis was the most common ab-normality in 10%. This was attributed by Moustafa et al.  to larger number of patients (about 5000) included in the study who underwent cardiac catheterization.
Mc-Goon’s ratio is an applicable method to evaluate the pulmonary arterial size and pulmonary blood flow. When the Mc-Goon ratio is adequate, it means enough pulmonary blood, and hence total surgical correction is planned. Otherwise, palliative operation is suggested in case of inadequate Mc-Goon’s ratio . In our study, 33 patients had ratio more than 1.2. This group are candidate for total repair. Chen et al.  who stated that MDCT with reconstructed oblique images are reliable methods for Mc-Goon ratio calculation when they compared it to the angiographic findings.
In this study MSCT entailed 19 patients with PDA versus 15 patients depicted by echocardiography. CT could also detect tight PDA pulmonary end in three patients, stenosed and obliterated aortic end in other two. However, echocardiography failed to detect them. This coincides with Moustafa et al.  and Shehata et al.  studies. They stated the superior role MDCT over transthoracic echocardiography in evaluation of PDA. Ishehara et al.  in their study also recommended MSCT for better evaluation of doubtful cases of PDA with normal echocardiography. However, Leschka et al.  in their study reported TTE as the method of choice for diagnosing PDA while MDCT has only a minor role. Regarding MAPCAs, MSCT had depicted 25 MAPCAs per 11 patients representing a percentage of 22%. Juneja et al.  in their study documented an incidence of 56% of MAPCAs in TOF patients. Their study included homogenous group of TOF patients with pulmonary atresia, however our study included different TOF variants with different pulmonary defects.
Echocardiography only detected 8 MAPCAs per 7 patients. In addition, CT detected stenosis in MAPCA in two patients besides its role in evaluation the origin, numbers, size and course and lung segments supplied by these MAPCAs, which agreed with study did by Hu et al. , they stated the superior role of CT in evaluation of number, origin, and supplied lung lobes of MAPCAs regardless of the size however TTE can only identify relatively large ones. Also Chandrashekhar et al.  stated that MDCT had the ability similar to catheterization in aortopulmonary collateral identification, while it is difficult to detect these collaterals by echocardiography mainly secondary to small field of view.
Being aware of anomalous pulmonary venous return in TOF patients prior to surgical correction is an important issue to avoid catastrophic outcomes . In our study TAPVR of supracardiac type was identified in one patient and PAPVR in another one with MDCT while missed by echocardiography matching with Türkvatan et al. , they stated more reliability of MDCT than TTE in delineation of the PAPVR. Chan et al.  in their study concluded that repaired TOF patients with unrepaired PAPVR were liable to more RV volume overload and more reduction in RV function.
Lack of conal rotation and conal malseptation is a characteristic anatomical feature of TOF which lead to dextroposed position of the aorta and significant RVOT narrowing . In this study, with thorough assessment of ascending aorta and aortic root by MDCT, Clock wise rotation (dextroposition) of aortic root was encountered in all patients. In addition, 38 patients showed dilated ascending aorta representing 76%.
In spite of the absence of functional significance of the right sided arch, it is important to tell surgeons beforehand to avoid complications .
In our study right sided aortic arch was found in 10 patients with percentage of 20%. This was comparable to documented incidence of 20–25% cases by Kemper et al.  and Siripornpitak et al. .
Regarding branching pattern of aortic arch, MDCT depicted normal branching pattern in 26 patients while the other 24 patients showed abnormal branching pattern; mirror image branching pattern in 10 patients, bovine aortic arch with common origin of right innominate and left common carotid artery in 7 patients, left vertebral artery origin from aorta in 5 patients and aberrant right subclavian artery in 2 patients. In addition, MDCT detected aortic coarctation in 2 patients and bicuspid aortic valve in another patient.
Preoperative assessment of coronary artery anomalies is essential for patients with TOF to avoid injury of aberrant coronary artery passing across the RVOT in ventriculotomy or trans annular repair when done to relieve the RVOT . Although Echocardiographic analysis remains the first step in the evaluation of patients with congenital heart defects, it might be limited to depict anomalous coronary arteries in patients with TOF .
In our study coronary artery abnormalities were identified in 8 patients with percentage of 16%. MDCT detected one patient with dual LAD where another LAD seen originating from RCA. Another patient showed anomalous origin of LAD from RCA. Echocardiography failed to detect both of them. 6 patients depicted by CT having prominent conus/RV branches. Echocardiography missed three of them. These findings were in accordance with Amzallag el al.  in their study wherein 3 of 7 cases, an anomalous coronary artery were missed by means of echocardiographic analysis. Superiority of MDCT in detection of coronary artery anomalies compared to Echocardiography coincides Moustafa et al.  as well as Shehata et al.  and Gotein et al. . Also Hu et al.  in their study described the coronary artery anomalies at 2 cases with sensitivity 100% and specificity 100% when compared to cardiac catheterization.
There is a high incidence of associated extracardiac vascular anomalies in pediatric patients with TOF, and these abnormalities might preclude certain types of surgical repair and may provoke late adverse outcomes [24, 40].
As regard to associated extra cardiac vascular anomalies, Echocardiography missed detection of 6 patients with right sided aortic arch out of 10 depicted by MDCT examination. It also failed in detection of bovine aortic arch, left vertebral artery origin from the aorta and aberrant right subclavian artery in other 14 patients. These results came in line with Eltatawy et al.  in their study where they confirmed the efficacy of MDCT in identifying different aortic arch branching pattern. This could be attributed to the small field of view during examination from the suprasternal direction could be the main reason why the great vessels could not be identified as accurately using TTE. Moreover, the short neck of pediatric patients, the overlying bone, and the aerated lung might also influence the diagnostic value of TTE in the depiction of these deformities.
Comparing MDCT findings with the data collected by cardiac catheterization and/or operation, MDCT could correctly depict all aortic, coronary and other associated extracardiac vascular anomalies including pulmonary aortic connections (PDA and MAPCAs) by axial, multiplanar and three-dimensional images with 100% sensitivity and 100% specificity.
These findings came in agreement with Hu et al.  they stated the superiority of MDCT over TTE in detecting extracardiac vascular anomalies in pediatric patients with complex CHD such as TOF.
A major limitation in this study was the relatively small number of the studied cases that rendered giving a full idea about the diagnostic efficacy of cardiac MDCT. Retrospective ECG gating was another limitation that relatively increase the radiation dose in infants and children. However, with performing low dose protocol in all cases complying with ALARA principle radiation exposure was minimized as much as possible.
We recommended careful selection of TOF patients for cardiac MDCT imaging and application of strategies for radiation exposure reduction. We also recommend Increasing the sample size in the future.