7.1 Imaging
Introduction
The tools of the radiologist include plain radiography, fluoroscopy (screening), intravenous, intracavity and gastrointestinal contrast media, angiography, nuclear medicine, ultrasonography, computed tomography (CT) and magnetic resonance imaging (MRI). Interventional radiology uses imaging for procedures such as abscess drainage, sclerosis of vascular malformations, biopsy and intravenous access.
There are many differences between imaging the child and imaging the adult. Radiologists rely heavily on the expertise of medical radiation technologists and sonographers in acquiring diagnostic images in sick or injured children. The child’s physical and psychological welfare during diagnostic imaging must be considered. Imagine how the typical 2-year-old child, scheduled to have a micturating cysto-urethrogram, would react when introduced to a stranger wielding a catheter. Cooperation from young children is not possible when procedures are long and/or invasive; sedation is needed for many.
Paediatric ailments often differ from those of the adult. Congenital disease as well as acquired disease must be considered in the differential list of diagnoses. History-taking and clinical examination of infants and children is not easy; thus, information from imaging is crucial in certain situations. Radiation protection is important both for the child and for society as a whole; a child is 2 to 10 times more radiosensitive than an adult. Some specific disease states and syndromes are associated with increased susceptibility to injury from radiation (e.g. ataxia telangiectasia).
Radiation protection is provided by the following measures:
1. Limiting examinations to those that are likely to provide diagnostic help and influence treatment decisions
2. Using optimal technical factors in order to provide the lowest dose of X-rays for a diagnostic study
3. Limiting fluoroscopic time and using ‘last image hold’ techniques when possible
Measures 2–4 are the responsibility of the radiologist and radiographer, but the clinician is responsible for choosing imaging studies carefully. The radiologist should be available for discussion of appropriate imaging for a given diagnostic consideration.
The equipment in the department of radiology should be calibrated to provide images requiring low-dose irradiation but still provide good diagnostic detail. Factors for image production from CT should be modified to suit the size and age of the patient. Dose measurements are expressed in units termed gray (Gy) (absorbed dose; 1 Gy equals 100 rad) or sievert (Sv) (equivalent dose; 1 Sv equals 100 rem in the old terminology).
Every person is exposed to background radiation from the world around them and from cosmic rays. Radon gas provides the major source of background dose. Total annual background dose, per person, therefore depends heavily on the geographical location, and is estimated to be 2–3 mSv (Table 7.1.1). This does not include medical exposure, predominantly from CT, which in the USA has recently doubled the average per person background dose to 4–6 mSv. For comparison, two radiographs of the chest give 0.02–0.08 mSv – less radiation than received on a round trip by air across the Pacific Ocean.
Table 7.1.1 Estimated equivalent background radiation for various imaging procedures
| Study | Estimated equivalent background assuming background of 2.5 mSv/year |
|---|---|
| Chest X-ray, 2 views | 3 days |
| Abdominal X-ray, 2 views | 1 week |
| Extremity X-ray, 2–3 views | 5 hours |
| Skull series X-ray, 3 views* | 3 weeks |
| Upper gastrointestinal series* | 6–12 months |
| Barium enema* | 8–16 months |
| MCU (VCUG) | 1–7 weeks |
| Chest CT | 12–18 months |
| Abdominal CT | 2 years |
| Cranial CT | 8–12 months |
CT, computed tomography; MCU, micturating cystourethrography; VCUG, voiding cystourethrography.
Conversely, advances in imaging have made diagnosis far more accurate and safe than in the past. Radiologists can often demonstrate the likely cause(s) for a child’s symptoms and signs, enabling timely medical or surgical treatment.
Paediatric radiologists play an important intermediary role between paediatrics and radiology, in both the conduct and the interpretation of an examination. They are the clinician’s friend and the patient’s advocate. Imaging has to be problem-oriented. The most important information on a requisition form, apart from the child’s name and age, is the question to be answered. The next most important items are the legible name and contact number of the person asking the question. In this age of computerization, conversation on the telephone or, better still, face to face is invaluable when a complicated situation arises. The paediatric radiologist should do the least investigation to achieve the most information about a child’s condition.
In concluding this introductory section, we note that reliable evidence-based information, to support many of the recommendations that are in print regarding appropriate algorithms for paediatric imaging, is difficult to access. There are few clinical situations and ethical guidelines that allow performance of several studies on a child simply to compare their utility. Clinical information is frequently incomplete; ‘comparable’ studies are rarely comparable. Furthermore, local traditions, practitioners in the area, available facilities and economic conditions usually prevail in decision-making. If you have any question about the appropriate imaging for your patient, ask for help from a radiologist who is experienced in paediatric diagnosis. With these caveats, and encouragement to you, the reader, to challenge algorithms when they seem less than sensible, the following sections outline appropriate imaging considerations for particular situations, and are arranged anatomically, for easy reference.
• Find out whether there are clinical guidelines for your department/institution. They may include guidelines for imaging.
• Generally, in a complicated case, it is best to begin with uncomplicated imaging, such as plain radiographs. They may provide the diagnosis; if not, they can help point the way to other studies.
• Provide relevant clinical information to the radiologist when requesting examinations.
• Consult a paediatric radiologist, or one experienced in paediatric diagnosis, when you have questions about the imaging of a specific clinical problem.
• Be prepared to answer questions from parents regarding ionizing radiation and know whom to ask for more information.
• Be familiar with the preparation, immediate complications and sequelae of invasive imaging procedures.
Neurology
Acute head trauma, all ages
• CT of the brain following the CHALICE (Children’s Head injury ALgorithm for the prediction of Important Clinical Events) rule (Fig. 7.1.1).
• Use NEXUS clinical prediction rule (altered neurological function, intoxication, midline posterior spinal tenderness, and/or distracting injury) to identify which children and adolescents should have imaging of the cervical spine.
• MRI when CT findings are negative in an infant or child with persisting abnormal neurological signs.
Fig. 7.1.1 Axial computed tomograms of the brain in an 8-year-old involved in a high-speed motor vehicle accident: (A) bone window; (B) brain window. The Glasgow Coma Scale score on arrival in the emergency department was 7/15. The scan reveals a large comminuted and depressed skull fracture involving right frontal and temporal bones with a small underlying extra-axial haematoma and midline shift to the left.
Notes
Radiographs of the skull are poorly predictive of intracranial pathology. They are of use in the situation of suspected inflicted injury (non-accidental injury) where multiple fractures or fractures of different ages are helpful in establishing the diagnosis.
For suspected injury to the neck, a lateral radiograph, in collar, can be followed by anteroposterior (AP) view of the cervical spine and odontoid view. Plain radiography can detect 94% of fractures. In infants and young children, the immature anatomy of the cervical spine along with the head being relatively large in proportion to the neck, results in the upper cervical spine (C1–C3 vertebral bodies) being more commonly injured than the lower spine, as seen in older children and adults. For this reason, careful scrutiny of the upper cervical spine on plain radiographs is needed. In some centres, the upper cervical spine is scanned at the time of brain imaging in the setting of significantly altered conscious state and major trauma. CT of the entire cervical spine, often routinely acquired in adults, includes the thyroid gland and is used in children only when there is credible utility.
Newborn
• Portable ultrasonography when screening for germinal matrix/intraventricular and intraparenchymal haemorrhage in the premature infant.
• CT for suspected acute extra-axial collections (bleeding, infection).
• MRI for suspected non-haemorrhagic parenchymal disease (e.g. hypoxic–ischaemic insult, neuronal migrational disorders).
Notes
Follow local protocols for timing/frequency of ultrasonographic screening for premature infants. Generally, a scan is performed at 3–4 days of age in infants who weigh less than 1500 g unless there is clinical concern that prompts an earlier study.
Timing of MRI for hypoxic–ischaemic insult depends on the availability of resources. One scan at 24–48 hours of life followed by a scan at 7–10 days of life is ideal for both assessment of timing of injury and severity of injury; a compromise is one scan at 3–4 days.
Seizures
For acute non-febrile seizure:
Notes
Because of the variable availability of MRI, local protocols should be consulted for neurological diagnostic work-up. Imaging following a single generalized non-febrile seizure in children is an area of controversy. Focal non-febrile seizures will usually be investigated by electroencephalography (EEG) and cerebral imaging. Temporal lobe lesions are more likely to be shown on MRI than CT.
Simple febrile seizure (single or recurrent) does not usually warrant imaging.
Altered conscious state, suspected tumour and stroke
MRI is the preferred diagnostic test for a child presenting with acute neurological signs (Fig. 7.1.2). If MRI is not available, CT can diagnose established infarction, haemorrhage and most tumours. MRI is the preferred test as it detects infarction earlier, can provide perfusion imaging in the setting of stroke and angiographic sequences in the setting of vascular disease. It also provides more detailed imaging (in multiple planes) for surgical planning in the setting of tumour.
Fig. 7.1.2 Magnetic resonance images of brain and spine in this 15-year-old shows a large tumour in the posterior fossa. Sagittal T2 (A), axial T2 (B), axial T2 level with the lateral ventricles (C), axial T1 post-contrast (D) and sagittal T1 post-contrast through the spine (E, F) are representative images from the study. On the T2 (water) weighted images, the tumour is predominantly bright in keeping with ‘fluid’ contents; soft tissue is present along the posterior surface. This soft tissue enhances on the contrast sequences, in keeping with mural nodules. The axial image at the level of the lateral ventricles reveals early ventricular dilatation and transependymal flow of cerebrospinal fluid, in keeping with early obstruction (due to pressure from the tumour on the cerebral aqueduct and fourth ventricle). No spinal metastases are identified. The tumour was a pilocytic astrocytoma.
Developmental delay
MRI is the preferred examination as it can assess white-matter volume and distribution along with grey-matter abnormalities (sulcation, heterotopias and other migration anomalies) better than CT. Timing for the examination needs to be considered, with children under 2 years undergoing progressive myelination (sometimes making white-matter signal assessment more difficult) and most children below 6–7 years (and often later when developmentally delayed) requiring general anaesthesia for the examination.
Spinal dysraphism
Notes
Spinal ultrasonography is unnecessary in cases of classical myelomeningocele; however, the accompanying Chiari II malformation of the brain can be assessed with ultrasonography or MRI, and the degree of ventricular dilatation after shunt placement or third ventriculostomy can be monitored.
For neonates with midline dermal abnormality, mass, abnormal sacral cleft/dimple, sacral agenesis, anorectal malformation or vertebral anomaly on plain radiographs, ultrasonography can assess the spinal canal, spinal cord and cauda equina. Ossification of posterior elements as the child grows obscures the sonographic window and, when this occurs, MRI is used to assess the anatomy.
Cardiology
Suspected congenital heart disease
(E.g. abnormal prenatal ultrasonography, cyanosis, murmur, unexplained oxygen requirement.)
• Posteroanterior (PA) and lateral chest radiographs to include upper abdomen (Fig. 7.1.3)
• CT or MRI for anatomical detail of vascular rings as necessary.
Fig. 7.1.3 The anteroposterior radiograph of this newborn, who presented in severe respiratory distress, has features that identify cardiac abnormality as the likely aetiology, although the heart is normal in size. Note that the nasogastric tube is entering a right-sided stomach, whereas the apex of the heart is left-sided. The tracheostomy tube is deviated leftward by a right-sided aortic arch and the umbilical artery catheter is in a right-sided descending aorta. This infant had heterotaxy syndrome with multiple cardiac anomalies. The venous congestion that is apparent in the lungs is from obstructed anomalous pulmonary venous return, the major cause of the respiratory distress.
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