Chapter Contents
Introduction 1243
Radiation safety 1243
Chest imaging 1244
Technical considerations 1244
Lines and tubes 1244
Endotracheal tube 1244
Umbilical venous and arterial catheters 1244
Long lines 1245
Enteral feeding tube(s) 1246
Heart size, position, aortic arch and pulmonary vasculature 1246
Lung appearances 1246
Chest ultrasonography 1248
Magnetic resonance imaging of the chest 1249
Abdominal imaging 1249
Radiological differential diagnosis for abdominal appearances 1249
Imaging in the neonate with abnormal liver function tests 1251
Imaging in the neonate with an abdominal mass 1254
Genitourinary imaging 1254
Musculoskeletal imaging 1254
Central nervous system imaging 1255
Introduction
Imaging in the neonate includes the whole range of imaging investigations available in the older child and adult. The cornerstone of imaging is still conventional radiography but ultrasound plays an important part. Most neonatal imaging is undertaken on very sick and usually very small neonates in the intensive care unit. This requires portable X-ray equipment of a very high specification that is capable of using very short exposure times. In the main computerised radiography has now largely replaced conventional film radiographs, maintaining high-quality imaging and providing several advantages, with postprocessing being one example. The advent of picture archiving and communication systems has allowed images to be available for viewing on monitors within the intensive care unit or transmitted electronically if a further opinion is required. Interpretation of intensive care radiology is often done on the spot by clinicians, who must receive appropriate training. Regular multidisciplinary meetings between clinical and radiology staff are essential for audit, training and maintenance of skills.
Radiation safety
All involved in the management of neonates should sign up to the ALARA (as low as reasonably achievable) principle with regard to radiation dose. In the European Union legislation has been enacted which, in the UK, has led to the IR(ME)R 2000 regulations ( ) governing both the justification, i.e. clinical needs, for an examination and optimisation (obtaining the best images with the lowest possible radiation dose).
Chest imaging
Almost every baby admitted to a neonatal intensive care unit will have one or more chest radiographs. It is most important that these answer the clinical question posed by the requesting doctor and that all those requesting such radiographs are able to interpret them. The film must be technically satisfactory with the appropriate radiation exposure so that the lungs, soft tissues and bones can be adequately visualised on the radiograph produced.
A systematic approach to reviewing the chest radiograph enables consistent interpretation, which should lead to accurate diagnosis. Routine review of any available older films is mandatory to place current radiological findings in context. A suggested routine for chest radiograph evaluation is as follows:
- 1
Check the patient’s name, date of birth and the date of the radiograph.
- 2
Assess the technical quality of the radiograph – degree of rotation, degree of penetration and degree of inspiration – all of which may influence the interpretation of the film.
- 3
Check the position of lines and tubes to ensure that they are in the expected place and have not become displaced since the last radiograph.
- 4
Check heart size and position, site of aortic knuckle and tracheal position.
- 5
Is there normal, increased or decreased vascularity? This can be quite subjective if not grossly abnormal.
- 6
Is there lung transradiancy? Is it equal or asymmetrical? If asymmetrical, evaluate for increased lucency versus increased density and try to determine which side you feel is abnormal. If there is increased transradiancy, the side with fewer visible vessels is abnormal.
- 7
Review the upper abdomen – check beneath both hemidiaphragms for free air and the liver shadow for evidence of portal venous gas.
- 8
Review the bones – the bone density, signs of metabolic bone disease, fractures and congenital malformations.
Technical considerations
The chest should be straight so that the hemithoraces in the normal situation would appear isodense. In the neonate evaluating the relative positions of the anterior ribs on both sides most easily assesses this. It is also important that the film is taken in full inspiration ( Fig. 43.1 ). Films that are grossly rotated, expiratory or underpenetrated can be very difficult to interpret and caution is advised when reviewing such radiographs.
Lines and tubes
Whilst most chest radiographs are taken to assess lung or intrapleural pathology or cardiac shape and size, a significant number are taken either solely for or additionally to demonstrate the positions of various lines and tubes inserted in the treatment of the baby. It is most important that all those managing such neonates are aware of the appearances of these.
Endotracheal tube
The endotracheal tube (ETT) is usually clearly visible on a plain X-ray. The tip of the tube should lie between the level of the seventh cervical vertebra (C7) (just below the larynx) and the level of the carina, which is usually around about the level of the fourth thoracic vertebra (T4). The vertebrae should be used as landmarks when assessing tube position rather than the clavicles, and the tip of the ETT is best placed opposite the body of the first thoracic vertebra ( ). Head flexion will effectively advance the ETT and head extension will withdraw the tube, hence an acceptable ETT tube position can be rendered apparently unacceptable by either of these manoeuvres.
Umbilical venous and arterial catheters
Umbilical venous catheters (UVCs) pass from the umbilical vein to the left portal vein and thence through the ductus venosus (up to 2 cm in length in term babies), middle or left hepatic vein, inferior vena cava (IVC) and into the right atrium. The ideal tip position is at the junction of the IVC and the right atrium. Advancement past the ductus venosus is not possible in 11% and the line terminates in the liver or below in 24% ( ). Based on radiographic correlation, optimum tip position is considered to be opposite T8–9 on an anteroposterior film, but the right atrium–IVC junction is not always consistent with these bony landmarks ( ). A line at the T6 or higher level is within the heart and if the tip lies below T11 the catheter is proximal to the ductus venosus ( ).
If the UVC is placed too far in, the tip may enter the superior vena cava, pass through the foramen ovale (and potentially into the pulmonary outflow tract) or through the tricuspid valve into the right ventricle. Low abnormal catheter positions can be seen with the UVC positioned within the portal venous system, the hepatic veins, ductus venosus or within the umbilical vein ( ) ( Fig. 43.2 ). For more information on complications of UVCs and umbilical arterial catheters (UACs), see Chapter 44 .
Either umbilical artery can be utilised for umbilical arterial line placement, with the catheter passing into the internal iliac artery, the common iliac artery and thence into the aorta ( Fig. 43.2 ). Owing to the course of the catheter, there is an apparent loop in the catheter on radiographs, passing into the pelvis before ascending. This ‘loop’ is the hallmark of a UAC and is important to establish its presence when checking UAC placement, which hence requires a chest and abdominal film. Optimal placement is away from the visceral aortic branch vessels (which usually lie opposite T12) and this can be achieved through either a ‘low’ or ‘high’ placement. With ‘low’ placement the line tip should lie between L3 and L5; ‘high’ placement should (ideally) leave the tip between T7 and T9 ( ) ( Fig. 43.2 ). A Cochrane Review concluded that the evidence is strongly in favour of a high catheter position ( ).
Long lines
Small peripheral venous catheters are frequently used for venous access and these may be inserted in any of the four limbs or sometimes via a scalp vein (see Ch. 44 ). The tip of such catheters should be in a large central vein but outside the heart. The tip of one of these catheters should not lie more centrally than the inferior limit of the superior vena cava or the superior limit of the IVC. As with a UVC, if the tip of the line lies opposite T6 it is within the heart and should be withdrawn. These catheters are very fine and are not usually radiopaque. Their position must be confirmed by radiography after insertion by injecting 0.5 ml of intravenous contrast medium (such as Omnipaque or Niopam) to confirm the position of the tip of the catheter ( Fig. 43.3 ). The question of how often the position of a catheter position should be checked after insertion is a difficult one, about which there is no consensus, but all clinicians must be aware that catheter migration both distally and proximally can occur. The tip of the catheter can be extremely difficult to identify without contrast medium, particularly over the chest or abdomen when overlying mediastinal structures or bowel can be extremely confusing. The positions of lines within the abdomen can also be determined by ultrasound and, whilst this is potentially an attractive option, it has not found favour owing to difficulties with overlying bowel gas.
Enteral feeding tube(s)
Frequently a nasogastric tube has been placed to assist feeding on the neonatal unit and the position of such tubes should be reviewed on each chest or abdominal film. The optimal position is within the stomach bubble. If the tube is not projected there consider the following:
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Is the tube simply short, within the distal oesophagus, or long, lying within the proximal small bowel?
- •
Is the tube projected away from the expected course of the oesophagus? Possibilities would then include intubation of the tracheobronchial tree (most likely: Fig. 43.4 ) or oesophageal perforation.
Fig. 43.4
Chest X-ray showing a malpositioned nasogastric tube, with the tip (arrow) lying within a right lower lobe bronchus in the costodiaphragmatic recess. Note how inferior the posterior costodiaphragmatic recess extends.
- •
Is the tube tip projected higher and more laterally than expected despite following an appropriate oesophageal course? If so, a congenital diaphragmatic hernia needs to be considered.
- •
Is the tube coiled up, projected over the cervical/upper thoracic region? If so, oesophageal atresia needs to be considered.
Heart size, position, aortic arch and pulmonary vasculature
Many congenital cardiac lesions are diagnosed prior to birth ( Ch. 28 ). However there are frequent occasions when a lesion is detected antenatally and the chest radiograph may provide some important clues to the presence of congenital heart disease.
Assessment of cardiac size may be challenging, given the widely variable appearances of the thymus in neonates. The thymus can fill the whole of the upper mediastinum or be prominent on the right or the left. Typical normal thymic features include an undulating margin or the ‘sail sign’. In preterm neonates the thymic shadow may be very difficult to identify or be apparently absent. A cardiothoracic ratio of 60% is the accepted upper limit of normal on a frontal radiograph. Apparent cardiomegaly caused by a prominent thymic shadow can be evaluated further using either a lateral chest radiograph or an ultrasound to look for prominent thymic tissue. When there is true cardiomegaly the heart shadow extends posterior to a line drawn from the trachea to the diaphragm on the lateral radiograph.
A right-sided aortic arch is associated with underlying congenital heart disease, including tetralogy of Fallot (25%), truncus arteriosus (35%) and pulmonary atresia with ventricular septal defect. Identification of the aortic arch position can be difficult. It may be possible to visualise the arch or the descending aorta or alternatively use leftward displacement of the tracheal position (which should normally be slightly to the right of the midline) or right-sided tracheal indentation to determine if a right-sided arch is present.
Assessment of pulmonary vasculature is challenging and can be quite subjective, with different observers having differing opinions! Essentially the aim here is to decide if there is increased, normal or reduced vascularity. For the differential diagnosis of congenital heart disease, see Chapter 28 .
Lung appearances
The lung parenchymal appearances should be evaluated not only by comparing each hemithorax with the other but also by comparing the overall appearances with a mental picture of what the normal appearances should be. This clearly requires a degree of experience, but with practice a mental ‘reference image’ will become easier to define. This is particularly important when assessing neonatal chest radiographs as many of the disease processes seen in this population lead to bilateral radiographic abnormalities. With improved perinatal care ‘classical’ appearances are less frequently encountered; however, the following is a rough rule of thumb.
Increased opacity
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Technical – an underpenetrated radiograph will lead to globally increased lung density.
- •
Respiratory distress syndrome (RDS) – classically small-volume lungs with a bilateral ground-glass infiltrate ( Fig. 43.5 ), often with a nodular pattern. Can have air bronchograms. May be asymmetric following administration of surfactant, with the right lower zone typically being better aerated than the remainder of the lung parenchyma.
Fig. 43.5
Chest X-ray showing classical respiratory distress syndrome pattern with bilateral, relatively symmetrical, nodular ground-glass infiltrates, with air bronchograms.
- •
Transient tachypnoea of the newborn – classically normal-volume lungs with increased vascular and interstitial markings. May have small effusions and mild cardiomegaly. Usually resolves within 72 hours.
- •
Meconium aspiration syndrome – coarse ‘rope-like’ opacities emanating from the hila. Overinflation and areas of atelectasis ( Fig. 43.6 ). There may be a pneumothorax present also.
Fig. 43.6
Chest X-ray showing typical features of meconium aspiration syndrome, with hyperinflation and coarse linear opacities.
- •
Congenital pneumonia – segmental or lobar consolidation, sometimes with effusions, which may be large.
- •
Pulmonary haemorrhage – varied appearance. Can mimic meconium aspiration or RDS.
- •
Pleural effusion – homogeneous increased density on a supine film. Without any distinguishing features (for example, air bronchograms) it may be impossible to differentiate fluid from air-space processes. When an effusion tracks laterally along the chest wall the lung edge may be displaced, allowing positive identification ( Fig. 43.7 ). Clearly both consolidation and effusion may coexist, further confusing the radiological appearances.
Fig. 43.7
Chest X-ray showing generalised increased density within the right hemithorax, with a definite fluid–lung interface (arrow), consistent with a pleural effusion.
- •
Lung aplasia/agenesis – increased density on the abnormal side, with volume loss, manifesting as mediastinal shift, elevation of the hemidiaphragm or both. May be difficult to differentiate from collapse – previous films are helpful to assess if the lungs previously appeared normal or not.
Increased transradiancy
- •
Technical – a rotated radiograph will lead to discrepancy between the transradiancy of the two hemithoraces. An overpenetrated film will cause both lungs to appear hyperlucent.
- •
Pneumothorax – as films are taken with the neonate in a supine position, free pleural air rises anteriorly, hence a lung edge may not be visible. Signs that suggest a pneumothorax are:
- ▪
sharp ipsilateral heart border or diaphragm ( Fig. 43.8 )
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