Paediatrics: from basic to advanced imaging

Normal variations of development of the skeleton are well documented when it comes to radiographs; with the introduction of MRI there are many more features to consider that are often less known. Normal variations are common and these can simulate disease. It is important for radiologist to be aware of this and an overview of cases will be presented. Protocols for MRI imaging in paediatric MSK imaging will be discussed.

Arthritis of the temporomandibular joint (TMJ) is common in children and adolescents with juvenile idiopathic arthritis (JIA). Early treatment is warranted to prevent severe growth disturbances and joint deformities. As TMJ arthritis is often clinically silent, MRI with contrast-enhancement has been considered to be the most reliable method to assess signs of inflammation. To reliably guide therapeutic decisions and monitor outcomes, it would be of utmost importance to clearly define the MR characteristics of a normal TMJ as a basis for the assessment of minor pathologies. However, similar to other small joints in children, we are just beginning to understand its developmental, physiological and anatomical characteristics as well as its reaction to inflammatory diseases and their treatment. Recent studies on normal TMJ in children have revealed age dependent changes in shape and angulation of the mandibular condyle as well as typical time-intensity curves of contrast-enhancement in the soft joint tissue and the condyle. To date, the differentiation between normal synovial findings and mild signs of synovitis remains challenging. This lecture presents typical MR images of normal and inflamed TMJs in children and adolescents, including age dependent anatomical variations. It discusses the available data on possible cut-offs between normality and pathology, the impact of the temporal dynamics of contrast-enhancement, and presents findings that can mimic arthritis. It summarizes the minimum requirements of image quality and spatial resolution, the best image orientation, as well as the advantages of fat suppression and subtraction analysis in contrast-enhanced imaging.

Musculoskeletal injuries are common in children. They account for 15-20% of admissions to the ED. Children have an immature skeleton with unique biomechanical features and a stronger, thicker and richly vascularized periosteum. Paediatric fractures may present with unique patterns including plastic deformation, buckled fractures, and greenstick fractures. Fractures in children include the ones involving the physis (epiphysiolysis) and we will review their classification and prognosis after treatment. We will review these injuries making special remarks on the imaging techniques used for their correct diagnosis and treatment planning.

Hypomyelinating leukodystrophies are frequent and have to fulfill both criteria of significant and permanent myelin deficit. Hypomyelination can be diagnosed by MRI. Care should be taken that, in children younger than 12 months of age, two MRIs at least 6 months apart are needed to make the diagnosis, in order to differentiate hypomyelination from the more frequent finding of delayed myelination. Severe atrophy in young children usually indicates a primary grey matter disorder with secondary myelin deficit and comes with its own differential diagnosis. MRI findings in hypomyelination consist of diffuse white matter signal hyperintensity on T2-weighted images and, depending on the amount of myelin deposited, hypo-, iso- or hyperintensity on T1-weighted images. Additional features may be cerebellar atrophy, signal and volume changes of deep grey matter structures and signal changes of the pyramidal tracts. These findings, when present, help differentiating the diverse hypomyelinating conditions. Some entities (e.g., 4H syndrome, H-ABC, HBSL, HCC, Salla disease, Pelizaeus-Merzbacher like disease⋯) can be easily diagnosed by MRI pattern recognition. This allows direct confirmation of the diagnosis by sequencing of the appropriate gene(s). Other hypomyelinating disorders do not come with extra features, necessitating a different genetic approach.

To understand congenital abnormalities of the brain, it is important to be familiar with the embryologic development. Neuronal proliferation, migration and cortical organization are important steps in the development of the cortex. Group I disorders of cortical development are disorders of neuronal and/or glial apoptosis or proliferation. Among these are congenital microcephalies (I.A), congenital megalencephalies (I.B), and diffuse or focal cortical dysgenesis or dysplasia (I.C). Microlissencephaly is characterised by a reduced gyration and microcephaly. Hemimegalencephaly is a hamartomatous overgrowth of one cerebral hemisphere or parts thereof. Group II disorders are disorders of neuronal migration. Among these are periventricular (subependymal) heterotopia (II.A), lissencephalies (II.B), focal subcortical heterotopia (II.C), or disorders of terminal migration, e.g. cobblestone lissencephalies (II.D). Heterotopia are defined as areas of grey matter in an ectopic location; they are isointense to cortex. Group III disorders are disorders of postmigrational development. Among these are polymicrogyria with schizencephaly (III.A), polymicrogyria without clefts or calcifications (III.B), focal cortical dysplasia (III.C), or postmigrational microcephaly. In schizencephaly, there is a cleft that extends from the ependymal to the cortical surface and that is lined by dysplastic grey matter. In polymicrogyria, there are too many too small gyri and sulci. Agenesis and dysgenesis of the corpus callosum are common disorders with a wide clinical spectrum. They may be associated with other congenital abnormalities. Important infratentorial congenital abnormalities include Chiari malformations, cystic abnormalities of the posterior fossa/the Dandy Walker spectrum, molar tooth malformations/Joubert syndrome and dysplastic cerebellar gangliocytoma/Lhermitte-Duclos syndrome.

Central nervous system tumours are the most common group of solid tumours in the paediatric age and the main cause of death from solid tumours in childhood. The clinical onset commonly includes signs and symptoms related to increased intracranial pressure, gait disorders, or cranial nerve deficits. Magnetic resonance (MR) imaging has an important role in characterizing these lesions, planning surgery, and follow-up. Advanced MR techniques, especially diffusion, help to predict the histological type and degree of malignancy, and facilitate the diagnosis of recurrence during follow-up. Identification of taurine within the tumour on spectroscopy is typical of medulloblastoma. Perfusion imaging, including dynamic susceptibility of contrast perfusion and arterial spin-labelling perfusion are particularly useful in differentiating recurrence versus radiation necrosis. Specifically referring to cerebellar tumours, they may be associated with diverse inherited cancer syndromes such as neurofibromatosis type 1 (pilocytic astrocytoma), Von Hippel Lindau syndrome (hemangioblastoma), Cowden syndrome (gangliocytoma), Li-Fraumeni syndrome (astrocytoma, medulloblastoma) and Gorlin and Turcot syndromes (medulloblastoma). In the differential diagnosis, conditions that can mimic neoplasms should be included, such as infections (e.g., rhombencephalitis), autoimmune processes (e.g., acute disseminated encephalomyelitis and multiple sclerosis), and radionecrosis. Proper recognition of these entities can help to prevent unnecessary surgery. Of note, certain previous histological classifications, such as that of medulloblastoma, are being replaced by new classifications based on genetic characteristics, which enable better prediction of tumour aggressiveness and help to guide therapy according to the specific tumour type. Current research focuses not only on prolonging survival, but also on improving long-term quality-of-life.

Age-related changes depending on neuronal migration, gyration and myelination have been described in the paediatric brain. These changes are responsible for the different imaging patterns as the paediatric brain matures. Conventional MR sequences along with diffusion tensor imaging (DTI) and functional MRI (f-MRI) offer important information regarding the structural and functional maturation of the brain. Congenital malformations of the brain can be assessed with conventional sequences, but DTI using ADC, FA maps and tractography and f-MRI offer valuable structural and functional information and help in the detection of additional malformations. The incidence and localization of different brain tumours depend on the age. Conventional MRI offers useful information, but further evaluation with DTI, susceptibility contrast-enhanced perfusion imaging, spectroscopy and f-MRI is very important in the diagnostic workup and before the application of any therapeutic scheme. A number of brain tumours may metastasize through CSF and additional imaging of the spinal canal to look for seeding metastases is necessary. MRI is the modality of choice for imaging the paediatric brain; nevertheless, in neonates and infants, brain ultrasound with colour Doppler should be the first imaging approach. Brain ultrasound offers valuable information provided that a state-of-the-art technique is applied.

Normal pulmonary and cardiovascular development will be reviewed leading on to a comprehensive overview of congenital disorders of the lung and mediastinum and their imaging features. This will include bronchopulmonary sequestration, congenital pulmonary adenomatoid malformations and thoracic duplication cysts along with an overview of congenital cardiac disorders. The role of imaging in neonatal respiratory distress syndrome will be reviewed along with a discussion of when and how to use CT in particular cases. Lastly, paediatric thoracic neoplastic disease will be comprehensively reviewed.

Paediatric abdominal diseases are highly age dependent. Newborn infants may have congenital diseases as Hirschsprung's disease or meconium ileus or may develop necrotising enterocolitis, incarcerated inguinal herniation and midgut volvulus. Infants and preschool children have intussusceptions, urinary tract infections and (rare) haemolytic uraemic syndrome. Children and adolescents have appendicitis, genito-urinary infections, ovarian torsion and Henoch Schonlein purpura. Abdominal neoplasms can occur at any age, even at birth. Considering radiation dose in children and the excess value of ultrasonography (US) in
small individuals, US plays an important role as initial diagnostic modality in paediatric abdominal emergencies. Sensitivity and specificity for US in diagnosing intussusception, midgut volvulus, urinary tract abnormalities and appendicitis is over 90%. Conventional abdominal radiographs or fluoroscopy are valuable in Hirschsprung's disease, meconium ileus, malrotation and necrotising enterocolitis. I consider CT as an additional technique when the initial techniques (US and conventional radiography) are inconclusive. MRI is seldom indicated in paediatric patients with abdominal emergencies because of motion artefacts in anxious children and sometimes limited MR capacity. However, in children with abdominal neoplasms it is the modality of choice.