Craniopharyngioma (Fig. 20)
Usually histologically benign locally aggressive lesions which arise from squamous epithelial rests along Rathke’s cleft, occurring in a bimodal age distribution, in children (<10 years) and adults (>40 years). Craniopharyngioma’s account for 3 % of all intracranial tumours. They are categorised into adamantinomatous and squamous-papillary types. The adamantinomatous type is more common and has a bimodal age distribution occurring in children and adults, whereas the squamous-papillary type usually occurs in adults. Although histologically benign, their insinuating pattern of growth makes complete surgical excision very difficult and not often possible.
The adamantinomatous type usually demonstrates circumscribed lobulated margins on MRI, with the suprasellar location seen at a greater frequency than intrasellar. They show variable low, intermediate and/or high signal on T1WI and T2WI, with or without a nodular or rim of contrast enhancement. They may contain cysts, lipid components and calcifications. In children, the cystic parts tend to predominate, and frequently contain highly proteinaceous fluid, which may be bright on T1WI [49].
Glioma (Fig. 21)
Lesions which may occur anywhere along the optic pathway and account for 4-6 % of brain tumours in children, with a median age of diagnosis from 5-9 years. Approximately half of patients with optic pathway gliomas have neurofibromatosis type 1 (NF1), with symptomatic lesions only occurring in 1-5 % of patients [50]. Most tumours are slow growing WHO grade I-II astrocytomas, often the pilocytic type, however tumours may show a wide spectrum of progression. On MRI, there is fusiform and/or nodular enlargement of the optic chiasm and/or optic nerves with thickening of the third ventricular floor and hypothalamus. They usually have low-intermediate signal on T1WI, intermediate-high signal on T2WI and variable contrast enhancement. Larger lesions may have cystic components and grow directly into the pituitary stalk [1].
Pilomyxoid astrocytoma (Fig. 22)
Rare neoplasms (WHO grade II) which contain prominent mucoid matrix and angiocentric pattern of bipolar neoplastic astrocytes. Typically, they occur in children in the first and second decades, with a mean age of 7 years [51]. They tend to occur at a younger age and are more aggressive with a higher rate of local recurrence than pilocytic astromcytomas. They have a strong predilection for the hypothalamus and suprasellar cistern [52]; however, they can also occur in the thalamus, cerebellum, brainstem, temporal lobe and spinal cord. The presence of intra-tumoural haemorrhage is the most prominent imaging characteristic to suggest a pilomyxoid astrocytoma over a pilocytic astrocytoma, 25 % versus 8 %, respectively [51]. MRI demonstrates a solid/cystic focal lesion with low-intermediate signal on T1WI, high signal on T2WI and FLAIR, variable contrast enhancement and may show increased ADC values.
Germ cell tumours—germinoma (Fig. 23)
Extra-gonadal germ cell tumours include: germinoma (most common), mature teratoma, malignant teratoma, yolk sac tumour, embryonal carcinoma and choriocarcinoma. They account for 0.6 % of primary intracranial tumours, with an incidence of 0.09 per 100,000. Peak incidence occurs between 10 and 14 years, with 90 % occurring in patients less than 25 years. They occur more frequently in males. Prognosis depends on the histological subtype, with a 10 year survival rate for germinomas of over 85 %. Other germ cell tumours have a lower survival rate, particularly those containing non-germinomatous malignant cells. Tumours often have intermediate signal on T1WI, slightly high signal on T2WI, and show contrast enhancement. They are typically centred on the pituitary stalk with the posterior pituitary bright spot often absent. They may look similar to Langerhans’ cell histiocytosis (LCH) or lymphocytic hypophysitis [53]. Contrast-enhancing disseminated subarachnoid and/or intraventricular tumour may occur through CSF seeding.
Germ cell tumours—teratoma (Fig. 24)
The second most common type of germ cell tumour behind a germinoma. They occur in children, with a greater frequency in boys, accounting for 0.5-1.5 % of all childhood brain tumours, and >50 % of intracranial tumours in infants less than 2 months [54, 55]. Histologically, they demonstrate benign or malignant types, composed of derivatives of ectoderm, mesoderm and/or endoderm. On MRI, they are relatively circumscribed lesions, most common in decreasing order frequency within the pineal region, suprasellar region, and third ventricle. On MRI, they demonstrate heterogeneous signal intensity on T1WI, T2WI and post-contrast sequences, with suppression of signal on fat-saturated imaging. Marked enhancement of the solid portions of the tumour is a key feature in distinguishing a mature versus a malignant teratoma, with malignant tumours demonstrating enhancement. Additionally, they typically present without perilesional high T2 signal due to intact capsules and an undamaged blood-brain barrier [55]. They may contain calcifications with low signal on T1WI and T2WI.
Tuberculous meningitis (Fig. 25)
Mycobacterium tuberculosis infection remains a major international concern, with increasing burden of disease in developed nations and young children being the most susceptible [56, 57]. MRI is increasingly being used to evaluate patients with suspected tuberculous meningitis, to aid in rapid early diagnosis [58]. MRI findings include basal leptomeningeal enhancement secondary to multiple granulomas and pial tuberculomas. The granulomas can be either non-caseating with homogeneous enhancement or caseating with rim enhancement (both T1 hypointense and T2 hyperintense) [59].
Osteosarcoma (osteogenic sarcoma)
Rare malignant bone tumour comprised of proliferating neoplastic spindle-cells which produce osteoid and/or immature tumoural bone. They usually involve the endochondral bone-forming portions of the skull-base and are locally invasive with high metastatic potential. Over 75 % of osteosarcomas are diagnosed in patients between 8 and 25 years, and account for less than 1 % of all head and neck malignant tumours in the paediatric population [60]. They occur in children as primary tumours and adults associated with Paget’s disease, irradiated bone, chronic osteomyelitis, osteoblastoma, giant cell tumour and fibrous dysplasia. MRI will show a destructive lesion involving the skull base, with low-intermediate signal on T1WI, mixed low, intermediate or high signal on T2WI. Usually contains matrix mineralisation/ossification with low signal on T2WI and heterogeneous contrast enhancement.
Ewing sarcoma
Bone tumour usually occurring between ages 5 and 30 years, with a greater prevalence in males and is the second most common primary bone malignancy affecting adolescents and young adults. It most commonly arises in the trunk and diaphysis of long bones, with only 4 % of cases within the head and neck region [61]. Tumours are locally invasive with a high metastatic potential. MRI will show a destructive lesion involving the skull base, with low-intermediate signal on T1WI, mixed low, intermediate or high signal on T2WI, with or without matrix mineralisation showing low signal on T2WI and heterogeneous contrast enhancement.
Esthesioneuroblastoma (olfactory neuroblastoma) (Fig. 26)
These malignant neoplasms of neuroectodermal origin, arise from olfactory epithelium in the upper nasal cavity and cribiform region. Tumours consist of immature neuroblasts with variable nuclear pleomorphism, mitoses and necrosis. Tumour cells occur in aneurofibrillary intercellular matrix. There is a bimodal age of occurrence in adolescents (11-20 years) and adults (50-60 years) [62], with a greater prevalence in males. On MRI, they present as a locally destructive lesion with low-intermediate signal on T1WI, intermediate-high signal on T2WI and prominent contrast enhancement. They are commonly located within the superior nasal cavity, ethmoid air cells with occasional extension into other paranasal sinuses, orbits, anterior cranial fossa, and cavernous sinuses. Positron emission tomography (PET)/computed tomography (CT) can be useful for staging of disease and detection of metastases [63].
Fibrous dysplasia (Fig. 27)
Benign medullary fibro-osseous lesion, which can involve a single site (mono-ostotic) or multiple locations (poly-ostotic), results from developmental failure in the normal process of remodelling primitive bone to mature lamellar bone with zones of immature trabeculae within dysplastic fibrous tissue. Craniofacial bones are involved in 10-20 % of patients with the monostotic disease and in 50 % of patients with the polyostotic form [64]. Patients range in age from <1 year to the eighth or ninth decades of life with 75 % occurring before the age of 30 years. MRI features depend on the proportions of bony spicules, collagen, fibroblastic spindle cells, haemorrhagic and/or cystic changes, and if associated pathological fractures are present. Lesions are usually well-circumscribed and have low-intermediate signal on T1WI and proton-density weighted imaging (PDWI). On T2WI, lesions have variable mixtures of low, intermediate and/or high signal often surrounding by a low signal rim of variable thickness. Internal septations and cystic changes are seen in a minority of lesions. Bone expansion with thickened and/or thinned cortex can be seen. Lesions show contrast enhancement varying in degree and pattern [65].
McCune-Albright syndrome
Refers to an uncommon disorder in which polyostotic fibrous dysplasia is associated with pigmented cutaneous macules (café-au-lait spots) with irregular indented borders, precocious puberty, and/or other endocrine disorders such as acromegaly, hyperthyroidism, hyperparathyroidism, and/or Cushing’s syndrome. McCune-Albright syndrome is uncommon and occurs in 2-3 % of patients with polyostotic fibrous dysplasia [66].
Langerhans’ cell histiocytosis (Fig. 28)
Disorder of the reticuloendothelial system in which bone-marrow-derived dendritic Langerhans’ cells infiltrate various organs as focal lesions or in diffuse patterns. Langerhans cells have eccentrically located ovoid or convoluted nuclei within pale to eosinophilic cytoplasm. Lesions often consist of Langerhans cells, macrophages, plasma cells and eosinophils. Prevalence of 2 per 100,000 children less than 15 years [67], with only a third of the lesions occurring in adults. Localised lesions (eosinophilic granuloma) can be single or multiple in the skull, usually at the skull base. There is a striking preference for LCH to involve regions without a blood brain barrier, with the hypothalamic-pituitary axis the most frequently involved intracranial region [68], with patients often presenting with diabetes insipidus. MRI will typically show a fusiform or lobulated lesion with intermediate signal on T1WI and T2WI involving the pituitary stalk. The pituitary stalk is usually >3 mm in thickness [69], and is often associated with a loss of high signal on T1WI of the posterior pituitary. Lesions involving the pituitary usually show contrast enhancement [70].
Lymphocytic hypophysitis (Fig. 29)
Rare autoimmune inflammatory process involving the pituitary gland confirmed by biopsy showing varying degrees of lymphocytic infiltration and plasma cells with fibrotic changes without multinucleated giant cells. They are more prevalent in women (80 %) who are in late pregnancy or postpartum but can also occur in children. Clinical findings include headaches and pituitary hormonal dysfunction, with deficiency of ACTH in adults and growth hormone in children [71]. Lesions can be associated with Hashimoto thyroiditis, polymyositis, pernicious anaemia, atrophic gastritis, psoriasis, systemic lupus erythematosus, adrenalitis and/or ovarian failure. MRI shows a slightly lobulated lesion with intermediate signal on T1WI, heterogeneous low-intermediate and high signal on T2WI involving the pituitary lobe with thickened pituitary stalk, and prominent homogeneous or heterogeneous contrast enhancement involving the pituitary gland, pituitary stalk and dura. A characteristic finding includes a parasellar T2 dark sign, which helps in determining lymphocytic hypophysitis from a pituitary adenoma [72].
Rosai-Dorfman disease
A rare benign histiocytosis, also referred to as sinus histiocytosis with massive lymphadenopathy. It usually occurs in children and young adults, with a male-female ratio of 3:2 [73]. Patients may present with painless adenopathy with one or more extranodal sites. When there is intracranial involvement, patients may present with headaches, seizures, visual disturbances, numbness and/or paraplegia. Involvement of the central nervous system (CNS) occurs in 4 % and usually involves the intracranial or spinal dura, rarely occurring in the sella and suprasellar region. On MRI, suprasellar/sellar masses tend to be isointense on T1WI and hypointense/isointense on T2WI with homogeneous enhancement, and may be mistaken as a meningioma. A central hypointensiity on T2WI, thought to be related to free radicals release by inflammatory macrophages, may help distinguish from a meningioma [74, 75].
Lymphomatous leptomeningial metastases (Fig. 30)
Primary CNS lymphoma is extremely rare in children, and usually occurs as intra-axial lesions and infrequently as leptomeningeal tumour. Most cases are non-Hodgkin, high-grade B-cell lymphomas. There is a strong link in patients with AIDS or primary immunodeficiency [76]. Leptomeningeal metastases can also be seen with acute lymphoblastic leukaemia, medulloblastomas, germ-cell tumours, ependymomas and malignant gliomas [77].
Vascular malformations (Fig. 31)
Numerous vascular malformations can be seen in the sellar/juxtasellar regions, including arterial aneurysms (saccular or giant aneurysms of the internal carotid artery [ICA]), cavernous carotid fistulas, arterial-venous malformations, haemangiomas or cavernous malformations. Cavernous malformations are the most common CNS vascular lesions in children, although the incidence is lower than in adults [78]. Compared to adults, these lesions are more aggressive in children with high rates of growth and haemorrhage, and a mean age of presentation of 9-10 years [79]. MRI findings will show single or multiple multilobulated intra-axial lesions with a peripheral or irregular zone of low signal on T2WI and T2* secondary to haemosiderin, with a central zone of variable signal on T1WI and T2WI depending on ages of haemorrhagic components. Contrast enhancement is usually absent.