Introduction 1 2 3 4 Technicalities for MDCT of the paediatric airways 3 Preparation of the child: is sedation always needed? Due to the faster scanning times possible with MDCT scanners, routine sedation is no longer required. In the neonate, recent feeding usually provides tranquillity while children over 3 years of age tend to cooperate after explanation of the procedure through play therapy and verbal reassurance. As a result, CT waiting lists have shortened. Which protocol should be applied? 1 2 3 4 5 Table 1 Imaging protocol for the paediatric chest using a 16-row MDCT scanner   Routine scan Combiscan/CTA Indication Stricture Cardiovascular anomalies Tumour Small tracheobronchial stenoses Tracheomalacia  Peripheral airways disease  Anatomical area Thoracic inlet to diaphragm Tube collimation (mm) 1.5 0.75 Slice width – reconstructed (mm) 5 3 Table feed (mm/rotation) 24 12 Exposure factors 100 kVp 100 kVp 20–75 effective mAs (dependent on patient weight) 20–75 effective mAs (dependent on patient weight) 0.5 s scan time 0.5 s scan time Respiration Suspended inspiration; single breath-hold where possible Three to five expiratory scans for tracheomalacia/small airways disease Contrast medium None Yes Triggering for CTA Algorithm Soft tissue Soft tissue plus reconstruction on bony algorithm for high-resolution CT of the lungs Table 2 Suggested delay times from the injection of contrast medium   Manual injection Pressure injector Scan initiationtime delay 10 s from termination of injection 25 s from start of injection Flow rate  2 ml/s Age range All age groups All age groups Table 3 Volumetric CT chest scanning parameters according to child’s weight when routine and Combiscan protocols are performed   <15 kg 15–24 kg 25–34 kg 35–44 kg 45–55 kg Volume Combi Volume Combi Volume Combi Volume Combi Volume Combi kVp 100 100 100 100 100 100 100 100 100 100 Effective mAs 20 20 25 25 35 35 55 55 75 75 Collimation (mm) 1.5 0.75 1.5 0.75 1.5 0.75 1.5 0.75 1.5 0.75 Scan slice width (mm) 5 5 5 5 5 5 5 5 8 8 Table feed (mm) 24 12 24 12 24 12 24 12 24 12 Scan time (s) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Calculated Effective Dose (mSv)–CT EXPO 0.9 1.0 1.13 1.31 1.58 1.75 2.48 2.75 3.38 3.75 Table 4 High-resolution CT chest scanning parameters according to child’s weight   <15 kg 15–30 kg >30 kg kVp 100 100 100 Effective mAs 20 30 55 Collimation (mm) 1 1 1 Scan slice width (mm) 1 1 1 Table feed (mm) 10 10 10 Scan time (s) 0.36 0.75 0.75 Calculated Effective Dose (mSv)–CT EXPO 0.21 0.32 0.59 Table 5 CT angiography scanning parameters according to child’s weight   <15 kg 15–24 kg 25–34 kg 35–44 kg 45–55 kg kVp 100 100 100 100 100 Effective mAs 20 25 35 40 50 Collimation (mm) 0.75 0.75 0.75 0.75 0.75 Scan slice width (mm) 1 1 1 1 1 Table feed (mm) 15 15 15 15 15 Scan time (s) 0.75 0.75 0.75 0.75 0.75 Calculated Effective Dose (mSv)–CT EXPO 1.0 1.26 1.76 2.0 2.5 Radiation dose 1 2 1 2 5 1 6 1 3 4 5 Applying a dose modulation function, where the system samples the patient thickness and adjusts (i.e. reduces) the exposure accordingly when the tube is in the AP/PA position, as patients are narrower in the frontal than in the side-to-side orientation. Reduction of the kilovoltage to 100 kVp when imaging the thorax. Further reduction to 80 kVp is possible for CTA, but as resolution of the parenchyma is not ideal this is applied only if lung pathology is unlikely. Selecting tube collimation of 1.5 mm. The 0.75-mm collimation improves spatial resolution but, as already mentioned, increases the radiation dose, and is therefore reserved for CTA or where thin-slice reconstruction is indicated. Selecting appropriate mAs selection dependent on the patient’s weight or cross-sectional diameter. Table 6 Dose comparison for different scanning protocols in a phantom study in our institution Weight range (kg) Effective dose (mSv) Volume (1.5 mm) Combi (0.75 mm) High-resolution CT CTA Chest radiography Male Female Male Female Male Female Male Female AP Lateral <15 0.77 0.90 0.9 1.05 0.36 0.42 1.30 1.51 0.00487 0.00799 15–24 0.93 1.09 1.13 1.31 0.36 0.42 1.62 1.89 0.00874 0.01086 25–34 1.34 1.56 1.58 1.84 0.54 0.63 2.24 2.62 0.01163 0.00968 35–44 2.11 2.46 2.48 2.89 1.00 1.17 2.57 3.0 0.01769 0.01452 Unlike the single-slice scanner, an increase or decrease in table feed time on the MDCT scanner only affects the overall scanning time. An increase in table speed results in a concomitant increase in mA and this has no effect on the dose delivered. The tube current is automatically compensated to ensure that the preset effective and total mAs is delivered, i.e. a fast table movement results in an automatic increase in the mA keeping the mAs constant. Anatomical coverage For imaging of the paediatric thorax, regular coverage extends from the thoracic inlet to the diaphragm. Greater coverage may be warranted in certain clinical cases, such as an extralobar pulmonary sequestration that may derive its blood supply from the upper abdomen. 2 Breathing during scan 6 6 Contrast medium 2 6 2 In children with complex vascular anatomy, the radiologist should be present for the examination. In those children, low-dose planning scans are performed at a predetermined anatomical level and the injection of contrast medium is triggered by the radiologist. The volumetric scan begins when the contrast medium has reached the relevant preselected vascular structure. Postprocessing There are four reconstruction displays available for postprocessing of the volumetric data, which are applied accordingly: multiplanar reformation or reconstruction (MPR), 3-D shaded-surface display (SSD), multiprojective volume reconstruction (MPVR) and 3-D volume rendering (VR). In fact, the axial images include all the information about the anatomy of the airways that is provided with 2-D and 3-D reformats. However, postprocessing gives added value to imaging since the axial scans that need to be studied are usually numerous, and oblique structures as well as interfaces and surfaces parallel to the axial plane are poorly demonstrated and sometimes occult. Multiplanar reformations 3 1 2 3 4 5 6 7 8 9 10 2 3 4 5 6 3 Fig. 1 arrow  Fig. 2 Coronal MPR of the chest in a child with a history of recurrent infections due to a congenital pulmonary airway malformation. A thin-walled multicystic lesion in the right lower lobe is shown  Fig. 3 a b c b c arrow d e curved arrows  Fig. 4 a b c arrows d arrowheads  Fig. 5 a arrow b c arrow  Fig. 6 a b c black arrow d white arrow  Fig. 7 a b c d  Fig. 8 a b  Fig. 9 Pneumocystis carinii  Fig. 10 arrow  3-D imaging This is a diagnostic tool only in certain cases as it usually requires more time and postprocessing skills to provide information already included and demonstrated in the axial images and the MPRs. There is no doubt, however, that the 3-D reformatted images may further increase the diagnostic confidence which eventually affects patient management, particularly pertinent in presurgical assessment. Communication with the referring clinicians is simplified as the images portray the spatial relationships of important anatomical structures. 3-D shaded-surface display techniques These are only applied in the imaging of the central airways and they are usually more visually impressive than clinically useful. Their generation from original data is time-consuming and they carry the risk of loss of density information due to problems with thresholding. Multiplanar volume reconstructions 2 3 11 2 7 Fig. 11 a b c d e  3-D volume rendering 2 3 2 3 6 7 11 12 Fig. 12 a b  Virtual bronchoscopy 4 13 1 4 Fig. 13 a b curved arrows  2 4 3 8 1 4 9 10 3 8 8 Dynamic and functional inspiratory and expiratory scanning 2 Peripheral airways 10 3 7 3 6 Clinical applications of volumetric imaging of the airways in children Selection of the most appropriate CT protocol for each individual case is paramount in imaging of the airways in children. Ultimately, the best protocol is the one that provides the most relevant information at the lowest radiation burden possible. The indications include: (1) congenital bronchial anomalies (e.g. accessory bronchi, bronchial hypoplasia and atresia, and bronchopulmonary foregut malformations), (2) tracheomalacia, (3) tracheobronchial strictures (congenital and acquired) or tumours, and (4) peripheral (small) airways disease. Tracheobronchial anomalies 1 11 1 12 Tracheal bronchus 13 1 13 13 V A C T R E L 11 Bronchial atresia 13 Bronchopulmonary foregut malformations These are anomalies of pulmonary development that are due to abnormal budding of the embryonic foregut and tracheobronchial tree. They include duplication cysts characterized by an isolated portion of lung tissue communicating with the upper gastrointestinal tract or the central nervous system such as bronchogenic cysts, enteric cysts, and neurenteric cysts. Symptoms are usually provoked by the size and location of the cysts, which may cause compression of the trachea or bronchi leading to distal collapse and air trapping. Infection is less commonly encountered. Pulmonary underdevelopment 1 13 Scimitar syndrome 13 Sequestration spectrum 2 3 3 Tracheomalacia 13 Tracheobronchial strictures Congenital tracheal stenosis 4 13 13 Congenital lobar emphysema/overinflation 14 Fig. 14 Axial CT of an infant with mediastinal shift shown on a chest radiograph showing hyperinflation of the left upper lobe without destruction of the alveolar walls, consistent with congenital lobar emphysema  Acquired tracheobronchial stenoses 5 Compression of the airways of cardiovascular origin 6 14 15 13 15 14 11 13 12 14 Foreign body aspiration 16 16 Local extension of neoplasms 7 8 3 Peribronchial air collections 9 3 Peripheral airways 10 6 6 Conclusion MDCT with 2-D and 3-D reconstructed imaging has enhanced the applications of CT in imaging of the chest in children. Obtaining high-quality scans should always be attempted at the lowest radiation dose possible. CT and bronchoscopy are supplementary examinations in the diagnostic work-up of children with tracheobronchial pathology. Although 2-D and 3-D rendering techniques are not the first-line diagnostic tools, they significantly reinforce the confidence in a diagnosis even in complex cases. In specific scenarios, they may supply information that is more easily interpreted among the different specialties than conventional axial scans. In the future, virtual tracheobronchial endoscopy is anticipated to be applied for interactive virtual-reality guidance in surgical procedures of the airways.